Suitability of Moist Olive Pomace as Soil Amendment

A laboratory experiment was performedto evaluate the suitability of moist olive pomace(MOP) as soil amendment. Moist olive pomace wasobtained from a new olive-oil industrial processcalled the `two-phases method'. Soil samples weremixed with MOP to approximate a field application of40 t ha^-1 and incubated under aerobic conditionsat 20 °C and 60% of soil water holdingcapacity. To estimate the effect of different loadingrates and N supply on mineralization, 40, 80, 120 and160 t ha^-1 of MOP and 200 ppm of N as(NH₄)₂SO₄ were used. CumulativeCO₂-C evolution, total microbial activity andbiomass-C were monitored during a 60-day period.Results indicate that the CO₂-C evolution fromMOP depends on soil type and is temporarily inhibitedin acidic soils. Evolution of CO₂-C increaseswith incremental addition of MOP, but the percentagesof the added C that were mineralized decreased withincreasing application rates. Mineral N supplementsresult in more efficiency of the mineralizationprocess. Among the kinetic models tested to describethe mineralization dynamics, a first-order exponentialmodel including a constant term provides the best fitto the experimental data. Both amount and activity ofsoil microbial biomass are enhanced by MOP added atthe 40 t ha^-1 rate, at least in the first periodof incubation. At higher rates of MOP addition, aconstant increase of biomass C during incubation isobserved, while the biological activity decreases atthe end of incubation. Following application ofmineral N, both amount and activity of microbialbiomass is enhanced.