Soil - 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 US, 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 (Cover crops or green manures for vegetable farms).
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.
Cover crops or green manures for vegetable farms.
|Crop||Seeding Rate(pounds)||Seeding Dates*||Tolerance to:|
|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
Approximate nutrient values for selected manures, animal products, composts and crop residues. There are many other sources of organic based nutrients. Always check the nutrient analysis to help determine application rates
|Nutrient Source||Total N||P2O5||K2O|
|Manures||lbs/ton wet weight|
|Compost - Manure Based||1.5-2.0||2||1|
|Compost - Plant Based||0.5 - 1.0||1||1|
|Animal Products||percent (%)|
|Crop Residues||lbs/ton (dry weight)|
|Sweet Corn Stover||30||8||25|
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 CC and GM should be location specific as each has different tolerances to cold, heat, or drought (Cover crops or green manures for vegetable farms).
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 program.
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 (Approximate Nutrient Values table).
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.