Managing soil pH for optimal soybean production

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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.

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Yorkton  |  April, 02, 2012 at 03:40 PM

This is great info for acidic soils, but nothing mentioned on higher ph soils. What are the effects on beans on soils with a PH of 8.0 and higher? Are there things that can be done to help beans perform at these high PH levels?

Benson, MN  |  April, 02, 2012 at 04:35 PM

It is nearly impossible to lower a high pH soil. IDC (iron chlorosis) is a problem on some high pH soils caused by high levels of carbonates or soluble salts in the soil. Using an iron chelate such as Fe-EDDHA as a starter application has shown very good response. Growers in our IDC areas are successfully overcoming the IDC problems with a starter application of Fe-EDDHA. Other types of iron chelates do not perform very well when compared to Fe-EDDHA.

colby, ks  |  April, 03, 2012 at 11:45 AM

we been using sulfur to off to side to lower ph for short period of time plus iron. Then in seaon over top iron treatment. On worse ph soils 8 to 8.5 getting yields to increase by 10 bu. These soils still lag behind are 7.3-7.7 ph soils by 10 bu as well. This is all irrigated.

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