Spectrum Agronomic Library
Knowledge is key to using your analytic results to their fullest. The Spectrum Agronomic Library provides you with useful information that will help you to better understand the complex science of agronomy. Our agronomists will be continually adding original and reprinted articles, so check the library regularly for new information.
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Building Up and Drawing Down Soil P and K Levels
It is common to hear people discuss fertilizer recommendations in terms of crop removal, plus or minus soil test buildup as if they were somehow disconnected from each other. In fact, they are two aspects of the same subject. Most of us fall into the habit of thinking that we are fertilizing plants. Except for foliar fertilizer or tree trunk injection, we do not fertilize plants… we fertilize soil. Because of this, soil chemistry will determine how much of the applied nutrients the plants will be able to take up. If a soil is low in phosphorus (P) or potassium (K), it will tie-up or “fix” much of the applied fertilizer P and K (P2O5 and K2O) into forms that are not available to the plants. This nutrient fixation is simply another way of saying that the soil is trying to build itself up in these nutrients… whether that is your intention or not. The soil is a reservoir for the nutrients that have been applied or generated by other means over the years. The nutrients that a plant gets in any one season are likely to be ones that have been in the soil for many years. Therefore you can think of fertilization as putting nutrients into one side of a reservoir while the plants are taking them out of another end. What happens inside of this nutrient reservoir is soil chemistry and microbiology. These processes, along with weather, determine how much access the plants have to the nutrients within the reservoir.
Spectrum Analytic Reporting Conversions
As of July 1, 2005, Spectrum Analytic began reporting P, K, Ca, and Mg results as unadjusted Mehlich 3 (M3) values in parts per million (ppm). Prior to this date, samples were extracted with the Mehlich 3 solution, and the results mathematically converted to their equivalents in the older tests of Bray P1 for soil P, and 1Normal ammonium acetate (1NAoAc) for K, Ca, and Mg.
The table below shows the correlation formula we used to convert M3-ppm to the lbs/a values.
| Table 1: Correlation Formulas | ||
|---|---|---|
| Nutrient | Formula | Result |
| M3-P | (ICP ppm x 0.7) x 2 | Bray P1 in lbs/a |
| M3-K | (ICP ppm x 0.84) x 2 | 1NAoAc K in lbs/a |
| M3-Ca | (ICP ppm x 0.75) x 2 | 1NAoAc Ca in lbs/a |
| M3-Mg | (ICP ppm x 0.88) x 2 | 1NAoAc Mg in lbs/a |
For a complete explanation, see the original announcement of the change in Spectrum Analytic Reporting Conversions