Mineralization of Soil-Incorporated Tomato Solid Waste

Solid waste from the processing of tomatoes (Lycopersicum esculentum Mill.) consisting primarily of fruit peels and seeds was incorporated into Yolo sandy loam soil, wetted, and incubated at 10, 20, or 30°C for 32 weeks. At 2-week intervals the soil was analyzed for NO₃-N, PO₄-P, SO₄-S, Cl, total N, organic C, and exchangeable Ca, Mg, K, and Na. Soil pH and electrical conductivity were measured at the beginning of the study and after 32 weeks. With each increment of dried tomato waste applied at rates of 2.5, 5.0, and 10.0% of air-dry soil, the total organic C and N content of the soil increased. The amount of NO₃-N released from 2.5% rate of waste at 20°C soil temperature was similar to that released from 60 ppm N applied as (NH₄)₂SO₄. Nitrogen released from waste applied at rates of 5.0 and 10.0% air-dry soil exceeded the level of N usually applied for crop production at about 76 kg N/ha. Significant interactions occurred on NO₃-N release among waste application rates, soil temperatures, and times. At 10°C, there was an increase in NO₃-N released with each increase in waste application. At 20°C, a NO₃-N increase occurred at 2.5 and 5.0% waste application rates, but the increase was less at 10.0 than at 5.0%. At 30°C, a marked reduction of NO₃-N release occurred between 16 and 32 weeks of incubation at all rates of waste application, but was more acute at 10.0% than at lower rates. A similar response, but of lesser magnitude, occurred with release of PO₄-P. Variability in release of NO₃-N and PO₄-P was attributable to the composition of tomato waste and to microbiological activity. Release of SO₄-S was a function of waste loading rate and time. Essentially, all Cl was released within the first week of incubation. An appreciable increase of K, slight increase of Ca, minimal increase of Na, and no change of Mg was found in the soil after 32 weeks of incubation. Although tomato waste is acidic, soil pH decreased only at the highest loading rate after 32 weeks of incubation.