A fresh look at soil testing

From the March issue of SportsTurf, Dr. Grady Miller of North Carolina State University answers this question about soil testing:

This is the time I normally turn in soil samples for my fields. It seems like every year I get about the same results back so I have been wondering if I can take a year off of testing. I have also read about a minimalistic approach to applying fertilizers based on soil testing is becoming more common with golf turf. Maybe I have been putting out more fertilizer than needed. Can you tell me more of what that is all about?

Answer: If you have been getting similar test results and are comfortable with your fertility programs, I see no harm in taking off a year. You are also correct that a group recently introduced a new approach to calculate fertilizer needs based on soil testing results. This new way of using soil testing results may be a real positive as most of our current soil testing theories were based on agronomic systems rather than calibrating results with turfgrass systems. So even if this approach is not used, it may encourage agronomists to give a second look at soil tests and fertilizer recommendations.

Since research on soil fertility for agronomic crops started back in the late 1800s, one may think we know all there is to know when it comes to soil fertility. In reality there has not been a great deal of research related to interpretation of turfgrass responses to soil nutrient additions based on soil testing. This is one of the reasons why you may get a wide range of recommendations by different laboratories on the same sample.

Soil analysis is largely based on the concept of how much nutrient will be available to a plant from a soil. Since in many cases elements are found within and/or attached to various soil particles, soil testing uses solutions to extract those elements from the soil particles so that their relative availability can be estimated. Extracting solutions may be something as simple as water but more commonly it is a weak acid solution. Probably the most common solution used is Mehlich III extractant, which is often noted on soil testing reports.

The amount of the element measured after extraction should then be interpreted so that it can be related to a plant response. As soil test level of a nutrient increases, the chance of getting a plant response to adding that nutrient decreases. In most cases soil nutrient levels are categorized as low, medium, high, etc. Therefore the amount of fertilizer recommended usually decreases with increases in soil test levels.

Interpretation of the numbers (i.e., fertilizer recommendations) can vary depending on philosophies of laboratories or individuals making recommendations. The most common approach is the sufficiency level concept in which the amount of available nutrient reported from the soil extract analysis is used to determine the need for fertilization. This philosophy is referred to as “sufficiency level of available nutrients” or SLAN. If the soil test indicates the nutrient level found in the soil is below a sufficiency level, then additions are recommended. Or in some cases higher recommendations may be suggested as a way to build up soil levels of particular nutrients.

A second philosophy is to evaluate the percentage saturation of base cations (K, Mg, and Ca) as a total of the cation exchange capacity. These percentages are used to determine if nutrients are in balance. Desired ranges could be 2-5% K, 5 to 15% Mg, and 60-80% Ca. A recommendation may also be related to their ratio of Mg to K and Ca to Mg. This philosophy is referred to as the “basic cation saturation ratio” or BCSR. Some salespeople used the BCSR concept to encourage fertilizer applications even though a soil test report indicated there were sufficient levels already in the soil.

This new philosophy is called the “minimum levels for sustainable nutrition” or MLSN. This method is based on the concept of managing nutrient levels by reducing fertilizer inputs to a minimal level while still maintaining desired turfgrass quality. It is a little more difficult to explain in a few words, but the general concept is to use the turfgrasses’ uptake for each nutrient in relation to its nitrogen uptake. The uptake values are obtained from expected leaf nutrient content. Nitrogen is used since it is the nutrient that primarily drives growth of the plant. Growth is a driving factor of uptake for all the nutrients. Minimum levels of each nutrient have been established through extensive sampling of turfgrasses. By using soil test values, considering the minimal levels needed, a bit of math is used to estimate how much of a nutrient needs to be added to account for depletion by the turfgrass while ensuring adequate amounts remain in the soil. In many cases, the amount needed is likely to be lower than would be recommended using the SLAN method.

That was a quick summary of the MLSN method, but if interested I would encourage you to investigate further. My best advice is to use your soil testing data along with your experience on site as a basis for developing your fertility program.