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Home  > Question of the Week

Question of the Week



Permalink What type of soil is best for organic tomato production?

Answer: A wide variety of soil textures can be used for tomato production. Ideal soil textures range from sandy to fine-textured clays as long as the soil is well drained (tomato roots will not tolerate being waterlogged), has good structure, and is well aerated. Planting dates can be determined by the type of soil a grower has. For example, sandy soils are preferred for early plantings in California due to adequate drainage during wet weather and rapid increase in soil temperature in the spring, which, in turn, can lead to early growth. However, loam and clay loam soils are the most productive soils for production. A mixture of one-quarter to one-half sand, one-quarter to one-half silt, and no more than 27% clay gives loam soils the best possible properties for plant growth. These mixtures supply adequate drainage and good soil structure. Conversely, heavier clays can be a serious problem due to poor soil drainage and excess soil moisture for extended periods, making tomatoes more prone to disease.

One of the foundations of organic farming is maintaining and building a microbial-active soil enriched with organic matter and a balanced mineral diet. Humus-building practices and addition of minerals not only supply plant nutrients but also increase tolerance to insects and diseases, help control weeds, retain soil moisture, and ensure produce quality.

A typical organic fertility system revolves around a combination of practices such as using crop rotation, forage legumes, cover crops, green manures, composted livestock manures, and lime, rock phosphate, and other rock minerals, as well as supplemental organic fertilizers. Depending on the soil type, soils with no history of organic management will probably need additional fertilization to be incorporated during field preparation and bedding operations or banded to the side of the row at planting.

Soil tests provide a baseline for your soil. Most state Extension services provide tests for a nominal fee. For information that is specific to organic production, see the ATTRA database Alternative Soil Testing Laboratories. In general, tomato plants have a high requirement for the macro elements potassium (K) and calcium (Ca) and the micronutrients iron (Fe), manganese (Mn), and zinc (Zn). Without a good supply of both K and Ca for plant uptake and utilization, the fruits will lack the recommended soluble solids content (sugars) and will be more susceptible to physiological disorders such as blossom end rot. Smaller requirements of the elements nitrogen (N), magnesium (Mg), phosphorus (P), boron (B), and copper (Cu) also are needed for proper plant development.

Tomatoes need moderate to high levels of P and K. On deficient soils, most needs can be met by applying rock powders such as rock phosphate, colloidal phosphate, untreated (mined) potassium sulfate, and sulfate of potash-magnesia in advance of planting. Supplementary P and K may be added as indicated by the soil test results. Always conduct a soil test to determine the proper amount of nutrients to apply.

Depending on the soil type, fresh market tomatoes also require about 75 to 100 pounds per acre of N. Most, if not all, can be supplied by legumes in rotation; composts or manures can also fill in the balance. If manure is chosen as the supplemental fertilizer, the USDA National Organic Program (NOP) regulations require that the manure be incorporated into the soil at least 120 days prior to harvest [NOP standard 205.203 (c)1]. Some farmers provide additional supplemental N at transplanting; a mixture of animal meal by-products, rock phosphate, and kelp meal is commonly used. If reliance is based primarily on supplemental fertilizers, about 50 pounds of actual N should be applied pre-plant and the remainder side-dressed when fruits are about nickel-size.

Tomato plants grow best when the soil pH is between 6.0 and 6.5. Liming to this range improves plant growth and optimizes fertilizer efficiency. If the pH is less than 5.5, Mg availability decreases significantly. Consequently, this increases the level of available Mn and aluminum (Al). Unless a deficiency of magnesium is noted, application of high-calcium (nondolomitic without magnesium) lime is advised. In addition, when a low pH is coupled with low Ca, blossom end rot is likely to occur. However, when soil tests are high in both P and pH levels, Zn will become insoluble, resulting in deficiency symptoms such as cupped leaves and splotchy chlorosis.

The ATTRA publication Organic Tomato Production will serve as a useful resource to learn much more on this topic. This publication addresses practical questions on organic tomato production. It focuses on the specific production challenges, including site selection (soil and climate), variety selection, sources of organic seeds and organic annual transplants, organic grafting, planting and training/staking arrangements, soil fertility and fertilization, crop rotation, and pest (insect, disease, and weed) management. Harvest and yield/productivity are closely related to marketing possibilities. While market conditions are extremely region-specific, this publication also addresses a few general principles on marketing and economics of organic tomatoes.

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