Managing soil pH for optimal soybean production
Managing soil pH in the optimal range is essential to producing high-yielding and profitable soybeans. The first step to managing soil pH is a good soil testing program. A representative soil sample measures the soil pH and the buffer pH or lime index. The soil pH identifies the need for lime and the buffer pH or lime index is used to determine how much lime to apply.
Soybeans will generally perform well at soil pH levels between 6.0 and 7.0. However, the optimal range is between 6.3 and 6.5 as this range maximizes nutrient availability and biological nitrogen fixation while minimizing soybean cyst nematode (SCN) population growth. When soil pH levels exceed 6.5, manganese deficiency symptoms can occur in lakebed and outwash soils and SCN populations have been shown to increase (Table 1).
Table 1. Relationship between soil pH and final soybean cyst nematode population density at harvest. When selecting a liming product, consider the magnesium soil test level. Dolomitic lime supplies magnesium to the soil and should be applied whenever any one of the following is true.
- Magnesium soil test level is below 35 ppm on coarse-textured soils and below 50 ppm on fine-textured soils.
- Magnesium is less than 3 percent of the exchangeable bases on an equivalence basis.
- Potassium exceeds magnesium as a percent of the exchangeable bases on an equivalence basis.
The lime recommendation on the soil test report should be adjusted whenever the lime material has a neutralizing value much different than 90 (less than 80 or more than 100). Use the following formula to make this adjustment:
(90 ÷ neutralizing value of your liming material) x lime recommendation listed on the soil test report = adjusted lime recommendation
The performance of all liming materials is improved when they are incorporated into the soil. Incorporation increases the distribution and soil contact of the lime particles. Surface applications should be limited to long-term, no-till sites as lime moves slowly through the soil (less than 1 inch per year).
The lime recommendation should also be adjusted for the effective tillage depth using the following formula and the information in Table 2.
Lime recommendation x the effective tillage depth ÷ 9 = adjusted lime recommendation
Table 2. Effective tillage depth for various tillage systems. Variable rate lime application based on grid sampling is an effective way to apply lime. This technology ensures that the lime is applied to only the areas where it is needed. This will help prevent over-liming, which can cause manganese deficiency symptoms to show up in soybeans.