TESTING AND JUDGING THE CONDITION OF COMPOST
There are many tests and checks by which various aspects of the composting process and the condition of compost may be judged. From the point of view of the overall operation and the final product there are three groups of tests:
a. test of the sanitary quality of the operation and of the finish product, i.e., pathogen and parasite destruction and absence of flies and odors;
b. test of fertilizer or agricultural or horticultural value, i.e., the amount of nitrogen, phosphorus, potash, and other nutrients, nutrient conservation, the C:N ratio, and compost value ; and
c. economic test, i.e., whether the total cost of producing the compost is less than its value as fertilizer plus the cost of disposal by other means, such as incineration or land fill.
The gardener, small farmer and other small compost operator usually will not be concerned with detailed tests other than those to confirm that the material is safe from a health standpoint. This will be judged from its temperature, and its satisfactory appearance as a soil additive.
Health organizations and laboratories can make tests for organisms of public health significance when necessary. Chemical tests for nitrogen in its different forms, phosphorus, potash and the organic character of the material can be made by standard techniques and are useful in analyzing the finished product and to determine the effect of different composting procedures. For routine day-to-day operations, temperature, appearance of material, odors, and the presence of flies are important tests. Cleanliness and the absence of flies at the site, as well as the absence of large numbers of larvae in the piles, are criteria of sanitary quality of the compost operation. Temperature is the best single indicator of the progress of aerobic composting and also the basis for determining whether pathogen, parasites, and weed seeds are being destroyed.
The temperature of compost can be checked by:
a. digging in the pile and feeling the temperature of the material;
b. feeling the temperature of a rod after insertion into the material; or
c. using a thermometer.
Digging into the pile will give an approximate idea of the temperature. The material should feel very hot to the hand and be too high to permit holding the hand in the pile for very long. Steam should emerge from the pile when opened. A metal or wooden rod inserted 2 feet into the pile for a period of 5-10 minutes for metal and 10-15 minutes for wood should be quite hot to the touch, in fact, too hot to hold.
These temperature-testing techniques are satisfactory for the smaller compost operations. Long stem metal thermometers are available for temperature testing.
When aerobic composting progresses in a typical manner, there will be a rapid rise in temperature to 140o-170o F. in the first three days. In small pits or piles, a pause in the temperature rise often occurs somewhere between about 85o and 135o F., during the transition from mesophilic (lower) to thermophilic (higher) decomposition. After the initial temperature rise, a high temperature is maintained for several days during the active decomposition period, provided that aerobic condition are maintained. Then a slow decline of temperature starts as the rate of heat generation falls below the rate of heat radiation of the material. During this period the rate of bacterial activity is dropping faster than the temperature indicates, owing to the insulating qualities of the composted material.
The failure of a compost pile to attain a high temperature in a period of 3-6 days indicates that the pile may be too small to retain the heat, that it may be too wet or dry, or that it has insufficient organic material and nutrients for rapid decomposition.
The temperature alone, however, cannot determine conditions within the composting mass. A temperature drop may result from the development of environmental conditions unfavorable to aerobic thermophiles, either through excessive heat, onset of anaerobic conditions, or lack of sufficient moisture. In rare instances, not usually encountered in composting municipal wastes, when some acid material has been added a low pH might also cause a lowering of the temperature.
Compost may be considered finished when it can be stored in large piles indefinitely without becoming anaerobic or generating appreciable heat. It can be safely spread because of its low C:N ratio or the poor availability of its carbon. The material, however, is still slowly active and will "ripen" somewhat in the large stacks. At this time it should be grayish-black or brownish-black in color, depending on what color of materials were used. However, color alone is not a good criterion of finished compost because the appearance of rich soil humus develops in a good compost long before the temperature decline signals the decrease in microbial activity.
Characteristic changes in odor during the period of composting help define stable compost. The material should be odorless, or have a slightly earthy odor or the musty odor of molds and fungi.
These approximate physical tests are adequate for most small compost operations.
Laboratory analyses for nitrogen, phosphorus, and potash are more precise and require more elaborate equipment, but are relatively simple chemical determinations to make. The determination of the C:N ratio, which is so important in regard to nitrogen conservation and for estimating the quality of the finished compost, is more of a problem, because the quantitative analyses of carbon is difficult, time consuming, and expensive.
If compost is modified by adding ammonium sulfate, phosphates, or other nutrients for special fertilizer purposes, percentages of these nutrients on a dry basis must be determined, so that users can compare them with other fertilizers.