**1. Introduction**

Sewage sludge or biosolids, which are one of the final products from wastewater treatment plants, are considered the most promising waste that can be utilized in an effective and environmentally friendly manner. Sewage sludge creates very little to zero environmental impact if utilized properly. Throughout the world, the safe disposal of the sewage sludge is one of the major environmental concerns. However, opinions on the utilization of the sewage sludge vary due to the possible positive and negative points associated with the handling and treatment. In fact, sewage sludge is increasing annually as the population increases and it is a renewable product that will never stop forming. It can be treated differently with various methods depending on the purpose of the treatment [1, 2]. Sewage sludge is composed of organic compounds, macro- and micronutrients, trace elements including toxic metals, microorganisms, and micro-pollutants. Micro- and macronutrients serve as a source of plant nutrients, whereas organic constituents serve as soil conditioner. It contains high concentrations of N, P, Ca, and Mg. Potassium is, however, deficient in sewage sludge [3]. Sludge amendment improves soil properties such as porosity, bulk density, aggregate stability, and water-holding capacity. Sewage biosolids are often used as a fertilizer on farms to grow corn and cereal crops such as wheat. Using sewage biosolids as a nutrient source for field or forage crops or for improved pasture (1) improves soil fertility—offsetting the need for commercial fertilizers; (2) reduces production costs; (3) improves soil fertility; (4) enhances soil structure, moisture retention, and soil permeability; (5) adds organic matter—enhancing soil structure, moisture retention, and permeability, while reducing the potential for wind and water erosion [3].

Higher level of heavy metals in sewage sludge may be a cause for problems when applied in field used for agriculture. Whether any problem actually takes place will depend on soil pH, soil organic matter content, cation exchange capacity, movement of heavy metals in the soil profile, and changes that take place in the forms of heavy metals [4]. It is always advisable to use sewage sludge in low doses to reduce bioavailability of toxic heavy metals [5]. Sewage sludge amendment increases the production of a variety of plants including vegetables, cereals, grasses, and trees. The use of sewage sludge also results in more robust plants with faster development and greater biomass production [6]. It has been observed that crops contain heavy metals at concentrations harmful to human health when such crops were grown in soil amended with extremely high level of sewage sludge [7]. However, the metal concentrations in the sewage sludge depend on several factors such as (i) sewage origin, (ii) sewage treatment processes, and (iii) sludge treatment processes.

In Oman, "Haya Water" is a government company that is responsible for building, operating, and managing wastewater projects in Muscat Governorate. Haya Water has developed its pioneering Kala Composting Plant to enable the efficient reuse of sewage biosolids and green waste enabling their conversion to a compost product that can be used for agriculture, landscaping, and for individual gardens. The use of Kala compost (KALA) has various benefits such as farmers reusing a waste product, municipal authorities reducing their dependence on chemical fertilizers, as well as reducing greenhouse gas emission due to the use of environmental friendly waste management process [8]. However, high application of sewage biosolids could result in heavy metal accumulation and many health problems. Therefore, sewage biosolids applied to agricultural land must be well treated and continuously monitored to avoid any environmental risk problems. The objective of this study is to evaluate the effect of different fertilizers especially Kala compost on the quality of soil and crops. Specifically to (1) conduct research to assess the performance of the tested crops under different fertilizers, (2) to determine the changes in physicochemical properties of the soil treated by different fertilizers, (3) to determine the amount of water that can be saved using Kala compost, (4) to determine the effect of Kala compost on plant growth and find out any heavy metal accumulation in soil and plant, and (5) to monitor characteristics and yield components of crops grown and treated by different fertilizers.
