VINYL-FILM MULCH - A PRACTICE FOR SWEET-POTATO (IPOMOEA-BATATAS LAM VAR EDULIS MAKINO) CULTIVATION TO REDUCE NITRATE LEACHING

Under conventional cultivation practices in Southern Kyushu, Japan, a considerable amount of nitrate-N is leached from agricultural fields i nto ground water. The leaching processes are accelerated by high perme ability of the volcanic ash soil, high rainfall and high summer temper atures. The present work was instigated to study the possibilities of controlling nitrate-N leaching by regulating the infiltration of rain water in the soil, the source of percolation below the root zone. Nitr ate-N leaching from the root zone of sweet potatoes to deeper soil lay ers was compared in three treatments: maximum soil surface mulching wi th 0.03 mm vinyl-film with holes at ridge tops to supply plants with e nough soil moisture to meet the crop water requirement (maximum vinyl- film mulch, MVM); applications of fertilizer containing a nitrificatio n inhibitor dicyaniamide (DCD); and conventional cultivation practices . NPK compound fertilizer (N8P5.2K16.6) containing the ammonium form o f nitrogen 900 kg.ha-1 and PK compound fertilizer P8.7K16.6 600 kg.ha- 1 were applied in the MVM and conventional plots. DCD-fertilizer (N15P 6.5K12.5), maintaining the same rate of absolute N with adjustment of P and K using fused magnesium phosphate (P8.6) and potassium chloride (K41.5), were applied to the DCD plots. MVM increased surface run-off, resulting in least percolation among the treatments. The highest nitr ate-N concentration, taken as an average for 2 years, in soil-water pe riodically sampled throughout the growing period at 210 cm depth below surface, was found 1.8 ppm in MVM, whereas in DCD and in conventional treatments the same was measured as 8.7 and 5.2 ppm, respectively, in dicating the lowest level of nitrate leaching to be in the MVM treatme nt. The highest availability of mineral nitrogen in the root zone of t he MVM treatment was confirmed by the highest plant-top-uptake of nitr ogen, Leaf Area Index and vine growth. DCD fertilizer did not contribu te to the control nitrate-N leaching. There was no significant differe nce in root yields among the three treatments.