High-pH soils are not as common as acid soils in Kansas, but they do occur in some areas. In fact, high-pH soils can be even more of a problem than acid soils because high pH levels are more difficult and expensive to correct. Soils with a pH of 7.5 or higher may result in reduced plant growth, depending on the crop species and the cause of the high pH level.
There are two categories of high-pH soil conditions: calcareous and saline/sodic soils.
In many cases, the high soil pH is the result of calcareous conditions, or excessive amounts of lime (calcium carbonate) in the soil. Calcareous soils occur most often in western Kansas, where soils are geologically quite young.
Soil pH levels increase with depth in most soils of Kansas, except for southeast Kansas. Subsoils typically have higher pH levels than topsoils. On eroded hillsides and side-slopes, the more calcareous layers of soil are closer to the surface.
Iron chlorosis can be a problem with some crops on high-pH, calcareous soils. Sorghum and soybeans are more affected by iron chlorosis than corn, wheat, or alfalfa. Marked differences in tolerance of calcareous soils also exist among horticultural crops.
To lower the pH of a calcareous soil, an amendment needs to be added to neutralize the excess lime. Natural processes can slowly lower the pH of a calcareous soil – including organic matter mineralization, crop removal of lime and other bases, and natural weathering. But these processes can take hundreds or thousands of years to have any effect on the soil pH. Applications of nitrogen fertilizer also act to lower the soil pH, but only in the topsoil. Nitrogen fertilizers will not correct a calcareous subsoil condition.
Elemental sulfur is one amendment that can be used to lower the pH of a calcareous soil. Even sulfur will take some time to be effective. Elemental sulfur is microbially converted in the soil to sulfate, and soil acidification is a by-product of this process. This acidity reacts with the excess lime to neutralize the soil. Unfortunately, it takes quite a bit of elemental sulfur to neutralize the excess lime in a calcareous soil. It takes a third of a pound of elemental sulfur to neutralize a pound of excess lime. A soil with one percent excess lime would have 20,000 pounds of excess lime per acre in the top 6-7 inches of soil. This would mean 6,600 pounds of elemental sulfur per acre would be needed. At $0.30 to $0.40 per pound, this is not economical in most agronomic situations. Many of the calcareous soils causing severe chlorosis on grain sorghum and soybeans have more than one percent excess lime.
In K-State research, banding sulfur to neutralize a small zone of soil near the seed has not been consistently effective in solving an iron chlorosis problem on calcareous soils. Neither has the application of inorganic iron fertilizer. Foliar application of chelated iron products have been effective, but this is expensive and repeated applications are often needed.
To manage for iron chlorosis problems on calcareous soils, producers can utilize crop selection, or varieties within a crop, to achieve some degree of crop tolerance. Manure applications sometimes help. And foliar applications of iron have also been effective in minimizing yield losses. Recent work in western Kansas with soybeans also suggests that some seed treatment products could be beneficial in severe cases.
For horticulturists wanting to lower the pH of a relatively small volume of soil, aluminum sulfate can be used effectively.
Saline and sodic soils.
A second reason for high-pH soils is the presence of high exchangeable sodium in the soil. High exchangeable sodium results from soils that formed from parent material with excessive sodium, use of poor quality irrigation water, application of manure or sewage sludge high in sodium, or spillage of brine water association with oil production. Saline and sodic soils are special cases, and are discussed in a separate article in this issue of the Agronomy eUpdate.
Dave Mengel, Soil Fertility Specialist
Dorivar Ruiz Diaz, Nutrient Management Specialist