In late fall and early winter, you might see farmers applying a fine white dust to their fields. So, what is it? And why do it?
That white dust is agricultural lime, sometimes called aglime. It is a soil conditioner made from crushed limestone. Once the lime dissolves, it releases a base that lowers the acidity of the soil. Farmers apply lime to increase yields. Homeowners and landscapers use it to improve the appearance of lawns that have acidic soils.
Making sure soil does not become too acidic is critical to good plant health. Soils that are too acidic can stunt root growth, limit nutrient availability, and reduce the effectiveness of fertilizer and herbicides. Most soils have a tendency to become more acidic over time for variety of reasons such as erosion, leaching, decomposition of organic matter, and fertilizer application.
Limestone is a sedimentary rock composed mostly of calcium carbonate. It is mined and mechanically crushed into varying degrees of fineness depending on its intended use. Limestone’s versatility, durability, and affordability make it a useful for many construction, industrial, home-improvement, and agricultural applications. Coarsely crushed limestone can be used to rock driveways, support railroad tracks, and prevent erosion on slopes and shores. Lime used as a soil amendment is ground into a very fine power so it easily dissolves in the soil. Soil amendments are organic or inorganic materials added to change the physical or chemical properties of soil and improve plant health.
Lime is a good soil amendment for acidic soils because it contains a high amount of calcium, which works to neutralize the soil’s pH level. Soil pH indicates the acidity or alkalinity of the soil. It is measured on a 14 point scale. A pH of 7 is neutral. Values below 7.0 indicate acidic soil, and values above 7 indicate alkaline, or basic, soil. A soil test is used to determine the pH of a soil. Farmers who practice precision agriculture often use grid sampling to determine where and how much lime to apply in specific parts of a field.
Lime can be applied any time after the previous crop is harvested. Lime is not lost by leaching, so farmers can apply it whenever practical. Agronomists recommend putting down lime several months before planting, so the lime has enough time to neutralize acidity.
It is common for farmers to hire a contractor to apply lime using a large truck-mounted spreader. It can also be applied with a smaller spreader pulled with a tractor. Lime can be incorporated into the soil or spread on top and left to dissolve and leach into the soil by rain and snowfall.
Iowa farmers do not lime fields every year. They only apply it when soil tests indicate the soil pH is too low. For corn and soybeans, Iowa State University Extension and Outreach recommends a soil pH of 6 or 6.5 to be sufficient, depending on the subsoil pH of the area. A higher pH is recommended for alfalfa and other acid-sensitive crops.
Now you may be asking, should I apply lime to my lawn or garden? You should only apply lime when recommended by a soil test. The optimal pH range for most turf grasses, flowers and vegetables grown in Iowa is 6.0 to 7.0, and most lawn and garden soils fall within that range. However, some plants like blueberries and azaleas prefer more acidic soils and others like lilac, peony, and salvia prefer more alkaline soils. If you are curious to know the soil PH of your soil, consider sending a soil sample to a soil testing lab on the Iowa Department of Agriculture and Land Stewardship list of certified labs.