All Things Organic - Soil testing may not tell the whole tale

By Ian Cushon
April 24, 2003

Soil testing can give important information to organic farmers. Knowing the nutrient needs of soil is an important management tool. Where farmers have access to acceptable organic fertilizers such as composted animal manure, then conventional soil tests can be used to tailor applications to soil needs. But most conventional soil tests measure only the macro and micronutrients available at a specific time, giving only a partial picture.

Conventional soil testing is inadequate in evaluating the ability of organically managed soil to supply nutrients throughout the growing season. This is especially true of nitrogen and less so with phosphorus. Most prairie organic farms use green manure and forage legumes to power their fertility needs. This system needs a different form of soil testing. Such a soil testing system would likely be a complex measurement of the soil organic matter and the ability of the soil to cycle the nutrients from organic matter into plant-available forms.

Healthy soils with lots of organic matter, good structure and a diverse range of micro-organisms provide nutrients by breaking down organic matter and parent material. This process occurs through the growing season, but is highly variable depending on soil type, cropping history and weather. Measuring nutrient availability at a particular time helps identify the potential for serious deficiencies. But when nutrients are only somewhat deficient, or marginal, then evaluating the soil's ability to supply the additional nutrients for optimal production would be valuable information for organically managed soils.

There are also other indicators of soil fertility. Crop evaluation by means of visual inspection and yield measurement are also a good indicator of soil health. However, when nutrient deficiencies occur, it can be difficult to clearly distinguish deficiencies from some types of insect damage, disease, drought, wind and frost damage. Tissue testing can also identify nutrient deficiencies. Plant material can be tested to find out critical, sufficient and toxic levels of various macro and micronutrients. Tissue sampling must be done carefully. Contact labs to learn how to sample properly for various circumstances.

Deficiencies identified by tissue testing may not always be due to nutrient deficiencies. Cold, wet soils may cause reduced nutrient uptake even when nutrients are sufficient. Imbalances in soil nutrients may also cause nutrient deficiency to occur. Diseases may interfere with nutrient uptake. A University of Saskatchewan survey of practices on our farm has revealed good news about our soil and fertility management. As part of a larger survey of organic farm practices, researchers took soil samples from two of our fields in 2002. They also surveyed weeds. Field five has been organic since 1989. Field eight has been organic since 1985. In 2002, field five showed 3.94 percent of the soil was organic matter while field eight had 4.34 percent. While these are not exceedingly high contents for our soil zone, they indicate organic matter is stable or increasing relative to tests taken in the late 1980s and early 1990s. Organic matter may be the most important measure of soil health in organic systems. If it is stable or rising, then soil management is likely good. But if the level is falling, concern is warranted.

Evaluation of organic matter in soil should be based on trends over several years of tests. There can be considerable variation among individual years because of cropping history or dry weather. Last year's tests showed that in that part of our rotation, nitrogen levels were good and phosphorus levels were OK. We have been concerned about possible deficiencies, but total available phosphorus levels were 37.5-44 pounds per acre, a marginal level, but not far off optimal levels. These levels did not take into account the phosphorus made available during the growing season. However, the amount of phosphorus that becomes available during the growing season is relatively small compared to the potential nitrogen that comes available. The nitrogen levels were 67 lb. per acre for field five and 93 lb. for field eight. In the end, our crop yields told the most important story. Field five was wheat planted on a green manure fallow. It yielded 33 bushels of 14.5 percent protein wheat. Field eight yielded 76 bu. per acre of oats, also planted on a green manure fallow.

Ian Cushon
Moose Creek Organic Farm Inc.
P.O. Box 85
Oxbow, Saskatchewan
Canada, S0C 2B0

Tel. 306-483-5034
FAX 306-483-2799
Cell 306-483-8257
E-mail: coldridge@sasktel.net