Spreading rock on your garden can help feed soil

If you feel like getting out in the garden, now is as good a time as any to spread rock on the ground.

Or not (more on that later).

You say your ground already has enough rocks in it? True enough, but the rock I'm talking about is a powder, and is likely a different kind of rock from what you already have.

But why put down more rock of any kind? The reason is that rock powders sold for garden use are particularly high in minerals.

For example, rock phosphate is, as the name implies, rich in phosphorous, one of the “big three” nutrients needed by plants. In fact, rock phosphate is the stuff, after being treated with sulfuric acid, that becomes the phosphorous in synthetic fertilizers.

Colloidal phosphate, also known as soft phosphate, is a similar product, this one ground up finer than rock phosphate.

Two other commonly used rock powders — granite and glauconite — are rich sources of potassium, another of the “big three” nutrients needed by plants. (The third, nitrogen, is not found in rocks.)

Glauconite is also called greensand, or Jersey greensand if that's where it was mined. And it is greenish.

Besides the major nutrients phosphorous and potassium, these rock powders are also sources of micronutrients. Micronutrients are needed in only minuscule amounts by plants, but nonetheless are essential to their health. A soil can be naturally deficient in micronutrients: For example, pockets of molybdenum deficiency exist in Nevada soils; natural cobalt deficiencies exist over much of Iowa and parts of the Northeast.

Synthetic (“chemical”) fertilizers generally supply no micronutrients at all.

Application for the long haulBecause they are merely ground-up rocks, rock powders do not readily dissolve in water to give up their goodness to plant roots. Release of their nutritional goodness takes time, as well as the work of bacteria, fungi and roots. Freezing and thawing opens up cracks in the soil so rock powders applied now at least get into the soil, even if they don't yet dissolve.There's no rush, though, to run outside and start spreading. What rock powders lack in quick action they make up for in long-term effect; they release their goodness over a decade or so.A typical application would be about 10 pounds per 100 square feet.Are ground rocks really needed?There's also no rush because you might have no reason to apply them in the first place. Rock powders are relatively expensive for the amount of phosphorous or potassium they offer. And unless some local garden store has rock powders for sale, you could pay as much or more for shipping as for the material itself.More to the point is whether rock powders are superfluous. If you constantly feed your soil an abundance and variety of compost, leaves and other organic materials #8212; as any good gardener does #8212; your soil already is rich in phosphorous, potassium, and micronutrients.This is especially true if you use plenty of compost made from all sorts of materials, including kitchen scraps. Orange rinds from Florida, old bread from Kansas-grown wheat, and banana skins from Costa Rica each contribute to the smorgasbord of micro- and macronutrients contained in homemade compost.So, do I ever use rock powders? Yes, about every decade or two, mostly as insurance and to supply micronutrients around trees and shrubs that don't get annual dressings of compost. But I'm not saying that using these rock powders is really necessary.