Nutrient Cycling in Pastures

Abstract

Good pasture management practices favor effective use and recycling of nutrients. Nutrient cycles important in pasture systems are the water, carbon, nitrogen, and phosphorus cycles. This publication provides basic descriptions of these nutrient cycles then provides guidelines for managing pastures to enhance nutrient cycling efficiency for productive forage and livestock growth, soil health, and water quality.

Table of Contents

• Introduction and Summary
• Publication Overview
• Chapter 1: Nutrient Cycle Components, Interactions, and Transformations
  • Water Cycle
  • Carbon Cycle
  • Nitrogen Cycle
  • Phosphorus Cycle
  • Secondary Nutrients
• Chapter 2: Nutrient Availability in Pastures
  • Soil Parent Material
  • Soil Chemistry
  • Prior Management Practices
  • Soil Compaction
  • Organic Matter
  • Soil pH
  • Timing of Nutrient Additions
• Chapter 3: Nutrient Distribution and Movement in Pastures
  • Pasture Nutrient Inputs and Outputs
  • Manure Nutrient Availability
  • Pasture Fertilization
  • Grazing Intensity
  • Diversity and Density of Pasture Plants
• Chapter 4: The Soil Food Web and Pasture Soil Quality
  • Diversity of the Soil Food Web
  • Organic Matter Decomposition
  • Primary Decomposers
  • Secondary Decomposers
  • Soil Organisms and Soil Health
• Chapter 5: Pasture Management and Water Quality
  • Risk Factors for Nutrient Losses
  • Pathogens
  • Nitrate Contamination
  • Phosphorus Contamination
  • Subsurface Drainage
  • Riparian Buffers
  • Riparian Grazing
• References
• Resource List
• Web Resources

Introduction and Summary

As a pasture manager, what factors do you look at as indicators of high production and maximum profitability? You probably look at the population of animals stocked within the pasture. You probably look at vigor of plant regrowth. You probably also look at the diversity of plant species growing within the pasture and whether the plants are being grazed uniformly. But do you know how much water seeps into your soil or how much runs off the land into gullies or streams? Do you monitor how efficiently your plants are taking in carbon and forming new leaves, stems, and roots through photosynthesis? Do you know how effectively nitrogen and phosphorus are being used, cycled, and conserved on your farm? Are most of these nutrients being used for plant and animal growth? Or are they being leached into the groundwater or transported through runoff or erosion into lakes, rivers, and streams? If so, do you know how to change your pasture management practices to decrease these losses and increase the availability of these nutrients to your forages and animals?

Figure 1. Nutrient Cycles in Pastures

Healthy plant growth provides plant cover over the entire pasture. Cover from growing plants and plant residues protects the soil against erosion while returning organic matter to the soil. Organic matter provides food for soil organisms that mineralize nutrients from these materials and produce gels and other substances that enhance water infiltration and the capacity of soil to hold water and nutrients.

Effective use and cycling of nutrients is critical for pasture productivity. As indicated in Figure 1 above, nutrient cycles are complex and interrelated. This document is designed to help you understand the unique components of water, carbon, nitrogen, and phosphorus cycles and how these cycles interact with one another. This information will help you monitor your pastures for breakdowns in nutrient cycling processes and then identify and implement productive pasture management practices to maximize effective nutrient cycling.

Water

Water is necessary for plant growth, for dissolving and transporting plant nutrients, and for the survival of soil organisms. Water can also be a destructive force, causing soil compaction, nutrient leaching, runoff, and erosion. Management practices that facilitate water movement into the soil and build the soil's capacity to store water will conserve water for plant growth and water recharge while minimizing its potential to cause nutrient losses. Water-conserving pasture management practices include:

• Minimizing soil compaction by not overgrazing pastures or using paddocks that have wet or saturated soils.
• Maintaining a complete coverage of forages and residues over all paddocks by not overgrazing pastures and by implementing practices that encourage animal movement across each paddock.
• Ensuring that forage plants include a diversity of grass and legume species with a variety of root systems capable of obtaining water and nutrients throughout the soil profile.

Carbon

Carbon is transformed from carbon dioxide into plant cell material through photosynthesis. It is the basic structural material for all cell life, and following the death and decomposition of cells it provides humus and other organic components that enhance soil quality. Plant nutrients such as nitrogen and phosphorus are chemically bound to carbon in organic materials. For these nutrients to become available for plant use, soil organisms need to break down the chemical bonds in a process called mineralization. If the amount of carbon compared to other nutrients is very high, more bonds will need to be broken and nutrient release will be slow. If the amount of carbon compared to other nutrients is low, fewer bonds will need to be broken and nutrient release will proceed relatively rapidly. Rapid nutrient release is preferred when plants are growing and are able to use the nutrients released. Slower nutrient release is preferred when plants are not actively growing (such as in the fall or winter) or if the amount of nutrients in the soil is already in excess of what plants can use. Pasture management practices that favor effective carbon use and cycling include:

• Maintaining a diversity of forage plants with a variety of leaf shapes and orientations to enhance photosynthesis and a variety of root growth habits to enhance nutrient uptake. A diversity of forages will provide balanced diet for grazing animals and soil organisms with a variety of food sources.
• Promoting healthy regrowth of forages by including a combination of grasses with both low and elevated growing points and by moving grazing animals frequently enough to minimize the removal of growing points.
• Maintaining a complete coverage of forages and residues over all paddocks to hold soil nutrients against runoff and leaching losses and ensure a continuous turnover of organic residues.

Nitrogen

Nitrogen is a central component of cell proteins and is used for seed production. It exists in several chemical forms and various microorganisms are involved in its transformations. Legumes, in association with specialized bacteria called rhizobia, as well as algae, are able to transform atmospheric nitrogen into a form available for plant use. Nitrogen in dead organic materials becomes available to plants through mineralization. Nitrogen can be lost from the pasture system through the physical processes of leaching, runoff, and erosion, the chemical process of volatilization, the biological process of denitrification, and through residue burning. Since nitrogen is needed in high concentration for forage production and it can be lost through a number of pathways, this nutrient is often the limiting factor in forage and crop production. Productive pasture management practices enhance the fixation and conservation of nitrogen while minimizing the potential for nitrogen losses. Practices that favor effective nitrogen use and cycling in pastures include:

• Maintaining stable or increasing percentages of legumes by not overgrazing pastures and minimizing nitrogen applications, especially in the spring.
• Protecting microbial communities involved in organic matter mineralization by minimizing practices that promote soil compaction and soil disturbance such as grazing wet soils, tillage, and cultivation.
• Incorporating manure and nitrogen fertilizers into the soil, and never applying these materials to saturated, snow-covered, or frozen soils.
• Avoiding pasture burning. If burning is required, it should be done very infrequently and then using a slow fire under controlled conditions.
• Applying fertilizers and manure according to a comprehensive nutrient management plan.

Phosphorus

Phosphorus is used for energy transformations within cells and is essential for plant growth. It is often the second most limiting mineral nutrient to plant production not only because it is critical for plant growth, but also because soil chemical bonds on soil particles hold the majority of phosphorus in forms not available for plant uptake. Phosphorus is also the major nutrient needed to stimulate the growth of algae in lakes and streams. Consequently, the inadvertent fertilization of these waterways with runoff water from fields and streams can cause degradation of water quality for drinking, recreational, or wildlife habitat uses. Regulations on the use of phosphorus-containing materials are becoming more widespread as society becomes increasingly aware of potential impacts agricultural practices have on water quality. Pasture management practices need to balance the need to ensure sufficient availability of phosphorus for plant growth with the need to minimize movement of phosphorus from fields to streams. Pasture management practices that protect this balance include:

• Minimizing the potential for compaction while providing organic inputs into the system to enhance activities of soil organisms and phosphorus mineralization.
• Incorporating manure and phosphorus fertilizers into the soil and never applying these materials to saturated, snow-covered, or frozen soils.
• Relying on soil tests, phosphorus index guidelines, and other nutrient management practices when applying fertilizers and manure to pastures.

Soil Life

Soil is a matrix of pore spaces filled with water and air, minerals, and organic matter. Although comprising only 1-6% of the soil, living organisms and decomposed organic materials are essential for providing plant nutrients, developing soil structure, holding water, and mediating nutrient transformations. Soil organic matter is composed of three components, stable humus, readily decomposable materials, and living organisms, also described as the very dead, the dead, and the living components of soil.(1)

Living organisms in soil include larger fauna such as moles and prairie dogs, macroorganisms such as insects and earthworms, and microorganisms including fungi, bacteria, yeasts, algae, protozoa, and nematodes. These living organisms break down the readily decomposable plant and animal material into nutrients, which become available for plant uptake. Organic matter residues from this decomposition process are subsequently broken down by other organisms until all that remains are complex compounds that are difficult to decompose. These complex end products of decomposition are known as humus.

Figure 2. Components of Soil Organic Matter

Soil contains 1-6% organic matter. Organic matter contains 3-9% active microorganisms. These organisms include plant life, bacteria and actinomycetes, fungi, yeasts, algae, protozoa, and nematodes.

Humus, as well as fungal threads, bacterial gels, and earthworm feces form glues that hold soil particles together into forms, called aggregates, that give soil structure, enhance soil porosity, and allow water, air, and nutrients to flow through the soil. These residues of soil organisms also enhance nutrient and water holding capacity of soil. Lichen, algae, fungi, and bacteria form biological crusts over the soil surface. These crusts are important, especially in more arid rangelands, for enhancing water infiltration and providing nitrogen fixation.(2)

Maintaining effective nutrient cycling in the soil is highly dependent on maintaining an active and diverse population of soil organisms. Maintaining a substantial population of legumes in the pasture insures biological nitrogen fixation by bacteria associated with legume roots. Pasture management practices that favor populations of soil organisms by maintaining the soil as a habitat favorable for their growth and multiplication include:

• Maintaining a diversity of forages that promote a diverse population of soil organisms by providing them with a varied diet.
• Adding organic matter, such as forage residues and manure, to the soil to provide food for soil organism and facilitate the formation of aggregates to enhance soil porosity and the infiltration, flow, and drainage of water through the soil profile.
• Guarding against soil compaction, soil saturation, and the addition of amendments that might kill certain populations of soil organisms.

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Publication Overview

This publication is divided into five chapters:

• Chapter 1: Nutrient Cycle Components, Interactions, and Transformations
• Chapter 2: Nutrient Availability in Pastures
• Chapter 3: Nutrient Distribution and Movement in Pastures
• Chapter 4: The Soil Food Web and Pasture Soil Quality
• Chapter 5: Pasture Management and Water Quality

The first chapter provides an overview of four nutrient cycles critical to plant production and water quality protection: the water, carbon, nitrogen, phosphorus, and sulfur cycles. Diagrams of each cycle are presented and the components of each cycle are explained, with emphasis on how these components are affected by pasture management practices. The description of each cycle concludes with a summary of pasture management practices to enhance efficient cycling of that nutrient.

The second chapter focuses on the effect of soil chemistry and mineralogy along with past and current land management practices on nutrient cycle transformations and nutrient availability. Management practice impacts discussed include soil compaction, organic matter additions and losses, soil pH, and method and timing of nutrient additions. The chapter concludes with a summary of pasture management practices for enhancing nutrient availability in pastures.

The third chapter discusses nutrient balances on grazed pastures and the availability of manure, residue, and fertilizer nutrients to forage growth. Factors affecting nutrient availability include nutrient content and consistency of manure, manure distribution as affected by paddock location and layout, and forage diversity. These factors, in turn, affect grazing intensity and pasture regrowth. A graph at the end of the chapter illustrates the interactions among these factors.

The fourth chapter describes the diversity of organisms involved in the decomposition of plant residues and manure in pastures. Included is a discussion of the impact of soil biological activity on the efficiency of nutrient cycles and forage production as well as impacts of pasture management on the activity of soil organisms. A soil health card developed for pastures provides a tool for qualitatively assessing the ability of soils to support healthy populations of soil organisms.

This publication concludes with a discussion of pasture management practices and their effects on water quality, soil erosion, water runoff, and water infiltration. Several topical water concerns are discussed: phosphorus runoff and eutrophication, nutrient and pathogen transport through subsurface drains, buffer management, and riparian grazing practices. A guide for assessing potential water quality impacts from pasture-management practices concludes this final chapter.

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Chapter 1: Nutrient Cycle Components, Interactions, and Transformations

The water, carbon, nitrogen, phosphorus, and sulfur cycles are the most important nutrient cycles operating in pasture systems. Each cycle has its complex set of interactions and transformations as well as interactions with the other cycles. The water cycle is essential for photosynthesis and the transport of nutrients to plant roots and through plant stems. It also provides the force and transport for nutrients lost through leaching runoff and erosion. The carbon cycle forms the basis for cell formation and soil quality. It initiates with photosynthesis and includes respiration, mineralization, immobilization, and humus formation. Atmospheric nitrogen is fixed into plant-available nitrogen by one type of bacteria, converted from the ammonical to nitrate form by another set of bacteria, and released back to the atmosphere by a fourth type of bacteria. A variety of soil organisms are involved in decomposition processes that release or mineralize nitrogen, phosphorus, sulfur, and other nutrients from plant residues and manure. Balances in the amount of these nutrients within organic materials, along with temperature and moisture conditions, determine which organisms are involved in the decomposition process and the rate at which this process proceeds.

Pasture management practices influence the interactions and transformations occurring within nutrient cycles. The efficiency of these cycles, in turn, influences the productivity of forage growth and the productivity of animals feeding on that forage. This chapter examines each of these cycles in detail and provides management guidelines for enhancing their efficiency.

Water Cycle

Water is critical for pasture productivity. It dissolves soil nutrients and moves them to plant roots. Inside plants, water and the dissolved nutrients support cell growth and photosynthesis. In the soil, water supports the growth and reproduction of insects and microorganisms that decompose organic matter. Water also can degrade pastures through runoff, erosion, and leaching, which cause nutrient loss and water pollution. Productive pastures are able to absorb and use water effectively for plant growth. Good pasture management practices promote water absorption by maintaining forage cover over the entire soil surface and by minimizing soil compaction by animals or equipment. Geology, soil type, and landscape orientation affect water absorption by soils and water movement through soils. Sloping land encourages water runoff and erosion; depressions and footslopes are often wet since water from upslope collects in these areas. Clay soils absorb water and nutrients, but since clay particles are very small, these soils compact easily. Sandy soils are porous and allow water to enter easily, but do not hold water and nutrients against leaching. Organic matter in soil absorbs water and nutrients, reduces soil compaction, and increases soil porosity. A relatively small increase in the amount of organic matter in soil can cause a large increase in the ability of soils to use water effectively to support plant production.

Infiltration and water holding capacity

Water soaks into soils that have a plant or residue cover over the soil surface. This cover cushions the fall of raindrops and allows them to slowly soak into the soil. Roots create pores that increase the rate at which water can enter the soil. Long-lived, perennial bunch grass forms deep roots that facilitate water infiltration by conducting water into the soil.(3) Other plant characteristics that enhance water infiltration are significant litter production and large basal coverage.(4) In northern climates where snow provides a substantial portion of the annual water budget, maintaining taller grasses and shrubs that can trap and hold snow will enhance water infiltration.

Soils with a high water holding capacity absorb large amounts of water, minimizing the potential for runoff and erosion and storing water for use during droughts. Soils are able to absorb and hold water when they have a thick soil profile, contain a relatively high percentage of organic matter, and do not have a rocky or compacted soil layer, such as a hardpan or plowpan, close to the soil surface. An active population of soil organisms enhances the formation of aggregates and the formation of borrowing channels that provide pathways for water to flow into and through the soil. Management practices to enhance water infiltration and water holding capacity include:

• a complete coverage of forages and residues over the soil surface
• an accumulation of organic matter in and on the soil
• an active community of soil organisms involved in organic matter decomposition and aggregate formation
• water runoff and soil erosion prevention
• protection against soil compaction

Soil saturation

Soils become saturated when the amount of water entering exceeds the rate of absorption or drainage. A rocky or compacted lower soil layer will not allow water to drain or pass through, while a high water table will prevent water from draining through the profile. Water soaking into these soils is trapped or perched above the hard layer or high water table. Soils prone to saturation are usually located at the base of slopes, near waterways, or next to seeps.

Impact on crop production. Soil saturation affects plant production by exacerbating soil compaction, limiting air movement to roots, and ponding water and soil-borne disease organisms around plant roots and stems. Because soil pores are filled with water, roots and beneficial soil organisms lose access to air, which is necessary for their healthy growth. Soil compaction decreases the ability of air, water, nutrients, and roots to move through soils, even after soils have dried. Plants suffering from lack of air and nutrients are susceptible to disease attack since they are under stress, and wet conditions help disease organisms move from contaminated soil particles and plant residues to formerly healthy plant roots and stems.

Runoff and erosion potential. Soil saturation enhances the potential for runoff and erosion by preventing entry of additional water into the soil profile. Instead, excess water will run off the soil surface, often carrying soil and nutrients with it. Water can also flow horizontally under the surface of the soil until it reaches the banks of streams or lakes. This subsurface water flow carries nutrients away from roots, where they could be used for plant growth, and into streams or lakes where they promote the growth of algae and eutrophication.

Artificial drainage practices are often used on soils with a hardpan or a high water table to decrease the duration of soil saturation following rainfalls or snowmelts. This practice can increase water infiltration and decrease the potential for water runoff.(5) Unfortunately, most subsurface drains were installed before water pollution from agriculture became a concern and thus empty directly into drainage ways. Nutrients, pathogens, and other contaminants on the soil surface can move through large cracks or channels in the soil to drainage pipes where they are carried to surface water bodies.(6)

Soil compaction

Soil compaction occurs when animals or equipment move across soils that are wet or saturated, with moist soils being more easily compacted than saturated soils.(7) Compaction can also occur when animals or equipment continually move across a laneway or stand around watering tanks, headlands, or under shade. Animals trampling over the ground press down on soils, squeezing soil pore spaces together. Trampling also increases the potential for compaction by disturbing and killing vegetation.

Soils not covered by forages or residues are easily compacted by the impact of raindrops. When raindrops fall on bare soil, their force causes fine soil particles to splash or disperse. These splash particles land on the soil surface, clog surface soil pores, and form a crust over the soil. Clayey soils are more easily compacted than sandy soils because clay particles are very small and sticky.

Compaction limits root growth and the movement of air, water, and dissolved nutrients through the soil. Compressing and clogging soil surface pores also decreases water infiltration and increases the potential for runoff. The formation of hardpans, plowpans, traffic pans, or other compacted layers within the soil decreases downward movement of water through the soil, causes rapid soil saturation and the inability of soils to absorb additional water. Compaction in pastures is remediated by root growth, aggregate formation, and activities of burrowing soil organisms. In colder climates, frost heaving is an important recovery process for compacted soils.(8)

Runoff and erosion

Runoff water dissolves nutrients and removes them from the pasture as it flows over the soil surface. Soil erosion transports nutrients and any contaminants, such as pesticides and pathogens, attached to soil particles. Because nutrient-rich clay and organic matter particles are small and lightweight, they are more readily picked up and moved by water than the nutrient-poor, but heavier sand particles. Besides depleting pastures of nutrients that could be used for forage production, runoff water and erosion carry nutrients and sediments that contaminate lakes, streams, and rivers.

Landscape conditions and management practices that favor runoff and erosion include sloping areas, minimal soil protection by forage or residues, intense rainfall, and saturated soils. While pasture managers should strive to maintain a complete forage cover over the soil surface, this is not feasible in practice due to plant growth habits and landscape characteristics. Plant residues from die-back and animal wastage during grazing provide a critical source of soil cover and soil organic matter. As mentioned above, forage type affects water infiltration and runoff. Forages with deep roots enhance water infiltration while plants with a wide vegetative coverage area or prostrate growth provide good protection against raindrop impact. Sod grasses that are short-lived and shallow rooted inhibit water infiltration and encourage water runoff. Grazing practices that produce clumps of forages separated by bare ground enhance runoff potential by producing pathways for water flow.

Evaporation and transpiration

Water in the soil profile can be lost through evaporation, which is favored by high temperatures and bare soils. Pasture soils with a thick cover of grass or other vegetation lose little water to evaporation since the soil is shaded and soil temperatures are decreased. While evaporation only affects the top few inches of pasture soils, transpiration can drain water from the entire soil profile. Transpiration is the loss of water taken up by plants through stomata in their leaves. Especially on sunny and breezy days, significant amounts of water can be absorbed from the soil by plant roots, taken up through the plant, and lost to the atmosphere through transpiration. A diversity of forage plants will decrease transpiration losses and increase water use efficiency due to differences among forage species in their ability to extract water from the soil and conserve it against transpiration.(9) Some invasive plant species, however, can deplete water stores through their high water usage.(4) Water not used for immediate plant uptake is held within the soil profile or is transported to groundwater reserves, which supply wells with water and decrease the impacts of drought.

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Carbon Cycle

Effective carbon cycling in pastures depends on a diversity of plants and healthy populations of soil organisms. Plants form carbon and water into carbohydrates through photosynthesis. Plants are most able to conduct photosynthesis when they can efficiently capture solar energy while also having adequate access to water, nutrients, and air. Animals obtain carbohydrates formed by plants when they graze on pastures or eat hay or grains harvested from fields. Some of the carbon and energy in plant carbohydrates is incorporated into animal cells. Some of the carbon is lost to the atmosphere as carbon dioxide, while energy is lost as heat during digestion and as the animal grows and breathes.

Carbohydrates and other nutrients not used by animals are returned to the soil in the form of urine and manure. These organic materials provide soil organisms with nutrients and energy. As soil organisms use and decompose organic materials, they release nutrients from these materials into the soil. Plants then use the released, inorganic forms of nutrients for their growth and reproduction. Soil organisms also use nutrients from organic materials to produce substances that bind soil particles into aggregates. Residues of organic matter that resist further decomposition by soil organisms form soil humus. This stable organic material is critical for maintaining soil tilth and enhancing the ability of soils to absorb and hold water and nutrients.

Carbohydrate formation

For productive growth, plants need to effectively capture solar energy, absorb carbon dioxide, and take up water from the soil to produce carbohydrates through photosynthesis. In pastures, a combination of both broadleaf plants and grasses allows for efficient capture of solar energy by a diversity of leaf shapes and leaf angles. Taller plants with more erect leaves capture light, even at the extreme angles of sunrise and sunset. Horizontal leaves capture the sun at midday or when it is more overhead.

Two methods for transforming carbon into carbohydrates through photosynthesis are represented in diversified pastures. Broadleaf plants and cool season grass have a pathway that is efficient in the production of carbohydrates but is sensitive to dry conditions. Warm season grasses have a pathway that is more effective in producing carbohydrates during hot, summer conditions. A combination of plants representing these two pathways ensures effective forage growth throughout the growing season. A diversity of root structures also promotes photosynthesis by giving plants access to water and nutrients throughout the soil profile.

Organic matter decomposition

Pasture soils gain organic matter from growth and die-back of pasture plants, from forage wastage during grazing, and from manure deposition. In addition to the recycling of above ground plant parts, every year 20-50% of plant root mass dies and is returned to the soil system. Some pasture management practices also involve the regular addition of manure from grazing animals housed during the winter or from poultry, hog, or other associated livestock facilities.

A healthy and diverse population of soil organisms is necessary for organic matter decomposition, nutrient mineralization, and the formation of soil aggregates. Species representing almost every type of soil organism have roles in the break down of manure, plant residues, and dead organisms. As they use these substances for food and energy sources, they break down complex carbohydrates and proteins into simpler chemical forms. For example, soil organisms break down proteins into carbon dioxide, water, ammonium, phosphate, and sulfate. Plants require nutrients to be in this simpler, decomposed form before they can use them for their growth.

To effectively decompose organic matter, soil organisms require access to air, water, and nutrients. Soil compaction and saturation limit the growth of beneficial organisms and promote the growth of anaerobic organisms, which are inefficient in the decomposition of organic matter. These organisms also transform some nutrients into forms that are less available or unavailable to plants. Nutrient availability and nutrient balances in the soil solution also affect the growth and diversity of soil organisms. To decompose organic matter that contains a high amount of carbon and insufficient amounts of other nutrients, soil organisms must mix soil solution nutrients with this material to achieve a balanced diet.

Balances between the amount of carbon and nitrogen (C:N ratio) and the amount of carbon and sulfur (C:S ratio) determine whether soil organisms will release or immobilize nutrients when they decompose organic matter. Immobilization refers to soil microorganisms taking nutrients from the soil solution to use in the decomposition process of nutrient-poor materials. Since these nutrients are within the bodies of soil organisms, they are temporarily unavailable to plants. In soils with low nutrient content, this can significantly inhibit plant growth. However, immobilization can be beneficial in soils with excess nutrients. This process conserves nutrients in bodies of soil organisms, where they are less likely to be lost through leaching and runoff.(10)

Populations of soil organisms are enhanced by soil that is not compacted and has adequate air and moisture, and by additions of fresh residues they can readily decompose. Soil-applied pesticides can kill many beneficial soil organisms, as will some chemical fertilizers. Anhydrous ammonia and fertilizers with a high chloride content, such as potash, are particularly detrimental to soil organism populations. Moderate organic or synthetic fertilizer additions, however, enhance populations of soil organisms in soils with low fertility.

Soil humus and soil aggregates

Besides decomposing organic materials, bacteria and fungi in the soil form gels and threads that bind soil particles together. These bound particles are called soil aggregates. Worms, beetles, ants, and other soil organisms move partially decomposed organic matter through the soil or mix it with soil in their gut, coating soil particles with organic gels. As soil particles become aggregated, soil pore size increases and soils become resistant to compaction. The organic compounds that hold aggregates together also increase the ability of soils to absorb and hold water and nutrients.

As manure and plant residues are decomposed by soil organisms, carbon dioxide is released as respiration and waste materials are produced, which are further decomposed by other soil organisms. Because carbon is lost to respiration at each stage of this decomposition process, the remaining material increases in nitrogen content. The remaining material also increases in chemical complexity and requires increasingly specialized species of decomposers. Efficient decomposition of organic matter thus requires a diversity of soil organisms. Humus is the final, stable product of decomposition, formed when organic matter is only broken down by soil organisms slowly or with difficulty. Humus-coated soil particles form aggregates that are soft, crumbly, and somewhat greasy-feeling when rubbed together.

Preventing organic matter losses

Perennial plant cover in pastures not only provides organic matter inputs, it also protects against losses of organic matter through erosion. Soil coverage by forages and residues protects the soil from raindrop impact while dense root systems of forages hold the soil against erosion while enhancing water infiltration. Fine root hairs also promote soil aggregation. In addition, a dense forage cover shades and cools the soil. High temperatures promote mineralization and loss of organic matter, while cooler temperatures promote the continued storage of this material within the plant residues and the bodies of soil organisms.

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Nitrogen Cycle

Nitrogen is a primary plant nutrient and a major component of the atmosphere. In a pasture ecosystem, almost all nitrogen is organically bound. Of this, only about 3% exists as part of living plant, animal, or microbial cells while the remainder is a component of decomposed organic matter or humus. A very small percent of the total nitrogen (<0.01%) exists as plant available nitrogen in the form of ammonium or nitrate.(12)

Nitrogen becomes available for the growth of crop plants and soil organisms through nitrogen fixation, nitrogen fertilizer applications, the return of manure to the land, and through the mineralization of organic matter in the soil. Nitrogen fixation occurs mainly in the roots of legumes that form a symbiotic association with a bacteria species called rhizobia. Some algae and free-living bacteria are also able to transform atmospheric nitrogen into a form available for plant growth. Fertilizer factories use a combination of high pressure and high heat to combine atmospheric nitrogen and hydrogen into nitrogen fertilizers. Animals deposit organically-bound nitrogen in feces and urine. Well-managed pastures accumulate stores of organic matter in the soil and in plant residues. Decomposition and mineralization of nutrients in these materials can provide significant amounts of nitrogen to plants and other organisms in the pasture system.

Plants use nitrogen for the formation of proteins and genetic material. Grazing animals that consume these plants use some of the nitrogen for their own growth and reproduction; the remainder is returned to the earth as urine or manure. Soil organisms decompose manure, plant residues, dead animals, and microorganisms, transforming nitrogen-containing compounds in their bodies into forms are available for use by plants.

Nitrogen is often lacking in pasture systems since forage requirements for this nutrient are high and also because it is easily lost to the environment. Nitrogen is lost from pasture systems through microbiological, chemical, and physical processes. Dry followed by wet weather provides optimal conditions for bacteria to transform nitrogen from plant-available forms into atmospheric nitrogen through denitrification. Chemical processes also transform plant-available nitrogen into atmospheric nitrogen through volatilization. In pastures, this often occurs after manure or nitrogen fertilizers are applied to the soil surface, especially during warm weather. Physical processes are involved in the downward movement of nitrogen through the soil profile during leaching.

Nitrogen fixation

Plants in the legume family, including alfalfa, clover, lupines, lespedeza, and soybeans, form a relationship with a specialized form of bacteria called rhizobia. These bacteria have the ability to fix or transform atmospheric nitrogen into a form of nitrogen plants can use for their growth. Rhizobia form little balls or nodules on the roots of legumes. If these balls are white or pinkish on the inside, they are actively fixing nitrogen. Nodules that are grey or black inside are dead or no longer active. Legume seeds should be dusted with inoculum (a liquid goo or powder containing the appropriate type of rhizobia) prior to planting to ensure that the plant develops many nodules and has maximal ability to fix nitrogen. Some algae and other microorganisms that live in the soil are also able to fix and provide nitrogen to plants.

Legumes require higher amounts of phosphorus, sulfur, boron, and molybdenum than non-legumes to form nodules and fix nitrogen. If these nutrients are not available in sufficient amounts, nitrogen fixation will be suppressed. When nitrogen levels in the soil are high due to applications of manure or nitrogen fertilizers, nitrogen fixation by legumes also decreases because nitrogen fixation requires more energy than does root uptake of soluble soil nitrogen. Nitrogen fixed by legumes and rhizobia is available primarily to the legumes while they are growing. When pasture legume nodules, root hairs, and above-ground plant material dies and decomposes, nitrogen in this material can become available to pasture grasses.(14)

However, while legumes are still growing, mycorrhizal fungi can form a bridge between the root hairs of legumes and nearby grasses. This bridge facilitates the transport of fixed nitrogen from legumes to linked grasses. Depending on the nitrogen content of the soil and the mix of legumes and grasses in a pasture, legumes can transfer between 20-40% of their fixed nitrogen to grasses during the growing season.(15) A pasture composed of at least 20-45% legumes (dry matter basis) can meet and sustain the nitrogen needs of the other forage plants in the pasture.(16)

Grazing management affects nitrogen fixation through the removal of herbage, deposition of urine and manure, and induced changes in moisture and temperature conditions in this soil. Removal of legume leaf area decreases nitrogen fixation by decreasing photosynthesis and plant competitiveness with grasses. Urine depositions decrease nitrogen fixation by adjacent plants since it creates an area of high soluble nitrogen availability. Increases in moisture in compacted soils or temperature in bare soils will also decrease nitrogen fixation since rhizobia are sensitive to wet and hot conditions.

Nitrogen mineralization

Decomposition of manure, plant residues or soil organic matter by organisms in the soil results in the formation of ammonical nitrogen. Protozoa, amoebae and nematodes are prolific nitrogen mineralizers, cycling 14 times their biomass each year. While bacteria only cycle 0.6 times their biomass, because of their large numbers in soil they produce a greater overall contribution to the pool of mineralized nitrogen.(17) Plants can use ammonical nitrogen for their growth, but under aerobic conditions two types of bacteria usually work together to rapidly transform ammonia first into nitrite and then into nitrate before it is used by plants.

Mineralized nitrogen is a very important source of nitrogen in most grasslands. As discussed in Carbon Cycle, above, efficient decomposition and release of nitrogen from decomposing residues depends on residues containing a ratio of carbon to nitrogen in balance with the nutrient needs of decomposing organisms. If the nitrogen content of residues is insufficient, soil organisms will extract nitrogen from the soil solution to satisfy their nutrient needs.

Nitrogen losses to the atmosphere

Under wet or anaerobic conditions, bacteria transform nitrate nitrogen into atmospheric nitrogen. This process, called denitrification, reduces the availability of nitrogen for plant use. Denitrification occurs when dry soil containing nitrate becomes wet or flooded and at the edges of streams or wetlands where dry soils are adjacent to wet soils.

Volatilization is the transformation of ammonia into atmospheric nitrogen. This chemical process occurs when temperatures are high and ammonia is exposed to the air. Incorporation of manure or ammonical fertilizer into the soil decreases the potential for volatilization. In general, 5-25% of the nitrogen in urine is volatilized from pastures.(11) A thick forage cover and rapid manure decomposition can reduce volatilization from manure.

Nitrogen leaching

Soil particles and humus are unable to hold nitrate nitrogen very tightly. Water from rainfall or snowmelt readily leaches soil nitrate downward through the profile, putting it out of reach of plant roots or moving it into the groundwater. Leaching losses are greatest when the water table is high, the soil sandy or porous, or when rainfall or snowmelt is severe. In pastures, probably the most important source of nitrate leaching is from urine patches.(18) Cattle urine typically leaches to the depth of 16 inches while sheep urination only leaches six inches into the ground.(19) Leaching may also be associated with the death of legume nodules during dry conditions.(20) Methods for reducing the potential for nitrate leaching include maintaining an actively growing plant cover over the soil surface, coordinating nitrogen applications with the period of early plant growth, not applying excess nitrogen to soils, and encouraging animal movement and distribution of manure across paddocks. Actively growing plant roots take up nitrate from the soil and prevent it from leaching. If the amount of nitrogen applied to the soil is in excess of what plants need or is applied when plants are not actively growing, nitrate not held by plants can leach through the soil. Spring additions of nitrogen to well-managed pastures can cause excessive plant growth and increase the potential for nitrogen leaching since significant amounts of nitrogen are also being mineralized from soil organic matter as warmer temperatures increase the activity of soil organisms.

Nitrate levels in excess of 10 ppm in drinking water can cause health problems for human infants, infant chickens and pigs, and both infant and adult sheep, cattle, and horses.(21) Pasture forages can also accumulate nitrate levels high enough to cause health problems. Conditions conducive for nitrate accumulation by plants include acid soils, low molybdenum, sulfur, and phosphorus content, soil temperatures lower than 55° F, and good soil aeration.(22)

This nitrate toxicity health problem is called methemoglobinemia, the common name being "blue baby syndrome" when seen in human babies. In this syndrome, nitrate binds to hemoglobin in the blood, reducing the blood's ability to carry oxygen through the body. Symptoms in human infants and young animals include difficulty breathing. Pregnant animals that recover may abort within a few days. Personnel from the Department of Health can test wells to determine whether nitrate levels are dangerously high.

Nitrogen loss through runoff and erosion

Runoff and erosion caused by rainwater or snowmelt can transport nitrogen on the soil surface. Erosion removes soil particles and organic matter that contain nitrogen; runoff transports dissolved ammonia and nitrate. Incorporation of manure and fertilizers into the soil reduces the exposure of these nitrogen sources to rainfall or snowmelt, thus reducing the potential for erosion. In pasture systems, however, incorporation is usually impractical and can increase the potential for erosion. Instead, a complete cover of forages and plant residues should be maintained over the soil surface to minimize raindrop impact on the soil, enhance water infiltration, help trap sediments and manure particles, and reduce the potential for runoff and erosion. A healthy and diverse population of soil organisms, including earthworms and dung beetles that rapidly incorporate manure nitrogen into the soil and into their cells can further reduce the risk of nitrogen runoff from manure. Since increased water infiltration decreases the potential for runoff but increases the potential for leaching, risks of nitrate losses from runoff need to be balanced against leaching risks.

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Phosphorus Cycle

Like nitrogen, phosphorus is a primary plant nutrient. Unlike nitrogen, phosphorus is not part of the atmosphere. Instead, it is found in rocks, minerals, and organic matter in the soil. The mineral forms of phosphorus are apatitite, which may be in a carbonate, hydroxide, fluoride, or chloride form, and iron or aluminum phosphates. These minerals are usually associated with basalt and shale rocks. Chemical reactions and microbial activity affect the availability of phosphorus for plant uptake. Under acid conditions, phosphorus is held tightly by aluminum and iron in soil minerals. Under alkaline conditions, phosphorus is held tightly by soil calcium.

Plants use phosphorus for energy transfer and reproduction. Legumes require phosphorus for effective nitrogen fixation. Animals consume phosphorus when they eat forages. Phosphorus not used for animal growth is returned to the soil in manure. Following decomposition by soil organisms, phosphorus again becomes available for plant uptake.

Mycorrhizae

Mycorrhizal fungi attach to plant roots and form thin threads that grow through the soil and wrap around soil particles. These thin threads increase the ability of plants to obtain phosphorus and water from soils. Mycorrhizae are especially important in acid and sandy soils where phosphorus is either chemically bound or has limited availability. Besides transferring phosphorus and water from the soil solution to plant roots, mycorrhizae also facilitate the transfer of nitrogen from legumes to grasses. Well-aerated and porous soils, and soil organic matter, favor mycorrhizal growth.

Soil chemistry and phosphorus availability

Phosphorus is tightly bound chemically in highly weathered acid soils that contain high concentrations of iron and aluminum. Active calcium in neutral to alkaline soils also forms tight bonds with phosphorus. Liming acid soils and applying organic matter to either acid or alkaline soils can increase phosphorus availability. In most grasslands, the highest concentration of phosphorus is in the surface soils associated with decomposing manure and plant residues.

Phosphorus loss through runoff and erosion

Unlike nitrogen, phosphorus is held by soil particles. It is not subject to leaching unless soil levels are excessive. However, phosphorus can move through cracks and channels in the soil to artificial drainage systems, which can transport it to outlets near lakes and streams. Depending on the soil type and the amount of phosphorus already in the soil, phosphorus added as fertilizer or manure may be readily lost from fields and transported to rivers and streams through runoff and erosion. The potential for phosphorus loss through runoff or erosion is greatest when rainfall or snowmelt occurs within a few days following surface applications of manure or phosphorus fertilizers.

Continuous manure additions increase the potential for phosphorus loss from the soil and the contamination of lakes and streams. This is especially true if outside manure sources are used to meet crop or forage nutrient needs for nitrogen. The ratio of nitrogen to phosphate in swine or poultry manure is approximately 1:1 while the ratio of the ratio of nitrogen to phosphate taken up by forage grasses is between 2.5-3.8:1. Thus, manure applied for nitrogen requirements will provide 2.5-3.8 times the amount of phosphorus needed by plants.(23) While much of this phosphorus will be bound by chemical bonds in the soil and in the microbial biomass, continuous additions will exceed the ability of the soil to store excess phosphorus and the amount of soluble phosphorus, the form available for loss by runoff, will increase. To decrease the potential for phosphorus runoff from barnyard manure or poultry litter, alum, or aluminum oxide, can be added to bind phosphorus in the manure.(24)

Supplemental feeds are another source of phosphorus inputs into grazing systems, especially for dairy herds. Feeds high in phosphorus increase the amount of phosphorus deposited on pastures as manure. To prevent buildup of excess phosphorus in the soil, minimize feeding of unneeded supplements, conduct regular soil tests on each paddock, and increase nutrient removals from high or excessively fertile paddocks through haying.

Phosphorus runoff from farming operations can promote unwanted growth of algae in lakes and slow-moving streams. Regulations and nutrient management guidelines are being developed to decrease the potential for phosphorus movement from farms and thus reduce risks of lake eutrophication. Land and animal management guidelines, called "phosphorus indices," are being developed across the U.S. to provide farmers with guidelines for reducing "non-point" phosphorus pollution from farms.(25) These guidelines identify risk factors for phosphorus transport from fields to water bodies based on the concentration of phosphorus in the soil, timing and method of fertilizer and manure applications, potential for runoff and erosion, and distance of the field from a water body.(26)

Although the total amount of phosphorus lost from fields is greatest during heavy rainstorms, snowmelts, and other high runoff occurrences, relatively small amounts of phosphorus running off from fields into streams at low water level in summer pose a higher risk for eutrophication. This is because phosphorus is more concentrated at low flows than during high flow periods.(27) Conditions for concentrated flows of phosphorus into low-flow streams include location near streams of barnyards or other holding areas without runoff containment or filtering system, extensive grazing of animals near streams without riparian buffers, and animal access to streams.

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Secondary Nutrients

Potassium and the secondary nutrients, calcium, magnesium, and sulfur, play a critical role in plant growth and animal production. Potassium, calcium, and magnesium are components of clay minerals. The soil parent-material prmarily influences the availability of these plant nutrients. For example, soils derived from granite contain, on the average, nine times more potassium than soils derived from basalt, while soils derived from limestone have half the amount. Conversely, soils derived from limestone have, on the average, four times more calcium than soils derived from basalt and thirty times more than soils derived from granite.(11)

Potassium

Potassium, like all plant nutrients, is recycled through plant uptake, animal consumption, and manure deposition. The majority of potassium is found in urine. Potassium levels can become excessive in fields that have received repeated, high applications of manure. Application of fertilizer nitrogen increases the potassium uptake by grasses if the soil has an adequate supply of potassium. Consumption of forages that contain more than 2% potassium can cause problems in breeding dairy cattle and in their recovery following freshening.(28) High potassium levels, especially in lush spring forage, can cause nutrient imbalance resulting in grass tetany.

Calcium and magnesium

Calcium and magnesium are components of liming materials used to increase soil pH and reduce soil acidity. However, the use of lime can also be important for increasing the amount of calcium in the soil or managing the balance between calcium and magnesium. Increasing the calcium concentration in the soil may enhance biological activity in the soil.(29) Managing this balance is especially important for decreasing the tendency for grass tetany, a nutritional disorder of ruminants caused by low levels of magnesium in the diet. Magnesium may be present in the soil in sufficient amounts for plant growth, but its concentration may be out of balance with the nutrient needs of plants and animals. When calcium and potassium have a high concentration in the soil compared to magnesium, they will limit the ability of plants to take up magnesium. Under these conditions, the magnesium concentration needs to be increased relative to calcium. Dolomite lime, which contains magnesium carbonate, can be used to both lime soils and increase the availability of magnesium. Phosphorus fertilization of tall fescue in Missouri was also shown to increase the availability of magnesium sufficiently to decrease the incidence of grass tetany in cattle.(30) This result was probably due to the stimulation of grass growth during the cool wet spring conditions that are conducive to the occurrence of grass tetany.

Sulfur

Sulfur increases the protein content of pasture grasses and increases forage digestibility and effectiveness of nitrogen use.(31) In nature, sulfur is contained in igneous rocks, such as granite and basalt, and is a component of organic matter. In areas downwind from large industrial and urban centers, sulfur contributions from the atmosphere in the form of acid rain can be considerable. Fertilizer applications of nitrogen as ammonium sulfate or as sulfur-coated urea also contribute to sulfur concentration in soils. However, as environmental controls for acid rain improve, other sources of nitrogen fertilizer are used, and forage production increases, pasture needs for sulfur fertilization increase.

As a component of organic matter, microbial processes affect sulfur availability. Like nitrogen, the sulfur content of organic matters determines whether nutrients will be mineralized or immobilized. Also, similar to nitrogen, the sulfur content of grasses decreases as they become older and less succulent. Thus, soil organisms will decompose younger plants more rapidly and release nutrients from this organic matter while they will decompose older plant material more slowly and may immobilize soil nutrients in the process of decomposition.

Chemical and biological processes are involved in sulfur transformations. In dry soils that become wet or waterlogged, chemical processes transform sulfur from the sulfate to sulfide form. If these wet soils dry out or are drained, bacteria transform sulfide to sulfate. Similar to nitrate, sulfate is not readily absorbed by soil minerals, especially in soils with a slightly acid to neutral pH. As a result, sulfate can readily leach through soils that are sandy or highly permeable.

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Chapter 2: Nutrient Availability in Pastures

Nutrient balances and nutrient availability determine the fate of nutrients in pastures. In the simplest of grazing systems, forage crops take up nutrients from the soil; haying and animal grazing removes forage crops and their associated nutrients; and animal manure deposition returns nutrients to the soil. Continuous nutrient removals deplete soil fertility unless fertilizers, whether organic or synthetically produced, are added to replenish nutrients. Nutrients may be added to pastures by providing animals with feed supplements produced off-farm. In addition, practices that erode topsoil and deplete soil organic matter decrease the ability of soils to hold or retain nutrients.

Chemical and biological interactions determine the availability of nutrients for plant use. Both native soil characteristics and land management practices affect these interactions. Phosphorus can be held chemically by iron or aluminum bonds while potassium can be held within soil minerals. All crop nutrients can be components of plant residues or soil organic matter. The type of organic matter available and the activity of soil organisms determine the rate and amount of nutrients mineralized from these materials. Nutrient availability and balance in forage plants affects the health of grazing animals. Depleted soils produce unhealthy, low-yielding forages and unthrifty animals; excess soil nutrients can be dangerous to animal health and increase the potential for contamination of wells, springs, rivers, and streams.

Soil Parent Material

Chemical, physical, geological, and biological processes affect nutrient content and availability in soils. As discussed in the previous chapter, soils derived from basalt and shale provide phosphorus to soils, granite contains high concentrations of potassium, and limestone is a source of calcium and magnesium. Some clay soils and soils with high percentages of organic matter contain a native store of nutrients in addition to having the capacity to hold nutrients added as manure, crop residues, or fertilizers. Soils formed under temperate prairies or in flood plains are fertile due to a long history of organic matter deposition and nutrient accumulation. Sandy soils and weathered, reddish clay soils contain few plant nutrients and have a limited ability to hold added nutrients. Soils formed under dry, desert conditions often are saline since water evaporating off the soil surface draws water in the soil profile upward. This water carries nutrients and salts, which are deposited on the soil surface when water evaporates. Tropical soils generally have low fertility since they were formed under conditions of high temperatures, high biological activity, and high rainfall that caused rapid organic matter decomposition and nutrient leaching.

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Soil Chemistry

Many clay minerals are able to hold onto water and nutrients and make them available for plant growth. The pH, or level of acidity or alkalinity of the soil solution, strongly influences the strength and type of bonds formed between soil minerals and plant nutrients. Soil pH also affects activities of soil organisms involved in the decomposition of organic matter and the dissolution of plant nutrients from soil minerals. Many clay soil particles are able to bind large amounts of nutrients because of their chemical composition and because they are very small and have a large surface area for forming bonds. Unfortunately, this small size also makes clay particles prone to compaction, which can reduce nutrient and water availability. Sandy soils are porous and allow water to enter the soil rapidly. But these soils are unable to hold water or nutrients against leaching. Organic matter has a high capacity to hold both nutrients and water. Soil aggregates, formed by plant roots and soil organisms, consist of mineral and organic soil components bound together in soft clumps. Aggregates enhance soil porosity, facilitate root growth, allow for better infiltration and movement of water and nutrients through soil, and help soils resist compaction.

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Prior Management Practices

In pastures, continual removal of nutrients through harvests or heavy grazing without return or addition of nutrients depletes the soil. Land management practices that encourage soil erosion—such as heavy grazing pressure, plowing up and down a slope, or leaving field bare of vegetation during times of heavy rains or strong winds—also deplete soil fertility. Some pasture management practices involve the use of fire to stimulate growth of native forages.(32) Burning readily mineralizes phosphorus, potassium, and other nutrients in surface crop residues. It also volatilizes carbon and nitrogen from residues and releases these nutrients into the atmosphere, thus minimizes the ability of organic matter to build up in the soil. Loss of residues also exposes soil to raindrop impact and erosion. Hot, uncontrolled fires increase the potential for erosion by degrading natural biological crusts formed by lichen, algae and other soil organisms and by promoting the formation of physical crusts formed from melted soil minerals.(33, 34) The continual, high application of manure, whey, sludge, or other organic waste products to soils can cause nutrients to build up to excessive levels. Pasture management practices that influence soil compaction, soil saturation, the activity of soil organisms, and soil pH affect both soil nutrient content and availability.

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Soil Compaction

Animal movement compacts soil pores, especially when soils are wet or saturated. Continuously trampling and foraging, especially in congregation areas and laneways, also depletes plant growth and produces bare spots.

Soil compaction reduces nutrient availability for plant uptake by restricting nutrient transport to plant roots and root growth through the soil profile. Treading and compaction can substantially reduce forage yields. One study showed that the equivalent of 12 sheep treading on mixed ryegrass, white clover, and red clover pasture reduced yields by 25% on dry soil, 30% on moist soil, and 40% on wet soil compared to no treading. On wet soils, root growth was reduced 23%.(35)

Compacted soils do not allow for normal root growth. This root grew horizontally when it encountered a compacted layer. Photo by: Scott Bauer 2004 USDA Image Gallery

Compaction also decreases the rate of organic matter decomposition by limiting the access soil organisms have to air, water, or nutrients. In addition, compacted soils limit water infiltration and increase the potential for water runoff and soil erosion. In Arkansas, observations of overgrazed pastures showed that manure piles on or near bare, compacted laneways were more readily washed away by runoff than were manure piles in more vegetated areas of the pasture.(24)

The potential for animals to cause soil compaction increases with soil moisture, the weight of the animal being grazed, the number of animals in the paddock, and the amount of time animals stay in the paddock. The potential for a paddock to resist compaction depends on the duration of forage establishment and the type of forage root system. Established forages with a strong and prolific root growth in the top 6 to 10 inches of the soil profile are able to withstand treading by grazing animals. Grasses with extensive fibrous root systems, such as bermuda grass, are able to withstand trampling better than grasses like orchardgrass that have non-branching roots or legumes like white clover that have taproots.(36) Bunch grasses expose more soil to raindrop impact than closely seeded non-bunch grasses or spreading, herbaceous plants. However, these grasses enhance water infiltration by creating deep soil pores with their roots.(3) Combining bunch grasses with other plant varieties can increase water infiltration while decreasing the potential for soil compaction and water runoff.

The risk of soil compaction can also be reduced by not grazing animals on paddocks that are wet or have poorly-drained soils. Instead, during wet conditions, graze animals on paddocks that have drier soils and are not adjacent to streams, rivers, seeps, or drainage ways. Soils that are poorly drained should be used only in the summer when the climate and the soil are relatively dry.

Compacted soils can recover from the impacts of compaction, but recovery is slow. Periods of wet weather alternating with periods of dry weather can reduce compaction in some clay soils. Freezing and thawing decreases compaction in soils subjected to cold weather. Taproots are effective in breaking down compacted layers deep in the soil profile while shallow, fibrous roots break up compacted layers near the soil surface.(37) Active populations of soil organisms also reduce soil compaction by forming soil aggregates and burrowing into the soil.

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Organic Matter

Nutrient release from organic matter decomposition

Manure and plant residues must be decomposed by soil organisms before nutrients in these materials are available for plant uptake. Soil organisms involved in nutrient decomposition require a balance of nutrients to break down organic matter efficiently. Manure and wasted forages are succulent materials that have high nitrogen content and a good balance of nutrients for rapid decomposition.

Dried grasses, such as forages that died back over winter or during a drought, or manure mixed with wood bedding, have lower nitrogen contents and require more time for decomposition. In addition, soil organisms may need to extract nitrogen and other nutrients from the soil to balance their diet and obtain nutrients not available in the organic matter they are decomposing. Composting these materials increases the availability of nutrients and decreases the potential for nutrient immobilization when materials are added to the soil. Temperature, moisture, pH, and diversity of soil organisms affect how rapidly organic matter is decomposed in the soil.

Nutrient release from organic matter is slow in the spring when soils are cold and soil organisms are relatively inactive. Many farmers apply phosphorus as a starter fertilizer in the spring to stimulate seedling growth. Even though soil tests may indicate there is sufficient phosphorus in the soil, it may not be readily available from organic matter during cool springs.

Nutrient holding capacity of organic matter

Besides being a source of nutrients, soil organic matter is critical for holding nutrients against leaching or nutrient runoff. Stabilized organic matter or humus chemically holds positively-charged plant nutrients (cations). The ability of soil particles to hold these plant nutrients is called cation exchance capacity or CEC. Continual application of organic materials to soils increases soil humus (38) and enhances nutrient availability, nutrient holding capacity, and soil pore space.

Soil aggregates

Soil humus is most effective in holding water and nutrients when it is associated with mineral soil particles in the form of soil aggregates. Soil aggregates are small, soft, water-stable clumps of soil held together by fine plant root hairs, fungal threads, humus, or microbial gels. Aggregates are also formed through the activities of earthworms. Research has shown that several species of North American earthworms annually consume 4-10% of the soil and 10% of the total organic matter in the top 7 inches of soil.(39) This simultaneous consumption of organic and mineral matter by earthworms results in casts composed of associations of these two materials. Earthworms, as well as dung beetles, incorporate organic matter into the soil as they burrow.

Besides enhancing the nutrient and water holding capacity, well-aggregated soils facilitate water infiltration, guard against runoff and erosion, protect against drought conditions, and are better able to withstand compaction than less aggregated soils. Since aggregated soils are more granular and less compacted, plant roots grow more readily through them and air, water, and dissolved plant nutrients are better able to flow through them. These factors increase plant access to soil nutrients.

To enhance aggregation within pasture soils, maintain an optimum amount of forages and residues across paddocks, avoid the formation of bare areas, and minimize soil disturbance. Grazing can degrade soil aggregates by encouraging mineralization of the organic glues that hold aggregates together. In areas with a good cover of plant residues, animal movement across pastures can enhance aggregate formation by incorporating standing dead plant materials into the soil.(40)

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Soil pH

Soil mineralogy, long-term climate conditions, and land-management practices affect soil pH. The acidity or alkalinity of soils affects the nutrient availability, nitrogen fixation by legumes, organic matter decomposition by soil organisms and plant root function. Most plant nutrients are most available for uptake at soil pH in the range of 5.5 to 6.5. Legume persistence in pastures is enhanced by soil pH in the range of 6.5-7.0. In low-pH or acid soils, aluminum is toxic to root growth, aluminum and iron bind phosphorus, and calcium is in a form with low solubility. In high-pH or alkaline soils, calcium carbonate binds phosphorus while iron, manganese, and boron become insoluble.

Application of some synthetic nitrogen fertilizers acidifies soils. Soil microorganisms involved in nitrification rapidly transform urea or ammonium into nitrate nitrogen. This nitrification process releases hydrogen ions into the soil solution, causing acidification, which decreases nutrient availability, thus slowing the growth of plants and soil organisms. Nitrification also occurs in urine patches when soil microorganisms transform urea into nitrate.

Another fertilizer that acidifies the soil is superphosphate. Superphosphate forms a highly acid (pH 1.5) solution when mixed with water. The impact of this acidification is temporary and only near where the fertilizer was applied, but, in this limited area, the highly acid solution can kill rhizobia and other soil microorganisms.(9)

The type and diversity of forage species in pastures can alter soil pH. Rangeland plants such as saltbush maintain a neutral soil pH, while grasses and non-legume broadleaf plants tend to increase pH and legumes tend to decrease it. The impact of plant species on pH depends on the type and amounts of nutrients they absorb. Rangeland plants absorb equal amounts of cation (calcium, potassium, magnesium) and anion (nitrate) nutrients from the soil. Grasses and non-legume broadleaf plants absorb more anions than cations since they use nitrate as their primary source of nitrogen. Legumes that actively fix nitrogen use very little nitrate. Consequently, they reduce the pH of the soil since they take up more cations than anions.(9) A combination of legumes and non-legumes will tend to stabilize soil pH.

Pasture soils should be tested regularly to determine soil nutrients, soil organic matter and pH. Based on test results and forage nutrient requirements, management practices can adjust soil pH. Lime and organic matter increase soil pH and decrease soil acidity. Soil organic matter absorbs positive charges, including hydrogen ions that cause soil acidity.(41) Lime increases soil pH by displacing acid-forming hydrogen and aluminum bound to the edges of soil particles and replacing them with calcium or magnesium. Limestone that is finely ground is most effective in altering soil pH since it has more surface area to bind to soil particles. All commercial limestone has label requirements that specify its capacity to neutralize soil pH and its reactivity based on the coarseness or fineness of grind.

Lime refers to two types of materials, calcium carbonate and dolomite. Dolomite is a combination of calcium and magnesium carbonate. Calcium carbonate is recommended for soils low in calcium; where grass tetany or magnesium deficiency is an animal health problem, dolomite limestone should be used. In sandy soils or soils with low to moderate levels of potassium, the calcium or magnesium in lime can displace potassium from the edges of soil particles, reducing its availability. Therefore, these soils should receive both lime and potassium inputs to prevent nutrient imbalances.

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Timing of Nutrient Additions

The timing of nutrient additions to fields or pastures determines how effectively plants take up and use nutrients while they are growing and setting seed. Different nutrients are important during different stages of plant development. Nitrogen applied to grasses before they begin flowering stimulates tillering, while nitrogen applied during or after flowering stimulates stem and leaf growth.(9) However, fall nitrogen applications for cool season grasses are more effective and economical than spring applica