MSc

Response to fertilizer application is manifested after a time lag in perennial crops like coconut. Continuous use of manures and fertilizers is likely to bring about changes in the soil properties which may influence the crop production in the long run. There had been some apprehensions regarding the deleterious effects of chemical fertilizer since the early days of its use. There is thus the need to study the changes in physical properties in a soil which is continuously fertilized and where a perennial crop like coconut is being grown.
The present investigation has been undertaken to study the effect of long term application of fertilizers on the soil physical properties in coconut garden. The study has been carried out at Coconut Research Station, Balaramapuram, Trivandrum. The samples for the investigation has been drawn from the 33 confounded factorial fertilizer experiment which has been systematically maintained since 1964. From the experimental site core samples and disturbed samples were collected from 0-30 cm depths at a distance of 90-100 cm away from the bole of the palms from two replications. They were analysed for the physical properties like bulkdensity, particle density, porosity water holding capacity, volume expansion, mean weight diameter, hydraulic conductivity, water retention characteristics and texture. In situ observation on soil temperature and soil compaction were also made using soil thermometers and penetrometer respectively. Physical properties of soil in all the treatment combinations were compared with the control. Main effects and interaction effects were studied. All the properties studied were then correlated with the yield characteristics of the palms.
The results obtained in the present study show that the organic matter status of the soil in the experimental site was not much influenced by the long term use of phosphatic and potassic fertilizers, but it was found to be increased with increasing levels of nitrogen. This might have been the cause for the significant increase in the water holding capacity of surface soil at higher levels of nitrogen. Other physical properties of soil like the bulkdensity, particle density, porosity, volume expansion, surface compaction, moisture retention characteristics, hydraulic conductivity, textural properties and soil temperature were found to be not much affected by the continuous use of chemical fertilizers to coconut.
The only parameter, which was found to be highly influenced by the long term use of NPK fertilizers was the structural index, viz. the mean weight diameter. With the increase in phosphorus and potassium levels, an increase in mean weight diameter was observed whereas there was a decrease in mean weight diameter with increasing levels of nitrogen.
Considering the yield attributes, phosphatic and potassic fertilizers had a profound influence. Increasing doses of phosphorus tended to decrease the yield to a considerable extent. Though not significant, nitrogen also showed the same trend. But on the contrary, with increasing levels of potassium, the yield was also found to be increased.
The yield characteristics of the palms and the physical properties of the subsurface soil were found to be related with each other. The treatment combination N2P0K2 (N at 680g, P at Zero and K at 900 g per palm per year) recorded the highest average yield for the year 1992-93. The optimum physical conditions were also observed in plots receiving the same treatment combination. So it can be concluded that the plots with the treatment combination N2P0K2 maintains the optimum ‘physical fertility’ in the experimental site where coconut has been fertilized continuously for more than a quarter of a century.