13.5: Advantages of Composting

Composted material is less bulky than the original feedstock, making it less costly to transport. It is also easier and more pleasant to handle. During the composting process, carbon dioxide and water are lost to the atmosphere and the size of the pile decreases by 30–60%. In addition, many weed seeds and disease-causing organisms may be killed by the high temperatures in the pile. Unpleasant odors are also eliminated. Flies, a common problem around manures and other organic wastes, are much less of a problem with composts. Composting reduces or eliminates the decline in nitrogen availability that commonly occurs when organic materials, such as sawdust or straw, are added directly to soil. Compost application can also lower the incidence of plant root and leaf diseases, as mentioned. Moreover, the chelates and the direct hormone-like chemicals present in compost often further stimulate the growth of healthy plants. Then there are the positive effects on soil physical properties that are derived from improving soil organic matter (figures 13.5 and 13.6).

The composting process also helps us address the concerns around nutrient flows we discussed in Chapter 7. When crops are sold off the farm, and sometimes transported over long distances, we remove carbon and nutrients from the fields that in many cases don’t get recycled for economic reasons. Composting allows us to use carbon and nutrients from waste materials and apply them to the soil in a safe and cost-effective manner, thereby reducing the nutrient loss and excess issues that are now inherent in our agricultural system. Sure, we aren’t able to recycle carbon and nutrients in corn or soybeans from an Iowa farm that ended up as manure from California beef or Chinese pigs—the logistics would be inhibitive. But composting that manure makes it easier and more economical to move off a farm with excess nutrients and to help improve nearby fields, gardens and landscapes with local organic resources that would otherwise mostly be a nuisance.

If you have a large amount of organic waste but not much land, composting may be very helpful and may create a valuable commercial product that improves farm profitability. Also, since making compost decreases the solubility of nutrients, composting may help lessen pollution in streams, lakes and groundwater. On many poultry farms and on beef feedlots, where high animal populations on limited land may make manure application a potential environmental problem, composting may be the best method for handling the wastes and removing the excess nutrients. Composted material, with about half the bulk and weight of manure, and a higher commercial value, can be economically transported over significant distances to locations where nutrients are needed. In addition, the high temperatures and biological activity during the composting process can help to decrease antibiotic levels in manures, which can be taken up by crops growing on manured land. Compost can also be stored more easily than the bulk feedstocks, so it can be applied when soil and weather conditions are optimal.

Without denying the good reasons to compost, there are frequently very good reasons to just add organic materials directly to the soil without composting. Compared with fresh residues, composts may not stimulate as much production of the sticky gums that help hold aggregates together. Also, some uncomposted materials have more nutrients readily available to feed plants than do composts. Plants may need readily available nutrients from residues if your soil is very deficient in fertility. Routine use of compost as a nitrogen source may cause high soil phosphorus levels to develop because of the relatively low N:P ratio. Finally, more labor and energy usually are needed to compost residues than when simply applying the uncomposted residues directly. In general, composting makes most sense when 1) the feedstock materials are difficult to handle, unsafe in the open environment, or have odor concerns (like livestock carcasses, or food processing waste), 2) the waste material cannot be used locally and needs to be transported over distances before field application (like urban tree leaves), 3) there are concerns with pathogens (like pet, zoo or human waste) or 4) there is a good market for the use of compost (like farms near urban areas).

Figure 13.5.Left: Compacted soil. Right: compost application prior mixing and planting.Photos by Urban Horticulture Institute, Cornell University