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Home  > Question of the Week

Question of the Week

Permalink What information can you give me on when I should apply compost?


Answer: Thank you for your recent request for information from ATTRA, the National Sustainable Agriculture Information Service. I am pleased to provide you with information on the application of compost.

There are several different thoughts on when the best time to apply compost is. Although several factors attribute to these differences, the main factor in deciding when to apply compost to the soil depends on the compost's condition, age, and degree to which the composting process is complete. Nutrients in the compost are released slowly and small amounts are available to germinating and young crops. If compost is applied too early before a crop is planted, nutrients may escape in to the air. Incorporating compost in to the soil close to planting time allows for the release of nutrients to take place in the soil.

Unfinished compost will continue to generate heat and decompose. This compost is best applied in the fall of the year so that it is ready to supply nutrients to spring planted crops. The preferred time to apply fully matured compost is around the time of seeding or transplanting. For a single crop, the compost can be applied a few weeks in advance. For succession plantings, it is best to apply the compost just before planting. It is recommended that the closer to the planting time the compost is applied, the finer it should be and more thoroughly incorporated into the soil.

Although compost is most often applied to an entire field or plot at one time, it is important to have a crop plan as some vegetable crops react differently to compost. For example, research suggests that tomatoes, cabbages, and most root crops yield better with compost applications taking place every-other-year, while beans, peas, corn, and squash do better with a compost application each year. In fact, applying compost to some crops should be avoided in the same season. This includes potatoes, which have a greater tendency to scab, and carrots as compost can cause them to become "hairy." A crop plan will allow you to plan your crop rotations accordingly. For example, you can apply compost prior to a planting of beans followed by tomatoes the following year that do not receive a compost treatment.

Based on the information you have provided regarding your plantings of fall cover crops, you can either apply the compost before the cover crops are seeded or in the spring once the cover crops are terminated and you are preparing for spring planting.

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Permalink What are some information resources on plans for aquaponics systems?


Answer: Thank you for your recent request for information from ATTRA, the National Sustainable Agriculture Information Service. I am pleased to provide you with information on plans for aquaponics systems.

Two authoritative guidebooks with details on system design and construction are:

The Aquaponics Guidebook
404.963.5998 or 404.421.1894
An online publication ($19.95 PDF download) covering the basic functional models of aquaponics, the potential for growth of a new industry, how to think about a contained balanced ecosystem, financial models and potential for business development, designing your own system (what you need to know), strategies for growing your system overtime, a step-by-step guide to getting your bacterial culture started, active links take you to key product websites, and a link to in-depth government knowledge bases about finance and science.

Aquaponic Food Production
Nelson and Pade, Inc
$29.95. The process, system components, system designs, daily operation and maintenance, and more.

Aquaponics Overview
Nelson and Pade, Inc
Detailed information on aquaponics, including instructions for building a mini-system available online at
Nelson and Pade also offers several systems for sale. Details are available at

Southern Regional Aquaculture Center
Recirculating Aquaculture and Aquaponic Systems publications
The SRAC offers the following publications as free PDF downloads:
• SRAC 0451 Recirculating Aquaculture Tank Production Systems: An Overview of Critical Considerations
• SRAC 0452 Recirculating Aquaculture Tank Production Systems: Management of Recirculating Systems
• SRAC 0453 Recirculating Aquaculture Tank Production Systems: Component Options
• SRAC 0454 Recirculating Aquaculture Tank Production Systems: Aquaponics Integrating Fish and Plant Culture
• SRAC 0456 The Economics of Recirculating Tank Systems: A Spreadsheet for Individual Analysis
• SRAC 4500 Partitioned Aquaculture Systems
• SRAC 4501 Constructing a Simple and Inexpensive Recirculating Aquaculture System (RAS) for Classroom Use

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Permalink What information can you give me on using molasses for weed control?


Answer: Thank you for your recent request for information from ATTRA, the National Sustainable Agriculture Information Service. I am pleased to provide you with information on the use of molasses for weed control.

Molasses is a byproduct from the processing of sugar from sugar cane and sugar beets. It is used as a food additive and in agricultural as a supplement in livestock feed, as a soil amendment, and as a natural herbicide. For agriculture purposes, there tends not to be a difference between cane molasses and beet molasses as both types have the same amount of calories as sugar; about 16 calories per teaspoon (1). The difference found in molasses and to consider for use as a soil amendment and for weed control is whether or not it has been treated with sulfur. Treating molasses with sulfur results in the fortification of iron, calcium, and magnesium. Some farmers feel that applying sulfured molasses is an organic way to add these nutrients to the soil. However, many researchers, such as Dr. Elaine Ingham (, have found that sulfured molasses can have significant effects on soil biology. This can have damaging effects when applying molasses for weed control.

There are two methods for using molasses for weed control. The first method focuses on applying molasses as a soil amendment that will feed soil organisms. By boosting soil microbial activity, more nitrates are taken up and biomass created. As a result, the soil is healthier which makes it harder for weeds to compete with desired plants. Studies have shown that the amount of molasses applied affects the activity of soil bacteria and fungi. These studies indicate that lower application rates promote more fungi while increased application rates favor bacterial activity (1). However, too much molasses increases the potash content in the soil. This can make calcium unavailable and help weeds thrive.

The second method for using molasses for controlling weeds is to apply it directly as an herbicide. This can be done by directly applying the molasses or by adding it to water or a calcium/water mixture. There are two trains of thoughts as to why the calcium mixture works. The first considers using molasses directly on the plants where the weed control is affective through the “burning” of the plants. The other train of thought considers the basis for this practice a modification of soil surface conditions that prevents weeds from germinating (2). The calcium and sugar solution reduces soil crusting and compaction, conditions that weeds flourish in. This practice tends to rely on being applied primarily to the soil surface as a pre-emergent, optimally following the drilling of a crop or at lay by following the last cultivation.

There are several recommendations for the amount of molasses to apply. As a feed for soil organisms, it is recommended to apply 2-4 oz. of liquid molasses per gallon of water or 20 lbs. of dry molasses per 1,000 square feet. As an herbicide, 40 lbs. of dry molasses per 1,000 square feet or one cup per gallon of water is suggested. For a calcium/molasses mixture, 2 gallons of liquid calcium and 2 gallons of molasses in 20 gallons of water is needed (2). Repeat applications may be necessary.


1. Moore, Robert. 2009. Molasses. Rockport, Texas: The Soil Guy. Retrieved July 9, 2010.

2. Diver, Steve. 2005. ATTRA Case Letter 62545. Fayetteville, AR: NCAT.

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Permalink What information can you give me on using high tunnels for vegetable production in warm season climates?


Answer: Thank you for your recent request for information from ATTRA, the National Sustainable Agriculture Information Service. I am pleased to provide you with information on using high tunnels for vegetable production in warm season climates.

High tunnels are used in southern climates to protect crops from pests, extreme solar radiation, and inclement weather. These high tunnels, which are commonly used throughout the Middle East, South America, and the southern U.S., differ in their primary use for season extension. This is opposed to the use of high tunnels in temperate regions where high tunnels provide warmer growing environments so that the growing season can be extended earlier in the spring, later in the fall, and possibly throughout the winter.

Due to the difference in use from temperate climates, high tunnels in more tropical weather conditions require somewhat different designs and construction. As with any high tunnel, the underlying factor in the design is ventilation. For a passively ventilated roof, it is suggested that the area of the roof vent be 20% of the floor area and located on the leeward side of the high tunnel. In addition, ventilation can be achieved through roll-up or roll-down sides. In fact, in many tropical areas, insect netting is used for the sides of the structure which allows for better ventilation.

One option to consider for reducing the amount of light and heat in a high tunnel is to use shade cloth rather than a plastic film. Shade cloth is a strong polyethylene fabric that can be purchased in different densities, usually between 30% to 80%. Shade cloth provides ventilation, improves light diffusion, and reflects heat, all of which help keep the high tunnel cool. A 30% density shade cloth can reduce the air temperature by 4 degrees Fahrenheit. The reduction in light intensity is beneficial to crops such as lettuce and greens. A further reduction in temperature can be achieved with the shade cloth by using sprinkler irrigation or mist.

Several growers have been able to reduce temperatures even more than with a shade cloth by using an aluminum reflective shade. These fabrics contain an anti-oxidation coating that cools the high tunnel. Some growers claim they can achieve a temperature reduction of 15 degrees Fahrenheit by using an aluminum-based shade cloth.

The web site high ( is a great resource for high tunnel growers. It provides information on growing in a high tunnel, resources and suppliers, as well as a list serve. You may be able to find information on using a high tunnel in your region and can also post any questions you may have to the group. In addition, the resource section contains several suppliers of shade cloth and aluminum fabrics, such as FarmTek ( One supplier you may want to investigate is Haygrove ( These unique three season high tunnels are becoming very popular amongst fruit and vegetable growers and may be of interest to you. Although based out of the UK, Haygrove does have a dealer and warehouse located in Pennsylvania (

Finally, I’d like to pass along the link to greenhouse consulting company that specializes in tropical vegetable production: This site contains information on constructing and growing in high tunnels located in tropical areas.

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