Author Topic: Introduction to Soil Supplements  (Read 1944 times)


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Introduction to Soil Supplements
« on: February 06, 2011, 03:48:23 PM »
Soil Biology:  Healthy fertile soil contains a wide variety of organisms: Bacteria, Fungi, Nematodes, amobae, insects, arthropods, and worms.  A cupfull of fertile soil contains many more individual organisms than the earths entire human population.

Microbes are the foundation of the soil's food pyramid and they feed on organic matter. To encourage high populations of microbes, we must supply organic matter. As noted above, the various microbes thrive on different components of organic matter. If we want to feed all of the different microbes, we must supply organic matter that is not fully decomposed, or we must apply biologically active compost tea. Well-aged compost has had most of the sugars and starches consumed during the composting process, so it will not stimulate a high level of bacterial activity. Nevertheless, well-aged compost is an excellent soil amendment because it improves soil structure, increases water and nutrient-holding capacity and provides nutrients. Lawn clippings are a complete microbial food that will stimulate bacterial as well as other microbial activity. So leave the clippings on the lawn

Microbes consume a lot of carbon (C), but also other nutrient elements such as nitrogen (N) contained in organic matter. They generally consume about 25 to 30 Cís for each N. If the soil contains C and N in this ratio (25:1 to 30:1), everything is fine. If the ratio is lower (e.g., 18:1), there will be more N than the microbes need and the excess will be available for plant uptake. If the ratio is higher (e.g., 50:1), there will not be enough N for the microbes and they will take what they need from the soil, creating an N deficiency for the plants that will require the addition of more N.

Generally, green materials have a surplus of N (they are low in C) and brown materials are low in N (high in C). Most finished compost has a C:N of 15:1 to 20:1 and will supply N.


Lime and Gypsum: pH, calcium and magnesium

Lime is used to raise the soil pH (reduce acidity). It also supplies calcium (Ca). Gypsum (calcium sulfate dihydrate) is used to raise Ca levels.

There are two types of readily available limestone for raising soil pH: calcitic (high-calcium) limestone and dolomitic limestone. Dolomitic limestone contains approximately 28 percent magnesium carbonate and approximately 35 percent calcium carbonate. Calcitic limestone contains no magnesium (Mg). Both of these products will elevate soil pH; however, they will affect the percent base saturation in the soil very differently. If dolomitic limestone is used continually, magnesium can accumulate to the point where it suppresses calcium levels in the soil.

Classic symptoms of calcium deficiency include necrosis around the base of the leaves and soft dead necrotic tissue at rapidly growing areas. Plants under chronic calcium deficiency have a much greater tendency to wilt than non-stressed plants.

If calcium or potassium is oversupplied and magnesium is deficient, turf can become chlorotic and appear yellowish green because it is unable to produce chlorophyll.

When using lime, it is important to know the calcium/magnesium ratio in the soil in order to choose the correct liming agent. This ratio can be calculated by dividing the exchangeable value (the amount that is free to move in the soil; the value returned by a soil test) of calcium by the exchangeable value of magnesium. For example, if the exchangeable value of calcium is 1,500 ppm and the exchangeable value of magnesium is 200 ppm, the calcium/magnesium ratio is 1,500/200 = 7.5. If the calcium/magnesium ratio is lower than 6, then calcitic lime (calcium carbonate) should be used. If the ratio is between 7 and 10 and magnesium is ranked lower than calcium on the soil test, dolomitic lime (calcium/magnesium carbonate) is preferred.

Many soil tests report the percent base saturation, which makes calculating the ratio of calcium to magnesium unnecessary. If percent base saturation is provided, the magnesium should be between 5 and 15 percent, and the calcium should be between 60 and 85 percent. If calcium is below this range, use calcitic lime, and if magnesium is needed, use dolomite. If they are both in the proper range, use either type and alternate with the other in succession. Ca and Mg do not need to be precisely within these ranges, but you should try to move toward the proper levels when using lime. If calcitic lime is not available, gypsum can be used to increase Ca levels. Gypsum does not affect soil pH and is more expensive than ground limestone.

Many people use pelletized lime on lawns. Pelletized lime consists of very finely ground limestone that is formed into pellets using a binder. Upon contact with moisture in the soil, the pellets disintegrate and the fine lime particles dissolve readily and provide for a rapid rise in pH. The pH increase is shorter lived than that produced by regular ground lime, which contains some larger particles that last longer. Pelletized lime is more expensive than ground limestone but is easier to apply and does not create a lot of dust.

Do not apply more than 50 pounds of ground limestone per 1000 square feet at one time, even if more than 50 pounds per 1000 square feet is called for. Apply pelletized lime at about half the rate of ground limestone, but apply it more often. Do not apply during hot or dry times of the season. Measure the pH a month or two after the first application to get an idea of its effect. Additional applications can be made in spring and fall until the desired pH is achieved. Keep in mind that lime does not move rapidly down into the soil, so the pH near the surface will be higher than it is a few inches below. Over time, the lime will move lower. If large applications are made, the difference between surface and sub-surface pH will be magnified. Therefore, small but frequent applications are suggested.


Nitrogen (N) is used in large amounts by plants and is especially important in keeping a lawn green. There are several forms of N in the soil. As organic materials decompose, N is released in the form of ammonium (NH3). Unless the soil is quite acid, bacteria convert ammonium to nitrite and then to nitrate (NO3). Nitrate-N is the form primarily used by most plants, including grass. However, it is also very soluble and subject to leaching.

Soil tests taken in late fall, winter or early spring usually show a low N level. As the soil warms in late spring and summer, microbes break down organic matter and release N. For this reason, a standard soil test is not a reliable indicator of N availability in the soil, and many soil labs do not measure N in a standard test. Under most conditions if a soil contains around 5 percent organic matter, there should be enough N. Many soil labs will perform a special nitrate-N test in the summer. This is a good way to determine if the organic matter is releasing enough N.

Compost is an excellent source of slow release N. Blood meal (dried animal blood) and some plant meals are also good sources.  White clover can be seeded into the lawn or included when establishing a new one. Clover is a legume, a plant that uses bacteria in its root nodules to take N out of the atmosphere and make it available to plants.  Be sure to inoculate the seed with the proper bacteria.  Some people like clover in a lawn; others do not.

« Last Edit: April 13, 2011, 04:03:49 PM by admin »