Organic Nutrient Sources

For farms with a focus on organic production, nutrient management is critical to maintain high levels of productivity. Depleted soils need to be regenerated and rebuilt so they can sustain crop yield and improve the foundation of the farm. In organic systems, nutrient levels need to be maintained or replaced through nutrient cycling, nutrient uplifting from deeper in the soil, or through the addition of nutrient from outside sources.

One of the keys to success will be creating a program that maintains and increases soil organic matter. Organic matter (OM) is the living component of the soil. It consists of plant and animal residues in various stages of decomposition and is an important storage site for nutrients. By increasing soil OM, you will also increase soil water storage, decrease runoff, erosion, and leaching as well as improve soil structure and porosity. In the western U.S., soils are low in OM. Soil OM breaks down quickly, particularly when there is intensive cultivation and frequent irrigation. Cover crops and green manures are good ways to recycle or lift nutrients already existing in the soil. Composts and manures can add new nutrients into the soil. While changing soil OM levels is a slow process, through the careful use of a variety of cover crops, manures (green or animal) and compost, organically managed farms can be highly productive and sustainable.

Cover Crops and Green Manures

Cover crops (CC) and green manures are commonly seeded after harvest, grown over a specific period of time and then incorporated into the soil. The winter-grown cover crops include wheat, barley, oats, rye, some brassicas and various legumes like alfalfa, vetches, clovers, or peas. Summer cover crops include warm-weather grasses like sudangrass, sorghum or millets, broadleaf plants like buckwheat and mustards, and legumes like beans or cowpea. Seeding rates for these crops vary, as do appropriate planting times.

Most CC are grown for several months before they are clipped or mowed, and then disked back into the soil. With all CC, care should be taken that they do not set seeds as this can lead to the cover crop becoming a weed problem. Sometimes the CC are strip tilled as the strips provide wind protection during the early part of the growing season. With proper management, CC can reduce nutrient loss during the winter and early spring. With all CC, they should be incorporated when the foliage is still green so they decompose rapidly and return the greatest amounts of nutrients to the soil. Most soils that are not productive due to poor physical properties can be restored and made to produce good crops through the use of a good cover crop rotation program. Also, if soil moisture is a limiting factor, growing CC can seriously deplete soil moisture levels in the spring or summer. Use of CCs and GMs should be location specific as each has different tolerances to cold, heat, or drought.

For more information on cover crops and green manures, refer to Managing Cover Crops Profitably, one of the many publications available at the Learning Center of the Western Sustainable Agriculture Research and Education (http://www.westernsare. org/Learning-Center) program.

Cover Crops or Green Manures for Vegetable Farms

Crop Seeding Rate(pounds) Seeding Dates* Tolerance to:
Cold Heat Drought
Barley 75-100 Sept 1 - Oct 31 G M M
Brassica (Mustard/rape/kale) 20-40 Aug 15 - Oct 31 G-E P M
Buckwheat 50-75 May 1 - July 31 P M M
Millet (various) 25-40 May 1 - July 31 P G P
Rye 75-100 Sept 1 - Oct 31 E M M
Sudangrass 30-60 May 1 - July 31 P E G
Oats 75-100 Sept 1 - Oct 1 M P P
Wheat 50-75 Sept 1 - Oct 31 E P M
Alfalfa 20-30 Mar 1 - Apr 30 G G G
Beans (various) 60-90 May 1 - Apr 31 P M M
Clovers (various) 15-30 Mar 1 - Apr 30 G M M
Cowpea  60-90  May 1 - July 31 P G-E P
Field Pea  75-100  Mar 1 - Apr 30 G P M
Soybean  75-100  May 1 - July 31 P G G
Vetch (various)  50-75  Sept 1 - Oct 31  M-g P M
**Seeding dates depend on location. Plant later in spring and earlier in fall in colder areas. Plant earlier in spring and later in fall in warmer areas. Dates are suggested ranges only.
P=poor; M=moderate; G=good; E=excellent

Compost and Manure

Application and incorporation of compost or manure to soils will increase soil organic matter and certain soil nutrient levels. Both compost and manures are widely used in crop production but differ in how they are used.

Composting is when plant tissue or animal waste is broken down into organic matter through heat and microbial action. Composting reduces bulk, stabilizes soluble nutrients, and hastens the formation of humus. Most organic materials (manures, crop residues, leaves, sawdust, etc.) can be composted. Finished composts provide relatively low amounts of readily available nutrients. They vary in their nutrient content depending on the original source of material. Even though most composts don’t supply large amounts of nutrients, they help improve soil fertility by increasing OM and by slowly releasing nutrients. Compost should be tested for nutrient content and for organic certification purposes.

Manure can supply the nutrients required by crops and replenish nutrients removed from soil during harvest. Since manure contains multiple nutrients, applications should consider not only what is needed for the crop, but also how the ratio of nutrients in manure could affect soil test levels. This ensures adequate nutrient supply and reduces potential for over- or underapplication and subsequent buildup or depletion of selected nutrients in the soil. Good manure nutrient management should consider short- and long-term impacts on crop nutrient supply and soil resources.

Manure has characteristics that make nutrient management different and sometimes more complicated than using fertilizer including;
A mix of organic and inorganic nutrient forms
Variation in nutrient concentration and forms
Variation in dry matter and resultant handling as a Liquid or solid
Relatively low nutrient concentration requiring large application volumes. Sampling and laboratory analysis are always needed since manure nutrient composition can vary significantly.
Timing of manure application
If applied far in advance of the crop, manure can be quite useful. When applied closer to when the crop is planted or at very high rates, damage may occur
Regulations associated with organic certification need to be followed

For more information on how to improve soil, refer to Building Soils for Better Crops: Sustainable Soil Management, one of the many publications available at the Learning Center of the Western Sustainable Agriculture Research and Education Program

Approximate Nutrient Values for Selected Manures, Animal Products, Composts and Crop Residues

Nutrient Source Total N P2O5 K2O
Manures lbs/ton wet weight
Cattle 18-22 14-18 22-26
Dairy 8-12 4-6 14
Horse 14 4 14
Pig 8-10 6-10 6-9
Poultry 35-55 40-50 30-35
Sheep 14-18 8-12 22-26
Compost - Manure Based 1.5-2.0 2 1
Compost - Plant Based 0.5 - 1.0 1 1
Animal Products percent (%)
Dried Blood 12 1 0.5
Bone Meal 3 15 0
Feather Meal 13 0 0
Fish Emulsion 4 2 0
Fish Meal 10 7 0
Crop Residues lbs/ton (dry weight)
Alfalfa hay 45-50 11 45-50
Buckwheat 10-15 1-5 45-50
Clovers 50-60 10-20 40-60
Sorghum/Sudan grass 20-30 5-10 10-30
Straws (barley/oat/wheat) 10-15 3-6 20-30
Sweet Corn Stover 30 8 25
Vetches (common/hairy) 40-60 15 45-55
There are many other sources of organic based nutrients. Always check the nutrient analysis to help determine application rates.

More Information