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.
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.
Use lime to raise the pH of your acidic soil. Several types are available, and which you should use depends on the soil's magnesium content. If a soil test indicates low to medium magnesium levels, use dolomitic limestone, which contains 46 to 51 percent calcium carbonate and 38 to 40 percent magnesium carbonate. Where magnesium levels are high, use a calcitic or oyster shell limestone. These products contain 65 to 80 percent calcium, but only 3 to 15 percent magnesium.
To lower pH of alkaline soils (above 7) apply sulfur. The sulfur products found in nurseries contain 90 percent elemental sulfur; apply them in the fall to have the correct pH by spring. The effect is delayed because soil-dwelling bacteria need time to break down the added sulfur and lower the pH. The bacteria oxidize the sulfur; it then combines with water to form sulfuric acid, which acidifies the soil. Because this process relies upon active soil bacteria, sulfur is best applied in spring or summer, when the bacteria are most active.
Nitrogen, phosphorus, and potassium (N-P-K) are the three nutrients plants extract from soil in greatest quantity. These are available in synthetic, organic, and mineral forms.
Nitrogen is a major player in the growth of stems and leaves. Too much causes delayed flowering and fruiting and too little can cause stunted growth. Suspect nitrogen deficiency when new growth is pale, yellowish, or otherwise weak. If your soil is deficient in nitrogen, the only rock mineral fertilizer to use is nitrate of soda (16-0-0), which is also called sodium nitrate. This mineral nitrogen, mined from the Atacama Desert in Chile, is almost immediately available to plant roots. It is short-lived in soil, so it is best applied just before planting time. Apply 4 pounds per 1,000 square feet.
Nitrate of soda is not recommended in the low-rainfall regions of the western U.S., where soil is normally alkaline and some soils may contain excess sodium. The fertilizer increases both sodium and soil pH - 1 pound of sodium nitrate raises pH about as much as 3 pounds of lime would.
Phosphorus plays a key role in germination, photosynthesis, and growth. Purplish stems and leaves, retarded growth and maturity, and poor fruit yields are all symptoms of soils low in phosphorus. Rock phosphates are mined in many areas of the U.S., including Idaho, Florida, and North Carolina.
The phosphates are generally grouped into hard and soft types. Hard-rock phosphate (0-30-0) has about 30 percent phosphorus and 48 percent calcium, even though only 2 to 3 percent is readily available in any given year. The remainder is left undissolved or "banked" in the soil for future use. Black rock phosphate, a form of hard rock phosphate, is denser, less dusty, and easier to use. Apply 2 pounds of either type per 1,000 square feet.
Colloidal or soft-rock phosphate has only 18 percent phosphorus (0-18-0) and 19 percent calcium. Although it has less phosphorus and calcium than hard-rock phosphates, it is recommended for sandy soil because of its colloidal clay content. The gluelike quality of this type of clay holds soil particles together which improves the water-holding capacity of sandy soil. Apply 4 pounds per 1,000 square feet.
Soft- and hard-rock phosphates dissolve best at a pH of 6.2. In alkaline soils where pH is greater than 7, the phosphorus will remain unavailable to plant roots. If your soil is alkaline and low in phosphorus, correct the deficiency with non-mineral bonemeal fertilizer. Phosphorus is long lasting in soil. Once levels are adequate, you shouldn't need to add more for five years.
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.
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.
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.
Apply in late spring or summer to lower pH of alkaline soils. The primary nutrient supplied is sulfur; longevity is 1 to 2 years. Sulfur is mined in the western U.S., but most is a by-product of oil refining.
Apply at planting time or during the growing season to supply magnesium and sulfate. It's fast-acting, but benefits last 1 or 2 growing seasons. Epsom salt is mined in Europe, but most that is available to gardeners is manufactured.
Apply in fall to raise pH of acidic soils that contain insufficient magnesium. Primary nutrients supplied are calcium and magnesium; longevity is 3 to 4 years. Dolomitic limestone is mined around the world.
Apply in fall or spring to supply calcium and sulfate, or to improve soils that contain excess sodium. Benefits last 1 to 2 years. Gypsum is mined primarily in the western U.S.
Apply in fall to raise pH of acidic soils that contain adequate magnesium. The primary nutrient supplied is calcium; longevity is 3 to 4 years. Calcitic limestone is mined in many regions around the world.
Apply in spring or during growing season to supply potassium, sulfur, and magnesium. Fast-acting so benefits are short-term, 3 to 6 months. Sul-Po-Mag is mined in New Mexico.
Apply in fall to supply trace nutrients such as aluminum, boron, iron, manganese, silica, and many others. Very slow acting, but provides long-lasting benefits, 5 years or more. Rock dusts are mined in all regions of the U.S., typically as a by-product of other quarry activities.
Apply both hard- and soft-rock phosphate in fall to supply phosphorus, calcium, and trace nutrients. Benefits are long-lasting, to 5 years or more. Har phosphate is mined in many regions around the world. Mined in Florida, soft-rock phosphate also improves soil moisture retention.
Apply in fall or spring to supply potassium and many trace nutrients. Greensand also improves soil moisture retention. Greensand is very slow acting; benefits last up to 5 years. Greensand is mined in New Jersey from a geologic marine layer.
Photography by Sabin Gratz/NationalGardening.com and John Goodman