«M.J. Depaz University of Florida Soil and Water Science Department Disposal of organic waste generated by humans is becoming a problem due to ...»
Temperature and pH throughout the composting process (Gray et al., 1971) Types of Composting Methods The main difference between compost operations is in the methods of composting. Passive piles, turned or aerated piles, and in-vessel systems are the three general types of methods. All these composting methods are relatively low in capital costs especially if these methods can be carried out in the open air (Schaub and Leonard, 1996). This process is time consuming and takes place at a much slower rate than any other method (Fernandez and Sartaj, 1997). Passive piles are compost materials that are piled into a heap and left undisturbed, relying on natural airflow for aeration. Windrow piles are similar except the piles are laid out into rows (Shaub and Leonard, 1996). Composting using passive piles can be accelerated by employing aeration systems that draw air through the pile and into perforated pipes to aid in oxygen delivery within the pile. Convection air currents are established by temperature gradients from the point where the air enters the pipes and center of pile containing the warm decomposing material (Fernandez and Sartaj, 1997).
Turned or aerated piles allow air to be mixed into the decomposing material through the use of the mechanical action of front loaders or manually by pitchforks. This method also allows for monitoring the conditions within the composting material (Schaub and Leonard, 1996).
The in-vessel composting method relies on containing the material in drums, bins, or channels to promote the optimum conditions for quick decomposition. The vessels have a means of turning, or stirring of the contents, and allow for control of aeration, moisture content, and temperature as well as containing odors. The benefit of the in-vessel system compared to the passive or turned pile method is that this system reduces the amount of time needed for the active composting phase from 3-5 weeks to 10-14 days. The disadvantage of the in-vessel system is the initial high capital cost and intense management (Schaub and Leonard, 1996).
Plant Available Nutrients from Biosolids, Manure, and Compost Amendments Methods of Nutrient Availability Application of biosolids directly to the field increase nutrient content of the soil by increasing organic matter and essential macro/micro elements to the soil. The availability of essential elements depends on soil properties, biosolid decomposition rate, and organic matter content (Banuelos et al., 2004). Applied biosolids are subject to changes by weathering and microbial processes that reduce leaching and allow for the elemental forms to be available for plant uptake (Banuelos et al., 2004).
Composting municipal solid wastes is beneficial because the unstablilized organic matter in uncomposted wastes has a high C:N ratio. The microbial decomposition can then affect the N mobility and cause it to become unavailable for the plant (Busby et. al., 2007). Applications of pig slurry, dairy manure, wastewater effluent and biosolids also affect the plant nutrient availability in the soil through microbial transformations of the nutrients. Nitrogen mineralization as well as nitrification rates have been found to increase through additions of sheep manure, dairy effluent, and cattle slurry (Habteselassie et al., 2006). Mineralization of N increases as temperatures increase and is greatest when the soil moisture is close to field capacity of the compost (Eghball et al., 2009).
The products of soil mineralization and nitrification are NH₄⁺ and NO₃⁻ which comprise the majority of N available for crops (Shi et al., 2004). Nitrification is the process where ammonium ions in the soil are enzymatically oxidized to nitrates (Brady and Weil, 2008). Manure compost provides soils nutrients that are beneficial to crops for many years. A study done by Eghball et al. (2000) found 12% of compost N was mineralized the first year, 12% the second year, and 8% the third year.
Benefits of Compost Use Composts change the soil chemical and physical properties. As surface mulches it increases organic matter, suppresses weeds, improves water holding capacity and provides plant nutrients (Levy and Taylor, 2002; Roe, 1998; Stofella ad Graetz, 2000). It can also be used as a means of reducing the frequency and rate of irrigation and inorganic fertilizer used on Florida sandy soils, and increase yields (Ozores-Hampton et al., 1998; Maynard, 2000).
Organic amendments such as MSW, increases K, Ca, and organic matter of sandy soils, promotes plant health, increases yields and increases soil pH (Courtney and Mullen, 2008; Roe, 1998). OzoresHampton et al. (1994) found that amending calcareous soils with municipal solid waste increase yields and growth of tomatoes compared to unamended soils. A different study conducted on barley growth by Courtney and Mullen (2008), showed that plant available K, Ca, Mg, Na, and P increased when spent mushroom compost was applied to the soil.
Multiple studies have found that using a combination of compost and fertilizer increases yields (Roe, 1998). Togun and Akanbi (2003) found that the use of compost alone or compost with mineral fertilizer was better than the control. The fortification of compost with a small amount of mineral fertilizer almost doubled the number of tomato fruits per plant compared to the control. The control in their 1997 trial produced 9.5 fruits per plant, the maize-stover with poultry manure compost produce
19.8 fruits per plant, and the maize-stover with poultry manure compost with 30 kg N/ha produced 18.1 fruits per plant. Maynard (2000) found that a combination of leaf compost and a reduced rate of 10-10fertilizer produced optimum yields of most vegetables including tomatoes.
Applications of yard compost had a cumulative effect on yields of onions (Maynard and Hill,
2000) and have higher yields of tomatoes than with unamended soils (Maynard, 1997; Maynard, 1999;
Maynard, 2000). Tomatoes that have been amended with leaf composts have shown a cumulative effect as well and have higher yields than with unamended soils showing positive results on tomato plants (Togun and Akanbi, 2003; Maynard, 1997). Sugar cane (Stoffella and Graetz, 2000) and hazelnut husk (Ozenc, 2006) composts have been used as alternative compost to traditional compost (such as yard waste and MSW) and have increased tomato yields by 1.8 and 1.74 times higher than the control.
The study by Stoffella and Graetz (2000) on sugar cane compost found higher marketable tomato fruit yield and fruit size in compost amended plots than unamended. The control was not amended with fertilizer or compost and produced a total of 2,156 kg/ha with an average fruit size of 108 g/fruit and the compost amended plots without fertilizer produced a total of 36,656 kg/ha with a fruit size of 177 g/fruit.
Affects on Weed Control In the past, composts have been used in place of polyethylene mulch, inorganic fertilizers, and weed control in alley ways (Stofella and Graetz, 2000). Immature compost can even be used in weed control because it possess phytotoxic compounds (acetic acid and propionic acid) that prevent weed growth (Leroy et al., 2008).
Soil Characteristics In sandy soils, the composted organic matter acts like a sponge, helping to retain water that drains down and out of the root zone (Golabi et al., 2007). In a study by Tambone et al. (2007), plots treated with compost showed a higher total organic carbon content than the control from the beginning to the end of the experiment. These researchers also found that the cation exchange capacity (CEC), which is the amount of exchange sites that are able to adsorb and release cations, correlated with the total organic carbon soil contents exhibiting the role that organic carbon plays in CEC determination.
Courtney and Mullen (2008) found that with the addition of spent mushroom compost, the organic carbon content increased with increasing application rates whereas, the application of inorganic fertilizer had no effect on soil organic carbon. Pandey and Shukla (2006) found that the repeated application of composted yard waste resulted in the increase of soil moisture, indicating increased organic matter increased soil water retention in the soil as well as increased the capillary rise from the water table.
In a study done by Leroy et al. (2008), application of fruit and garden waste (VFG) compost and raw cattle slurry maintained aggregate size and distribution and the combined application of both VFG and slurry resulted in the highest aggregate size and distribution. The hydraulic conductivity, which is the ease that water flows through the soil pores in response to a potential gradient, increased when both VFG and slurry were applied (Brady and Weil, 2008; Leroy et al., 2008). Both the aggregate stability and hydraulic conductivity can be attributed to an increase in soil organic matter which forms larger and more stable aggregates and increases the total pore volume. Organic matter can also prevent leaching by retaining water that would normally drain down beyond the root zone allowing sufficient residence time within the root zone for plant uptake of available nutrients (Golabi et al., 2007).
Composting is a low energy input decomposition process that is easy and cost-effective. Types of composting materials include municipal solid waste, manure, and vegetable and yard waste. Studies show that adding compost to Florida sandy soils can provide benefits to crops and soil for many years by reducing the amount of irrigation and application rates of inorganic fertilizer required to maintain or increase growth and yields. Florida has very sandy soils with little residual fertility due to nutrient runoff and leaching, organic matter loss, and erosion. Adding composts to soils in Florida is beneficial by increasing the macro/micro nutrient content and organic matter content of the soil. The addition of compost can also suppress weeds, increase water holding capacity, and reduce leaching of essential plant nutrients.
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