The Seven Types of Soil Amendments

Except in a very few cases, all soils need amending if you expect to establish and maintain a superb landscape. The better soils are found in areas of moderate rainfall of around 30 inches per year, with temperate conditions and under good management practices resulting in a moderately high soil organic matter content. California, for example, has few good soils like this. Common problems encountered in native California soils are high levels of harmful salts, poor water infiltration and drainage, low aeration, compaction and crusting problems. Almost all soil amending requires improvement in the physical properties of soil.

The productivity of soil for landscaping is affected by ‘its structure or the arrangement of soil particles in aggregates or crumbs. Soils lacking these larger composites made from sands, silts and clays are termed structureless. The function of soil conditioning materials is to cement smaller soil particles into larger “water-stable” aggregates. Otherwise the aggregates break down during irrigation or rain. Soils containing stable aggregates maintain good water infiltration, retain plant-available water, provide for a large amount of pore space needed for excellent aeration and temperature control of the soil. They also favorably influence nutrient availability and help prevent accumulation of undesirable salts.

Classes of Soil Amendments

As a matter of convenience, soil amendments and soil conditioners can be categorized into seven groups as follows:

I. Inorganic Minerals that React with Soil

These are materials such as gypsum, sulfur, sulfuric acid and iron sulfate. These minerals help to control excess sodium and help to control the soil alkalinity. Calcium from the gypsum or from acidification of lime in the soil helps to form clay into the more porous block form. Sulfur produces sulfuric acid. This oxidation process of sulfur is slow, especially in poorly aerated soils. Only a few types of bacteria can oxidize sulfur and these bacteria are not common in non-acid soils.

II. Inert Materials that do not React with Soil

These are materials such as sand, gravel, expanded minerals (perlite and vermiculite), non-decomposable wood materials (redwood and cedar sawdust) etc. used to dilute soil particles. Often a very large quantity is needed such as 75 percent to 90 percent of the volume of soil to be amended. In most instances, this is not cost effective. It does not take much clay to cement sand into a concrete-like material. This is the reason for the  large quantities needed. Concrete is made from sand, gravel and cement.

III. Decomposable Organic Materials

These come from animal and plant origins and are products such as composts, manures, sludges , hardwood sawdusts, dried bloods etc. These products affect the soil indirectly. Soil microbes use decomposable organic materials as an energy source. In the process, biological gums and mucilages are formed. These gums are well known as natural soil cements which form “water-stable” soil aggregates. The more decomposable organic materials added to soil, the more gums are formed. However, too much organic materials may cause toxicity from salts in manures or heavy metals from sludges. Because the gums and mucilages are also decomposable, the cementing action is temporary only. The warmer the soil, the shorter the period of soil improvement.

IV. Synthetic Organic Cements

An example of a synethic soil conditioner is Complete Green Soil Drain/P.A.M. This functions like the natural gums and mucilages in stabilizing soil aggregates. These materials do not have the means to make soil structure. Rather, they stabilize existing soil aggregates already formed from other procedures such as from tillage operations. Because these synthetic cements are 100% active, a few pounds of these are equivalent to several tons of organic materials. In addition, the synthetic gums are not decomposable. They will work for many years. Also, Soil Drain/P.A.M. is nontoxic.

V. Wetting Agents

These are chemicals to lower the surface tension of water in order to make water wetter.” They are surfactants, soaps, detergents, and penetrants. They do not modify the soil structure. They help to move water through the soil in the existing pores. Some disadvantages include, moisture holding capacity of soil is also decreased, their effect is not permanent, they are not cost effective, and some of these materials are toxic to plants.

VI. Soil Inoculates and Microbial Cultures

These are microbes such as legume bacteria (Rhizobia) that have a symbiotic relationship with legumes for making atmospheric nitrogen available to plants. Mycorrhizae are beneficial fungi which help plants to assimilate nutrients and ward off pathogens. Enzymes and vitamins applied to soil have not been proven valuable for enhancing plant growth.

VII. Mechanical Cultivation

Tillage and soil ripping are valuable at the right soil moisture in improving the physical properties of soil, particularly soil structure and pore space. Unless stabilized, however, soil crumbs formed by tillage do not endure the presence of water. Water dissolves the weak binding material causing the aggregates to disintegrate unless stabilized by organic gums (several months duration) or by synthetic cements such as Soil Drain/P.A.M. (persists for many years). Naturally, earthworms cultivate the soil.

The best soil preparation procedures use a combination of several of these techniques. Amendments can not be incorporated unless the soils are loosened and tilled. Often, rototillage is ineffective unless the soils are ripped. Even disc plows will not cultivate compacted soil more than a few inches deep unless the soil is first ripped. For smaller sized areas, back hoes can be used. Mechanical tillage is essential.

Some organic amendments should not be used in excess, especially uncomposted or partially composted organic materials. However, uncomposted or partially composted organic materials will form more gums and mucilages than highly composted materials. If too much decomposition takes place in the soil and if the volatile decomposition products do not readily escape from the soil, toxicity and plant injury can occur.

The bacterial content of partially decomposed organic matter can limit plant pathogenic fungi due to competition from other microbes. Typically, saline and/or alkaline soils need some inorganic minerals, especially if the soil is low in soluble calcium.

For long-term stabilization of soil aggregates, synthetic mucilages such as Soil Drain/P.A.M. are highly valuable and essential unless large amounts of decomposable organic materials are applied to the soil several times a year preferably with a source of calcium.

In addition to soil conditioning, proper mineral nutrition in correct proportions is critical. Soil testing and plant tissue testing in concert are invaluable in guiding a good fertilizer program.