Soil fertility in contour hedgerow systems on sloping oxisols in Mindanao,Philippines

Although contour hedgerows are widely advocated to sustain food crop produc tion in cereal-based farming systems in the tropics, little is known about the nutrient dynamics of these systems or the spatial gradients in soil fer tility that may develop in the terraces that evolve behind the vegetative b arriers. We studied the effects on soil chemical properties of four contour hedgerow systems: double rows of a tree legume Gliricidia sepium (Jacq.) ( G); a row of G, sepium and a row of a native pasture grass Paspalum conjuga tum (GPas); a row of G, sepium and a row of a productive fodder grass Penis etum purpureum (napier grass) (GPen); and a double row of P. purpureum (Pen ). An open-field control (C) without hedgerows was included. The research w as conducted in Mindanao, the Philippines on two Oxisols (Ferralsols) with slopes ranging from 20% to 30%; average alley width was 4.7 m and the avera ge hedgerow width was 0.8 m. The prunings of Gliricidia were mulched on the alley (food-crop planting) area, the clippings of Penisetum were removed f rom the plots, and no pruning was done for Paspalum. Crop residues were dis tributed uniformly on the alley area as mulch. Each of the two crops in the rotation, rice (Oryza sativa L.) and corn (Zea mays L.), received 20 kg ha (-1) each of P and K and 60 kg ha(-1) of N. After 4 years of experimentatio n exchangeable Mg was about 40% lower in the Pen treatment compared to that in the C treatment. There was a tendency for the Pen treatment to have the lowest exchangeable K, Ca, and Mg and Bray-2 extractable P Exchangeable Ca and Bray-2 P increased while exchangeable AI decreased linearly with a dec rease in soil elevation in the alleyways in the hedgerow treatments, althou gh these gradients were not always significant at the p = 0.05 level. Excha ngeable K decreased linearly with the decrease in the soil elevation in the Pen treatment (p = 0.12). The use of Penisetum grass as hedgerow crop and at the same time for supplying fodder, mined the soil nutrients indicating that the system is unsustainable unless accompanied by fertilization or man uring. (C) 1999 Published by Elsevier Science B.V. All rights reserved.