Navigating Fertilizer Prices Part 2: Interpreting Soil Test Results
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In order to best describe this process, we will work through the example below.
This sample was taken in Fall 2021 by a producer who was establishing tall fescue in some pastures, and maintaining tall fescue stands in others. For this specific sample, we know they were maintaining a tall fescue pasture because of the listed crop code. This code is Fescue/OGrass/Tim, M, which means tall fescue, orchardgrass, or timothy, maintenance. The code for establishment is Fescue/OGrass/Tim, E– E for establishment.
pH and Lime
Lime is arguably the most important soil amendment of them all. Lime is applied to increase soil pH. Nutrient availability to plants is highly dependent on soil pH. pH is a measure of the soil’s acidity, and is an index of 0-14, with 0 being most acidic and 14 being basic, or least acidic. The ideal pH range for cool-season perennials, like tall fescue and orchardgrass, is 5.7-6.5. As seen in the graphic below, this range is when crucial nutrients are most available for plant use. The further the pH out of this range, the less available the nutrients become. With this being said, application of fertilizer is naive if soil pH is not correct.
This producer had also applied 1 ton/acre of lime in November of 2020. It is important to include this information to ensure the most accurate lime recommendation possible is made. The lime recommendation from the NCDA&CS laboratory is based on a combination of the soil’s pH, and Ac value. Ac is exchangeable acidity, which is the combined total of hydrogen and aluminum cations in the soil. The exchangeable acidity impacts the amount of lime needed to neutralize the pH. In our example sample, the Ac is 1.8 and the pH is 5.2, slightly lower than the ideal range. Because of this, and the liming history, the recommended lime application is 0.8/ton acre. Generally, it is recommended to apply no more than 1 ton/acre of lime at once. In situations where more than 1 ton/acre is recommended, it is advisable to split the amount into 2 applications, one in the Spring and again in the Fall. Lime is currently $35-$40/ton spread, making it the most economical of the soil amendments, especially due to its importance to nutrient uptake.
Now, onto the “big three” nutrients, or macronutrients, nitrogen (N), phosphorus (P), and potassium (K). The nutrients are needed in the greatest amounts for plant growth, and will most likely need to be applied more often to hay fields than pastures. When hay is mowed, nutrients are removed with the forage, and there are no nutrients being cycled back into the soil. This is in contrast to pastures, where nutrients removed in forage as animals graze are reapplied in the form of animal waste. This is not to say that it is never necessary to fertilize pastures; however, it is likely that well managed pastures will need fertilizer less often than hay fields.
It is important to note that the NCDA & CS laboratory does not measure available nitrogen in the soil. This process is very extensive, and would increase the laboratory’s costs, and therefore the price of sample submission. With this in mind, the standard nitrogen application recommendation for the maintenance of tall fescue, orchardgrass, and timothy will be 120-200 lbs/acre. This recommendation is for the full year, so it is recommended to split the applications (60 lbs/acre during Spring growing period, 60 lbs/acre in the Fall growing period). This is advisable as these rates are more likely to be fully utilized, rather than a one time application of 120 lbs/ac, which would be difficult for the plants to utilize. On field visits, I often compare this to humans eating meals. It would be hard for us to eat a whole cake in one sitting, but we could easily eat a slice. The same concept applies to plants. Nitrogen prices may make this recommendation unrealistic this year. Alternatively, managed grazing can be implemented to disperse animal waste more evenly throughout the paddock. By doing this, nitrogen from the waste is distributed throughout the paddock to be available to a greater number of plants. This can also somewhat offset phosphorus and potassium fertilizer needs as well.
Phosphorus is the second macronutrient listed on the soil test report. Recommendations for phosphorus are made in the form of P2O5, or phosphorus pentoxide. This is because this is the phosphorus compound most commonly used in commercial fertilizers. The recommendation for this sample is 130 lbs/acre. It is also notable that the phosphorus index, listed as P-I on the report, is 7, which is very low and indicates that there would be a large plant response to P fertilization. This is in great contrast to the mean phosphorus index of soils in NC at 137, which is very high and indicates there would likely not be a plant response to P application.
Potassium is the final macronutrient on the report, and application recommendations are made in the form of K2O, or potash, because of the reasoning listed above for P. The report above recommends 70 lbs/acre of potash. As with P, there is also a potassium index on the soil report, listed as K-I. The K-I value for this sample was 36, falling into the “low” range, indicating that there would be a great plant response to potash.
Stay tuned for an explanation on choosing amendments for the example report.
For more information on interpreting NC soil reports, review Understanding the Soil Test Report.