Problems of low soil pH are common throughout central and south-central Kansas. Even well-drained, productive soils under good management often become acidic over time due to high crop yields. This problem typically starts in sandier soils and is exacerbated by high rates of nitrogen (N) fertilizer. As a result, long-term continuous wheat production in these areas is particularly vulnerable. However, long-term application of N can cause acidic soils in other regions of the state with different soil types.
How acidic soils impact wheat production
Strongly acidic soils may present several problems for wheat production, including aluminum (Al) toxicity and, in some cases, manganese toxicity. Deficiencies in phosphorus, calcium, magnesium, and molybdenum can also develop. These problems are often difficult to distinguish and largely stem from root damage caused by Al toxicity (Figure 1).
Figure 1. Soil pH stratification after long-term surface nitrogen application. Aluminum concentration in solution increases with a decrease in soil pH. Data from Dorivar Ruiz Diaz, K-State Research and Extension.
Typical symptoms of Al toxicity include thin stands, poor plant vigor, and purpling (Figure 2). High Al levels reduce root development, resulting in short, stubby, and often brown-colored roots.
Figure 2. Wheat growing on very acidic soils, such as this soil in Harper County with a pH of 4.6, is often spindly and has poor vigor. Photos by K-State Research and Extension.
In general, wheat grain yield potential starts to decline when soil pH drops below 5.5 and KCl-extractable (free aluminum) levels are greater than 25 parts per million (ppm). If Al levels are not high, pH levels in this range are not as much of a problem for wheat. When soil pH levels are 5.0 or less, yields start dropping off rapidly in most cases. A minimum soil pH of approximately 6.0 is needed to maximize wheat fall forage production for most wheat varieties.
Where acid soils are limiting wheat production, applying lime to raise soil pH to optimum levels can provide significant improvement to plant growth and yield.
Common questions about lime applications for wheat
If I apply a half-rate of lime now, or in late August, will it benefit wheat planted in early to mid-October?
Lime applications may require time to react and increase soil pH. However, most of the change in pH will occur in the first 4-6 weeks after lime application.
Is incorporation needed to be effective in the above situation?
If the lime is incorporated, the effect in the upper soil profile will be relatively quick. With a lower application rate to the surface in no-till systems, the effect on pH would be limited to the upper 2-3 inches and would require more time to have a significant effect, depending on factors such as soil texture and moisture.
What kind of yield increases can I expect?
Several studies in Kansas have shown significant increases in yield and test weights when liming acid soils (Figure 3 and Table 1). In some cases, yields can easily double depending on the severity of the problem.
Figure 3. Effect of lime on wheat yields at four locations in Reno and Rice Counties. Yields averaged over two varieties – one susceptible and one tolerant to acid soils. Initial soil pH varied from 4.8 to 5.1, and lime application rates varied from 5,000 to 11,000 lbs/acre ECC. Source: Olsen, C.J. et al. Kansas Fertilizer Research 1999, SRP847.
Should I apply less lime than the K-State Soil Testing Laboratory recommendation?
It can be expensive to apply the full recommended rate of lime to soils. The yield increases from an application of the full rate of lime are likely to hold up for up to 8 years or more. But the initial cost can be quite high. Lime is a long-term investment that many producers are reluctant to make for several reasons.
If the cropping system consists of some combination of wheat, grain sorghum, corn, or sunflowers, without a legume in the rotation, then it’s not critical to use the full recommended rate of lime, particularly during years of lower grain prices. These crops can tolerate somewhat lower pH levels than soybeans and alfalfa. Producers may realize some benefit by applying less-than-recommended rates of lime as long as they are willing to make more frequent applications. If soybeans or alfalfa will be grown on the field in question, and if the pH level is less than 6.0, then the full rate of lime should be applied.
Table 1 below shows the effect of a lower-than-recommended rate on wheat yield and test weight. Lower rates can provide near-equivalent yield boosts but require more frequent application. With current crop prices, using tools like variable-rate technology allows producers to target lime more cost-effectively, especially since soil pH can vary widely within a single field.
Table 1. Effect of lime rate on wheat yield and test weight, Sedgwick County.
|
Lime rate |
Yield |
Test weight |
|
0 |
23 |
46 |
|
3750 (half rate) |
42 |
60 |
|
7500 (full rate) |
46 |
61 |
Variety: Karl (susceptible to acid soils). Initial soil pH: 4.7. Lime recommendation: 7500 lb ECC/acre (full rate). Source: Suderman, A.J., et al. Kansas Fertilizer Research 1994, SRP719.
Choosing the right lime source
The Effective Calcium Carbonate (ECC) content of the lime depends on both its purity (relative to pure calcium carbonate) and its fineness. Lime can be from various sources and with different qualities, but all materials must guarantee their ECC content and are subject to inspection by the Kansas Department of Agriculture. To ensure a standardized unit of soil-acidity neutralizing potential, we use units of ECC.
Research has clearly shown that a pound of ECC from ag lime, pelletized lime, water treatment plant sludge, fluid lime, or other sources is equal in neutralizing soil acidity. However, since all lime sources have very limited solubility, they must be incorporated and given time to react in the soil to raise the pH.
When selecting a lime source, the cost per pound of ECC should be a primary factor in source selection. Also consider the rate of reaction, uniformity of spreading, and availability, but the total ECC applied ultimately determines the final pH change.
Final thoughts
Other strategies to increase yields in acidic soils include using aluminum-tolerant wheat varieties and applying phosphate fertilizer with the seed to tie up aluminum and reduce toxicity. These management practices can certainly help to maintain yields and may be the best alternatives for some producers. However, there is only one long-term solution to low soil pH levels: liming.
Dorivar Ruiz Diaz, Nutrient Management Specialist
ruizdiaz@ksu.edu