**9. Conclusion**

**Figure 6.** Hot pepper plants grown in soil amended with chicken manure applied at 15 t acre-1 at Kentucky State Uni‐

**Figure 7.** Invertase activity expressed as mg glucose released g-1 dry soil and urease activity expressed as mg NH4–N

Tabatabi and Bremner [114] used a simple method for the quantification of urease activity in soil. In their method, they used a few grams of soil (about 5 g) and added 10 mL of 0.1 M phosphate buffer in a volumetric flask kept in water bath at 30°C for 1 h to allow the soil

method [115]. For standardization, a series of standard solutions of NH4 Cl covering the concentrations of 0.1–100 μg NH4–N mL-1 of water was prepared. In this method, urease

<sup>+</sup> ions were determined by the selective electrode

versity Harold Benson Research and Demonstration Farm (Franklin County, KY, USA).

174 Organic Fertilizers - From Basic Concepts to Applied Outcomes

released g-1 dry soil h-1.

temperature to equilibrate. The liberated NH4

Organic waste used as fertilizer must be safe for the environment and wildlife, and safe for all who apply and consume the food product. The simultaneous use of recycled waste to enhance soil physical, chemical, and microbial conditions could also enhance soil fertility and crop yield. Organic farming, farming without chemicals, requires organic fertilizers. While such a definition is concise and clear, it is unfortunately untrue and misses out on several character‐ istics which are of fundamental importance. All materials, living or dead, contain chemical compounds; therefore organic farming utilizes chemicals. The United States Department of Agriculture (USDA) has framed a handy definition of organic farming: "Organic farming is a production system which avoids or largely excludes the use of synthetically compounded fertilizers, pesticides, and growth regulators." The potential health hazards of pesticide residues, nitrates, and phosphates, resulting from conventional agriculture are now receiving attention.

Thirty percent of US streams have high levels of N and P contamination and drinking water violations due to nitrates and phosphates that have been doubled in the last 8 years. Organic wastes are usually rich in carbon and nitrogen, and their addition increases the soil content of labile carbon, accelerates the activity of soil microbes, increases nitrification and denitrification rates. With the growing interest in recycling organic amendments, it is important to monitor the activity of urease, invertase, and phosphatase since these three enzymes play a significant role in the soil N, C, and P cycles, respectively. In addition, the activity of these three enzymes and other soil enzymes can be used as a direct indicator of soil health and soil microbial population and activity in the rhizosphere of growing plants (a zone of increased microbial and enzyme activity where soil and root make contact). With increasing emphasis on fertility sustainability and environmental friendliness, restoration of soil microbial ecology has become important. In agricultural practice, composting of soil with sewage sludge, chicken manure, or yard waste provides an organic amendment useful for improving soil structure and soil nutrient status and generally increases soil organic matter and stimulates soil microbial activity. In the US, about 11.4 million tons of poultry litter was produced and about 90% of this amount was used as fertilizer in agricultural production [121]. It has been found that poultry litter contains many essential plant nutrients (N, P, K, S, Ca, Mg, B, Cu, Fe, Mn, Mo, and Zn) and has been reported as excellent fertilizer [122]. It is expected that significant chicken manure generation will become available in increasing quantities because of the increasing growth in the poultry industry. In addition, as more sewage sludge treatment districts turn to composting as a viable means of sludge stabilization, sewage sludge will also become available in increas‐ ing quantities. Sewage sludge and chicken manure contain significant amounts of trace elements that may impact soil microorganisms and the enzymes they produce by blocking of either the enzyme or substrate when present in excessive concentrations. Trace elements are among the major contaminants of food supply. They are not biodegradable, have long biological half‐lives, and have the potential for accumulation in edible plants grown under this practice [123] that requires environmental measurement and mitigation [124]. Trace elements may also accumulate in the different human and animal body organs leading to potential adverse effects on human health. The rate of release of trace elements from sewage sludge into soil solution and subsequent uptake by plants could also result in phytotoxicity and/or bioaccumulation. Regarding the use of horse manure as organic fertilizer, typically, a ton of horse manure contains 11 pounds of N, 2 pounds of P, and 8 pounds of K [125]. Horse manure contains about 60% solids and 40% urine [126]. During cleaning, soiled bedding removed with the horse manure may account for another 8–15 pounds of waste per day. The volume of soiled bedding removed during cleaning is almost twice the volume of manure removed but varies widely depending on management practices. As described earlier, field application of horse manure is also based on fertilizer needs of a particular crop. The approximate fertilizer value of manure from bedded horse stalls based on its dry matter content, which is about 46%, is 4  lb ton-1 ammonium‐N, 14 lb ton-1 total N content, 4 lb ton-1 P2O5 (phosphate), and 14 lb ton-1 K2O (potash), whereas the fertilizer value of horse manure at 20% moisture without bedding is approximately 12–5–9 lb ton-1 (N–P2O5–K2O). Overall, organic amendments from animal manure are excellent fertilizers. However, there is an emerging concern regarding the impact of endocrine disrupting compounds (EDCs) in reclaimed water and sewage sludge. Most livestock grown in the US and worldwide are raised in large‐scale concentrated animal‐feeding operations. This high population densities require heavy use of antibiotics that can cause severe local soil, air, and water pollution.
