Organic Field Crop Production and Marketing in North Carolina

Chapter 6: Soil Management

Carl Crozier, Soil Science Extension Specialist, NC State University

Keith Baldwin, Extension Specialist, NC A&T State University

David Howle, Assistant Professor, Clemson University

Soil Management & the Organic Standards

In an organic farming system, rotation and tillage practices must provide an appropriate seedbed and pest control while minimizing erosion. Soil fertility must be sustainable without application of prohibited substances. Soil management practices must be developed in consultation with the certifying agent who judges compliance with organic standards, approves inputs, and specifies needed documentation.

Crop Rotation, Tilth, Fertility, and Pest Management

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. 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.

Soil Fertility Management

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, past erosion, and residual nutrient contents. Periodic soil testing is the only way to understand the current fertility level and maintain the fertility status of each field. Plant tissue analysis can also be used to verify soil fertility status, 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 www.ncagr.com/agronomi/ptaflyer.htm or contact your county Extension center).

North and South Carolina have numerous sources of plant and animal manures and by-products. This region also has a favorable climate for growing a diversity of rotational and green manure cover crops that can provide needed nitrogen and other nutrients. Farmers should study their crops to fully understand production requirements, nutrient needs, and common production problems. Crops differ in their nutrient removal rates (Table 6-1), and nutrient sources differ in their nutrient contents (Table 6-2).

Table 6-1. Nutrient removal (in pounds) by different crops. Missing values indicate no data are available.

Table 6-2. Nutrient content of selected natural sources. These are general values and may not accurately represent the content of any specific source. Laboratory analysis should be performed prior to utilizing these materials. Missing values indicate no data are available. Use of any specific source should be approved by the certifying authority prior to application on an organic farm.

1 Plant available N values shown represent estimate for material incorporated into the soil unless specified otherwise.

2 Sprinkle-irrigated and not incorporated.

Certain inputs are allowable on organic farming systems, if applied according to guidelines. These include mostly natural and a few synthetic materials. The National List of Allowed and Prohibited Substances under the National Organic Program is available online: http://www.ams.usda.gov/nop/NationalList/FinalRule.html. This list specifies synthetic substances allowed for use and nonsynthetic substances prohibited for use in crop production. The Organic Materials Review Institute (OMRI) was developed to review materials for approval and to simplify the National List. OMRI classifies materials as either allowed (A) or regulated (R), and lists generic and brand names of materials. OMRI can be accessed via the Web at www.omri.org; or by mail or phone at Box 11558, Eugene, OR 97440, (541) 343-7600. 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 for certification as an organic farm.

Critical aspects of soil fertility management include pH, major nutrients (nitrogen, phosphorus, potassium), secondary nutrients (sulfur, calcium, magnesium), and micronutrients (especially boron, copper, manganese, zinc; but also iron, molybdenum, chlorine, selenium, and cobalt). A summary of soil fertility parameters and organic management options is given in Table 6-3.

Table 6-3. Soil fertility parameters and management option.

1 Inputs are limited to materials approved on the National Organic Program List or on the OMRI-approved source list after consultation with the certifying agent regarding need to document deficiency and application records.

2 Avoid over-application of micronutrients since toxicities can occur.

3 See restrictions in text.

4 Documentation of nutrient deficiency required.

5 Deficiencies of Co, Mo, and Se are not common in North Carolina, and these elements are not included in routine tissue analysis performed by the North Carolina Department of Agriculture and Consumer Services. Consult a Cooperative Agricultural Extension office for information regarding private agricultural laboratories.

Soil pH is important because it influences nutrient solubility, microbial activity, and root growth. The low pH levels common in native Carolina soils continue to be the most common limiting factor for plant development seen in samples submitted to the NCDA&CS Agronomic Division Laboratory. Since most agricultural lime is from naturally occurring minerals of relatively low solubility, its use is generally allowed in organic farming systems. Hydrated limes and burnt limes are not allowed. Pelletizing agents should be evaluated to determine that they are not prohibited materials.

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 by-products, plant-processing by-products, 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 ensure no other factors limit legume growth. Inoculums, however, must not be genetically engineered. Nitrogen-fixing cover crops for summer in North Carolina include cowpeas and soybeans. Winter N-fixing cover crops include hairy vetch, Cahaba vetch, Austrian winter peas, and many clovers. 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 by-products are available in many parts of the state. Mined nitrates, such as sodium nitrate (NaNO3, bulldog soda, or Chilean nitrate) may be used, but are limited to a maximum of 20 percent of the crop’s total N requirement. Constantly relying upon NaNO3, a restricted substance in organic agriculture, will be questioned by a certification agency.

Other nutrients. Phosphorus, potassium, calcium, magnesium, sulfur, copper, manganese, and zinc can generally be supplied in adequate amounts through additions of lime (calcium, magnesium), animal or plant by-products or wastes (phosphorus, potassium, sulfur, 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 by-products are available to provide essential crop nutrients (Table 6-2). It is important to check with the certifying agency before using any input.

Tillage Practices

Management of soil tilth, organic matter, and fertility is an important aspect of a successful organic farming system. Current organic systems usually require tillage prior to planting and cultivation after planting, especially for corn and soybean production, to control weeds and reduce the incidence of seedling diseases and insect pests. However, tillage destroys the organic matter that is critical in improving soil fertility and soil water-holding capacity. The use of rotations with cover crops where the soil surface is covered with a growing crop for most of the year is important in maintaining organic matter content during periods when corn is not grown. Tillage should be performed when soil moisture is low enough to prevent compaction. Since primary tillage operations are usually performed at least a month before a crop is planted, this requires careful planning and the ability to take advantage of periods of dry weather. No-till agriculture in organic systems is starting to be used in parts of the country. The Rodale Institute has experimented with no-till organic using cover crops and tractor-mounted rollers to kill the cover just before planting into it.

Documenting Crop Nutrient Deficiencies and Soil Quality Maintenance

Since use of some soil amendments is limited to cases of nutrient deficiency, organic producers should maintain records of soil test results and plant tissue analyses to document specific nutrient deficiencies that need correction. Soil test records can also be useful in documenting soil quality maintenance because they will show changes in humic matter and nutrient levels over time. It is important to avoid the topsoil erosion that could result from excessive cultivation for weed control (declines in humic matter indicate erosion losses) and to avoid accumulation of excess phosphorus and micronutrients following application of manures and composts.

Composts and Manures

Specific guidelines must be followed when applying composts and manures in organic farming systems. Materials must be applied at agronomic rates in compliance with any applicable nutrient management guidelines and in ways that avoid excess nutrients (see http://www.soil.ncsu.edu/nmp/ncnmwg/ or contact your local Soil and Water Conservation District office). Raw animal manures must be

• composted according to specific criteria,

• applied to land used for a crop not intended for human consumption,

• incorporated into the soil at least 90 days prior to the harvest of an edible (human-consumed) product not contacting soil or soil particles, or

• 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 131º and 170ºF 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, since this could lead to accumulation of excess phosphorus, copper, and zinc in soils. For example, based on the general nutrient contents shown in Tables 6-1 and 6-2, 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 P 2O 5 equivalent) would be 360 pounds per acre, while crop removal would only be 53 pounds per acre. Similarly, 2.8 pounds per acre of zinc would be added, while crop removal would only be 0.15 pounds per 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 plant nutrients in appropriate amounts.

NC State University Soilfacts bulletins describe specific types of manures (such as swine, poultry, and dairy) at http://www.soil.ncsu.edu/publications/Soilfacts/). Since nutrient composition of animal manures and composts can vary widely, it is wise to submit a sample to the Plant and Waste Analysis Laboratory of the NCDA&CS Agronomic Division before use. Sewage sludge and composted municipal wastes are not allowed on organic fields.