Compost contains high portions of the humus-C, the portion of carbon that contributes to the humus reproduction. It accounts for 51% of the total organic carbon and is therewith higher than in any other humus fertilizer. Compared with compost, straw and liquid manure contain 21% carbon and green-fertilizers contain only 14%. The effectiveness on the humus reproduction with compost is by factor 4 higher than with straw and by factor 20 higher than with liquid manure (Kehres 2008). This fact is very relevant from the view of sustainable agriculture, because agriculture cultivation is connected with losses of humus-C in the soils and the fertilization with compost could compensate it.
Organic material and their decomposition products can reduce P fixation in soils by the complexation of Al and Fe by organic acids, by the competition between organic acids and orthophosphate for adsorption sites and release of P by organic material during decomposition (Mnkeni and MacKenzie 1985; Sibanda and Young 1986; Iyamuremye et al. 1996; Kwabiah et al. 2003). Also composted organic material has been reported to reduce P fixation in soils: Ogaard (1996) studied the effect of fresh and composted cattle manure on P retention in soil and found that both reduced P fixation compared to inorganic phosphate. Buchanan and Gliesman (1990) reported that composted spent mushroom, bedding material, horse manure, and hay residue decreased P fixation in soils compared to inorganic fertilizer. Hue et al. (1994) also reported similar findings using yard-waste compost and attributed this to the release of P during the decay process and the competition between organic anions (released by compost) and P for adsorption sites in the soil complex. Guisquiani et al. (1988) reported that the addition of urban-waste compost increased soil P solubility. They postulated that the increase in soil P solubility was caused by the formation of phosphohumic complexes that minimize immobilization process, anion replacement of P by the humate ion, and coating of sesquioxide particles by humus to form protective cover. Mkhabela and Warman (2005) studied the effect of municipal solid waste (MSW) compost on soil P availability and uptake by potato and sweet corn crops. Three rates of MSW compost, one rate of chemical NPK fertilizer, and a mixture of MSW compost and chemical NPK fertilizer were used on both crops according the soil P test results. The NPK and mixture treatments produced significantly higher yields in the first year, while in the next year the yields were not significantly different for all treatments. All the MSW compost treatments had lower tissue N compared to the inorganic fertilizer and mixture treatments. The MSW compost and mixture treatments generally results equivalent concentration of Mehlich-3 extractable soil P compared to the inorganic fertilizer. In addition, the application of both inorganic fertilizer and MSW compost decreased P adsorption by the soil up to 30%. MSW compost may be a good source of P for both potatoes and sweet corn. However, the low availability of compost-N means that supplementary N in the form of inorganic fertilizer may have to be added together with compost in order to enhance N availability to crops. Alexa et al. (2004) studied the effect of compost fertilization on the NPK content of a sandy soil. Three rates of compost were used and all of them produced significantly higher contents of Nmin, AL-P2O5, and AL-K2O. The compost treatments had also a positive effect on the soil pH. Erhart et al. (2005) found that yields in treatments with compost fertilization (9, 16, and 23 t/ha/annum on average of the first 10 years) increased 8%, 7%, and 10%, respectively when compared to the unfertilized control. Yield response to the compost applications was very low in the beginning and increased slightly with the duration of the experiment. The analysis of the yield components of the cereals showed that the plants in the compost treatments were sufficiently supplied with nitrogen in the early growth stages and after pollination, but at booting, when N uptake is highest, the N supply in the compost treatments was comparable to that with mineral fertilization at up to 30 kg N/ha. Crop quality was not affected by compost fertilization, but in some cases even improved. The results suggest that on fertile soils the fertilizer effect of compost is small, but it increases with time. Chan et al. (2007) evaluated the short-term effects of selected composted soil conditioners applied in 0, 25, 50, and 100 t/ha in radish growth in a pot experiment. They found that the pot trial results indicated lack of growth response of radish at application rates up to 100 t/ha of unblended soil conditioners from garden organics. The blended soil conditioners were more variable in quality and as confirmed by pot trial results produced highly variable plant responses. Maynard (2000) showed that for optimum yield of most vegetables, a combination of compost and 10-10-10 fertilizer is preferred, although the full rate is unnecessary. Half the rate of fertilizer plus compost is sufficient for optimum yields on loamy soils and for most years on sandy soils, additional side dressings of nitrate supplying fertilizer may be required after heavy rainfalls during the growing season. Additional soil tests can determine nitrate levels during the growing season. Organic fertilizer with compost can also be used on loamy soils, but it may not provide satisfactory yields on sandy soils. This experiment demonstrated the importance of knowing soil type when using compost.
The available nitrogen of compost is predominantly dependent on the following parameters (Amlinger et al. 2003):
• Composting conditions (mainly aeration, agitation)
• Decomposition/stabilization rate, duration of composting (immature or mature compost)
• Posttreatment of compost (sieving, blending)
• Time of application (time before cropping or leaching phase)
• Compost quality parameters, such as C/N ratio, degradable amounts of C and N fractions
To evaluate the mineralization (availability) of compost N ratio to estimate the amount available for plants over a certain period several experimental and calculation methods are possible (Amlinger et al. 2003):
• Laboratory incubation tests under constant conditions (temperature, water content of the substrate with and without cultivation of crops) with or without periodical percolation of the mineralized N
• Field trials to evaluate the N efficiency in terms of comparative yield or N uptake, respectively, relative to control or mineral fertilized plots
• N-leaching tests on field scale with lysimeter or suction cups, as well as regular measurements of mineral N in the leaching horizon
• Model-based calibration of N-mineralization kinetics
For the assessment of long-term effects of compost Fuchs and Larbi (2004) applied on one half of a field each year 10 t dry weight/ha compost, while the other half was used as a control. After 5 years, soil samples were taken from the different field plots. The disease receptivity of the soils was lower in the plots with compost compared to the control plots. Elherradi et al. (2005) found that the quantities of mineralized N of the compost varied between 15% and 24% of the compost total N. Scheurell and Mahaffe (2002) reported that compost extracts could improve plant health. Larbi et al. (2006) found that green waste compost extracts protect apple plants against scab and grapevines against downy mildew under controlled conditions. The efficiency of the compost extract against scab was not affected by the compost to water ratio and extraction duration. But in many cases, different compost to water ratios have reduced the incidence or disease severity of pathogens.
Soil quality and health play an important role in sustainable agriculture: Soil quality assessment provides a basic means to evaluate the sustainability of agricultural and land management systems (Doran and Parkin 1994). Soils that are healthy have homeostatic properties, within broadly set limits (Coleman et al. 1998). The application of compost can results soil quality and health because of the organic matter content and humus-producing properties.
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