MSU Agronomists (Richard Engel, Clain Jones, and Simon Fordyce) have put together a document on Soil Acidification Management. The document is available online at the following URL: http://landresources.montana.edu/fertilizerfacts/html/FF79.html. Paper copies are available at the Chouteau County Extension office.

The study concluded that adding 90 pounds of seed-placed fertilizer-phosphorus (0-45-0) can mitigate aluminum toxicity and increase growth and grain yield of cereal crops in acid-affected soils. In addition, responses to fertilizer-phosphorus in acid-affected soils occur even if the test is very high in available-phosphorus.

Soil acidification is a recent development in Montana occurring because of long-term use of ammonium-based nitrogen fertilizers. Acidification has led to cases of aluminum toxicity of field crops where the surface soil pH has fallen below 5.0. Remediation of acid-affected soils will ultimately require the application of liming materials. However, aglime can be quite expensive, exceeding $100 per acre. We conducted this study to determine if high rates of seed-placed fertilizer-phosphorus could mitigate aluminum toxicity and increase yield of cereal grains similar to aglime in an acid-affected Montana soil.

We found durum grain yield increased with phosphorus fertilization in 2018 and 2019, but only where soil pH was not increased with the addition of aglime. The interaction of lime and fertilizer-phosphorus was particularly impressive in 2018. In 2018, seed-placing 60 lb P2O5/ac ($23 cost @ $0.38 per lb of P2O5) increased grain yield by 22.2 bu/ac over the zero phosphorus controls where lime was not applied. Conversely, yields did not increase with fertilizer-phosphorus when lime was applied to correct soil acidity. In 2019, we found a similar interaction between lime and fertilizer-phosphorus, but durum growth and grain yield were greatly

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reduced by two hailstorm events. The second hailstorm occurred during grain-fill and caused considerable numbers of shattered heads and broken tillers. Still, the results of our study indicate that seed-placed phosphorus fertilizer can mitigate aluminum toxicity and related phosphorus deficiency at field sites with very acidic soils (pH < 4.5 in top 4 inches).

How fertilizer-phosphorus mitigates aluminum toxicity in crops is still a bit unclear. One possible mechanism suggests that fertilizer-phosphorus ties up dissolved aluminum so it can’t be taken up by the plant. Another theory is fertilizer-phosphorus uptake by crops results in aluminum-phosphorus being tied up on root surfaces, in root cell walls or within root cells that decrease aluminum toxicity. Whatever the response mechanism, use of high rates of fertilizer-phosphorus should be viewed as a short-term approach to manage acidic soils. Although all plots in this study were tilled, we expect the benefit of seed-placed fertilizer-phosphorus would be similar under no till management.

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