Photo credit: Patrick Drohan
Soil pH is the measure of how acidic or alkaline (basic) a soil is and the optimum pH for every forage species is different. This is an often overlooked aspect of forage production and it can be one of the primary determinant factors on stand life and overall forage productivity of a stand – whether that be pasture or hay. If the pH is not optimum according to the species of forage present, then nutrients applied to the crop (fertilizers and manure) could not be fully available and will be lost through the soil – causing nutrient runoff issues as well as an economic loss of losing valuable nutrients.
Several factors can inherently affect soil pH, including soil parent material, microorganisms and organic matter decay, as well as temperature and rainfall. High clay and silt soils are better able to resist a sharp drop or rise in soil pH because the larger cation exchange capacity of these soils buffers the soil pH. High yielding crops absorb basic soil-neutralizing elements (calcium, magnesium, and potassium), and as crop yields increase, more of these nutrients are removed from the soil, causing a reduction in soil pH. In general, grain contains less of these minerals than leaf components, so typically high-yielding forage crops affect soil acidity more than harvesting crops for grain. Rotating crops can reduce the acidification of soil.
Soil sampling every three years, or before establishing a new forage stand, can help to ensure that the optimum soil pH required for that forage species is present. Liming is the quickest and most effective form of raising the soil pH towards neutral. Whenever possible, incorporate lime after application, as it takes several months for the lime to react and act as a neutralizer. Application of 3-6 months before planting is ideal.
When determining the best lime to use, consider coarseness of the lime as well as the Calcium Carbonate Equivalent (CCE). Research has shown that the finer the texture of lime, the faster the reaction occurs to neutralize the soil. In Pennsylvania, liming materials graded as “fine-sized” provide a reaction speed that is usually acceptable for agronomic purposes. The CCE is the chemical neutralizing capacity relative to pure calcium carbonate. Limestone recommendations on a soil test report are usually given as a rate of limestone with 100% CCE, but the actual CCE of different liming materials varies with the type of material. Therefore, the application rate of your particular liming material needs to be adjusted for the CCE. For instance, a material with 25% CCE will require 4 times the recommendation on a soil test report, since it only has one quarter of the neutralizing capacity as pure calcium carbonate.
Legumes typically require a higher soil pH than grass species. For optimum production, alfalfa requires 6.6-7.0 pH, while clovers and birdsfoot trefoil can withstand slightly more acidic conditions ranging from 6.5-6.5. Cool-season perennial grasses require a minimum of 5.8 for their optimum production, with warm-season and cool-season annuals typically requiring a minimum of 6.0 for their optimum production potential.
Research has shown that by increasing the soil pH of an alfalfa stand from 5.8 to 7.0, nearly 4 tons more of dry matter can be expected throughout a single growing year.