Organic Soil Fertility & Amendments

Organic soil fertility management is fundamentally different in philosophy and practice from conventional approaches — and understanding that difference helps organic producers make better decisions about their fertility programs.

Conventional approach: Feed the plant.

Conventional fertility management focuses on directly supplying the nutrients a plant needs in readily available, soluble forms — synthetic nitrogen, phosphorus, and potassium applied to match crop removal. The soil is largely treated as a physical medium that holds nutrients and roots.

Organic approach: Feed the soil.

Organic fertility management focuses on building and maintaining a living, biologically active soil system that makes nutrients available to plants through natural processes — mineralization, biological nitrogen fixation, mycorrhizal associations, and organic matter decomposition. The goal is a self-regulating, increasingly fertile soil ecosystem rather than crop-by-crop external nutrient inputs.

Practical implications:

• Organic fertility programs are built on long-term practices: crop rotation, cover cropping, compost application, and reduced tillage

• External organic inputs (approved fertilizers, compost, manure) supplement the biological system rather than replacing it

• Soil testing and organic matter monitoring are essential management tools

• Response to organic fertility inputs is often slower and more cumulative than synthetic inputs — planning horizons are longer

Allowed soil amendments and fertility inputs in organic production are governed by the National List of Allowed and Prohibited Substances (7 CFR §§ 205.601–205.602) and the general principle that synthetic fertilizers are prohibited.

Always allowed (naturally derived):

• Compost: Produced from approved materials following NOP composting standards (specific temperature and turning requirements if using raw animal manure)

• Raw animal manure: Allowed with restrictions — must be incorporated at least 90 days before harvest of crops for human consumption not in contact with soil; 120 days for crops in contact with soil

• Plant-based materials: Crop residues, plant-based composts, kelp meal, alfalfa meal, cottonseed meal (from non-GMO source)

• Mined minerals: Rock phosphate, greensand, limestone, gypsum, sul-po-mag — must not be chemically altered

• Organic nitrogen sources: Blood meal, feather meal, fish meal, fish emulsion, soybean meal — from non-GMO sources

Allowed with conditions (on the National List):

• Potassium sulphate (nonsynthetic source only): Allowed as mined mineral

• Elemental sulphur: Allowed for pH adjustment and disease management

• Copper-based materials: Allowed with restrictions due to soil accumulation concerns

• Soluble boron products: Allowed when soil tests demonstrate deficiency

Prohibited:

• Synthetic nitrogen fertilizers (urea, ammonium nitrate, anhydrous ammonia)

• Synthetic phosphorus fertilizers (triple superphosphate, DAP, MAP)

• Synthetic potassium fertilizers (muriate of potash/potassium chloride — prohibited)

• Sewage sludge / biosolids

• Any input not clearly covered by the National List must be reviewed by your certifier before use

Compost is one of the most valuable organic soil amendments — and NOP has specific requirements governing what qualifies as NOP-compliant compost and how it can be used (7 CFR § 205.203).

NOP compost requirements:

For compost made from manure or other materials:

• The composting process must achieve a temperature of 131–170°F (55–77°C) for a minimum of 15 days (in a windrow system) with a minimum of 5 turnings during the process — OR equivalent time and temperature in an in-vessel or static aerated pile system

• This process destroys weed seeds and pathogens and is what distinguishes 'compost' from raw manure under NOP rules

Compost that meets these standards:

• Can be applied at any time, including close to harvest

• Is not subject to the 90/120-day raw manure restrictions

• Is considered a processed organic material, not raw manure

Vermicompost:

• Vermicompost (worm castings) produced from approved feedstocks is allowed

• Must document inputs used in vermicompost production — no prohibited materials in the feed stock

On-farm compost production:

• Many certified organic producers produce their own compost from crop residues, manure, and cover crop materials

• Document your composting process: inputs used, date started, temperatures achieved and dates measured, number of turnings — your certifier will review this during inspection

Soil testing is the foundation of an informed organic fertility program — and for certified organic producers, it serves both agronomic and compliance functions.

What to test for:

• Macronutrients: Nitrogen (though pre-plant nitrate testing is more practical than standard N tests), phosphorus, potassium

• Secondary nutrients: Calcium, magnesium, sulphur

• Micronutrients: Boron, zinc, manganese, iron, copper — particularly important if you're observing deficiency symptoms or planning specific crop amendments

• Soil pH: Critical for nutrient availability — most organic crops perform best at pH 6.0–7.0

• Organic matter content: Track over time — organic matter should be trending upward in a well-managed organic system

• Cation Exchange Capacity (CEC): Important for understanding your soil's nutrient-holding capacity

Testing frequency:

• Annual testing for actively managed fields, especially during transition

• Every 2–3 years once your system is stable and nutrient levels are well-characterized

• Test fields separately — don't average across your whole farm

Choosing a lab:

Select a soil testing laboratory experienced with organic systems and familiar with interpreting results in the context of organic amendments. Some conventional fertility recommendations assume access to synthetic inputs — request organic-system-specific interpretations.

USDA NRCS can assist certified organic producers with developing formal nutrient management plans (EQIP Practice 590) that satisfy both NOP and conservation program requirements.

A crop nutrient management plan is a field-by-field plan that matches nutrient applications to crop needs based on soil test data, realistic yield goals, and the nutrient contribution of organic amendments and cover crops. For certified organic producers, it is both a good management practice and a compliance asset.

NOP requirement: NOP § 205.203 requires certified organic producers to select and implement tillage and cultivation practices that maintain or improve the physical, chemical, and biological condition of the soil — and to manage deficiencies and excesses of plant nutrients. A nutrient management plan is the practical tool for demonstrating compliance with this standard.

Benefits of a formal nutrient management plan:

• Optimizes organic input use — avoids over-application of expensive organic amendments and the environmental risks of nutrient loading

• Provides documented evidence of responsible nutrient management for your certifier and for USDA NRCS program compliance

• Tracks organic matter trajectory over time

• Required documentation for some EQIP practice payments

USDA NRCS support:

NRCS offers both technical and financial assistance for developing formal nutrient management plans through EQIP and Conservation Stewardship Program (CSP) contracts. Contact your local NRCS Service Center for information about nutrient management planning support in your area.