LOSS OF NUTRIENTS IN A HIGH YIELDING COCONUT PLANTATION THROUGH REMOVAL OF PLANT MATERIALS FROM THE FIELD

High yielding coconut palms absorb large quantities of nutrients from the soil. A high proportion of these nutrients are removed from the field along with the harvest and other plant components. The objective of this study was to determine the rate of macro and micro nutrient removal from the field by Typica x Typica coconut palms, yielding an average of 17,380 nuts per ha. per year, in order to assess the adequacy of the current fertilizer recommendations, to prevent a serious depletion of the nutrient reserves in the soil.

Harvested nuts, fallen fronds and inflorescence parts were collected for a two-year period from September 1998 to August 2000 from 10 palms located in a "Madampe series" soil at Bandirippuwa Estate. Their nutrient content was determined by chemical analysis of samples collected from harvested nuts, fallen fronds and fallen inflorescence parts. The reserves of plant nutrients in the soil were determined by chemical analysis of representative soil samples taken from the manure circle and the centre of squares at 0-15 cm and 15-100 cm depths.

The amounts of macronutrients lost through the removal of plant components from the site were 116.79 kg N, 14.02 kg P, 245.43 kg K, 40.47 kg Ca, and 33.66 kg Mg per ha. per year. In addition, removal of Na was also observed at the rate of 55.79 kg/ha. Inputs at the recommended rate (4.5 kg of Adult Palm Mixture (APM) and 1.5 kg of dolomite per palm per year) supplied 87.21 kg N, 17.07 kg P, 188.90 kg K, 104.80 kg Ca and 28.59 kg Mg per ha per year (no sodium containing fertilizers were used). The difference between the removal and input should be supplied from reserves of nutrients in the soil. The study showed that except for exchangeable potassium (363 kg/ha), the other macronutrients were present in reasonably high quantities in the experimental site. As the potassium reserves in the soil are low and fertilizer inputs supply only about 77% of the requirement of high yielding coconut (17,380 nuts per ha. per year), soil potassium will deplete rapidly at the current production level. Hence, the depletion should be compensated for either by increasing the 12 quantity of chemical fertilizer or recycling organic products of the palm itself or both. 

The amounts of micronutrients lost were 1.14 kg Fe, 0.63 kg Mn, 0.13 kg Cu, 0.44 kg Zn and 0.26 kg B per ha. per year. The soil reserve of available iron in the site was so high that a significant depletion of Fe would take more than 300 years. However Mn, Cu and Zn depletion may need attention since reserves in the soil were not very high compared to the rate of removal.