Composts, Peat, and Sewage Sludge

H. W. Reuszer.

Organic materials once were the only fertilizers used by farmers. They were mainly plant and animal products high in protein and were used for their nitrogen-supplying value. The demand for many of them in making feed and the lower cost and greater availability of plant nutrients in mineral fertilizer have led to the replacement of most of them as fertilizer.

Other organic materials, such as composts, peat, and sewage sludge, continue to be used to improve soil. They are called soil amendments rather than fertilizers because of their low content of plant nutrients.

They may be incorporated into the soil or used as mulches. Heavy rates of application are the rule. Thus they have the double effect of contributing some plant nutrients and improving the physical condition of the soil.

Sometimes the amendments represent utilization of materials that otherwise would be wasted. Some have an unusual composition, and special practices are needed to use them successfully.

IN COMPOSTING, a microbiological process, organic materials are partially decomposed by the activity of microbes. Hemicelluloses (the gumlike substances), cellulose (the plant fiber), and lignin, (the woody material) make up 50 to 85 percent of mature plant materials. The lower percentages occur in the leguminous plants, the intermediate ones in nonleguminous crops, and the higher amounts in wood. The rest of the plant is largely water-soluble substances and protein and small amounts of fat and ash.

Microbes readily attack the water-soluble substances, hemicelluloses, and cellulose, which rot quickly.

Lignin is quite resistant to attack. Its nature changes somewhat, but it disappears only slowly.

From the readily decomposable substances, microbes get energy to carry on their activities and the carbon they need for building their cells.

About 20 percent of the carbon in the decomposed part may be synthesized into microbial cells. The remainder enters the air as carbon dioxide and becomes available for photosynthesis by new generations of plants.

About one-half of the total dry matter originally present is decomposed by the time the compost is ready for use. Three-fourths of this loss is represented by a decrease in hemicelluloses and cellulose.

Microbial cells contain 5 to 10 percent of nitrogen. So, if large amounts of energy substances are present, considerable nitrogen is needed for synthesis of cells. The amount of energy available and consequently the amount of nitrogen needed depend on the amount of material susceptible to decomposition by the microbes.

Materials like sphagnum plants and highly lignified wood tissues, which resist decomposition, have low nitrogen requirements. For the usual farm crop residues, a nitrogen content of 1.5 percent is enough for a maximum rate of decomposition. Microbes do not assimilate all the nitrogen in materials that have higher nitrogen values, and the excess is subject to loss by volatilization, leaching, or denitrification. Actually, values of 1 to 1.25 percent of nitrogen are adequate.

Mature nonleguminous plant residues are low in nitrogen and high in substances that supply energy for microbial growth. When they are incorporated into soil, microbes assimilate available nitrogen from the soil and cause a shortage of nitrogen for crop growth.

This nitrogen-depleting effect can be overcome by adding enough available nitrogen to supply the needs of the microbes. The nitrogen may be supplied by commercial fertilizer added with the organic matter if it is turned under directly. If used as bedding for livestock, the feces and urine of animals supply the nitrogen. A third (but more expensive) way to overcome the nitrogen-depleting effect is to compost the material.

Two main objectives are accomplished by composting.

First, readily decomposable substances are removed, and the percentage of nitrogen content is increased. Thus there is no danger that a nitrogen shortage will be induced when composts are added to soils.

Second, the physical nature of the material is changed. The decomposition of cellulose causes the plant material to lose its strength and to break easily. It becomes friable, crumbly, and easier to handle and incorporate into the soil. That is important when hand tools or small tillage implements are used.

With some materials, such as manure or municipal garbage, a third result of composting is the removal of obnoxious odors.

THE COMPOSITION of composts is variable. The moisture content is usually in the neighborhood of 75 percent, but it may be as low as 40 percent. A high moisture content makes the finished compost weigh more than the dry weight of the material originally placed in the heap. That is the basis of statements that 1 ton of plant residues will produce 2 tons or more of compost. Its value, of course, is in the dry matter.

Composts commonly contain 2 percent of nitrogen, but the content may be 1.5 to 3.5 percent in the dry matter. The phosphorus content of dry composts is about 0.5 to 1.0 percent. Potassium values probably are twice as high. These values will be correspondingly higher if phosphate and potash are added to the compost.

The nitrogen of composts is only slowly available and never approaches that of inorganic sources of nitrogen. Its slow availability lowers the possibility of leaching and extends availability over the entire growing season. Presumably the availability of phosphorus and potassium in composts approaches that of inorganic sources.

Composts are essentially low-analysis fertilizers, and large amounts must be used to obtain adequate additions of plant nutrients to soils.

THE MAXIMUM EFFECTS of Composts On soil structure increased aggregation, pore space, and water-holding ability and on crop yield usually occur only after several years of use.

Composts increase crop yields as much as do equal additions of manure from the bedding of horses and cattle. Composts should be used in much the same way as manure with regard to amount and method of application and reinforcement. Because compost is like farmyard manure in physical nature, composition, and value, we sometimes call it synthetic manure.

Composts are good to use as mulches in gardens or around shrubbery. Applied 2 or 3 inches deep, they conserve soil moisture, lower soil temperatures in hot weather, help control weeds, and contribute nutrients.

Applications on small areas of large amounts of compost may supply the entire nutrient needs for the successful production of crops. If the composted materials come from a large area, the land from which they come loses its share of organic matter. One can overcome some of that loss by using rotations of sod crops whose roots restore the physical condition of the soil. The amount of organic matter that can be returned to the soil over any large area of land can be no larger than the amount produced on it. Because, furthermore, some is used by animals and man, only moderate rates of compost applications can be attained over any large area.

Before you decide whether to practice composting in practical farming operations, you should compare the soil-improving value of compost and that of the fresh residues from which it is made.

In 12 years of comparisons at the Rothamsted Experimental Station in England, turning under fresh straw to which nitrogen was added gave 10 to 20 percent greater yields of potatoes, barley, and sugar beets than composts prepared from the same amount of straw and nitrogen.

At the New Jersey Agricultural Experiment Station, fresh residues, applied on an equal organic-matter basis, produced double or triple the aggregation of silt and clay particles produced by composts prepared from the same materials.

One must also realize that (since one-half the organic matter is lost in composting) fresh residues applied at the same rate will cover twice the area that can be covered by composts. When it is feasible to do so, one should return plant residues directly to the soil in preference to composting; doing so leads to greater soil improvement and saving of labor. Sufficient nitrogen and other nutrients in the form of commercial fertilizers should be added to meet the needs of the crop.

In some situations, however, composting meets a need and is a highly desirable practice.

The first is in areas where commercial fertilizers are expensive, labor is cheap, and implements are simple. Composts prepared from plant, animal, and human wastes have been used extensively for many centuries in India, Japan, and China. More than one-half the nitrogen and a higher proportion of the phosphorus and potassium returned to the soil in Japan in 1946 were supplied by composts. Composting practices in some countries include the use of town garbage and night soil; a supplementary benefit thus is improved sanitation.

Composts also are used when soil is used intensively as in market gardening, in which frequent tillage and almost complete removal of crops (sometimes even the roots) may lead to soil deterioration. Composts are used to overcome this effect.

Special composts are needed for growing mushrooms. They used to be prepared from horse manure, but more and more they are made from definite mixtures of plant products and commercial fertilizers, which supply nitrogen and potash.

The most prevalent composting in the United States is by gardeners who save garden residues, weeds, tree leaves, lawn clippings, and kitchen wastes.

COMPOST is produced commercially in many places.

A few operations in the United States use manure from stockyards or large dairies. An installation at the stockyards in Chicago uses a mechanized process and can treat 50 tons of manure daily. Plans have been made to compost all of the 75,000 tons of manure produced annually at the stockyards. These preparations command a premium price and are used on gardens and lawns.

A plant at Wyster, Holland, produces 120,000 tons of compost a year from municipal refuse. The annual production of compost is sold at a low price to farmers, and the demand for it is great.

In general, it may be said that commercial production of compost is limited to situations where the cost of assembling the material is not charged to the composting operation itself.