NGA Articles: Minerals for Soil

Minerals for Soil

By: Charlie Nardozzi

Like most gardeners, I spend a lot of time adding organic materials, such as compost, to my soil. I also spend time culling small rocks and stones, rarely thinking of them as the essence of soil. Yet soil is about 45 percent minerals weathered from bedrock, 50 percent air and water, and only 5 percent organic matter. So although organic material plays a vital role in your garden's general health and productivity, depleted mineral content might account for lackluster plant performance.

If your plants have pale, yellow-green leaves or are stunted, or have small or few flowers and fruits, it may be time for a soil test. That's the only way to rule out a mineral deficiency as a cause of poor growth. This can be done at a cooperative extension office or a private laboratory. Look in the yellow pages under "Soil Testing." The cost of an analysis varies but generally shouldn't run more than $30.

Once you know which minerals are lacking, you have two choices: rock-based or synthetic fertilizers. Although both types have their pros and cons, rock-based mineral fertilizers and amendments have these specific advantages: They release the nutrients slowly, so one application can benefit the soil for years. Many also include essential trace minerals that soils and plants need in only minute quantities.

Below, I describe the most common soil mineral deficiencies and how to remedy them using the different rock mineral fertilizers.

Minerals to Adjust Soil pH

The measure of a soil's acidity or alkalinity is called pH, and the symptoms of a pH too low or too high for your plants are so many it makes sense to lump them under the heading "poor growth." More specifically, a too low or too high pH will reduce flower and fruit production, cause stunted growth, and promote various plant diseases. Most gardeners keep a close eye on the pH of their soil for these reasons.

The best pH range for most garden plants is 6 to 7. Neutral soil is 7. Soils to the east of the Mississippi River andin the Pacific Northwest tend to be acidic (below 7), while soils in the Plains states, Rocky Mountain region, and Southwest are usually alkaline (above 7). Adjusting your soil's pH is easy. If it is below 6, add limestone to raise it. If it's above 7, add sulfur to lower it. The necessity of adjusting pH depends upon the kinds of plants you want to grow. The quantity of sulfur or lime to apply depends on your soil's present pH, the desired pH, and the soil type.

Once your soil is at the desired pH, you may not need to intervene for another three to four years, though in some situations more frequent attention is required. One example is maintaining acidity in strongly alkaline western soils. Yearly addition of organic matter will also help keep the pH stable.

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