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Table of Contents: Soil Testing Nutrient Requirements Sources of Nutrients Nitrogen (N)|Phosphorus (P)|Potassium (K)|Sulphur (S) Secondary and micronutrients Nutrient Loss Soil Fertility Animal Manure|Green Manure|Legumes in the rotation Rotating high and low nutrient demand crops|Crop Residues Acceptable Fertilizers|Summerfallow Conclusion References

Fertile soil is essential to successful organic crop productions systems.   Synthetic fertilizer use is not allowed, therefore organic farmers must use various other means to replace nutrients and improve soil fertility.  Organic soul management techniques build organic matter and humus, protect the soil from erosion, reduce nutrient loss, and maintain soil in a condition that supports diverse life-forms.  Crop rotations are an essential component in fertility management, pest control and long-term sustainability.

Soil Testing

Soil testing is an important practice in managing an organic farm and is also a requirement of many organizations providing organic certification.

Proper soil sampling and testing should be done every two or three years to provide the producer with a record of soil nutrient status. A soil test will include information on texture, pH, organic matter content, cation exchange capacity, salinity and electrical conductivity.

Soil testing allows a producer to make informed decisions while measuring the benefit of various agronomic practices. It may be advantageous to sample at the same time every year (early October) and to use the same laboratory in order to track changes over time.

Plant tissue analysis is also useful in evaluating soil management practices and crop nutrition. It provides a reliable reading of nutrients take up by a crop and it can be used to indicate what nutrients are available. To interpret tissue analysis using standardized nutrient sufficiency levels, samples must be taken according to laboratory instructions regarding timing and plant portion.

Nutrient Requirements

There are 17 essential nutrients required for plant growth.

The essential macronutrients are nitrogen (N), phosphorus (P), potassium (K) and sulphur (S).

Calcium (Ca) and magnesium (Mg) are considered secondary nutrients.

Micronutrients include iron (Fe), manganese (Mn), boron (B), molybdenum (Mo), copper (Cu), zinc (Zn), nickel (Ni) and chlorine (Cl).

The remaining nutrients, carbon ©, hydrogen (H), and oxygen (O) compose more than 90% of the dry matter weight of the plant and are supplied from air and water.

The soil often provides the majority of the nutrients, however, the remainder must be supplemented.

The "law of the minimum" states that yield is determined by the factor that is most limiting. For example, if only enough N is available to grow a 30 bushel per are crop, yet there is enough of all the other essential nutrient to grow a 40 bushel per acre crop, the yield will be limited to 30 due to the limited N. The "law of the minimum" also applies to other factors such a light, heat, moisture and crop variety.

Sources of Nutrients

Nitrogen (N)

• The plant uses nitrogen:
  as a main component of amino acids and proteins, which are required for growth and yield
• in the production of chlorophyll

The primary source of N on an organic farm is atmospheric N fixed by legumes. Forage legumes and plow-down of legume green manure can provide most of the N required to grow crops. Nitrogen s provided to the organic farm through the application of composted manure and by incorporating straw and organic wastes back into the land.

Phosphorus (P)

Phosphorus is important in many plant metabolic processes:

• Photosynthesis and respiration
• Energy storage and transfer
• Protein and carbohydrate metabolism
• Cell division and enlargement
• Structure of DNA
• Component of cell membranes

phosphorus is important in stimulating root growth, promoting early maturity, kernel development and increasing winter survival, particularly in perennial legumes.

Manitoba soils are naturally low in P, so supplementation is usually required.

Organically-acceptable sources of P include composted manure, crop residues, green manure and rock phosphate fertilizer. Green manure can increase the availability of P. Legumes, buckwheat and mustard provide acidity around their roots which assists in stabilizing soil P and increasing uptake.

Rock phosphate is a low-analysis organic source or P. It is only practical for perennial hay and pasture fields because the breakdown or rock P is very slow on the high pH soils found across Manitoba. Annual crops benefit very little from rock phosphate applications.

Potassium (K)

Potassium is involved in several plant processes:

• Production of structural components like lignin and cellulose which impart stalk strength and lodging resistance
• Influences CO2 uptake, photosynthesis and opening of stomata in leaves
• Influences water uptake
• Influences starch and sugar content, enhancing storage quality
• Aids in disease and insect resistance

 The K requirement is high for perennial crops, forages, potatoes and tomatoes. Most of Manitoba’s soils are naturally high I K and can meet the needs of the crop. The exceptions are sandy soils and those with a high organic matter content.

Sources of K include composted manure, but K is soluble and care must be taken to minimize leaching during storage. Much of the K is in livestock urine and will be held in the livestock bedding. Potassium can also be added to the soil in the form of composted straw and hay, powdered basalt, granite dust, clay minerals, langbeinite, greensand (glauconite), kelp meal, wood ashes and a variety of the materials.

Sulphur (S)

Sulphur also has many important roles:

• As a constituent of amino acids to form proteins
• Develops enzymes and vitamins
• Involved in N-fixation by legumes
• Aids in seed production
• Needed for chlorophyll formation

Certain crops such as brassicas and forage legumes require higher levels of S as compared to cereal crops.

The primary sources of S are gypsum (calcium sulphate) and composted manure. Several S-containing soil amendments (such as Epsom salts and langbeinite) may be permitted, but approval from your certification body is required prior to application.

Although composted manure is low in S, it is generally the best amendment since it contains a balanced supply of other nutrients, as well as organic matter and micro-organisms.

Secondary and micronutrients

Although these nutrients are only required by plants in trace amounts, they are important to plant growth and development, as well as to the livestock that may consume the plants.

In a biologically active soil with good physical properties and a balanced, pH, micronutrients deficiencies are rare. Sandy soils with high pH and low organic matter contents are more likely to have micronutrient deficiencies. Because the range between deficiency and excess is small, certain micros can be toxic to plants if they exceed trace levels. It is therefore recommended that micros not be applied unless a deficiency is confirmed by leaf analysis or by visible plant symptoms. Compost and some seaweed products can supply micros.

Nutrient Loss

Conserving nutrients is an important part of any farm operation. Nutrient loss may harm the environment, in addition to the loss of money, time and resources. For example, nutrients leaching into ground or surface water may cause excessive algae growth and oxygen depletion, harming natural flora and fauna.

Nitrate leaching increases when certain factors exist:

• Coarse-textured soils or soils with large deep cracks
• Recharge areas in the landscape
• Significant precipitation while crops are not using water
• Limited plant root zone to intercept nitrate due to shallow-rooted or immature plants

Nutrient run-off increases when certain factors exist:

• Fine-textured (clay) soils with low infiltration rates
• High rainfall
• Excessive tillage and crop residue incorporation

Nutrient loss can be reduced with effective use of catch crops, crop rotations and good manure management techniques.

Soil Fertility

Nutrients to meet both the needs of the crop and organic certification standards may be supplied by several management tools:

Animal Manure

• Manure from any source must be composted for a specific period before application on organic fields (check with your certification body for specific requirements).
• The compost must not be allowed to pollute water sources and the pile must be turned regularly to allow effective decomposition
• The levels of the various nutrients in the manure vary according to the type of animals, the nature of the feed and how the manure was stored. Manure generally contains all macro-and micronutrients, but rarely in the proportion needed by crops. Manure or compost analysis is essential to identify which nutrients may need supplementation from another source.
• Its physical and biological characteristics make manure an excellent amendment for low organic matter, eroded, saline and other poorly-structured soils.

Green manure plays a role in soil improvement, nutrient management and pest management.

• It is effective in controlling erosion, adding organic matter, improving soil structure, stimulating biological activity in the soil and reducing compaction.
• Legumes such as alfalfa and sweet clover can fix over 200 lb. per acre of nitrogen.
• Effective green manure smothers weeds, breaks insect and disease cycles and provides a habitat for bees, parasitic wasps, and other beneficial organisms. Fall rye and oats are particularly competitive. Some crops such a s yellow sweet clover and mustard are allelopathic and produce natural chemical toxins that retard germination and inhibit the early growth of weed species.

The value of green manure can vary with the type of crop and the timing of the plow-down process (see Figure 1). For example, most legumes turned under as green manure at the blossom stage will contribute in excess of 100 lb. of nitrogen per acre. A mixture of grass and legumes turned under at the blossom stage will contribute 50-100 lb. or N per acre. Grass and legume residue after harvest will add less than 50 lb. of N per acre. The rate of decomposition also varies with soil and climatic conditions.

Incorporating green manure with a discer into the top 3-4 inches (7.5-10 cm) of soil allows a favourable rate of decomposition. Deeper levels of incorporation will slow down the rate of decomposition. Incorporation levels below 6 inches (15 cm) should be avoided. Tillage in early summer may leave a considerable portion of nitrogen in the nitrate form by winter, and vulnerable to leaching or denitrification losses. Fall tillage will keep N in the organic form over winter, allowing N to mineralize during the next season.

As microbes break down the green manure residue, the micro-and macronutrients from these plants are made available over a number o years. An added bonus is that organic acids are released in the breakdown process, resulting in lower soil pH and increased plant-available phosphorus.

 Legumes in the rotation

When properly inoculated before planting, annual legumes such as peas and lentils will fix 50-90% of the N they require from the air (see Table 1).

• Legume residue breaks down more quickly than non-legume residue, which allows N to be available sooner to subsequent crops when the residue is worked into the soil  (see Table 2).

• Perennial legumes such as alfalfa supply substantial amounts of N to the soil from their root systems, even though much of the top growth may be removed as hay or grazed pasture.

Rotating high and low nutrient demand crops

Different crops require different amounts of the various essential nutrients. Rotating high and low nutrient demand crops may avoid depleting one of more of those essential nutrients in the soil. Knowing the nutrient demands of various crops is essential to the producer and many resources are available to provide this information.

Crop Residues

Returning crop residues to the soil contributes tremendously to the organic matter and the nutrient pool available for new plant growth. Crop residues also prevent soil erosion and improve the water-holding and infiltration properties of soils (see Figure 2).

The straw from a wheat crop yielding 40 bushels per acre can contain 25 lb. of nitrogen, 9 lb. of phosphorus, 55 lb. of potassium and 5 lb. of sulphur per acre.

Acceptable Fertilizers

Organic producers often use certain commercially-available fertilizer to address specific nutrient deficiencies identified by soil test.

Rock phosphate, certain types of elemental sulphur and gypsum, borax, microbial inoculants and other products derived from natural sources are often applied to the soil, to the seed or to the plant as nutrient sources.

It is important that the producer contact the appropriate certification body to find out which products are acceptable for use. All measures must be taken to avoid jeopardizing the organic status of the land.

Commercial amendments are typically used only as "last resort" remedies when other methods have not achieved the desired results.

Summerfallow

On the positive side, summerfallow exposes soil organic matter to air and can stimulate the activity of micro-organism that break down organic matter. This can speed organic matter breakdown and nitrogen release. Because summerfallow also discourages plant growth, few nutrients are used while it is in effect. This means more nutrients will be available to the next crop.

On the negative side, summerfallow puts the soil at risk from loss of soil moisture and erosion by wind and water. Lost soil equals lost nutrients. Organic matter lost form the soil due to erosion can expose the soil to even more risk that can result in a reduced nutrient pool.

Conclusion

The health of the soil is essential for a successful organic cropping system. Any management practice that increases biological activity in the soil will enhance the productivity of the soil environment.

All living things depend on a healthy soil – plants, animals and humans alike. Proper attention to fertility in an organic production system will be the key to the farm’s sustainability.

References

• "Organic Field Crop Handbook", 2nd edition, Canada Organic Growers, Ottawa, Ontario.  Published 2001.
• "Farm Facts - Organic Crop Production: An introduction to organic fertilization", Saskatchewan Agriculture, Food and Rural Initiatives.  Published January 1995.
• "Organic Agriculture Industry Wed Sites", Manitoba Agriculture, Food and Rural Initiatives.   Published 2001. http://www.gov.mb.ca/agriculture/crops/organic_links.html
• "Soil Fertility Guide", Manitoba Agriculture, Food and Rural Initiatives.  Published March 2001.
• "Switching to a Sustainable System", Fred Kirschenmann, Northern Plains Sustainable Agriculture Society, Windsor, N.D.  Published 1988.