7/2/2013 - By Harrell's

For years, turf managers have been taught various methods of interpreting soil and tissue analysis which would then correspond to an appropriate nutrient application. Commonly, ratios have been used (such as base saturation, N:K, etc.). While these methods serve a purpose, in the last 15 years or so, many scientists have moved more towards the “sufficiency” philosophy when providing nutrient recommendations. The objective of this article is to discuss the value of the sufficiency theory for turfgrasses.

When I was an undergraduate student in the 1990’s, we were taught that fertilizer applications should take into consideration the existing balance of certain nutrients. Base saturation (sometimes called basic cation saturation ratio or BCSR) is a ratio commonly discussed and is still provided on various soil analyses. BCSR is defined as the percentage of cation exchange capacity (CEC) occupied by the cations Ca²⁺, Mg²⁺, K⁺, and Na⁺. The way BCSR works is turf managers receive a soil report that states something like: BCSR = 80%, Ca = 45%, Mg = 24%, K = 10% and Na = 1%. The turf manager would then adjust his fertilizer program based upon the balance of these elements to each other and to the CEC.

It is somewhat analogous to the food triangle which tells us we should eat 6 servings of bread, 4 servings of vegetables, 2 servings or meat, etc. Unfortunately, ratios like BCSR do not take into consideration the quantity of a given element necessary for optimum plant development. In the above example, the K⁺ saturation is 10% which, according to the BCSR philosophy, is properly balanced. However, if the sample was taken from a low CEC soil (let’s say a CEC of 4), the total amount of extractable K⁺ would not be sufficient to meet the needs of most turfgrasses. Inversely, if you have a K⁺ saturation of 3% you may believe you need to apply more K⁺ in order to re-balance the BCSR. However, if the soil has a CEC of 100, it is highly probable that extractable K⁺levels would be sufficient to meet plant demands. Using our food analogy, it would be as if you ate 6 grains of rice, 3 peas, 1 grape, and thimble of milk. Based upon ratios, you are eating correctly, but I would be starving!

The sufficiency theory basically asks, “Are you eating enough food or are you hungry?” In other words, the SLAN (sufficiency level of available nutrients) theory places a much lower importance on the ratio of one nutrient to another and asks the simple question, “Are extractable nutrients in sufficient quantities to meet plant demands or are they not?” While this is commonly used with soil samples, for turfgrasses it may be more appropriate to use this process with tissue analysis. While soil samples provide an estimate of the plant available nutrients, tissue analysis provides an exact ‘snap-shot’ of plant nutrition. General sufficiency ranges for various turfgrasses are listed in Table 1.

So what does all this mean and why should a turf manager use the SLAN method? Briefly put….simplicity and accuracy. The SLAN is simple because it simply states, if the plant is deficient in K⁺ then you should apply more, regardless of any nutrient ratios. Moreover, the inverse is also true. If your K⁺ levels are sufficient you may be able to lower your K⁺ input which makes room to apply more N, Fe, etc. (assuming they are deficient). Accuracy is drastically increased because the SLAN method is not subject to an accurate CEC analysis, as is the case with the BCSR. This may sound minor but in fact it can be the source of egregious error especially in highly acidic soils when CEC is measured using NH₄OAc (ph 7.0). Accuracy is further achieved using SLAN with tissue analysis because it allows you to account for differences between similar species. As seen in Table 1, the amount of Mg required by bermudagrass can be twice that required by St. Augustinegrass. Soil analysis cannot be used in this manner because, currently, we do not have a reliable correlation between soil extractable nutrients and the need of those nutrients between different grass species.

While certain nutrient ratios still serve a purpose in soil fertility and plant nutrition, the SLAN method provides a more reliable assessment of the plant’s nutrient status. By utilizing the SLAN method, you can easily and reliable adjust your fertility program to more efficiently meet your plant’s nutrient demands.

Sources

• Havlin, J.L., J.D. Beaton, S.L. Tisdale, and W.L. Nelson. 1999. Soil fertility and fertilizers 6^th ed. Prentice Hall. Upper Saddle River, NJ.
• Carrow, R.N., D.V. Waddington, and P.E. Rieke. 2001. Turfgrass soil fertility and chemical problems. Ann Arbor Press, Chelsea, MI.