The USDA rules for certified organic farming include specific objective criteria such as allowable and prohibited practices and inputs, and more subjective criteria such as minimization of erosion and maintenance or enhancement of soil quality. Producers should note that international guidelines can differ from USDA guidelines. Certain inputs are allowable, and some inputs will be required to sustain soil fertility, but these must be applied in a manner that avoids nutrient excesses and minimizes runoff. Management of weeds, insects, and diseases often depends on several cultural practices. Crop rotation and tillage practices must provide an appropriate seedbed and pest control while minimizing erosion. Soil management practices must be developed in consultation with the certifying agent who interprets subjective aspects of the guidelines, approves inputs, and specifies needed documentation.

Crop rotation is critical to the maintenance of soil tilth (physical condition), fertility, organic matter, and as a preventive practice to minimize pest problems. No specific rotations are mandated, but suggested crops to include are sods, cover crops, green manures, and catch crops. The rotation adopted must resolve any relevant problems with soil organic matter content, deficient or excess plant nutrients, soil erosion, and pest management (for perennial crops). Defining a rotation is also a key component in designing soil sampling and tillage management schemes. For short rotations (two to three years), soil samples can be collected once per rotation. For longer rotations, soil samples may still need to be collected every two to three years, preferably prior to planting the most intensively managed crops. Conservation tillage reduces soil erosion and can improve soil tilth and may be alternated with more intense cultivation needed at other times during the rotation.

Although crop nutritional requirements are the same for organic and conventional farms, organic producers need to be more creative due to the limitations on allowable inputs. Soils throughout the Carolinas differ in texture, organic matter, and past erosion and management history. Additionally, fluctuations in weather patterns and crop yields result in unpredictable residual nutrient status. Periodic soil testing is the only way to understand the current fertility level and sustain the fertility status of each field while avoiding excess nutrient accumulation. Plant tissue analysis also can be used to verify the adequacy of soil fertility management, particularly for nutrients not easily measured in routine soil tests (nitrogen, sulfur, boron). With tissue testing, the appropriate plant part must be collected at the proper growth stage as specified by laboratory guidelines. See Plant Tissue Analysis or contact your county Extension center for more details.

North and South Carolina have numerous sources of plant and animal manures and byproducts. This region also has a favorable climate for growing a diversity of rotational and green manure cover crops. Farmers should study their crops to fully understand production requirements, nutrient needs, and common production problems. Crops differ in their nutrient removal rates (Table 9-1), and nutrient sources differ in their nutrient contents (Table 9-2).

Certain inputs are allowable on organic production systems, if applied according to guidelines. These include many (not all) natural and certain synthetic materials. The National List of Allowed and Prohibited Substances under the NOP is available at USDA's National Organic Program. This list specifies synthetic substances allowed for use and nonsynthetic substances prohibited for use in crop production. A list of commercially available sources of these materials which have been reviewed by the OMRI and classified as either allowed (A) or regulated (R) is available at OMRI Lists. Other materials should be considered prohibited until further notice. In all cases, input use should be included in the farm plan and confirmed by the certifying authority prior to application.

Critical aspects of soil fertility management include pH, major nutrients (N, P, K, S, Ca, Mg), and micronutrients (especially B, Cu, Mn, Zn; but also Fe, Mo, Cl). A summary of soil fertility parameters and organic management options is given in Table 9-3.

Soil pH

Soil pH is important because it influences nutrient solubility, microbial activity, and root growth. Low pH levels common in native Carolina soils can kill developing root tips and prevent root colonization of the soil. Low soil pH continues to be the most common limiting factor for plant development seen in samples submitted to the NCDA&CS Agronomic Division Laboratory. Correcting low soil pH should be based on soil test recommendations that consider both the soil pH and the soil residual acidity levels, crop pH requirement, target pH based on soil class, and whether calcitic or dolomitic lime should be used. Because most agricultural lime is from naturally-occurring minerals of relatively low solubility, its use is generally allowed in organic production systems. Hydrated limes, burnt limes, lime-stabilized biosolids, and industrial wastes or slags are not allowed. In rare cases when soil pH is too high to permit optimum crop growth, elemental sulfur can be used to lower soil pH. An approximate rate can be based on the estimation that the amount of acidity generated by 640 pounds of sulfur is the same as that neutralized by 1 ton of lime.

Nitrogen

Nitrogen (N) is the most frequently limiting nutrient for crop production. Organic farms need to supply N through sources such as legumes, animal wastes or byproducts, plant processing byproducts, or limited additions of mined mineral deposits. It is possible for a nitrogen-fixing legume or legume-and-grass mixture cover crop to provide adequate nitrogen for certain cash crops. A seed inoculum is recommended for legumes unless adequate native inoculum is present, and adequate soil fertility is needed to insure no other factors limit legume growth. Inoculums, however, must not be mixed with any prohibited substances such as pesticides or inorganic fertilizers.

Some promising legume cover crops for North Carolina are described in Table 9-4. Besides the species listed, there are numerous other clovers, lupines, grasses, mustards, and species mixtures that could be useful.

Many farmers in North Carolina use composted or uncomposted poultry litter to supply the nitrogen needs for their organic field crops. Poultry litter and poultry byproducts are available in many parts of the state. Mined nitrates, such as sodium nitrate (NaNO_3, often referred to as Chilean nitrate), may be used in certain places, but are limited to a maximum of 20 percent of the crop’s total N requirement. Constantly relying upon NaNO_3, a restricted substance in organic agriculture, will be questioned by a certification agency. Because many international certifiers, including those in the European Union, Japan, Australia, and even some U.S. certifiers prohibit any use of Chilean nitrate, producers should verify its acceptability to their certifier and to their prospective markets.

Other nutrients

Other nutrients (P, K, Ca, Mg, S, Cu, Mn, Zn) can generally be supplied in adequate amounts through additions of lime (Ca, Mg), animal or plant byproducts or wastes (P, K, S, micronutrients), or permissible mineral inputs. Naturally-occurring minerals of relatively low solubility are generally allowed (lime, gypsum, rock phosphate, rock dusts, mined humates).

In addition, the following naturally-occurring minerals of relatively high solubility may be applied if used in compliance with the National List.

• Magnesium sulfate (Epsom salt), with a documented soil deficiency.
• Sulfate of potash and potassium magnesium sulfate, if from an approved source and with a documented soil deficiency.
• Muriate of potash, if derived from a mined source and applied in a manner that minimizes chloride accumulation in the soil. This may be acceptable for most crops in the Carolinas with a soil test to document the deficiency and recommend an application rate.
• Many micronutrient salts, with documented soil deficiency and if not in the form of nitrate or chloride salts. This includes various soluble boron products and sulfates, carbonates, oxides, or silicates of zinc, copper, iron, molybdenum, selenium, and cobalt.

Numerous animal and plant byproducts are available to provide essential crop nutrients (Table 9-2). It is important to check with the certifying agency, and the prospective market if possible, before using any input.

Management of soil tilth, organic matter, and fertility is an important aspect of a successful organic production system. Organic farming systems traditionally relied on tillage before planting and cultivation after planting to control weeds and reduce the incidence of seedling diseases and insect pests. However, tillage destroys organic matter and increases soil erodibility, both critical aspects of soil fertility, water-holding capacity, and general soil quality. Following annual grain crops with cover crops where the soil surface is protected by a growing crop for most of the year makes it easier to maintain soil organic matter content. The use of roller-crimpers to kill cover crops, and heavy residue cultivators for additional weed control, also provides soil and water conservation benefits. Where tillage is used, it should occur when soil moisture is low enough to prevent compaction. Because primary tillage operations are often a month before a crop is planted, tillage requires careful planning and the ability to take advantage of periods of dry weather. The effect of soil moisture on tillage also means that fields with soils that are poorly drained or that have low spots may not be good choices for organic production. Recommendations on the type of primary and secondary tillage practices for specific soil types and field conditions can be found elsewhere in this guide.

Because use of some soil amendments is limited to cases of nutrient deficiency, organic producers should maintain soil testing and plant tissue analysis records documenting specific nutrient deficiencies that need correction. Soil test records can also be useful in documenting soil quality maintenance as they indicate changes in humic matter and nutrient levels over time. Of particular concern are avoiding topsoil erosion, which could result from excessive cultivation for weed control (declines in humic matter are indicative of erosion losses), and avoiding excess phosphorus and micronutrient accumulation following application of manures and composts.

Specific guidelines must be followed when applying composts and manures in organic production systems. Materials must be applied at agronomic rates that comply with any applicable nutrient management guidelines and that avoid excess nutrients. Raw animal manures must either be: (1) composted according to specific criteria, (2) applied to land used for a crop not intended for human consumption, (3) incorporated into the soil at least 90 days prior to the harvest of an edible product not contacting soil or soil particles, or (4) incorporated into the soil at least 120 days prior to the harvest of an edible product that does contact soil or soil particles. The guidelines for compost production for organic agriculture state that the initial C:N ratio must be between 25:1 and 40:1, and a temperature between 131°F and 170°F must be achieved. This temperature must be maintained for at least 3 days for in-vessel or static aerated pile systems, or for at least 15 days during which there are at least five turnings for windrow systems. Composts not meeting these criteria must be applied based on other raw manure criteria, which also apply to lagoon liquids, lagoon solids, and stockpiled poultry litter. Ashes of manures may not be used, but ashes from other untreated plant and animal materials may be applied if not combined with any prohibited substances.

Avoid over-reliance on animal manures because this could lead to accumulation of excess P, Cu, and Zn in soils. For example, based on the general nutrient data shown in Tables 9-1 and 9-2, stockpiled turkey litter applied at a rate of 5 tons per acre would supply approximately the amount of N removed by a 150-bushel-per-acre corn crop. Note that the amount of phosphorus added (as P₂O₅ equivalent) would be 360 lb/acre, while crop removal would be only 53 lb/acre. Similarly, 2.8 lb/acre of zinc would be added, while crop removal would be only 0.15 lb/acre. Sporadic use of manures in conjunction with more frequent use of legume cover crops, green manures, or other N sources is an excellent way to supply several plant nutrients in appropriate amounts.

Other NC State Extension publications describe specific types of manures (such as swine, poultry, dairy). Because the nutrient composition of animal manures and composts can vary widely, submit a sample to the Plant and Waste Analysis Laboratory of the NCDA&CS Agronomic Division before using such material. Sewage sludge and composted municipal wastes are not allowable on organic fields.

Looking Back Farms, located in northeastern NC, is a 350-acre organic grain farm located mostly on Arapaho fine sandy-loam soil. This farm has been certified organic for nearly 15 years. Currently, the farm employs a two-year rotation of corn–wheat–soybeans, with wheat and soybeans double-cropped.

Corn is planted in May and is seeded at a rate of 32,000 seeds per acre on 36-inch rows. This rate is about 10 percent higher than a conventional rate for the area. The higher plant population reduces weed competition and allows the population to compensate for any plants lost during cultivation. Soybeans are planted on the same row spacing, and wheat is broadcast-seeded at a rate of about 120 to 150 pounds per acre.

Fertility Management: Looking Back Farm applies chicken litter from a neighboring farm on corn and wheat. The litter is applied preplant at 3 to 4 tons per acre. Mushroom compost (2:2:4 analysis) is used when available before soybeans (applied on top of wheat stubble when double cropping) and is eventually applied to all fields over two years. The compost contains liming agents and helps raise the soil pH, which keeps soil life and nutrients in balance, which in turn allows maximum crop health and growth. Cover crops are also used on this farm as part of the fertility management plan. Rye, crimson clover, Austrian winter pea, and triticale have all been grown as cover crops after corn. The cover crops increase soil organic matter, soil microbiology, water infiltration, and water-holding capacity. The legume crops (Austrian winter pea and crimson clover) also fix nitrogen, adding it to the soil so it is available for subsequent crops. The cover crops are broadcast-seeded in October after corn has been harvested and fields tilled. The cover crops are cut and tilled into the soil in March. This gives enough time for the cover crop residue to break down before soybeans or corn are planted.

The farmers try to avoid a buildup of phosphorous and heavy metals from poultry litter in their soil by rotating the compost and litter fertility amendments and by using cover crops. They see benefits of one application of a fertility amendment for up to three years after the application because the organic amendments take much longer than conventional fertilizers to decompose and release N and other nutrients. They often do not see the benefits of applying litter or compost in the spring to their wheat crop because of the decomposition time. Nitrogen from the litter or compost becomes available slowly, as the amendment breaks down, and the plants cannot take advantage of a nitrogen boost because the nutrients are released more slowly than conventional fertilizers. However, their yields do not suffer. It is likely that the organic amendments applied at planting (and years before) provide enough fertility as they slowly break down for the crop. No lime has been applied in many years, and the pH of soils in all fields is 6.0 to 6.5.