Making it work on organic vegetable farms



Managing soil nutrients, or chemistry, should not be separated from management of the physical and biological condition of soil. A good nutrient management program supports and improves all three aspects of soil health, since they are interrelated.

Nutrient management on organic farms is part of promoting healthy soil with good physical, biological and chemical properties. The key to building healthy soil is organic matter management. Building up and maintaining soil organic matter contributes to nutrient management through better soil tilth and thus root exploration, enhanced biological activity which increases mineralization and nutrient availability, and greater cation exchange capacity which enhances nutrient retention. Soil organic matter also promotes an abundance of microorganisms that can stimulate root growth and help solubilize nutrients.

The main practices involved in nutrient management and soil organic matter management are crop rotation, cover cropping, additions of compost and/or manure, and supplemental applications of organically approved amendments and fertilizers.

Besides maintaining soil health, the goal of a nutrient management plan should be to meet a crop’s nutrient needs as economically as possible while avoiding application of excess nutrients. It can be a challenge to build up and maintain soil organic matter levels without large annual applications of nutrient-rich amendments, and that can lead to excess nutrient levels. In fact, it is not uncommon to find organic vegetable farms with very high levels of soil P and K that continue to apply large quantities of compost. These farms need to consider other methods for adding carbon and nitrogen to their soils.

PHOTO BY VERN GRUBINGER.
Hairy vetch plus winter rye being plowed down in early spring. The vetch, a winter annual, can provide significant N to a subsequent crop without taking land out of cash crop production.

Crop rotation

It all starts with crop rotation because this provides so many benefits to a farming system. Good rotations not only help with pest management, they also benefit soil structure and provide free nitrogen when legumes are included. Rotating with legume cover crops or forages like alfalfa, clover, hairy vetch and field pea is especially important on farms that make (or should make) limited use of manure or compost. Legumes also help avoid excessive reliance on bagged fertilizes to meet crop nitrogen needs, which gets expensive and is not in keeping with the organic farming principle of optimizing on-farm inputs.

Including nonlegume soil-improving crops in a rotation is also important, since they add carbon to the soil that helps maintain organic matter and soil structure. Nonlegumes sown after cash crops can also mop up available nutrients, preventing them from leaching, thus keeping them on the farm for future use.

Small grains like rye, oats or wheat are typically used as winter covers; summer smother crops include sorghum-Sudangrass or Japanese millet; perennial grasses are sometimes used for hay production in rotation with vegetables.

The extent to which vegetable growers can rotate with legumes and other soil-improving crops depends on the land they have available. Some growers try to rotate fields so they are in cash crops one year and cover crops the next year. On farms with limited land for rotation out of cash crops, soil-improving crops may take up a much smaller part of the rotation, but I recommend that at least one-quarter of your land be resting in cover crops or forages at any given time.

On farms where all land must be cropped for economic reasons, winter cover crops, along with occasional summer smother crops and interseedings, are usually viable options.

Tables 1 and 2  are two examples of crop rotations that include legumes to provide most of the N needed for vegetable crop production on an ongoing basis.

 

Nitrogen credits from a previous crop can be estimated using the following table, adapted from the New England Vegetable Management Guide at www.nevegetable.org.

Compost and manure

Compost and manure are great for enhancing the physical condition of soil while building soil organic matter that serves as a slow-release reservoir of nutrients. But, as noted above, care must be taken with these soil amendments to avoid adding excess nutrients, especially with repeated applications over time. Research by Tom Morris at the University of Connecticut has demonstrated that it is not uncommon for organic farms to have soils that are very high in N, P and K, yet they continue to add compost annually (see: www.newenglandvfc.org/pdf_proceedings/SoilOrganicAmend.pdf).

The nutrient content and rate of release varies among composts and manures. Fresh manures have more available nutrients than aged manures, and while mature compost is a good soil conditioner, it is not a good source of short-term fertility since it contains relatively low levels of available nutrients.

Testing compost or manure prior to application is the only way to determine its nutrient status, pH and salt content. It takes a little extra effort to send in a sample, but the information is useful to nutrient management since variability is so high. For example, in a survey of 20 on-farm composts, UMass researchers found the pH to range from 5.4 to 7.9, and the total N content to vary from 8 to 47 pounds per ton. They estimated that adding 20 tons per acre of a typical compost would add a total of 320 pounds of N; however, only about 42 pounds of this would likely be available to crops in year one. See www.umassvegetable.org/soil_crop_pest_mgt/soil_nutrient_mgt/
compost_use_soil_fertility.pdf
 online.

Most land grant universities offer manure and compost testing at a modest cost. Alter-natively, you can estimate the nutrient content using tables like Table 3, but keep in mind these are only general estimates because manure properties are variable and the only way to really know their nutrient content is to have them tested.

Organic fertilizers

There are many kinds of organic fertilizers made from plant meals, minerals and/or animal products. The following table, adapted from the University of Maine soil testing lab, lists some common bagged fertilizers, their nutrient content and the quantities needed to provide different amounts of available nutrients.

 
 
 

If you are on a certified organic farm, always be sure to check with your certifier whether the material, as well as the brand of fertilizer you plan to apply, is allowable.

Soil organic matter

The total amount of N in the plow layer of soil is surprisingly large; it can be estimated by multiplying soil organic matter content by 1,000. Thus, a soil with 4 percent organic matter contains about 4,000 pounds total N per acre. However, very little of the total N is mineralized annually into the mineral forms plants can use—typically from 1 percent to 4 percent each year, depending on the soil and environmental conditions. For soil with a total of 4,000 pounds N per acre, a 1 percent to 4 percent conversion would produce 40 to 160 pounds N per acre.

The mineralization rate depends on microbial activity, which is favored by warm soils with adequate, but not excessive, moisture and a pH above 6. On well-managed soils used for vegetable production, a 2 percent mineralization rate is a reasonable estimate, so that 20 pounds of N per acre can be credited for each percentage of soil organic matter.

Putting it all together

It requires some record-keeping and calculations, whether putting pencil to paper or fingers to keyboard. It’s not possible to maintain an effective nutrient management plan in your head. Soil test results and recommendations are also essential. Based on these, you start with a target level for each major nutrient and then subtract the available nutrient credits contained in soil amendments and soil organic matter.

An example

You will be growing an acre of cabbage. The New England Vegetable Guide suggests a total of 160 pounds of N per acre (60 of it as a sidedress) and your soil test results show high P levels with zero P recommended and medium K levels with 130 pounds K2O per acre recommended. The field you’ll be planting has 3 percent organic matter and a pH of 6.5, and there’s a fair stand of red clover that will be turned in a week or so prior to planting as soon as you spread and promptly incorporate 5 tons per acre of dairy manure with bedding.

Your fertilizer options to meet the crop’s need are limited because you do not want to apply a blended fertilizer that will add excess P. If this field did need some P, say 50 or 60  pounds an acre, then you could apply 750 pounds of pelletized bagged poultry manure (4-4-4) to provide 30 pounds each of N, P2O5 and K2O, then add another 45 pounds of K2O.

That is not the case in this example, so you need to apply N and K separately. To provide 30 pounds of N you could sidedress 230 pounds an acre blood meal (but this might cause excess ammonia to burn plants), 200 pounds per acre feather meal, or 187 pounds per acre Chilean nitrate (the 30 pounds of total N per acre is still less than 20 percent of the recommended crop need). Whatever material or combination of materials you choose, split the total application into two, or even three, sidedressings (when cultivating for weeds anyway) so as to optimize plant uptake of the applied nitrogen as the crop grows.

Potassium is a little easier to figure out in this example. To apply 85 pounds K2O per acre, you can broadcast and incorporate 170 pounds of potassium sulfate, if your soil already has sufficient magnesium. If magnesium is needed, then the equivalent amount of K would be in 386 pounds of sul-po-mag, which would also add some Mg. But don’t worry about such precision when it comes to applying major nutrients; in this case 350 or 400 pounds per acre would be fine. The goal is to be in a reasonable range with your nutrient applications: avoiding excesses of nutrients in the soil while meeting the needs of your crops.

The author is Vegetable and Berry Specialist with University of Vermont Extension based at the Brattleboro office.