Manure and Compost
Compost
Unlike manure, compost is very stable and generally not a readily available source of nitrogen, but it is important to recognize that phosphorus and potassium remains highly available. The composting process uses heat and microbial activity to quickly decompose simple compounds like sugars and proteins, leaving behind more stable complex compounds such as lignin and humic materials (Figure 13.5). The addition of compost to the soil increases available water holding capacity by improving organic matter content and pore space that holds water. It also improves cation and anion exchange capacities, and thus the ability for nutrients to be stored and released for plant use. Quality compost contains more humus than raw manure because primary decomposition has occurred during the composting process. As a result, compost is less effective at building soil aggregation than fresh manure, because the readily-degradable organic compounds have already been decomposed, and it is the microbial process of decomposition that helps build aggregates. Properly produced composts are safe to use on human food crops with respect to pathogens. Composted manure is also easier to handle, less bulky, and better smelling than raw manure.
Compost Nutrient Content
Compost is not as good a source of readily available plant nutrients as raw manure. But compost releases its nutrients slowly, thereby minimizing losses. Generally, compost contains relatively low concentrations of nitrogen, phosphorus, and potassium compared to manure. Just like manure, the nitrogen content of compost will vary according to the source material and how it is composted. In general, nitrogen in the form of ammonium (NH4⁺) or nitrate (NO3¯) is low in compost compared to manure.
Compost and High-Value Crops
Compost, given its cost, is more suited for use in growing high-value crops such as vegetables. Most vegetables are intensively cultivated and require high levels of nutrients and water. Moreover, many vegetables are produced on soils with low native organic matter. An advantage of using composted rather than uncomposted animal manures as soil amendments in vegetable production systems is consumer safety and biosecurity.
Cover Crops as an Alternative
There is an increased interest among vegetable farmers to incorporate cover crops into crop rotations in addition, or as an alternative, to applying compost. Using fast-growing annual cover crops such as different types of grasses, mustard, sunn hemp, clovers, vetches, rye, and radishes, to mention a few, can also build soil health by increasing water holding capacity, increasing organic matter and trapping excess nitrogen (Chapter 16).
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