Rate and Method of Application of Gypsum As a Soil Amendment.

The use of gypsum as a nutrient source and a soil amendment to improve soil quality and crop growth is rapidly expanding in the United States. However, relative to other soil amendments such as lime and NPK fertilizers, the amount of information on the proper use of gypsum is much less. There are a number of sources of gypsum, but source is generally not a variable in terms of setting application rates. Instead, the end purpose of the gypsum is the basic determinant. Where S deficiencies are anticipated, an application rate of 20-30 kg/ha of SO4‐S is generally recommended on fine‐textured soils and a rate of 35-50 kg/ha on coarse‐textured soils. This is equivalent to about 110 to 275 kg/ha gypsum from the lowest to the highest rate. To apply these rates is difficult unless the gypsum has first been pelletized. If applied at higher rates to affect soil physical properties, e.g. at 500 to 2000 kg/ha or more, the amount of S applied as gypsum is sufficient to carry over to later years. For peanuts and root crops, it is the Ca that is most required from gypsum and gypsum application rates are often in the range of 500 to 2000 kg/ha. Gypsum is also applied to improve soil physical properties such as aeration, water infiltration, water percolation and more recently to provide benefit to water quality by reducing soluble P losses from soil into receiving lakes and rivers. Application rates for these purposes often are in the 2000 to 4000 kg/ha initially and then are generally adjusted downward based on the soil’s cation exchange capacity and the percentage of Ca saturation on the exchange site. Where Na is a problem in irrigation water, gypsum can be dissolved and applied in irrigation water and application rates are determined based on the Na content in the water and the amount of water that is irrigated onto the soil. Methods of applying solid forms of gypsum depend on whether the gypsum is in a pellet form or fine granular (powder) form. Pelleted gypsum is more expensive, but can be blended and applied like other solid fertilizers. Fine granular gypsum can be applied using various types of lime spreaders with steep sides and wide delivery chain or belt drives. Soluble gypsum can also be applied to soil using a “gypsum machine” that consists of a tank with agitation paddles and a pump to meter out the gypsum. In summary, gypsum rates and application methods vary and are most often based on the desired end use effect and cost.