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4.2.4 Bioremediation of Organic Contaminants, Heavy Metals, and Pesticides in Soils
Soils are contaminated with harmful chemicals. Potential bioapproaches are biostimulation, i.e., fertilizers and bioaugmentation by adding organic amendments with active microorganisms in it, or the compost. Recently, compost has been used for soil bioremediation. Compost addition can comprise “super bioaugmentation” with a mixture of degrading microbes, combined with “biostimulation” (Kastner and Miltner 2016).
Compost is used in bioremediation to break down toxic pollutants and other harmful substances, minimizing the toxicity of metallic contaminants in organic residues, wastes, and by-products. After the composting process stabilizes the waste, it can be disposed to landfills or farmland (Ankidawa and Nwodo 2011). The stabilized organic matter can also form complexes and restrict metal flow and enhance absorption in plants.
The microbial activity during composting helps in the decomposition of pesticides in the soil. A report suggests that the pesticide content is present in composts following the breakdown of pesticides during composting (Wu and Ma 2000). The addition of compost is known to enhance the degradation of two herbicides viz., benthiocarb S-4-chlorobenzyl diethylthiocarbamate (benthiocarb) and 4-chloro-2-methylphenoxyacetic acid (MCPA) in soils. The soil amended with compost improved the breakdown of a fumigant (1, 3-dichloropropene) in comparison with the soil without compost. The concentration of pesticides was at safe levels for plants in soils due to the application of compost.
4.2.5 Compost in Horticulture as the Supply of Minerals to Crops and Other Plants
Compost application is beneficial in horticulture as an effective means of recycling; it also minimizes the use of chemicals and harmful fertilizers. Compost has been trialed for the grapevine (Powell et al. 2007). Based on the benefits of compost, it can enhance water absorption and minimize the need for irrigation. Compost is also an exciting peat substitute. Compost added to soil has improved the biomass of crops and has increased N, P, and K uptake (Miller et al. 2015). There is a report on a significant increase in the growth and biomass of Anemone coronaria L. cv. Red meron (Aydinsakir et al. 2009).
When compost nitrogen is used in the soil, organic nitrogen is converted to readily available nitrate–nitrogen for growing plants and microbes as well. Compost usually contributes 10–15 kg of nitrogen per 10 cubic meters applied per crop. This nitrogen from the compost helps plants grow. High NH3 emission is due to NH4+–N content in the compost (Nicholson et al. 2017). Nitrogen emissions from green and food compost are relatively more minor, and they can be used to build nitrogen reserves and improve soil condition (Nicholson et al. 2017). The study’s findings have revealed around a 30% reduction of N2O emission due to manure compost in andosol soil (Mukumbuta et al. 2017).
Phosphorus is a crucial central element for plant growth. Compost usually provides 6–8 kg of plant-available phosphorus, which is around 65–90 kg super-phosphate. Compost use increases soil phosphorus supply by releasing inorganic phosphorus through the mineralization of organic phosphorus (Wang et al. 2010).
Compost normally contains 0.8–1.2% potassium. Therefore, with 1.0% (w/w) potassium content, 10 m3 of mature compost will contain 40 kg of potassium, providing 36 kg of plant-available potassium, equivalent to 90 kg of potassium sulfate. When applied to soil, municipal waste compost increased the amount of nitrogen, phosphorus, and potassium available for plants (Soheil et al. 2012). Compost-amended soils contain nutrients (macro and micronutrients) for plant growth (Brown and Cotton 2011).
Compost is used in orchards and vineyards. There has been a report on improved growth and yield of citrus, avocado, apples, and table and wine grapes due to the application of compost during planting (Buckerfield and Webster 2003). The application of compost and compost mulches in fruit and vineyards can be advantageous when growing conditions are less stringent; soils are more impoverished, especially when less water is available. Thus, compost can be used to increase uniformity and, hence, enrich the entire orchard and vineyard.
Adding compost to vegetable cultivation has reduced the use of chemicals and harmful pesticides by 80%. It has also minimized the detrimental effects of intensive cropping on soil performance and quality of water. Compost also increased the ability to hold cations, viz., potassium due to which there was a 20% reduction in the requirement of potassium for vegetable crops.
Compost has improved the yield of lettuce, carrot, broccoli, etc. There is a report on the increase in red pepper yield due to compost application (Park et al. 2018). The soil mixed with 25% compost showed an increase in the yield of cabbage and onion (Smith et al. 1992). Increased growth on the use of compost as fertilizer has been reported on onion and lettuce (Brechin and McDonald 1994), tomatoes (Manios and Kapetanios 1992), barley (Hountin et al. 1995), and rice (Charfen 1981).
4.3 Use of Compost in Mulching
The application of compost in mulch can be used to control weeds and improve soil fertility (Ozores-Hampton et al. 2002). Mulching is mainly found in areas abounding in greenery. The effect of compost on mulching, inhibiting weeds is achieved by minimizing the light reaching the seeds of the weeds, which often germinate in the topsoil. Before compost is applied in mulching, the place must be cleared of all established weeds. Woodchip waste from young trees and compost prepared using poultry manure as mulch have been used in an organic apple orchard to control weeds in California (Smith et al. 2000). The thickness of the compost layer required is around 10 cm to be effective against germinating weeds (Hanslin et al. 2005). Mulch consists of two layers of composts: nutrient-rich and nutrient-poor, which is the top that has particles of size 20 mm. Trees and shrubs get nutrients from fine particles, and weeds can be controlled as they cannot grow on a coarse and nutrient-poor layer of the mulch. Compost is applied as mulch in orchards, and this improves the water balance and physical properties of the soil, reduces erosion, and decreases the use of inorganic fertilizers.
4.4 Perspectives
The compost-turning machines market is estimated to reach around US$ 150 million by 2028. These developments will also increase the volume of sales of waste-management equipment. The drum type compost machine will become the dominant compost machine in the market. It is expected that there will be a tremendous demand for compost-turning machines. Agriculture will benefit due to the use of compost-turning machines. Different types of waste generated in cities can be used to make value-added products, i.e., compost by solid-state fermentation (SSF) method (Farrell and Jones 2009). These value-adding products (composts) are considered to be eco-friendly, economical, and non-toxic, and they help promote sustainable agriculture without causing any pollution and not causing any harm to plants and the environment.
4.5 Conclusion
Compost is an excellent soil conditioner and can enhance the growth of crops and many plants due to the supply of various nutrients, viz., nitrogen, phosphorus, and potassium, and minerals, and this will thus be very helpful to many farmers. The use of chemical fertilizers can be minimized, thanks to the application of compost to the soil, and it is also very cost-effective. Also, compost increases microbial diversity, which is vital for the soil ecosystem. The application of compost will be beneficial for organic farming and agricultural sustainability.
References
1 Adugna, G. (2016). A review on impact of compost on soil properties, water use and crop productivity. Academic Research Journal of Agricultural Science and Research 4: 93–104.
2 Anastasi, A., Varese, G.C., and Marchisio, V.F. (2005). Isolation and identification of fungal communities
