THE EFFECT OF ROCK FRAGMENTS ON WHEAT BIOMASS PRODUCTION UNDER HIGHLYVARIABLE MOISTURE CONDITIONS IN MEDITERRANEAN ENVIRONMENTS

The effects of soil properties that affect water availability to plant s such as soil depth, rock fragments, parent material and degree of er osion on biomass production of rainfed wheat were studied on hilly soi ls under different weather conditions. Total above ground biomass prod uction (TAGBP) of wheat was measured along catenas of marl and shale-s andstone formations in two successive growing periods. During the firs t growing period which was extremely dry (total amount of rainfall R = 95 mm versus 370 mm in an average year), the soils on marl, free of r ock fragments, produced an average TAGBP of 1100 kg ha-1 despite their great soil depth, whereas on shoulders and footslopes of the stony so ils on shale-sandstone formations TAGBP values of 4620 and 11,100 kg h a-1 were measured, respectively. The opposite occurred the following g rowing period which was exceptionally wet (R = 663 mm). Soils on marls were more productive due to the higher fertility status than the soil s on shale-sandstone formations. Rock fragments, more responsible for the lower exploitable soil volume and the generally lower biomass prod uction under wet climatic conditions, play an important role on soil w ater conservation under dry climatic conditions.