**Acknowledgements**

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

176 Organic Fertilizers - From Basic Concepts to Applied Outcomes

The author would like to thank Kentucky State University and the University of Kentucky farm crew for maintaining the field plots. This investigation was supported by a grant from the USDA/NIFA to Kentucky State University under Agreement No. KYX‐10‐13‐48P.
