Source: Ohio State University
By Ed Lentz
Sept. 25, 2013 — Fall is an excellent time to test soil pH and determine whether any lime needs to be applied for future crops. Proper soil pH is important for nutrient availability, herbicide activity, and crop development. For most soils, additional lime is not needed every year. Consider these points before liming your fields:
Do I need lime? Each year we hear stories of people adding lime to their fields without a soil test. The grower has a source of free waste-product lime that they pick up and apply to their fields. In many cases their soil pH was fine, but they did not want to pass up a "good deal". Without knowing the soil pH, a grower may inadvertently raise their soil pH to the high 7's. At this elevated pH, certain nutrients may become limited and the productivity of their crop may be reduced and require special management practices. Northwest Ohio has the greatest risk of elevating soil pH from careless applications of lime. A soil analysis is the best step to determine if a field needs lime.
What is the pH of my subsoil? Generally a laboratory recommends lime when the soil pH drops two to three units below the desired value. The desired value depends upon the crop and the pH of the subsoil. In parts of Ohio where the subsoil pH is less than 6.0 for mineral soils (eastern Ohio), additional lime is recommended after the soil pH drops to 6.2 for corn and soybean, and 6.5 for alfalfa. In other parts of the state (generally western Ohio), the subsoil pH for mineral soils is greater than 6.0 and lime is not needed until the soil pH drops below 6.0 for corn and soybeans, and 6.2 for alfalfa. Private laboratories may not take in account the subsoil pH and use recommendations based on a subsoil pH less than 6.0 for all parts of the state, possibly recommending lime applications several years earlier than needed for some areas.
What is the Effective Neutralizing Power of my lime source? An important item from a lime analysis report is the Effective Neutralizing Power (ENP) value, which is required for material sold as lime for agricultural purposes in Ohio. This value allows a producer to compare the quality among lime sources because ENP considers the purity, neutralizing power (including fineness) and moisture content. In other words, the ENP tells you how much of that ton of lime actually neutralizes soil acidity. The unit for ENP is pounds/ton (be careful not to use %ENP, which may also be on a lime analysis report). The ENP allows a producer to compare different lime sources because they can now determine price per pound or ton of actual neutralizing material.
Should I use “hi cal” or dolomitic lime? In most situations it does not matter, so a producer can select the least expensive of the two lime sources. Transportation is often the largest cost of a lime material, so generally the closest lime source (quarry) is often the most economical.
Several parts of the state are historically low in soil magnesium (eastern and extreme southern Ohio). Adequate soil magnesium is important to reduce the risk of such problems as grass tetany for grazing animals. Soil test magnesium levels need to be greater than 50 ppm (100 lb) for optimal corn, soybean, wheat, and alfalfa production. Often areas low in magnesium also need lime, which has made the application of dolomitic lime an economic solution for both concerns.
The ratio between calcium and magnesium is important. Soils should contain more calcium than magnesium. Extensive research has shown that crops yield the same over a wide range of calcium to magnesium ratios and will not affect crop production as long as the calcium to magnesium ratio is larger than 1. High calcium lime should be used in situations where the soil test calcium to magnesium ratio is less than 1, or in other words, the soil magnesium levels are greater than the soil calcium levels. I have not observed any Ohio soil tests where the magnesium levels are above the calcium levels. Also keep in mind that almost all dolomitic lime sources will contain more calcium than magnesium. Unfortunately, some producers have been led to believe that magnesium levels in dolomitic lime may be undesirable. The level of magnesium is unimportant as long as the calcium level is above magnesium. The focus should be selecting lime on its Effective Neutralizing Power (ENP) rather than its calcium level.
In summary, make sure you take a soil test to determine if lime is needed, determine if magnesium is needed, know the historic pH of your subsoil, and then use the ENP to select the most cost effective lime material. A soil test every three to four years will determine the lime requirements for your fields. Additional information on ENP and lime sources may be found at the following location: Ohio State Factsheet on effective neutralizing power and lime sources.