Soil Amendments
By Bob Chapman
From the time when man graduated from following the herds to a rudimentary form of agriculture, he has sought to improve his precious soil, the lifeblood of his existence.
Unhappy with soils that were too heavy to work in the early spring, or so sandy that they would scarcely hold water, man learned to add ingredients to the soil to increase workability, hold scarce water or increase the yield. Most likely, the first amendments were animal manures, followed by crop residues. With their limited knowledge about soils and manures farmers were able to grow the crops that fed the family. Sometimes, as a result, there was extra food that could be traded for other goods needed by the farmer.
Amendments are needed
Amendments are vital to the well being of our soils. They are added because they loosen the soil, prevent compaction, add needed nitrogen, hold water and allow air to penetrate to the root zone of plants. There are many types on the market, organic and inorganic. Following is a listing of commonly used amendments and their benefits or drawbacks.
Animal manures
Modern gardeners can buy bags of steer manure. Sometimes you can even find a local source of manures. Manures, when properly aged, do a marvelous job of adding humus and nitrogen to the soil. Humus derived from decomposed organic matter, coats the soil particles, helping them hold water and keep them from sticking tightly together. Although low in nitrogen (the average is about 2%), manures will keep plants growing, but not always at an optimum rate.
Unless aged for several years, manures can create problems for the gardener. Fresh manures are high in toxic salts, which retards growth and can accumulate in the soil. Excess salts must be leached below the root zone. The winter rains usually leach these salts out, but if there is not enough rain, flooding may be needed. Fresh manures often contain undigested weed seeds. You may be bringing in alien weeds into the garden. Fresh manures added to the soil may be "hot" enough to harm the root systems of plants.
Mushroom compost
Mushrooms are grown commercially in many areas of the country. They are usually grown in a mix that includes straw and horse manure. Mushroom compost (not a true compost) has a pleasing color, is relatively inexpensive and may be available in bags or bulk. It is a long-lasting amendment, as the straw takes a long time to break down in the soil. Although the process of growing mushrooms uses some nutrients, there is still some left in the manure. Adding nitrogen to the soil when using mushroom compost will keep the plants from starving until the microorganisms can convert the manures to nutrients.
Gardeners should be cautious about the use of relatively fresh manure with its high salt content. Adding salts to the soil may not be in the best interest of the soil you are trying to amend. They often are difficult to leach from the soil. Horse manure can have a high number of weed seeds in it depending on the type of feed given the animal.
Peat Moss
This soil amendment is partially decomposed sphagnum moss. It is harvested in the Northeast and Canada and is sold in compressed bales. Peat moss is widely used by commercial growers of acid-loving plants such as azaleas. It is also a prime ingredient in many synthetic soil mixes. Peat moss, when moist, holds large quantities of water. When dry, peat moss sheds water. Peat moss is partially decomposed but it takes many years for complete decomposition by bacteria. It is commonly used to loosen heavy soils. It is more expensive than other, more locally available amendments. Few gardeners purchase enough to do an adequate job as an amendment
Homemade Compost
Compost, in its completed form, is a soft, crumbly, black or brownish substance resulting from the decomposition of organic material. Lawn clippings, yard trimmings and kitchen scraps are placed in containers and microorganisms digest and break down the materials. The process takes time, effort and space. To make compost requires a certain quantity of the right kinds of materials, moisture and air. The compost pile must be turned regularly. Compost piles undergo three stages of decomposition, each with its own set of microorganisms, before the end product is produced. The degree of decomposition at each level is determined by the amount of moisture, air and heat. Attaining the third and final stage of decomposition is the goal of every composter.
Compost has limited value as a source of nutrients, but great value as an organic soil amendment. Compost that has gone through the three stages of decomposition is very rarely offered on the market. Commercially produced compost may be offered in the future as a part of the recycling programs currently being used by many municipalities.
Inorganic or physical amendments
These amendments are mined, then altered, and are useful in special situations. Because they provide no nourishment for the bacteria, they cannot be considered as substitutes for organic amendments. The physical amendments are perlite, pumice and vermiculite.
Perlite and pumice have excellent moisture-holding capacities. They hold water like a sponge, but are rock-like in texture.
Vermiculite is soft-textured, expanded mica that can absorb nutrients as well as water.
These physical amendments improve the soil texture of heavy soils. They greatly increase the moisture-holding ability of sandy soils. The high cost of these products usually limits their use to container growing and plant propagation. Inorganic amendments include lime and gypsum. Both will loosen clay-based soils but are most commonly used to modify soil acidity or alkalinity. In areas of high rainfall, these materials are used as a nutrient supplement.