Wednesday, March 21, 2012

Which Fertilizer Recommendation is Best?

Lloyd Murdock, Extension Soil Scientist, University of Kentucky

There are two concepts for making fertilizer recommendations. Both take the same soil test but they usually result in two very different fertilizer recommendations. One is the Sufficiency concept and the other is the Cation Saturation Ratio (CSR).

The Sufficiency concept is based on local or regional research on similar soil types that find a level of each plant nutrient resulting in maximum yield. When a soil test is below this Sufficiency level, crops respond to the addition of the nutrients.

The CSR was developed in the 1940’s to create an ideal balance of calcium (Ca), magnesium (Mg) and potassium (K) that would result in maximum crop yields.

A lot of research, by many land grant universities comparing these two concepts, has shown that CSR ratios can be much broader than what is held “sacred” by many CSR users. The research has also found that CSR recommendations do NOT improve crop yield relative to the Sufficiency concept used by land grant universities. The research also found that the CSR concept always results in significantly higher quantities of recommended fertilizer materials and much higher costs.

The research done in Kentucky, as well as that done at other universities, is found in the links below. 

The Sufficiency concept is superior to the CSR because the land grand universities have done a lot of research to refine the fertilizer recommendations for their state and their soil types. The research supporting the Sufficiency method allows farmers to use less fertilizer (relative to the CSR method) and maximize yield.

If there is little research on soil tests and fertilizer response, then the CSR has a place. If recommendations are made over a very broad range of states, or countries, where the recommender does not use/have local research, or where local research is not available, the CSR approach is a safe method of making recommendations. The CSR method especially protects the recommender from making recommendations that would result in inadequate nutrition. The recommendations could/should be refined with local research if or when such is available. This is one of the reasons the CSR concept remains active.

One of the frequently asked questions is “How much magnesium is too much”? The answer is when the magnesium on the soil test is greater than the calcium. This is extremely hard to find in Kentucky since our liming programs use limestone that is mostly calcium based. Use of dolomitic limestone would also not give this result since our dolomitic materials contain more calcium than magnesium.

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