Some basic concepts of liming
Correcting acid soil conditions through the application of lime can have a significant impact on crop yields, especially with alfalfa. Liming is one of the most essential, but often overlooked, management decisions a producer can make.
One important aspect of liming is the time required for pH change to occur. It can take some significant time for lime to react and raise soil pH. The exact amount of time is generally a function of lime particle size, and soil moisture. Smaller particles have more surface area, and react faster in the soil. So when time is of the essence, using finely ground lime materials will result in a quicker pH increase.
Research has shown that the fine particles in ag lime, generally those which are smaller than 60 mesh, will react within 30 days. As particle size increases, the rate of reaction slows, since the relative surface area decreases as particle size increases. Particles between 30 and 60 mesh size may take as long as 1-2 years to react, while those between 8 and 30 mesh may take as long as 5 years or more. This is especially important when liming for new alfalfa seedings. Since seeding alfalfa is expensive and a stand is expected to last for several years, getting lime applied early enough to get the acidity problem corrected before seeding is critical.
Unfortunately lime is not always available close to where it may be needed. In many cases trucking and spreading costs may be more than the cost of the lime itself. Lime quality can also vary widely and no one wants to apply more than is necessary. So to make the best decisions on how much and what kind of lime to apply, it is useful to know how lime recommendations are made.
Crop sensitivity to pH and regional differences in target pH
A routine soil test will reveal the pH level of the soil, and this will determine whether lime is needed on the field. Crops differ in their sensitivity to low pH. In most cases, our crops are tolerant to pH levels in the higher ranges, as long as they don’t exceed pH 7.0. So it is generally best to lime to satisfy the needs of the most acid-sensitive crop commonly grown on that field.
Alfalfa is the crop most sensitive to acidity, and requiring the highest pH. Soybeans and red clover are intermediate, doing best at pH 6.0-6.4 in most areas; and wheat and corn are the most acid-tolerant crops. Generally, east of the Flint Hills, lime is recommended for alfalfa if the pH drops below 6.4, with a target pH for liming of 6.8. In the Flint Hills and west, lime is recommended for alfalfa and all other crops when the pH drops below 5.8, with a target pH of 6.0. Target pH is simply the pH goal once the lime reacts with the soil.
Why are the target pH levels different for the two areas of Kansas? They differ because of the pH of the subsoil. East of the Flint Hills, especially south of the Kansas River, the subsoil tends to be acidic, and a higher target pH is used to assure adequate pH conditions in the surface root zone, and provide sufficient amounts of calcium and magnesium. From the Flint Hills west, most soils have high-pH, basic subsoils that can provide additional calcium and magnesium to meet crop needs.
Lime rates are given in pounds of effective calcium carbonate, ECC, per acre, Soils with more clay and organic matter (higher cation exchange capacity) will have more reserve acidity at a given pH, and will require more ECC to reach a target soil pH, than will a sandy soil. This is why two soils may have the same soil pH but have quite different buffer pHs, and different lime requirements.
So, how does ECC relate to ag lime and how much lime to apply? Lime materials can vary widely in their neutralizing power. All lime materials sold in Kansas must guarantee their ECC content and dealers are subject to inspection by the Kansas Department of Agriculture. The two factors that influence neutralizing value and are used in determination of the ECC content are the chemical neutralizing value of the lime material relative to pure calcium carbonate, and the fineness of crushing, or particle size, of the product. The surface area of the particles is critical for neutralizing to occur.
Expressing recommendations as pounds of ECC allows fine-tuning of rates for variation in lime sources, and avoids under or over applying lime products. This is important for two reasons; excessively high pH can lead to micronutrient availability problems, especially iron and zinc in Kansas. It is not universally true that all high-pH soils will have iron chlorosis or zinc deficiency. The availability of these metals is also strongly influenced by soil organic matter and other factors. But with little or no crop response to raising pH above the recommended target pH, why take a chance of creating a problem which is very difficult to undo?
Lowering soil pH is a natural process which we accelerate with nitrogen fertilizers. But to decrease pH rapidly can be very expensive.
Sources of lime and their effectiveness
Research has clearly shown that a pound of ECC from any lime source -- ag lime, pelletized lime, water treatment plant sludge, fluid lime, or other sources -- is equally effective in neutralizing soil acidity. All lime sources have a very limited solubility and must be incorporated and given time to react with and neutralize the acidity in the soil.
What about the calcium and magnesium contents? Most ag limes found in Kansas contain both calcium and magnesium, though the relative concentrations of the two essential plant nutrients varies widely. While the advantages and disadvantages of using a dolomitic, magnesium-containing, lime versus a calcitic lime (low-magnesium, high-calcium lime) have been cussed and discussed for years, the differences are very, very slight unless your soil is deficient in magnesium. In Kansas, both dolomitic lime and calcitic lime are suitable for use on cropland.
Therefore under most circumstances, the cost per pound of ECC applied to your field should be a primary factor in source selection. Such factors as rate of reaction, uniformity of spreading, and availability should be considered, but the final pH change, and subsequent crop growth, will depend on the amount of ECC applied.
With no-till or limited-till systems, lower rates of lime have been shown to be cost-effective in many cases. This is because lime is relatively immobile and will only react with the top 2 or 3 inches of soil. Current K-State lime recommendations suggest that “traditional” rates should be reduced by 50 to 60% when surface applied in no-till systems, or when applied to existing grass or alfalfa stands.
Liming to reduce the toxic effects of soil acidity on crop growth is important in many areas in Kansas. Determining the appropriate lime rate requires soil testing, and sampling should take into consideration soil variability since soil texture and organic matter content will impact the lime need. Lime rates are given in pounds of effective calcium carbonate per acre, which adjusts for differences in chemical purity, calcium carbonate equivalent, and fineness. Finally, changing pH takes time. So be sure to allow plenty of time for lime to react before seeding acid-sensitive crops such as alfalfa on acid soils.
For more information, see K-State publication “Soil Test Interpretations and Fertilizer Recommendations,” MF-2586: http://www.ksre.ksu.edu/bookstore/pubs/MF2586.pdf
Dave Mengel, Soil Fertility Specialist
Dorivar Ruiz-Diaz, Nutrient Management Specialist