Does a Tomato Plant Need Rock Dust?

Rocks into Soil

• Soil is formed primarily from two substances: rocks and organic matter. Chemical and biological processes eventually break down parent rock materials and mix them with organic matter to form topsoil. Newly formed soils contain an abundant diversity of balanced plant nutrients, but this mineral fertility gradually declines, especially when soils are subjected to intensive agriculture. Rock dust is intended to restore the mineral content of soil by supplying fresh parent materials that are finely ground to accelerate nutrient release.

Find the Right Dust

• Many rock types can be made into rock dust, but materials such as granite and basalt have more experience and research behind them to support their efficacy in the garden. Ground limestone products, such as agricultural lime or dolimitic lime, do not qualify as rock dust because these materials do not supply a wide variety of minerals and because they rapidly alter soil pH. Coarsely ground rock dissolves very slowly in the soil, so look for rock dust with particles small enough to pass through a mesh screen with 200 openings per linear inch.

Supply Minerals the Easy Way

• Scientists currently identify at least 18 nutrients that are essential for plant growth and reproduction; these include such obscure elements as molybdenum, cobalt and boron. Other elements -- such as vanadium and silicon -- are recognized as beneficial but perhaps not strictly necessary. It is difficult, if not impossible, to individually supply all known essential and beneficial nutrients in the correct amounts and proportions. Rock dust is a safer and more effective approach, because the mineral content of rock dust is the same as the mineral content of the rocks from which soil originally formed. Balanced mineral fertility is especially important for tomatoes and other garden crops that are expected to grow vigorously and produce an abundance of flavorful, well-formed fruit.

Look for Mineral Deficiencies

• If you have supplied your tomato plants with plenty of major nutrients but the quality of the crop is not satisfactory, consider amending the soil with rock dust, which provides many minor nutrients that have a significant effect on productivity and fruit quality. For example, an inadequate supply of boron can lead to tomato plants that set unevenly ripened fruit, and molybdenum deficiency interferes with leaf development. Additional indications that your tomatoes need rock dust are high susceptibility to pests or disease, poor leaf coloration and fruits that lack flavor or storage life.