Organic Soil Fertility

Organic Agriculture April 23, 2014|Print

eOrganic authors:

Michelle Wander, University of Illinois

Nick Andrews, Oregon State University

John McQueen, Oregon State University

This article provides an overview of key concepts in organic fertility management, a review of essential macro and micronutrients, and a listing of nutrient amendments approved for use in organic farming systems. It summarizes strategies used to build and manage fertility on organic farms and provides tips on soil testing and use of nutrient budgets.

Introduction: Soils as the Foundation of Organic Farming

Soil health is the foundation of organic farming systems. Fertile soil provides essential nutrients to plants, while supporting a diverse and active biotic community that helps the soil resist environmental degradation. Organic producers face unique challenges in managing soil productivity. Current guidelines on nutrient management for organic farmers are fairly general in nature. Organic farmers rely on intuition and observation, advice from vendors, conventional soil tests, and their own experience to make decisions about the quantity and types of soil amendments to apply. As a result, there is tremendous variability in both the quantities of nutrients applied and the resulting soil fertility status on organically managed farms. Organic farmers seek to "build the soil" or enhance its inherent fertility by using crop rotations, animal and green manures, and cover crops. Crop rotation and tillage practices must provide an appropriate seedbed and pest control while minimizing erosion. Nutrient stocks are maintained through use of natural (non-synthetic) substances and approved synthetic substances listed on the National List of Allowed and Prohibited Substances. This list includes a few approved synthetic fertility inputs, such as elemental sulfur, aquatic plant extracts, and liquid fish products. Many substances on the National List have restrictions, or annotations, on their use, source, or rate of application. Organic farmers are advised to check with their certifying agent before purchasing or applying any synthetic inputs. See Can I Use this Input on My Organic Farm? for more information. In addition, organic growers must document their soil management practices in their organic farming system plan as part of their certification, and keep records of all inputs purchased and applied.

Although the following sections address nutrient management and soil building practices separately, these two apects of management are intimately connected through a system of management. Organic farms that achieve their goals maintain soils and protect the environment while using modest amounts of inputs. Soil tests and simple budgeting tools can help producers maintain balance to achieve success.

Supplying and Managing Nutrients

Although crop nutritional requirements are the same for organic and conventional farms, organic producers apply natural materials and emphasize practices that retain and recycle nutrients within the soil. Sixteen elements are consistently found to be necessary for plants to complete their life cycles (Tables 1 and 2). Additional elements (Table 3) are listed as essential for some species and for animals relying on plants for their nutrition. Carbon, hydrogen, and oxygen--which account for about 95% of plant biomass--are supplied from carbon dioxide and water. The other macronutrients with concentrations greater than 500 micrograms/g plant include nitrogen, phosphorus, potassium, sulfur, calcium, and magnesium (Table 1). Micronutrients taken up in lower abundance are no less neccessary but are not limiting to growth in most situations. Sandy soils with inherently low nutrient contents are an exception. Micronutrients include iron, zinc, manganese, copper, boron, chlorine, and molybdenum.

Table 1. Macronutrients essential for plant growth.

Element Cationic Anionic Other
Nitrogen NH4+ NO2-, NO3- organic
Phosphorus - HPO4-2, H2PO3-, polyphosphates organic
Potassium K+ - -
Calcium Ca+2 - -
Magnesium Mg+2 - -
Sulfur - SO4-2, S2-2 organic

Table 2. Micronutrients essential for plant growth.

Element Cationic Anionic Other
Iron Fe, Fe+2, Fe+2 - organic-chelated
Manganese Mn, Mn+2 - organic-chelated
Copper Cu, Cu+2 - organic-chelated
Zinc Zn, Zn+2 - organic-chelated
Molybdenum - Mo, MoO4- -
Boron - Bo, B(OH)4-, H3BO3 H3BO3
Chlorine - Cl- -
Nickel Ni, Ni+ - -

Table 3. Micronutrients essential to support animal health.

Element Cationic Anionic Other
Cobalt Co+2 - -
Selenium - SeO4-2, SeO3-2, Se-2 organic
Sodium Na+ - -
Silicon - SiO2-2 -

Organic farmers use natural materials or, when possible, exploit biological processes to supply needed nutrients to soils. Organic fertilizers are needed in larger quantities than are conventional fertilizers because nutrient concentrations tend to be lower. Organic fertilizers can be more expensive, more bulky and less uniform than conventional counterparts. Before applying anything to your field, you should know what the nutrient analysis of the material is, and be certain the substance is allowed by your certifier. See Can I Use this Input on My Organic Farm? for more information.

Table 4. Some of the most commonly used NOP-approved amendments are summarized below. Nutrient contents are listed as proportion of N:P2O5:K2O.

Fertilizer Type Description Comments/Issues
Alfalfa meal or pellets Contain around 3 percent nitrogen and are commonly used as an animal feed. Commonly used for high-value horticultural crops but rather expensive for field crops.
Ash Wood ash (0–1–3) contains P and K, is a good source for micronutrients and acts as a liming agent. Commonly used in gardens; avoid over-application which can cause alkalinity and salt build up; avoid ash from treated wood or from the burning of manure.
Biological amendments While not fertilizers per se, there are a number of biological amendments used to promote biological activity or microbial associations between plants and soils with the intent of increasing plant nutrient uptake. A separate article covering this is under development.
Bone meal Typically a mixture of crushed and ground bone that is high in phosphorus. N contents vary depending upon handling. Range from 4:12:1; 1:13:0; 3:20:0.5. Permitted as a soil amendment but can not be fed to animals in certified production. Blood, bone and meat meal are prohibited in many countries in Europe and Japan because of BSE transmission risk.
Blood meal Dried blood, is a soluble source of nitrogen. Typical N:P:K contents are 13:1:0. Solubility can vary. Should be used carefully, release of ammonia can burn plants and lead to loss through volatilization. Use limitations are the same as bone meal above. Recently Canada, with the support of IFOAM, proposed to prohibit cattle wastes as fertilizer at the UN Codex Alimentarius session in Montreal, 2004. Allowed under National Organic Program (NOP) regulation.
Calcium sulfate (Gypsum) CaSO4.2H2O. Contains about 23% Ca, is a mined deposit that is used to reclaim alkali soils, lower soil pH, and adjust cation balance. Good source for sulfur; useful for alkaline soils with high sodium content. Avoid gypsum from recycled sheetrock.
Cocoa Shells Cocoa shells (1:1:3) are available in some regions. They are used as a source of potassium and are popular due to their slow release properties. Also used as a mulch.
Dolomitic lime (Calcium-magnesium carbonate) CaCO3–MgCO3 is about 24% Ca and 20% Mg, is a very effective lime source. Over application is perceived to be a problem in horticultural systems. Under application is an issue in some field crop systems. Has a lime equivalent of 1900 lb/ton. Labs following the cation balance theory avoid the use of dolomitic limes, KCl, and oxide forms of trace elements.
Feather meal (13:0:0) a by-product of the poultry processing industry, which contains 15% N as non-soluble keratin has been promoted as a slow release N source. Feather meal can transmit the Avian flu, A(H5N1) virus, which is relatively easily transmissible to animals and people.
Fish emulsion (Ranges in content from 4:1:1 to 9:3:0); suitable for foliar feeding of starts and the spot treatment of transplants; is reputed to prevent stress, stimulate root growth and provide cold protection. Fish emulsion may be fortified with chemical fertilizer, so be suspicious of any product with a phosphorus content in excess of 4%. Fish products may also contain synthetic preservatives, stabilizers and other products prohibited under the NOP. Fish meal can also contain high levels of PCB’s.
Granite Dust Granite dust is available in some regions. It is used as a source of potassium that is popular due to its slow release properties. Availability varies regionally.
Greensand (Glauconite) A mined sandstone deposit (typically 0:0:3 or 0:0:6) used as a source of potassium. Also contains iron, magnesium, silica and other trace minerals. Is a common ingredient in potting mixes.
High calcium lime (Calcium carbonate) Limestone containing 0–5% magnesium carbonate. Rapid reacting due to high solubility, valued source of Ca and liming where magnesium abundance is a concern and soil is not alkaline.
Hydrated lime High quality Ca(OH)2 is a dry powder produced by reacting quicklime with a sufficient amount of water to satisfy the quicklime's natural affinity for moisture. The National Organic Standards Board approved use of calcium hydroxide as a component of Bordeaux mix and lime sulfur for fungicide use, but does not allow its use as a soil amendment.
Manures and composts Nutrient contents vary widely, it is recommended they be applied on the basis of phosphorus need. Use as an N source leads to over application of P. Manure- and compost-based P has high plant availability, ranging 70–100% available. Compost, if produced according to NOP requirements, can be applied any time during the growing season. Animal manure can only be used on crops for human consumption if it is incorporated into the soil at least 120 days prior to harvest for crops that contasct the soil or 90 days prior to harvest for crops that do not contact the soil.
Potassium Sulfate K2SO4 is a mined fertilizer not widely available. It has been used as a food preservative. This is allowed under the NOP rules if you can prove you are using a mined source that has not been treated with acid or any other chemical reaction to make the potassium more available. This is a good choice for high Mg soils, but it is fairly reactive and must be used carefully.
Rock phosphates Rock phosphates are frequently divided into hard rock and colloidal or soft rock forms. Rock phosphate typically has lower availability than colloidal P, which is low (2%) compared to materials like bone meal (11%). Marine sediments are typically ground and cleaned. Availability is low where soil pH is above 6 and biological activity is low. Addition of manures can increase solubility. Contains Calcium and acts as a liming agent. Phosphate rock is most effective at supplying P in soils with low pH (less than 5.5) and low calcium concentrations. Phosphate rock applications made to soils with pH greater than 5.5 may not be effective because of reduced solubility.
Sea weed and Kelp (Ranges from 1:0.2:2 to 1.5:0.5:2.5) Also high in micronutrients, Fe, Cu, Zn, Mo, Bo, Mn, Co. and Alginic acid (26%). Is used as a soil conditioner. Several kinds of sea weed and kelp are on the market. Kelp meal can be applied directly to the soil or in starter fertilizer. Can be high in salts and metals. Other reputed benefits are hormones or hormonal activity. Claims to protect plants from stress:  cold, drought and insect pressure. Expensive, so best suited for high value crops.
Seed meal (Ranges from 6:1.5:2 to 6:2:2); cotton seed and soybean seed meal have been popular. Now that generically modified crops are so wide-spread sourcing GM free meal can be difficult. Check with your certifier about the needed documentation.
Sodium nitrate (16:0:0) Historically an important component of fertilizers, and a raw material for the manufacture of saltpeter. It is a mined product that is about 16–20 percent nitrogen and highly reactive. It acts more like a synthetic fertilizer and can cause sodium buildup in the soil. Can contain medium to high levels of Boron. The NOP stipulates that the nitrogen obtained from sodium nitrate must account for no more than 20 percent of the crop’s total nitrogen requirement. This can be used cautiously when rapidly available nitrogen is needed. It is prohibited by the Farm Verified Organic and Organic Crop Improvement Association-International Federation of the Organic Agriculture Movements accredited levels of certification. European organic standards consider it to be the equivalent of a synthetic fertilizer because it is highly soluble and leaches readily from the soil. Check with your certifier before using.
Soybean meal (8:0.7:2) Useful to augment N and P. Often used as a feed additive; medium N release rate; may inhibit germination of small seeds. Check with your certifier before using, due to widespread use of GM soybeans.
Sulfate of potash (sul-po-mag and K mag or langbeinite) (0:0:21 with 11 Mg) Naturally occurring crystalline product commonly used to supply potassium. This and calcium sulfate are allowed under the NOP if you can prove you are using a mined source that has not been treated with acid or any other chemical reaction to make the potassium more available. Potassium sulfate is the better choice for high Mg soils, but it is fairly reactive and must be used carefully.

One of the simplest things a producer can do is maintain optimal soil pH levels. This is critical as pH influences nutrient solubility, microbial activity, and root growth. High pH favors weathering of minerals and increases the release of cations but reduces the solubilty of salts including carbonates and phosphates. Lower pH values favor fungi while high pH favors bacteria. Soil pH can also affect the plant’s ability to take up nutrients directly. At very low pH values (<3), cell membranes are impaIred and become leaky. For most crops, soil pH levels are optimal between 6.0 and 7.0. Lime can be applied to raise the pH of acidic soils (pH <6) and supply calcium. Alkalinity, which is more difficult to correct, typically requires the use of sulfur; this remedy is typically temporary and more expensive than liming. When adjusting the pH, it is important to know the crop’s pH requirement since optimum pH levels vary by crop.

Nitrogen (N) is abundant in the environment yet remains the most frequently limiting nutrient for crop production. Organic farms frequently acquire N through nitrogen fixation by legumes. Legume cover crops, green manures, and legume sods can be an excellent sources of N. Vigorous stands of alfalfa, red clover, crimson clover, or hairy vetch can provide between 100-200 lbs N, which should be most, if not all, of the needed N for the subsequent crop. About half the N in a green manure is released during decomposition following incorporation. Nitrogen needs are often supplemented by the addition of animal manures, either composted or raw, or other more concentrated sources of nitrogen. These include blood meal, fish emulsion, fish protein, kelp and seaweed, and vegetable meals. 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. Certifiers frown on use of imported N sources because these share the problems of conventional N sources. Ideally, organic systems will rely on rotations that supply most, if not all, of their N needs.

Phosphorus (P) is another macronutrient that is frequently limiting in sandy soils and/or where systems do not receive additions of animal wastes. Soil P is found in organic and in inorganic forms that are slowly available. Phosphorus availability is sensitive to soil pH and organic matter decay rates. Important sources of P include manure, bone meal, fish and poultry meal, and rock phosphate. High levels of phosphorus are a risk associated with use of manures and some composts.

Potassium (K) is taken up from soil solution and is abundant in soils rich in illitic clays. Mineral weathering can be an important source of K in some soils. Potassium is weakly held on the exchange and so can be depleted where leaching rates are high. Manure and plant meals are good sources for K.

Common sources for nutrients are:

  • Potassium: manure, alfalfa meal, kelp meal, greensand, wood ash, potassium sulfate, and granite dust.
  • Sulfur: acid rain, manures.
  • Calcium: lime, colloidal phosphate, bone meal, gypsum, and wood ashes.
  • Magnesium: dolomitic lime and langeinite.
  • Micronutrients: mineral weathering, manure, compost, and liming amendments. The National Organic Program requires that micronutrients not be used as defoliants, herbicides, or dessicants. Micronutrients made from nitrates or chlorides are prohibited. Soil deficiencies must be documented by soil or tissue testing.

Methods Used to Build Soils

Bare fallow

Bare fallow can be used with fallow periods occurring between harvested crops. Fallows commonly occur over the winter in temperate zones or during the dry season in Mediterranean or tropical zones. Use of bare fallow to accumulate water and, at times to control weeds only works to enhance the soil where it concentrates resources enough to increase overall crop productivity. If bare fallow is used, soil erosion must be prevented.

Crop Rotation

Crop rotation varies plant species in time and space and is an important strategy for organic farmers. Goals are to keep the soil surface covered with a growing crop for most of the year. Key elements of rotations include the breaking of disease and pest cycles and the inclusion of soil building cover crops or cropped fallow periods. By selecting effective cover crops or perennial crops farmers can maintain or increase soil organic matter content and nutrient availability during periods when cash crops are not grown. For most organic farmers, fertility is based on the rotation and not the amendment.

Cover Crops

Cover crops include annual, biennial, or perennial herbaceous plants grown in pure or mixed stands. Annual covers occupy the rotation for part of the year. Perennial crops may be referred to as ley or pasture phase or as a plant-fallow. Cover crops provide soil cover and can help loosen compacted soil through the growth of roots. They enhance soil physical condition and improved water filtration. Legume cover crops provide nitrogen while non-legumes can increase nutrient availability to subsequent crops by taking up nitrogen, phosphorus, and potassium that might otherwise leach or become unavailable to plants.

Diversification

Diversification through rotation and use of covers or ley crops can reduce crop insect pests and diseases, if the cover crops are not alternate hosts. Both covers and perennial ley covers help maintain or increase soil organic matter if they are allowed to grow long enough to produce sufficient biomass. These also help prevent soil erosion caused by both water and wind, and suppress weeds. The management of residues within rotations can be quite sophisticated. For a good example, see the video of a living mulch system for soil fertility featuring Helen Atthowe of BioDesign Farm.

Helen Atthowe of BioDesign Farm describes using a living mulch to achieve slow release fertility
Video 1. Helen Atthowe of BioDesign Farm describes using a living mulch to achieve slow release fertility. Video credit: Alex Stone, Oregon State University, Weed Em and Reap Part 2 Living Mulch System Soil Fertility video.

For more information on rotation and cover crops, see the ATTRA publication Overview of Cover Crops and Green Manures and the SARE publication Managing Cover Crops Profitably.

Judicious Use of Tillage

Tillage is an integral part of many organic systems. 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. 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 farming using cover crops and tractor-mounted rollers to kill the cover just before planting into it. Ron Morse at Virginia Tech and Nancy Creamer at North Carolina State University have been adapting these systems for organic vegetable production. Watch Weed Em and Reap Part 2 for more information.

Organic Amendments

Organic amendments can be an important resource. Soil fertility and physical condition can be effectively maintained with rotation and appropriate use of organic amendments. Application should be made based on soil testing and/or use of budgets. Manures and composts are the most common organic resources where livestock is in the vicinity. Estimating nutrient contents and availability is necessary for organic materials. For a farmer's perspective on using organic amendments, watch the video of Steve Pincus of Tipi Produce.

Steve Pincus of Tipi Produce in Wisconsin explains how fertility is not a matter of NPK

Video 2. Steve Pincus of Tipi Produce in Wisconsin explains how fertility is not a matter of NPK and how bulky organic matter is managed to improve tilth and maintain nutrient supply on his farm. Video credit: John Marlin, Agroecology and Sustainable Agriculture Program, University of Illinois.

Problems Associated with Nutrient Over-Addition

There is such a thing as too much of a good thing. Off-site problems caused by over-application of nutrients are better recognized than are problems caused on-site. Conventional agriculture is the primary source of non-point source and P pollution that contribute to a myriad of environmental and health risks. Problems of over-application in organic systems vary; probably P over-additions are most widespread where manure is readily accessible. This is because the ratio of P to N in manure exceeds that required by the plant. Avoid over-reliance on animal manures, in addition to accumulation of excess phosphorus, concentrations of copper, and zinc, which may accumulate in soils. Over-addition of N, particularly in readily available forms, is a common problem. Over-addition of N and P in organic systems can occur in situations where leaching is restricted (eg., in greenhouses) or after N rich cover crops or manures are applied. The notion that N surplus promotes microbial activity and works against organic matter storage and suppresses plant-microbe associations is finally being accepted as an additional downside of over-fertilization. Excess nutrients can also increase plant susceptibility to pathogens and arthropod pests and can also lead to increased weed competition. Tendency toward nutrient leaching and ability to hold and retain nutrients varies with soils and climatic conditions. Texture and CEC are related to this, with nutrient storage capacity increasing with soil clay and silt contents and cation exchange capacities.

Soil Tests and Nutrient Budgeting

To manage nutrients effectively you can use soil testing and nutrient budgeting. Soil tests are used by organic farmers for several reasons:

  1. to ensure that pH and nutrient levels and proportions are in appropriate ranges;
  2. to justify the application of micronutrients and other approved fertilizers to a certifier, or to comply with other certification requirements;
  3. to identify soil nutrients in excess, so soil and fertility management strategies can be manipulated to a) reduce levels of excess nutrients over time, or b) mitigate the impacts of the excess nutrients on crop, soil, and environmental quality; and/or
  4. to track trends in nutrient content, pH, and soil organic matter content over time to ensure soil improvement is taking place.

The National Organic Program regulations require that micronutrients and other fertilizers be applied only when soil or tissue tests indicate a deficiency. Because of this language, some certifiers may require soil testing and possibly other tests. Contact your certifier for its testing requirements. See the related article, Organic Certification of Vegetable Operations.

Frequency of soil testing will depend on your purpose in testing and your situation. To track trends in macronutrients, pH and soil organic matter content, testing once every two or three years, or at a specific point in your rotation cycle, may be sufficient. However, if you are just starting to manage your soil’s fertility with organic practices, or adopting new soil or nutrient management practices, you might want to test more frequently.

Timing of testing will also depend on the purpose of the test. To determine nutrient status of your soil for the upcoming season, test your soil in early spring. To test contents of nutrients with potential to leach over the winter, test in late summer.

A typical soil test evaluates your soil’s pH, CEC, and content and proportion of macronutrients—calcium, magnesium, potassium, phosphorus, and sulfur—which are required by plants in relatively large quantities. Soil organic matter content may not be a routine test, but can be requested. Soils can also be tested for micronutrients (nutrients required by plants at relatively small quantities).

Soil testing laboratories use different soil testing methods that may generate different results. It is important to understand the methods used to generate your test results and use interpretation information that corresponds to that testing method. In addition, using the same testing laboratory for all your testing over time will allow you to compare your test results from year to year and track trends. A soil test is only as good as the soil sample it evaluated. It is important to take a representative sample of the field and the soil volume the crop plant roots will explore to obtain nutrients.

References and Citations

Further Reading

  • Agricultural Marketing Service—National Organic Program [Online]. United States Department of Agriculture. Available at: http://www.ams.usda.gov/nop/ (verified 10 March 2010).

 

This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.

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