Understanding your soil test report

By Desiree Jans, Ph.D.

Soil testing can be a useful tool for organic farmers to evaluate management practices. Some organic certification bodies even 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:

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

Options for field soil analysis include

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. 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; above 7 is alkaline. Soil microbes are affected by soil pH as is availability of essential nutrients.

Nitrogen (N): Reported as nitrate-nitrogen (NO3-N), which is the plant available form. Because soil NO3-N is so mobile, it often travels into the soil profile. It’s a good idea to sample soil at depths of 0-15 cm and 15-30 cm. If the previous season was quite wet, a sample at 30-60 cm may be useful. The recommended rate of additional nitrogen will be lower based on analysis of the 0-60 cm soil samples. Be aware that plowdown of legume biomass 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.

Phosphorus (P) and Potassium (K): Reported as available P2O 5 or K2O. P and K are reported as oxides, P2O5 and K2O, because phosphate and potash fertilizers are sold on this basis. A rating for each nutrient indicates the amount of the nutrient that is in soil solution at that time.

Sulphur (S): Reported as sulphate-sulphur (SO4-S). SO4-S is a mobile nutrient and, as for NO3-N, sampling to 30 cm, or even 60 cm in a wet year is a good idea.

Calcium (Ca) and Magnesium (Mg): The available amount of these nutrients is reported and a rating given.

Micronutrients: Boron (B), Copper (Cu), Iron (Fe), Manganese (Mn), and Zinc (Zn): Ratings may be given if the crop specified is known to have a special demand for one or all of these nutrients. Micronutrients should only be applied if a specific deficiency is identified.

Cation Exchange Capacity (CEC or EC): 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.

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.

 

Posted on the OACC website, November 2005