Lime and gypsum are valuable soil amendments in certain areas of the United States. Many gardeners in the eastern U.S. apply lime to their soils to improve plant performance. Gypsum can help overcome problems associated with sodic soils in arid regions of the west. In Iowa, however, the addition of lime to gardens and lawns is often unnecessary. Gypsum provides little or no benefit to most Iowa soils.
Lime is applied to acid soils to raise the soil pH. Soil pH is an important chemical property of the soil. The pH scale runs from 0 to 14 and is used to indicate the relative acidity or alkalinity of the soil. A pH less than 7 is acid, while that above 7 is alkaline. A pH of 7 indicates a neutral soil. The pH is important because it influences the availability of essential nutrients. Most horticultural crops will grow satisfactorily in soils having a pH between 6 (slightly acid) and 7.5 (slightly alkaline). Most soil nutrients are readily available in this pH range. Since the pH of most Iowa soils ranges from 6 to 7.5, soil pH causes few problems for area gardeners.
Gardeners should apply liming materials to gardens and lawns only when recommended by a soil test. A soil test will indicate the current soil pH and, if necessary, the amount of lime to apply to the area. Liming materials include ground limestone which is mainly calcium carbonate (CaCO3) and dolomitic limestone which contains CaCO3 and magnesium carbonate (MgCO3). Applying lime to neutral or alkaline soils can actually create problems. The addition of lime can raise the soil pH to excessively high levels, reducing the availability of plant nutrients and leading to poor plant growth.
Advertisements for gypsum often claim the addition of gypsum will help loosen heavy, clay soils and improve soil drainage. The addition of gypsum to Iowa soils, however, is of little benefit. Gypsum is chiefly used to amend sodic soils. Sodic soils are found mainly in arid regions of the western United States. They are characterized by poor soil structure and drainage. As a result, sodic soils support very little plant growth. The problems associated with sodic soils are caused by high levels of sodium in the soil. Sodic soils can be improved by applying gypsum, then leaching the sodium out of the soil with large quantities of water.
Poorly drained soils in Iowa typically contain large quantities of clay and small amounts of organic matter. The best way to improve these soils is to incorporate large amounts of organic matter. Manure, compost, and peat are excellent sources of organic matter. In vegetable gardens and flower beds, work the organic matter into the top 8 to 12 inches of soil. Before establishing a new lawn in poor soils, bring in fertile, well-drained topsoil or organic matter and incorporate the additions into the top 6 to 8 inches of soil. In established turf areas, it is extremely difficult to correct problems caused by heavy, clay soils. Compacted soils can be improved by aerating the lawn in the spring and fall.
Additional information on the use of lime and gypsum can be found in Pm-1487 Modifying Soils in Iowa Lawns and Gardens .
This article originally appeared in the July 21, 2000 issue, p. 93.