NC State Extension Publications

Soil Management and the Organic Standards

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 (Tilth, Fertility, and Pest Management)

Crop rotation is critical to the maintenance of soil tilth (physical condition), fertility, and 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.

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, 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 can also 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 local county Extension center for more details.

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. 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).

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 online. 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 www.omri.org/OMRI_brand_name_list.html. 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.

Table 6-1. Nutrient removal (in lb) by different crops. Approximate removal rates can be adjusted based on comparison with the crop yield level shown. Missing values indicate no data available.

Nutrient

Corn, grain 150 bu

Wheat 60 bu

Soybean 50 bu

Tobacco, flue-cured 3000 lb

Irish potato, 15 tons

Sweet potato, 300 bu

Fescue 3.5 tons

Ryegrass 5 tons

Sorghum-sudan 8 tons

N

112

75

188

85

90

40

135

215

319

P2O5

53

38

41

15

48

18

65

85

122

K2O

40

22

74

155

158

96

185

240

467

S

10

4

23

12

7

6

20

46

Ca

2

2

10

75

5

4

60

Mg

8

9

10

15

7

4

12

40

47

B

0.1

0.05

0.06

0.05

0.08

0.05

Cu

0.06

0.04

0.05

0.03

0.06

0.02

Mn

0.08

0.14

0.06

0.55

0.14

0.06

Zn

0.15

0.21

0.05

0.07

0.08

0.03

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 available. Use of any specific source should be approved by the certifying authority prior to application to an organic farm.

Source

Units

Plant available N 1

(total N if specified)

P2O5

K2O

S

Ca

Mg

B

Cu

Mn

Zn

Swine lagoon liquid

lb/acre-inch

109 1

68 2

37.1 2

93.1 2

10

26

8.3

0.18

0.3

0.34

1.5

Broiler, fresh manure

lb/ton

15.6

17

11

2

10

4

Broiler, stockpiled litter

lb/ton

21.6

80

34

12

54

8

0.04

0.27

0.59

0.55

Turkey, fresh manure

lb/ton

16.2

25

12

27

2

Turkey, stockpiled litter

lb/ton

21.6

72

33

9.5

42

6.8

0.05

0.34

0.62

0.56

Blood, dried

lb/ton

240 to 300 (total N)

60

6

Bone meal, raw

lb/ton

70 (total N)

440

4

440

12

Shrimp process waste

lb/ton

58 (total N)

200

Cotton motes

lb/ton

40 (total N)

10

60

12

80

14

Peanut hull meal

lb/ton

24 (total N)

12

16

Wood ash

lb/ton

0.0

40

120

400

20

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 6-3.

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, as well as 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 lb sulfur is the same as that neutralized by 1 ton of lime.

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-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 6-4. 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 by-products are available in many parts of the state. Mined nitrates, such as sodium nitrate (NaNO3, 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 NaNO3, 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: P, K, Ca, Mg, S, Cu, Mn, Zn can generally be supplied in adequate amounts through additions of lime (Ca, Mg), animal or plant by-products 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 minimizeschloride 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, and the prospective market if possible, before using any input.

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

Effect on Plants

Problem Documentation

Supply Options 1

Not Allowed

pH

Nutrient solubility,

Root development,

Microbial activity

Soil test

Lime (standard, ground calcitic or dolomitic carbonate source)

Hydrated or burnt lime

[Ca(OH) 2, CaO]

Major Nutrients

Nitrogen (N)

Component of proteins, chlorophyll

Tissue analysis

N fixation by legumes, manures 3, animal by-products (blood, fish), plant by-products (cotton (restricted due to pesticide use), apple, fermentation wastes), mined sodium nitrate (NaNO3)3

Synthetic fertilizers, sewage sludges, municipal waste composts

Phosphorus (P)

Component of nucleic acids

Soil test, tissue analysis

Manures 3, rock phosphate, animal by-products (bone meal; fish, shrimp, & oyster scraps; leather)

Processed rock phosphates

Potassium (K)

Water, salt, & pH balance; enzyme activation; protein synthesis; photosynthesis

Soil test, tissue analysis

Manures 3, plant by-products (ash, dried seaweed), greensand, sulfate of potash (K2SO4)4, possibly muriate of potash (KCl)3,4

KCl if excess chloride

Secondary Nutrients

Sulfur (S)

Component of proteins; volatile compounds of mustard, garlic, onion

Tissue analysis

Manures 3, plant by-products (cotton motes, peanut meal), elemental sulfur 4, gypsum (CaSO4), Epsom salt (MgSO4) 4, sulfate of potash (K2SO4) 4

Synthetic fertilizers

Calcium (Ca)

Cell wall & membrane stabilization, cell growth, osmoregulation

Soil test, tissue analysis

Lime (mined carbonates), gypsum (CaSO4), bone meal, ash

Ca(OH)2, CaO, calcium nitrate [Ca(NO3)2]

Magnesium (Mg)

Component of chlorophyll, cell pH and cation balance, enzyme activation

Soil test, tissue analysis

Dolomitic lime, Epsom salts (MgSO4) 4, sulfate of potash magnesium, bone meal, plant by-products (cottonseed meal, wood ash)

Synthetic fertilizers

Micronutrients 2

Boron (B)

Cell wall & membrane stabilization, cell growth, carbohydrate & protein metabolism, pollen germination

Tissue analysis

Manures, animal and plant by-products, soluble boron fertilizers 4

Copper (Cu)

Enzyme component, photosynthesis, respiration, cell wall lignification, pollen formation

Soil test, tissue analysis

Manures, animal and plant by-products, sulfates & oxides 4

chlorides

Manganese (Mn)

Enzyme activation, protein component, photosynthesis, cell growth

Soil test, tissue analysis

Manures, animal and plant by-products, sulfates & oxides 4

chlorides

Zinc (Zn)

Enzyme component & activation, protein synthesis

Soil test, tissue analysis

Manures, animal and plant by-products, sulfates & oxides 4

chlorides

Cobalt (Co), Iron (Fe), Molybdenum (Mo), Selenium (Se)

Tissue analysis5

Manures, animal and plant by-products, sulfates, carbonates, oxides, or silicates 4

Chlorides, nitrates

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.

Tillage Practices

Management of soil tilth, organic matter, and fertility is an important aspect of a successful organic production system. Organic farming systems traditionally relied on preplant tillage 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 so that 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, this requires careful planning and the ability to take advantage of periods of dry weather. This 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 (see Chapter 7, “Weed Management”).

Documenting Crop Nutrient Deficiencies and Soil Quality Maintenance

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 these records 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.

Composts and Manures

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 and 170°F must be achieved. This temperature must be maintained for at least three 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 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 P2O5 equivalent) would be 360 lb/acre, while crop removal would only be 53 lb/acre. Similarly, 2.8 lb/acre of zinc would be added, while crop removal would only be 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.

Extension fact sheets are available online that describe specific types of manures (such as swine, poultry, and dairy). Because 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 a manure or compost. Sewage sludge and composted municipal wastes are not allowable on organic fields.

Species

Type

NC Planting dates*

Seeding rates** lb/ac

Seeding depth in.

Comments

Crimson clover

winter annual legume

Mtns: 8/10 – 9/15 (10/15?)
Piedmont: 8/25 – 10/1 (10/15?)
Coastal Plain: 9/1 – 9/30 (10/30?)

15 – 20 d
20 –25 bc

¼ to ½

50-150 lb N/ac or more possible. On very-poorly drained soils often <25 lb N/ac.

Hairy vetch

winter annual legume

Mtns: 8/10 – 9/15 (10/15?)
Piedmont: 8/25 – 10/15 (10/30?)
Coastal Plain: 9/1 – 9/30 (10/30?)

15 – 20 d
20 – 30 bc

½ to 1 ½

100-150 lb N/ac or more possible on most soils. Avoid weedy persistence by killing before seed set.

Austrian winter pea

winter annual legume

Mtns: 8/10 – 9/15 (10/ 15?)
Piedmont: 8/25 – 10/1 (10/15?)
Coastal Plain: 9/1 – 9/30 (10/30?)

20 – 25 d
25 – 35 bc

¼ to ½

50-100 lb N/ac possible. Reportedly susceptible tosclerotinia fungal disease.

Cahaba white vetch

winter annual legume

Coastal Plain: 9/1 – 9/30 (10/30?)

15 – 20 d
20 – 30 bc

½ to 1 ½

Not adapted to NC mountain or piedmont region.

Rye

Wheat

Triticale

winter annual grass

Mtns: 8/15 – 9/30 (10/ 30?)
Piedmont: 9/15 – 10/15 (11/15?)
Coastal Plain: 9/30 – 11/15 (12/15?)

90

½ to 1 ½

Most vigorous winter growth of annual grasses.

Avoid wheat as a cover crop in areas with wheat grain production.

Oats

winter annual grass

Piedmont: 9/15 – 10/15 (11/15?)
Coastal Plain: 9/30 – 11/15 (12/15?)

120

½ to 1 ½

Not adapted to Mountain region.

Sunn hemp

summer annual legume

May-August

30 – 50 d
40 – 60 bc

½ to 1

Up to 125 lb N/ac reported in southeast U.S. Nematode resistant.

Forage soybean

summer annual legume

mid-April to July

90 – 120

1 to 2

Up to 250 lb N/ac possible. For forage, higher plant populations result in thinner, more palatable stems.

Cowpea

summer annual legume

May to August

30 – 90 d
70 – 100 bc

1 to 2

Up to 130 lb N/ac reported in eastern U.S., relatively drought tolerant

Sorghum-sudan

summer annual grass

May to July

35 – 40 d
40 – 50 bc

1 to 2

High biomass, multiple hay cuttings possible, possible chemical weed & nematode suppression effect

Buckwheat

cool season annual

September

50

½ to 1

30-45 day growth period, not tolerant of flooding or hardpans

Forage radish

cool season annual

September

10

¼ to ½

Potential to break up hardpans

Mustard

cool season annual

September

10 to 15

½

Roots may break up hardpans, incorporated residues may suppress soil-borne diseases

* Planting dates shown are preferred dates, with the possible latest planting date with a question mark in parentheses.

**Seeding rates are shown when planted with a drill (d), or when broadcast (bc).

Farm Profile: Looking Back Farms

Looking Back Farms, located in northeastern North Carolina, 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/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 lb per acre.

Fertility Management: Looking Back Farms applies chicken litter from a neighboring farm on corn and wheat. The litter is applied preplant at 3 to 4 tons/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 build-up 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 this. 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.

Return to North Carolina Organic Grain Production Guide

Author:

Cooperative Extension Soil Science Specialist
Crop and Soil Sciences

Publication date: Feb. 10, 2014
AG-660

North Carolina State University and North Carolina A&T State University commit themselves to positive action to secure equal opportunity regardless of race, color, creed, national origin, religion, sex, age, veteran status or disability. In addition, the two Universities welcome all persons without regard to sexual orientation.