EFFECTS OF FRESH AND HUMIFIED WOOD RESIDUES ON POTATO YIELDS AND N AVAILABILITY IN A SANDY SOIL

Wood residues and soil productivity. Intensive potato cropping has been reported as a major cause of degradation of sandy soils. Wood residues from tree clipping applied to soils can substitute for crop residues to maintain adequate organic matter levels and crop productivity. However, this ligneous material presents a C:N ratio varying from 50 to 175 and, when applied in large quantities, may induce N immobilization at the expense of the crop. The objectives of this study were to quantify the importance of this N immobilization following addition of residues and its effect on yield and quality of potato crops. Fresh or humified residues (50 t ha-t) were applied to the soil and incorporated. The third treatment received no residue. Each treatment was subdivided into subtreatments which received either 0, 150, 200 or 250 kg N ha-1, P, K, Mg and s were applied at the same rate on all treatments. Following the application of wood residues in 1987, 46 kg N ha-1 were immobilized in amended plotsl. N recovery from fertilizer was 51%. To obtain comparable yields of similar quality to those measured on unamended plots, an additional 1.9 kg N t-1 residues added or 100 kg N ha-1 were required. In 1988, N immobilization was considerably reduced; yield and quality of potato tubers in plots that received wood residues the previous year were comparable to those of the unamended plots with appropriate fertilization. Water retention was significantly improved in plots that received wood residues. Partial decomposition of the residues for 1 yr prior to application on the soil did not significantly improve plant growth compared to fresh residues.