Intensive cultivation and the failure to implement effective soil conservation practices have led to soil degradation and a decline in productivity because of excessive soil erosion, nutrient run-off and a decrease in stable soil organic matter. Efforts must be made to halt the decline in soil productivity and to restore the productivity of degraded soils in the shortest possible time. This could be achieved through proper management and recycling of organic wastes on land to protect agricultural soils. The efficient and effective use of organic wastes as soil conditioners and fertilizers constitutes one of the best means for maintaining and restoring soil productivity (Passarini et al., 2014, Kumar et al., 2014).

The utilization of organic wastes in agriculture depends on several factors, including the characteristics of the waste such as its organic matter, nutrient and heavy metal content, its energy value, the odor generated by the waste, its benefits to agriculture, its availability and the transportation costs and regulatory considerations. Although the importance of these factors can vary by type of organic waste, the considerations for use are similar for most organic wastes. Organic amendments affect soil properties in numerous and variable ways. These effects can be due to the intrinsic properties of the organic amendment (direct effect) or as a consequence of the beneficial effect of the organic amendment on the physical, chemical and biological properties of the soil (Stewart et al., 2000, Tejada et al., 2006, Tejada et al., 2009). Organic wastes do several things to benefit the soil that synthetic fertilizer cannot do. First, they add organic matter, which improves the way water interacts with the soil. In sandy soils, organic wastes act as a sponge to help retain water in the soil that would otherwise drain down below the reach of plant roots, thus protecting the plant against drought. In clay soils, compost helps to add porosity to the soil, making it drain more easily so that it does not stay waterlogged and does not dry out into a brick-like substance. Organic wastes also inoculate the soil with vast numbers of beneficial microbes (bacteria, fungi, etc.) that promote the biological activity of the soil (Siddiqui et al., 2009, Jain et al., 2014). These microbes are able to extract nutrients from the mineral part of the soil and consequently make them available for plant uptake. Furthermore, when properly processed, organic wastes reduce soil-borne diseases without the use of chemical control (Pascual et al., 2000, Garcia et al., 2004, Suarez-Estrella et al., 2013). Beside nutrients, organic amendments add organic matter to the soil, contributing to the improvement of soil quality and fertility, as compared to the use of mineral fertilization alone.

The management of soil organic matter by using composted organic waste is the key for sustainable agriculture (Nyamangara et al., 2003). Several works have highlighted the beneficial effects of organic waste application for crop production. In addition to its slow release nutrient capability (Eghball, 2000, Choi et al., 2014), organic matter is largely responsible for aggregation, as well as for the improvement of various soil physical properties, including soil moisture holding capacity (Aggelides and Londra, 2000, Borken et al., 2002, Cuevas et al., 2003, Basso and Ritchie, 2005, Tejada et al., 2006). Therefore, increasing soil organic matter content must be the first step in any farming practice. If productivity is to be maintained, it is essential to develop an agricultural system able to preserve satisfactory physical conditions in the soil. Organic matter additions are the only means of making some soils economically productive (Rathod et al., 2013).

Nevertheless, organic waste application as a substitute for conventional mineral fertilization is sometimes problematic because some crops have high nutrient needs or punctual needs throughout their growth cycle. As a result, large quantities of organic material would be necessary to satisfy the overall needs of the crop, and/or the organic wastes would not supply sufficient quantities of nutrients at the right moment. Bazzoffi et al. (1998) found that urban refuse compost produced a lower maize grain yield than mineral fertilization, whereas maize produced lower yield when compost was applied, as compared to mineral fertilization (Businelli et al., 1990).

Combining organic amendment applications with a nitrogen mineral fertilizer with the aim to meet crop N needs can be a suitable alternative for replacing conventional mineral fertilizer. The use of treated organic wastes as a fertilizer and soil amendment not only results in economic benefits for the small-scale farmer, but it also reduces pollution due to reduced nutrient run-off and N leaching (Nyamangara et al., 2003).

The objective of this study was to assess the feasibility of using combined organic (compost) and inorganic fertilizers as an alternative to the conventional inorganic fertilization used for the nutrition and production of tomato plants.