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Gardening Articles: Care :: Seeds & Propagation

Minerals for Soil (page 3 of 4)

by Charlie Nardozzi


Symptoms of potassium deficiency include low yields and mottled, spotted, or curled leaves (they have a scorched look). It is common in the East but less likely in the West. Two important mineral sources provide potassium.

Correct soil potassium deficiency with sulfate of potash magnesia, also known as Sul-Po-Mag or K-mag (0-0-22). Sul-Po-Mag is mined from a crystalline rock material, langbeinite, and contains 22 percent sulfur, 22 percent potassium, and 11 percent magnesium. Unlike most other rock-based fertilizers, the potassium in Sul-Po-Mag is readily available to plants so it should be applied in spring just before planting. It has only a short-term effect. Apply 6 pounds per 1,000 square feet.

For a slow-release form of potassium, greensand is the best choice. Greensand is mined in New Jersey from ancient sea deposits of iron potassium silicate (glauconite). It has 5 percent to 7 percent potassium and 32 trace minerals, and will increase the water-holding capacity of the soil. Greensand is slow to work, so it's best used to build up reserves of potassium when a soil test indicates an average level of potassium and not when levels are low and a quick dose is needed. Apply 17 pounds per 1,000 square feet. One application can last up to five years.

Other Main Nutrients

Plants need smaller quantities of calcium, magnesium, and sulfur than nitrogen, phosphorus, and potassium. Nevertheless, most soil tests will indicate quantities of nutrients. Correcting low pH with limestone also increases calcium, and if you use dolomitic limestone, it will also increase magnesium levels. If the pH is correct, use gypsum to add calcium without altering pH. Mined from calcium sulfate, gypsum contains 23 percent calcium and 18 percent sulfur. It also can improve the structure of alkaline soil by removing excess sodium. Apply 10 pounds per 1,000 square feet. Its effects last for 1 to 2 years.

In soils that need magnesium and sulfur but not calcium, apply Sul-Po-Mag as described for potassium deficiencies, or use epsom salts. Mined from magnesium sulfate, epsom salts contain 10 percent magnesium and 13 percent sulfur. Apply 6 pounds per 1,000 square feet. Beneficial effects last for one to two years after application.

How to Apply Rock Minerals

Remember three rules: When you apply rock mineral fertilizers, 1) Mix them thoroughly into the soil; 2) smaller-sized particle will work quicker; and 3) use them with lots of organic matter.

It's important to work mineral fertilizers into the soil so the nutrients they contain can easily contact roots. Some minerals, such as phosphorous, do not move around well in the soil, while others, such as sulfur, can form a hardpan layer if not thoroughly mixed with the soil.

Smaller-sized mineral particles will be more readily available to plants. This is especially true for rock minerals, such as greensand, that release nutrients very slowly. Rock minerals have been crushed fine enough to pass through a 200-mesh screen (and it should say so on the bag). This rule doesn't apply to pelletized fertilizers, such as pelletized lime. Although these particles are larger than the crushed material, they are held together by a water soluble compound that quickly breaks down in the soil. Pelletized fertilizers are less dusty and easier to use, but more expensive.

If you add rock mineral fertilizers to your soil, it is important to add organic matter as well. Organic matter helps stabilize the soil pH, which, in turn, increases microorganism activity. Microorganisms are key to breaking down the rock mineral fertilizers to usable form for the plant.

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