Understanding your soil test report

By Desiree Jans, Ph.D.

Soil testing can be a useful tool for organic farmers to evaluate management practices. The 2005 draft of the National Standard of Canada for Organic Production Systems recommends soil testing every five years. But have you ever had trouble making sense of your soil test report? You’re not alone!

Here’s what your soil test report will tell you

1. the level of nutrient in your soil sample,
2. the rating for that level with regard to the crop you plan to grow (High, Medium, Low), and
3. the recommended rate of additional nutrient required to meet the specified crop’s demand.

Standard field soil analysis includes

Organic matter (OM): Reported as a percentage of the dry soil sample. OM is important for holding nutrients and water. It stabilizes soil structure and supplies energy and habitat for soil organisms. Organic farmers should aim for at least 4% OM and watch for stable or increasing values as an indicator of sound soil management.

pH: A measure of the soil's acidity. pH below 7 is acidic (sour); above 7 is alkaline (sweet). Soil microbes are affected by soil pH as is availability of essential nutrients. The optimum range is usually between 6 and 6.5. The soil test report includes a recommendation for lime application (t/ha) to raise the soil pH to a given value (usually 6.5).

Phosphorus (P) and Potassium (K): The available amount of these plant nutrients reported as ppm (parts per million) P2O 5 or ppm K2O. P and K are reported as oxides, P2O5 and K2O, because phosphate and potash fertilizers are sold on this basis. However, P and K do not usually occur as oxides in the soil. A rating for each nutrient indicates the potential of the soil to supply P or K to the specified crop during the growing season.

Calcium (Ca) and Magnesium (Mg): The available amount of these nutrients reported as ppm. A rating may be given.

Boron (B), Copper (Cu), Zinc (Zn), Sulphur (S), Manganese (Mn) and Iron (Fe): The available amount of these nutrients reported as ppm. Ratings may be given if the crop specified is known to have a special demand for one or all of these nutrients.

Cation Exchange Capacity (CEC): Reported as milliequivalents per 100 grams of soil. CEC represents the ability of the soil to retain positively charged nutrients (cations) like Ca+2, Mg+2, K+ and NH4+. Clay and organic matter are negatively charged. Soils with high amounts of clay and/or organic matter have high CEC and the potential to "hold" large amounts of nutrients.

% Base Saturation: A measure of the proportion of total CEC that is taken up by the basic cations, Ca, Mg, K, and sodium (Na). 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 your local soil-testing lab. Adding nutrients in an attempt to reach an ideal ratio is not recommended (or economical) because you may add more than you need to achieve a crop response. However, consider your % Base Saturation when choosing amendments and try to move the soil toward a balanced state.

Recommendations for nutrient application are based on extractable nutrients in your soil and predicted crop yield response to additional nutrients. At some point, the yield return to adding more nutrients does not cover the cost of additional amendment. Nutrient recommendations attempt to optimize economic return.

Because soil nitrogen (N) is so mobile, it is not part of standard soil analysis. Recommendations for N application assume that most of the readily available soil N has moved past the root zone. Be aware that plowdown of a legume sod adds slowly available N to the soil and reduces the requirement for additional N. The lab’s recommendation will take this into account if you clearly explain your management practices upon sample submission. Similarly, manure application should be reported to the lab so that recommended levels of additional N, P, and K can be adjusted.

Soil testing is often done in the fall which leaves plenty of time to plan amendments or alter rotations before spring planting. Spring sampling is also possible, however. To best evaluate your management practices, always sample at the same time of year. In a following article, I will discuss amendments organic farmers can use in response to soil test recommendations.

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.ca.

Posted on the OACC website, November 2005