In many agricultural areas, one widely-recognised marker of the passing seasons is a cloud of white dust in the fall. This marks the season when farmers apply lime to their fields, increasing their fertility by raising the pH to a level more friendly to plants. Newer varieties of lime are processed to reduce the amount of dust and make the lime easier to apply.
Soil pH and Plants
Soils vary widely in their degree of acidity, as plants do in their tolerance for acidity. In general, most plants prefer a neutral pH. Some, such as rhododendrons and blueberries, prefer an acidic soil, while roses like theirs to be alkaline. Lime is a common soil amendment used to raise the pH, either to make an acidic soil neutral for general plant growth, or to make a neutral soil alkaline. Neutral pH has numerous other benefits to plants, including improved nutrient availability.
Lime is nothing more than limestone that has been ground up, to make it easier to spread on soil. The fineness of a given grind is expressed in mesh sizes, with 20 mesh representing a grid of 20 squares per inch, and 100 mesh representing a grid of 100 squares per inch. The smaller the particle size, the faster the lime is absorbed into the soil. Most agricultural lime is a mixture of mesh sizes, providing some lime that will act quickly to amend the soil, and some that will act over time.
Granulated and Pelletized Lime
Agricultural lime is a dusty powder, convenient to apply with large machinery but less so for home users distributing it manually. Several manufacturers have created forms of pelletised or granulated lime for the lucrative consumer market. It is finely ground lime that has been processed into small, quick-dissolving pellets with a water-soluble binder. Pelletised lime can easily be applied by hand broadcasting, or with a small wheeled spreader. There is no physical difference between "granulated" and "pelletized" lime. Some manufacturers use one term, some use the other.
For agricultural purposes, lime is applied in large quantity from the same spreaders used to distribute chemical fertilisers and other soil amendments. Use of bulk agricultural lime is most common, because of the difference in cost, though pelletised lime is occasionally used. Homeowners more often used pelletised or granular lime, because it is neater and more convenient to apply. Autumn is the traditional time to apply lime, since it will have the entire winter to dissolve and neutralise acid in the soil.