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Gardening Articles: Care :: Seeds & Propagation

Minerals for Soil (page 2 of 4)

by Charlie Nardozzi


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.

Mineral N-P-K

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.

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