By Daniel Kaiser
There are a lot of ideas out there on how to manage pH of soil to optimize plant growth. One major issue with high pH soils is iron deficiency chlorosis (IDC) in soybean. Soybean growers have been looking for the magic bullet for managing IDC and while there are options out there, nothing is long term.
Some wonder if soil acidification to lower pH would help solve IDC. In most Minnesota soils, which are poorly buffered, adding nitrogen fertilizers over time will acidify soils. Of the nitrogen sources, ammonium sulfate has the greatest potential for acidification but is mostly applied for sulfur and rarely used as the sole nitrogen source. Elemental sulfur has been used in horticultural crops, but it’s required in high amounts and might not be effective if oxidation is slow.
Buffering capacity is an important process to understand when it comes to liming. The buffering capacity is a measure of the reserve acidity or alkalinity in the soil. This dictates how easy it is to change the pH of the soil. Soil test labs run buffer pH, a measure of the buffering capacity which helps determine application rates for limestone. Soils with carbonates present already have a high buffering capacity so it is difficult to lower the pH of those soils.
Soil pH can affect nutrient availability, nodulation in legumes, herbicide carryover, and other soil chemical processes. Soils in Minnesota range from basic to acidic. Acidic soils need lime from time to time to neutralize acidity. Basic soils in Minnesota developed in areas of the state with less rainfall, which means the carbonate materials deposited by glaciers remain near the soil surface. While we can lime acidic soils to improve soil pH, dealing with basic soils is much more difficult.
The past winter, John Lee from Agivse labs in Northwood, ND presented data from a long term site they had established on a soil with pH over 8.0 and a calcium carbonate equivalency of 1.5%. They applied elemental sulfur at a rate of 10,000 pounds per acre and monitored the site over the span of 12 years. Soil pH decreased by 0.5 initially but within a span of five years returned back to the original level. This example shows how resistant soils can be to change and why acidification is not feasible in highly buffered situations.
Managing nutrients, and in the case of managing IDC, variety selection are the key to managing crops for higher yield. High pH soils do present challenges but modifying the pH is likely not feasible. The cost to do so would not provide a workable ROI, especially compared to other yield-improving strategies.