What are the benefits of applying gypsum?
Improves Structure. Gypsum, and particularly its calcium component improves soil structure. Soluble calcium (calcium in the soil water) enhances soil aggregation and porosity to improve water infiltration. And soils that have too much sodium or too much magnesium relative to calcium can deflocculate or disperse. The addition of soluble calcium helps overcome the dispersion effects of magnesium or sodium ions.
Flocculation and aggregation improves soil structure which creates a better environment for root growth and air and water movement. Any time clay particles disperse, soil structure can collapse which decreases porosity and can cause surface crusting. Gypsum has been used historically to improve particle aggregation.
Improves water infiltration. Once you improve structure, you improve porosity, infiltration and aeration. A better structure enhances soil drainage and it will be less likely to become waterlogged either from dense soil layers or a combination of high sodium, swelling clay and excess water. It will also reduce the tendency of clay particles on the surface to coalesce and cement together and seal the surface.
Better infiltration and porosity means the soil is able to capture and store more water. Gypsum creates and support good soil structure, which means more water for the crop and better aeration for the roots.
And the more water the soil can take in and hold, the less likely that water will run off carrying away valuable soil particles and fertilizers, like nitrogen and phosphorus, that end up in surface waters. Gypsum can a BMP (Best Management Practice) to reduce soluble phosphorus losses.
Counters the Impact of Acid Soils. While lime (calcium carbonate) can raise the pH of soil, gypsum does not lower soil pH directly. However it can improve acid soils by remediating aluminum toxicity. While lime can remediate the surface soil, it does not raise the pH of the subsoil and at pH of 5.5 or lower, aluminum becomes more soluble and its ion is toxic to new root growth. Gypsum will dissolve and leach down in the subsoil. In the subsoil the calcium displaces the aluminum, which will then leach deeper into the soil resulting in improved root growth.
While gypsum is very soluble, dissolves quickly in the soil and leaches down into the profile, routine rates of gypsum of a 500 lbs. a year or a ton every 4 years does not immediately improve soil structure. However over time (starting in year 1 and gradually improving over the subsequent years) you will notice a change and by 5 or 10 years after applying it, you will notice a dramatic change. Measuring tilth isn’t clear-cut and requires personal observation of change over time. So take a spade, dig down 8 inches and see if the soil is more crumbly. Over time you will see a change.
Have you tried gypsum as an amendment? If so, have you noticed if soil became mellower and more crumbly?