When is the composting process completed? In large, well-prepared heaps, a drop in the interior temperature to values near air temperature and easy crumbling of the materials in the hand indicate completion.

Full composting in small heaps usually requires 3 months under favorable conditions of moisture and temperature. Composts prepared late in the fall in regions of cold winters may not be ready for use until early the next summer. Under commercial conditions, with large heaps and frequent turning, composting time may be shortened to 2 or 3 weeks. With some mechanized processes, only 10 days are required for fresh material, and that may be shortened to 3 days if the beginning material is already partly decomposed.

Bins of simple construction are desirable for home or garden-scale composting. A bin will help to maintain moisture at the edges of the heap and prevent blowing. It should be about 4 feet wide, 5 feet high, and as long as needed to hold the material available for composting. No floor is needed.

It is well to have two bins side by side with one common wall. The compost may be forked then from one bin to the other for turning and mixing. The compost that is ready for use may be kept in one bin while fresh compost is started in the other.

Snow fencing with posts at the corners makes a satisfactory bin. A variety of timbers, arranged in log-cabin fashion, or boards nailed to corner posts provide satisfactory enclosures. Only narrow cracks should be left between the timbers or boards. More permanent structures may be built of concrete blocks or bricks. Small openings should be left near the bottom of such walls to permit penetration of air. One end of the bin should be closed with removable boards to permit access for mixing and removal of the compost.

PEAT is a widely used organic soil amendment. It is made up of plant remains that have accumulated over the centuries under relatively airless conditions in bogs.

Peats may be divided into two main types, according to the kind of plants from which they were formed. One is sphagnum peat or peat moss, or highmoor peat which is derived from species of the sphagnum plant. The other, formed from the sedges, reeds, mosses, or trees is called lowmoor peat. Peat derived from trees is sometimes grouped separately as forest peat, or peat mold. It is intermediate in composition between sphagnum peat and that derived from sedges and reeds. Forest peat contains many finely divided particles of wood and is often used for mulching.

Sphagnum peats on a dry-matter basis have an ash content usually below 5 percent, nitrogen 1 percent or less, and phosphorus and potassium below 0.1 percent. They are very acid, with pH values between 3.0 and 4.5. Fresh sphagnum peats have a high water-holding capacity equal to 15 to 30 times their own weight, but that is cut in half following drying. Cellulose and hemicellulose make up about 40 percent of sphagnum peats, but they are resistant to decomposition by microbes.

Lowmoor peats are more variable than sphagnum. Their dry matter contains 5 to 40 percent of ash, 1.5 to 3.5 percent of nitrogen, and less than 0.1 percent of phosphorus and potassium. They can hold 3 to 8 times their own weight of water. Their pH values range from 3.5 to 7.0. Because most of the cellulose and hemicellulose in them has been decomposed, they have a high amount of lignin-like substances.

Peats improve the water-holding ability of most soils and give better physical structure to fine soils. Heavy applications equal to 25 to 50 percent of the volume of the soil often are made with that in mind. They are used mostly on specialty crops and home grounds.

Undecomposed or slightly decomposed forms of sphagnum, if incorporated into the soil, require small amounts of nitrogen. Acid peats are used for acid-loving plants as a direct growth medium or by mixing into the soil or as a mulch on the place where they are grown. The acidity of such peats may need to be neutralized with ground limestone if they are to be used for ordinary plants.

Peats, especially the coarser grades of sphagnum, are good livestock bedding and poultry litter. In 1950 in the United States an estimated 161,000 tons of peat were used for soil improvement and 31,000 tons for stable bedding and poultry litter.

SEWAGE SLUDGE is the solids remaining from the treatment of sewage in disposal plants. Various methods of digestion and removal of the solids reduce the organic matter in the plant effluent to a safe point. The resulting sludge is filtered off and may be burned or sold or given away for use as fertilizer.

The value of the sludge for soil improvement depends on the method used for treating the sewage.

Activated sludge comes from disposal plants in which aerobic treatment is obtained by bubbling large quantities of air through the digesting sewage. The sludge is then allowed to settle in large settling tanks, drawn off, and filtered. The filtered material still contains 80 to 85 percent of water. If it is to be sold as fertilizer, it is dried by heat to a moisture content of 5 to 10 percent. Activated sludge contains 30 to 40 percent of ash, 5 to 6 percent of nitrogen, and 1 to 3.5 percent of phosphorus.

Digested sludges come from disposal systems in which solids are allowed to settle out and are then digested anaerobically. On a dry-matter basis, they contain 35 to 60 percent of ash, 1 to 3 percent of nitrogen, and 0.5 to 1.5 percent of phosphorus. They are allowed to air-dry on sand filter beds outside or in greenhouses where they are protected from rain. Because of their low content of plant nutrients, they are seldom sold for fertilizer.

Activated sludge has a higher nutrient content, lower moisture, better physical condition, and no odor. Available nitrogen in activated sludge is almost equal to that in cottonseed meal and costs about the same. When it is added to soil, about one-half the nitrogen is nitrified in 4 weeks. More than 50,000 tons of activated sludge are produced annually by the sewage disposal plant of Milwaukee, Wis. There is a wide demand at good prices for this product for fertilizing grass in lawns, parks, and golf courses.

All sewage sludges are low in potassium because compounds of potassium dissolve readily in water. They must then be supplemented with a potash fertilizer when used on soils that have too little potassium. Additional phosphate also is needed on some soils, depending on the amount of sludge used.

Sludges contain appreciable quantities of the minor elements, copper, boron, manganese, molybdenum, and zinc. A few experiments indicate that they are available for plant growth.

Sanitary aspects must be considered when digested sludges are applied. Pathogenic organisms may escape the treatment process. It is not advisable to use digested sludge on root crops or low-growing vegetables that are to be eaten raw. Incorporation into the soil 3 months ahead of planting leads to destruction of the disease organisms. Digested sludges give rise to bad odors, which can be overcome by immediate incorporation into the soil. Activated sludges have no bad odor and microbes are killed in the heat treatment.