How Soil pH Affects Wheat and Corn Yields
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Collapse ▲Soil pH is an important factor in determining crop performance in North Carolina. Low pH levels affect nutrients by converting them into forms that are not readily available to the crop. In addition, low pH levels can increase the solubility of plant toxic metals such as aluminum resulting in stunted growth and a general lack of plant vigor. Because the soils commonly found in the southeastern US tend to be highly weathered and average yearly rainfall is high, pH levels in the surface and subsoil horizons tend to be low. In the sandy soils of the coastal plain, native pH levels often average less than 5.5, and on organic soils pH levels below 3.8 have been found. Therefore, to obtain optimum plant growth it is critical that soils be limed to increase pH levels.
Current recommendations for soil pH levels depend upon the amount of aluminum in the soil. This varies by soil type. Following are the recommendations made by the North Carolina Department of Agriculture concerning target pH levels on different soil types:
| Soil Type (Crop) | Target pH |
|---|---|
| Mineral Soils | 6.0 |
| Mineral Soils | 6.2 |
| Mineral/Organic Soils | 5.5 |
| Organic Soils | 5.0 |
In planning when to apply lime and how much lime to apply, it is critical to understand the effect of pH levels on crop performance. Figures 1 and 2 show the response of wheat and corn to different pH levels. Yield reductions in both of these charts are expressed as the percent of maximum yield expected under the prevailing environmental conditions. It is important to recognize that potential crop yields are determined by the weather conditions experienced during the year. Therefore, it is difficult to determine the absolute yield reductions caused by low pH levels without considering the weather conditions. To determine the impact of pH levels in any given situation, first, determine the potential yield of the crop and then multiply that yield by the percent of maximum found for a given pH on the two figures presented here.
Fig. 1. The relationship between soil pH levels and wheat yields on mineral or organic soils in North Carolina. 
Fig. 2. The relationship between soil pH levels and corn yields on mineral or organic soils in North Carolina.
Lime recommendations are a function of soil class, target pH, current pH, level of acidity (Ac) and residual credit (RC).
Tons lime/acre = Ac x [(target pH – current pH)/(6.6 – current pH)] – RC
To calculate the lime recommendation, you need the following information. Target pH can be found in the table shown in this paper. Current soil pH and Ac values appear on the soil test report. RC is any amount of lime recently applied, reduced by a certain percentage for each month that has elapsed since application: 8% for mineral soils (MIN), 16% for mineral-organic soils (M-O) and organic soils (ORG). Residual credit decreases as the amount of time after the last lime application increases. After 12 months have passed since the last application of lime, the residual credit is 0.