The Effects of Pineapple Residue ("Trash") on N Mineralization and Early Growth of Pineapple

Although limited studies by the Pineapple Research Institute of Hawaii indicated that mineralized Nin the soil during a 3-year pineapple cycle represented a significant amount of available N to the crop, the effects of crop residue management on available N in pineapple soils have not been clearly determined. The studies presented here were undertaken to evaluate the effects of incorporated pineapple plant residue on soil N mineralization and on early growth of pineapple with different applications of NH4N03. Soil N mineralization during incubation in the laboratory was studied for four different pineapple soils from Central Oahu, Hawaii. The soils were incubated with and without 1.0% residue containing 1.0% N. Mineralization was linear with time between 30 to 210 days of incubation in those soils without residue treatment. After 30 days of incubation, soils treated with 1.0% residue had a deficit of 9 to 61 ppm mineral N relative to untreated soil samples. Following the initial residue-induced N immobilization period, an increased rate of mineralization compensated for the immobilized N. Pineapple was grown in the glasshouse in two experiments of identical design for 4 and 10 months. The same four soils that were used in the incubation experiment were prepared for planting with 0.0 and 1.0% residue and O and 100 ppm N applied to the soil as NH4N03. The plants were grown with and without foliar applied NH4N03. All treatments were superimposed in a complete factorial design. During the 4-month interval after planting, plant growth appeared to be more closely related to the structure, moisture-holding characteristics, and base status of the soils than to the N regime. While residue incorporation significantly reduced the amount of available soil N during this initial 4-month interval, it significantly improved the soil-plant moisture status and resulted in significant increases in plant dry weights. For plants harvested 10 months after planting, both the initial levels of soil N03-N, ranging from 15 to 105 ppm, and the amount of N applied to the soil or the leaves were major determinants of plant N uptake and plant growth. Residue incorporation reduced the final N uptake from the soil at 10 months, but by small amounts and only for soils having 105 ppm initial N03-N or 100 ppm applied soil N. There was no correlation between N uptake in the glasshouse and N mineralized in the laboratory. In both the 4- and 10-month glasshouse experiments the application of NH4N03 to the soil tended to be superior to its application to the leaves as far as plant dry weights were concerned. There was no evidence in these studies that the removal or incorporation of pineapple residue has any direct effect on the N regime during a pineapple cropping cycle in Hawaii