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Compost is organic matter that has undergone partial thermophilic, aerobic decomposition. This environmentally safe process is called composting. The combination of raw materials and the chosen composting method yields a wide range of characteristics, such as organic matter (OM) content, nutrient content, potential for disease suppressiveness and other physical, chemical, and biological properties. The objectives of this review are describing the horticultural outlets for composts, defining compost characteristics important for the above uses, and describing composting procedures and raw materials leading to these characteristics. The two main horticultural uses of composts are as soil amendment and as an ingredient in container media. Soil-applied composts improve soil fertility mainly by increasing soil organic matter (SOM) that activates soil biota. Compost's nutrient content, and especially that of nitrogen (N), should be high (>1.8%). Composts having these characteristics are produced of raw materials rich in both OM and N, while minimizing their loss during composting. Typical raw materials for this purpose include animal manures, offal, abattoir residues, sewage sludge, and grass clippings. Various composting methods can yield the required results, including turned windrows, aerated static piles, and in-vessel composting. Composts are also used for substrates as low-cost peat substitute, potentially suppressive against various soilborne diseases. These composts must be stable and non-phytotoxic. Physical properties of compost used as substrate are important. Hydraulic conductivity, air porosity, and available water should be high. Reconciling the physical and biological demands may be difficult. Materials such as softwood bark, wood shavings, various types of shells or hulls, and coconut coir are characterized by good physical properties after composting. However, being relatively resistant to decomposition, these materials should be subjected to long and well-controlled composting, which may be shortened using N and N-rich organic matter such as animal manures. High temperatures [>65 °C (149.0 °F)] may cause ashing, which leads to reduced porosity. In addition to ligneous materials, composts serving as growing media may be produced from numerous organic wastes, such as manures, food industry wastes, etc. These materials are better composted in aerated static piles, which tend to minimize physical breakdown. Animal excreta are of special value for co-composting as they contain large, diverse populations of microorganisms, which accelerate the process.

Fruit growers apply fertilizer at high rates with soil organic matter (SOM) below 2.0%. As organically certified fertilizers are costly, our objective was to compare the effects of two modes of organic nitrogen nutrition to conventional control on plum tree yield and soil properties. The orchard is located on a Vertisol soil, and planted to plums in 1998. The tested treatments were: A) conventional control, receiving an average of 350 kg of N/ha per year; B) fertilization using cattle manure compost (15 tons/ha per year) + feather meal (1 ton/ha per year); and C) a combination of the same amount of compost + 500 kg of feather meal/ha per year + leguminous cover crop (alfalfa, Medicago sativa cv. Gilboa). By 2003, the SOM of treatment A remained stable and that of treatments B and C increased by 36% and 91%, respectively. As a result, soil bulk density of treatments B and C declined with no change in A. During the first year, levels of soil nutrients were lower in treatments B and C than those in A, but they became higher after 2 to 3 years. Nitrification capacity of the soils of treatments B and C was higher than that of treatment A. This enabled a drastic reduction of the application rates of organic amendments without resulting a decline in the soil's nutrient content in the organic treatments over the next 3 years, due to continued mineralization of the SOM pool. Various soil microbial characteristics (microbial counts, total hydrolytic activity, functional richness, and diversity) were determined. In all these parameters the organic treatments showed higher levels than treatment A. Stem circumferences and yields were similar for all treatments. It can be concluded that soil productivity is affected by SOM, so that after achieving a threshold level of SOM, fertilization needs are reduced considerably.

Physical characteristics of two media were studied concerning water availability to roots, as reflected in specific transpiration rate, stomatal conductance, and specific growth rate of very young leaflets of `Kardinal' rose (Rosa ×hybrida L.), grafted on Rosa canina L. `Natal Brier'. Plants were grown in UC mix [42% composted fir bark, 33% peat, and 25% sand (by volume)] or in coconut coir. Water release curves of the media were developed and hydraulic conductivities were calculated. Irrigation pulses were actuated according to predetermined media moisture tensions. Transpiration rate of plants was measured gravimetrically using load cells. Specific transpiration rate (STR) was calculated from these data and leaf area. STR and stomatal conductance were also determined using a steady-state porometer. Specific growth rate (R_SG) of young leaflets was calculated from the difference between metabolic heat rate and respiration rate, which served as an indicator for growth potential. Low STR values found at tensions between 0 and 1.5 kPa in UC mix suggest this medium has insufficient free air space for proper root activity within this range. Above 2.3 kPa, unsaturated hydraulic conductivity of UC mix was lower than that of coir, possibly lowering STR values of UC mix-grown plants. As a result of these two factors, STR of plants grown in coir was 20% to 30% higher than that of plants grown in UC mix. STR of coir-grown plants started to decline only at tensions around 4.5 kPa. Yield (number of flowers produced) by coir-grown plants was 19% higher than UC mix-grown plants. This study demonstrated the crucial role of reaching sufficient air-filled porosity in the medium shortly after irrigation. It also suggests that hydraulic conductivity is a more representative measure of water availability than tension.