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COG Organic Field Crop Handbook


1.6 Purchased Soil Amendments


Mineral fertilizers, micronutrient fertilizers and biological agents are most important during the period of transition while the soil is withdrawing from chemical inputs and adapting to organic

agriculture. Once the soil fertility and nutrient cycles have been established, amendments are seldom used.


1. Mineral fertilizers

Organic certification programs do not allow the use of synthetically-produced, highly-soluble fertilizers. Also prohibited are some naturally-occurring products with undesirable characteristics. One example is muriate of potash; it is highly- soluble and has a salinizing effect. And, some materials are restricted. Check the standards of the certification agency in your area. Most ecologically-acceptable sources of mineral fertilizers are slow-acting rock powders which have less energy-intensive methods of production. Rock powders are non-toxic to soil organisms, and there is less danger of burning plant roots or of salt build up. Unrefined rock powders also contain micronutrients. The more finely-ground the rock minerals, the greater the surface area from which nutrients can be liberated by the biological processes in the soil.

Rock powders either substitute for mineral nutrients which are naturally-deficient in the soil or replace the nutrients that leave the farm as cash crops. When only the grain or seed leaves the farm, the majority of nutrients used by the plant can be returned to the soil. If hay crops are sold, however, there is a major loss of organic matter and nutrients from the farm system. Replacing these nutrients may be difficult and costly.

Long-range planning is necessary to obtain the most effect from rock powders. One of their disadvantages is that large quantities are needed to provide nutrients for the crop's immediate need. They are much more effective as a long-term reserve of nutrients. For example, an application of one ton of hard rock or colloidal phosphate per acre should satisfy crop requirements for 4-5 years. When deciding whether to buy rock powders, keep in mind that they are ineffective in soil that has a low level of biological activity and that the minerals become unavailable in alkaline soils.

Rock powders can be added to compost to help increase the nutrient availability or be spread directly on the field. For greatest effectiveness, apply rock phosphate prior to seeding clovers, alfalfa or buckwheat because these plants are able to make good use of phosphate in this form.


[photo 6.1: Spreading colloidal rock phosphate]

Acceptable sources of mineral fertilizers are given in Table 4. Basalt is not commercially available but is a good source of minerals if there is a local source. There is also increasing interest in the use of glacial rock dusts to remineralize depleted soils.


[Table 4 or on adjacent page]

2. Micronutrient fertilizers

It is possible for plants to absorb a limited quantity of micronutrients through the cuticle (skin) of the leaf. During the transition period, organic farmers use foliar sprays such as liquid seaweed extract to correct certain micronutrient problems. The foliar sprays supply small quantities of nitrogen, calcium, magnesium, boron, zinc and iron to the plants.

The effectiveness of the spray depends on air temperature, light levels, surface pH, energy availability and leaf surface moisture. For example, foliar sprays should be applied to soybeans during the early mornings or evenings when temperatures are below 26C, to take advantage of the natural processes that assist the intake of nutrients. When temperatures are moderate, spraying is best done in the evening or during days of high humidity. If the foliage is dry and water vapor is leaving the cells rapidly, there is little chance of the nutrients being absorbed.



3. Biological agents

Organic farmers are well aware of the crucial part played by microbial soil life in building soil structure, in nutrient release and in disease and weed control. There are products on the market that can accelerate the development of these beneficial bacteria. While some of these products are advantageous in the transitional years, most long-time organic farmers agree that using compost along with green manures, appropriate tillage and an adequate crop rotation will maintain bacterial activity at its optimum level without purchased biological agents.

If a legume crop is being grown in a field for the first time, or if the soil has a low bacteria population, adding Rhizobia bacteria to legumes will encourage nitrogen fixation. Each Rhizobium species is effective with only one legume species. When legumes are inoculated with the appropriate bacteria, cell division is stimulated, and nodules which have active bacteria populations form on the roots of the legumes. The nitrogen fixation occurs directly within the nodule. Rhizobia inoculant is available in powder, liquid or granular form and should be stored in a cool area; the bacteria are killed by heat. It is important to have the seed well mixed with the inoculant so that every seed is inoculated.

A different symbiotic relationship at the biological level, this time between plants roots and fungi, has also been investigated with a view to developing a commercial product. A group of fungi called vesicular-arbuscular mycorrhizal fungi (VAMs), have been shown to penetrate the inner area of plant roots and extend the surface area of the roots. In this way, VAMs increase the roots' ability to absorb moisture and nutrients. VAM fungi are thought to dramatically increase the ability of some plants to absorb normally unavailable phosphorus. Natural populations have been severely depleted in soils where applications of phosphorus fertilizer have been high. A Canadian company has developed a VAM inoculant that should be available on the Canadian market in the near future. Imported products are also likely to become more readily available.


Further reading

Gershuny, G. & Smillie, J., The Soul of Soil: A guide to Ecological Soil Management, Gaia Services, Erle, Que., 1986, 109 pp.

Parnes, R., Fertile Soil: A Grower's Guide to Organic and Inorganic Fertilizers, AgAccess, Davis, CA, 1990, 190 pp.



Copyright 1992 Canadian Organic Growers. Inc

Reprinted with permission. All rights reserved.


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