Soil Testing: A Basis for Making Fertilizer Recommendations or Just a Balancing Act?

Soil testing is a valuable management tool that can be used to make fertilizer recommendations. One approach used to make recommendations based on soil testing is the "sufficiency level" approach. This approach is built on the concept that there are certain levels of plant nutrients in the soil that are defined as optimum. This is the approach that the Manitoba Provincial Soil Testing Lab used. Usually a crop will respond to a fertilizer application when the soil test value is below the defined level for a particular nutrient, but response will be limited if above the critical level.

Another approach that you may have heard of and is not commonly used in most soil testing labs is the "basic cation saturation ratio" (BCSR). In this approach, it is thought that maximum yields can only be achieved by creating an ideal ratio of calcium (Ca), magnesium (Mg), and potassium (K) in the soil. Little emphasis is placed on other nutrients. These ratios have changed over the years but the first recommendation was that 65% calcium, 10% magnesium, 5% potassium, and 20% hydrogen should occupy the exchange complex. These percentages calculate to 13 parts of Ca to 2 of Mg to 1 of K. Fertilizer recommendations are very different between the two approaches.

The basic cations (Ca, Mg, K) are known to have antagonistic effects on each other. This means that if there is a very high level of one of these nutrients it may reduce the availability of one of the others. However, these interactions between nutrients are only important when one of them is approaching deficiency.

The concept of optimum cation ratios has been debated over time, but there is very little research evidence to show that these ratios have either a positive or negative effect on crop production. There is evidence that common field crops will tolerate a wide range of cation ratios, but has no effect on yield as long as one of the nutrients was not limited.

A major drawback of the BCSR approach is that a nutrient deficiency may exist even though the ratio of cations is considered optimum. For example, the total amount of exchangeable cations for sandy soils that are low in organic matter is small. Although these soils may have the ideal BSCR, the actual total amount of nutrients available for plant growth is not sufficient, and therefore a deficiency may result. Another important point is that on soils that have a high level of carbonate minerals, an unrealistically high potassium recommendation may result. This results from the fact that if these minerals are dissolved by the soil test extractant, this will release calcium and magnesium into the extractant, thereby inflating the calculated CEC (cation exchange capacity) and the Ca and Mg percentages.

The most important consideration in making economic fertilizer recommendations is to ensure that there is an adequate supply of available cations in the root zone.

Here is some typical base saturation ratios from Manitoba:

It is likely that soils 1 and 3 would receive a recommendation to apply K if the BCSR approach was used. However, if the sufficiency level approach were used, K would not be recommended, as the level of K in these soils was over 200 ppm or 400 lb/acre (very high).