Soil property changes in contour hedgerow systems on sloping land in the Philippines

The impact of contour hedgerow systems on soil sustainability under acidic conditions has been widely criticized. A study was undertaken to determine the effects of management and hedgerow species on soil properties. Cassia s pectabilis (a non-N-fixing tree legume), Gliricidia sepium (an N-fixing tre e legume), Pennisetum purpureum (a forage grass), and Stylosanthes guyanens is (a forage legume) contour barriers were compared with an open field (non -hedgerow treatment) over 1 cowpea and 2 rice seasons. Three types of manag ement viz.: prunings applied + N0P0K0, prunings applied + N20P20K20 and pru nings removed + N20P20K20 were used as subplot treatments. The soils were s trongly acidic (pH 4.5) and classified as clay Orthoxic Palehumult. Cassia performed better than the other species in terms of pruning biomass, N and P contributions over a period of 20 months. There was a combined positive e ffect of pruning biomass and fertilizers on rice and cowpea yields in Penni setum and Gliricidia systems, while a tendency towards a positive effect of pruning biomass on rice was found in the Cassia system. The pruning biomas s and/or fertilizer application did not significantly influence the top soi l organic C, N and available P in the hedgerow systems. Soil bulk density w as significantly reduced by the application of Cassia prunings after 12 mon ths. Organic C, N and P dynamics indicated that in situ pruning biomass was not sufficient to maintain their level in the soil. But the cassia systems with prunings applied + N20P20K20 experienced the lowest degradation in so il organic C (2.1 t ha(-1)) followed by the Gliricidia systems (4.1 t ha(-1 )). The overall results imply that the application of pruning and inorganic fertilizer is imperative to conserve soil resources, and non-N-fixing tree species can exert a significant advantage in biomass and thereby in soil N -recycling under acidic soil.