Understanding your soil test report – Organic Matters

Soil testing can be a useful tool for organic farmers to evaluate management practices. Some organic certification bodies require soil testing as part of the certification process. But have you ever had trouble making sense of a soil test report? You’re not alone.

Here’s what a soil test report will tell you:

  • The level of nutrient in a soil sample.
  • The rating for that level with regard to the crop you plan to grow Ð deficient, marginal, sufficient or excessive.
  • The recommended rate of additional nutrient required to meet the specified crop’s demand.

Options for field soil analysis include organic matter, pH and nitrogen availability.

Organic matter is normally reported as a percentage of the dry soil sample. It is important for holding nutrients and water. It stabilizes soil structure and supplies energy and habitat for soil organisms. Watch for stable or increasing values as an indicator of sound soil management.

A pH level below seven denotes acidic land while a level above seven denotes alkalinity. Soil microbes are affected by soil pH as is availability of essential nutrients.

Nitrogen is reported as nitrate-nitrogen, which is the plant available form. Because nitrate-nitrogen is so mobile, it often travels into the soil profile. It’s a good idea to sample soil at depths of zero to 15 centimetres and at 15-30 cm.

If the previous season was wet, a soil sample at 30-60 cm may be useful. The recommended rate of additional nitrogen will be lower based on analysis of the zero to 60 cm soil samples.

Plowdown of legume biomass adds slowly available nitrogen to the soil and reduces the requirement for additional nitrogen. The lab’s recommendation will take this into account if you explain your management practices upon sample submission.

Similarly, manure application should be reported to the lab so that recommended levels of additional nitrogen, phosphorus and potassium be adjusted.

Normally, phosphorus and potassium are reported as oxides because phosphate and potash fertilizers are sold on this basis.

A rating for each nutrient indicates the amount of the nutrient that is in the soil solution at that time.

Other important nutrients include sulphur, magnesium, calcium and micronutrients such as copper, iron, boron and zinc.

Sulfur, which is reported as sulfate-sulfur and is a mobile nutrient so sampling to a depth of 30 cm, or even 60 cm in a wet year, is a good idea.

Ratings are also given for calcium, magnesium, boron, copper, iron, manganese and zinc.

Another indicator of soil health is cation exchange capacity.

It represents the ability of the soil to retain positively charged nutrients.

Clay and organic matter are negatively charged so soils with high amounts of clay and organic matter have high CEC levels and the potential to hold large amounts of nutrients.

Base saturation is a measure of the proportion of total CEC taken up by the basic cations such as calcium, potassium and sodium.

Ideally, levels of these cations should be balanced. Excessive levels of one may impede plant uptake of another, even when soil test ratings indicate that each nutrient is present in sufficient quantity.

The ideal ratio varies with soil type so check with the soil testing lab. Adding nutrients in an attempt to reach an ideal ratio is not recommended or economical because you may add more than needed to achieve a crop response.

Soil testing is often done in the late fall once microbial activity has slowed.

This leaves plenty of time to plan amendments or alter rotations before spring planting. However, spring sampling is also possible. To best evaluate management practices, always sample at the same time of year.

For information on soil testing services in Canada check http://www.certifiedorganic.bc.ca/rcbtoa/services/soil-testing-services.html.

Desiree Jans, Ph.D., is a Web Course Instructor for the Organic Agriculture Centre of Canada (OACC). For information about OACC courses or to send comments, contact us at 902-893-7256 or oacc@nsac.ns.ca. One month after publication, articles are archived at www.oacc.info.

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