Composts generally represent a high organic matter medium containing all of the nutrients needed by plants, in approximately the same quantities that are found in plants. However, the most common mistake made in utilizing compost as a soil amendment is to treat it as a fertilizer. Most composts are quite nutrient deficient, and must be amended with plant nutrients, particularly with available N. Compost manufacturers are often to blame for the misconception that compost alone is a suitable plant growth medium. One can stop by nearly any garden-supply store and see bags of recycled compost products with the total N-P2O5-K2O label written across the front. The implication is clearly that this is a fertilizer product despite claims in the 'directions for use' that it is not. Most of the nutrients in compost are in an unavailable, organically bound form. For instance, a typical compost may be 1-3% total N by weight, but the ammonium and nitrate (available forms) of N are typically less than 0.05% by weight.
Like compost, the total N concentration of peat moss, another commonly utilized soil amendment, is generally less than 1%, with an available N fraction of less than 0.01% by weight. When used without added nutrients, peat moss alone is a very poor plant growth medium.
The availability of nutrients from compost varies significantly with respect to the composition ofthe compost, and can be similar or considerably higher or lower than that of a soil. Composts derived from high nutrient materials such as municipal biosolids or animal wastes are likely to have relatively high nutrient availability while compost derived primarily from cellulosic plant material (i.e., yard waste) are more likely to be nutrient poor. For instance, municipal biosolids compost is often used directly as a plant growth medium with generally good success, but yard waste or municipal refuse composts almost always need to be amended with fertilizer nutrients in order to obtain good plant growth.
The nutrient value of composts will generally be realized over a long period of time as the organic matter in compost decays, releasing inorganic (and available) nutrients. However, a severe short-term nutrient deficiency generated by addition of large amounts of low-nutrient composts can prove fatal for new plants, and an initial planting might fail. One method ofassessing the potential for availability ofN is the C:N ratio (g total C divided by g total N) of the compost. It is generally accepted that below a C:N ratio of 20:1, release of available N should occur, and above 30:1, immobilization of available N by soil microbes should occur. Thus, if the C:N ratio of the compost to be used is >20:1, the use of fertilizer amendments is likely to be essential.
Over a long term, the release of plant-available nutrients from composts can be substantial, and the lower need to refertilize is seen as a major benefit of compost use. For example, nitrogen is stored in soil almost entirely in the organic form. The quantity and nature of organic matter and its decomposition in the soil has a significant influence on the long-term availability of nitrogen to plants. For instance, an addition of 100 t ha-1 of compost with a N:P:K analysis of 2:1:1 would result in a total addition of 4000:2000:2000 kg ofN:P:K ha-1. If 2% of the total nutrients in this compost were to mineralize every year, this would release 80 kg of available N, 40 kg of available P, and 40 kg of available K each year.
The availability of P, K, and other nutrients is generally higher for most composts than the availability of N. However, the availability of P, in particular, is highly variable. It is quite important to establish by analysis and field trial the potential for nutrient availability from composts due to the high variability of these materials.
The addition of compost can make the utilization of fertilizer nutrients more efficient. First, compost may allow the long-term retention of nutrients through enhanced ability for soil to retain nutrients chemically and biologically. Second, compost applications may improve other soil factors that limit plant growth other than fertility, allowing plants to achieve more of their genetic growth potential. However, a wide body of experience has shown that in most cases compost should not be used as the sole source of plant nutrients.
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