Standardisation of soil sampling and fertilizer recommendation techniques for coconut gardens

Abstract

An investigation was carried out during 1994-95 to standardise soil sampling technique for coconut and to work out fertilizer recommendation system considering physical/textural nature of soil as well as nutrient levels. Two standing populations of WeT palms grown under good and average management practices (situation A and B respectively) located at Mulamkunnathukavu (Thrissur district) was utilised for the study. From each plot twenty healthy palms were selected for the study. The coconut gardens were separated by a distance of about 1 km and the soil of the site was laterite (Oxisol). Composite soil samples were collected from three depths from the surface and from three radial positions from the bole of the palm. Samples were collected in May, October and January. Leaf samples were collected from the 10th frond and observations on yield and yield attributes were also recorded along with soil samples. Physico-chemical properties of soil, root activity pattern, leaf nutrient content and yield and yield attributes of both situations were compared. Correlation between physical and chemical properties of soil, correlation between soil parameters, leaf nutrient content, yield and yield attributes were also worked out. Both situations showed much variation in physico-chemical properties of soil. Coarser soil fractions were more in situation B while, in situation A finer fractions dominated. High yield situation (situation A) recorded higher values for organic carbon and available K compared with situation B. Depthwise as well as radial distance wise differences were also noticed in the nutrient content. In both

situations soil nutrient content decreased as the sampling depth from the surface as well as radial distance from the bole of the palm increased. A perusal of seasonal variation of nutrients in soil indicated that maximum organic carbon, available P and available K content was attained in the second season (in October) which declines continuously through the third and first season except organic carbon of situation B. Comparison of leaf nutrient status of two situations revealed that palms of situation A recorded higher N, P and K content in their leaves. Corresponding differences were also reflected in the production of nuts. Root fractions collected from different depths have indicated that in both situations maximum root activity was confined to a depth of 0-30 cm. Hence conventional method of fertilizer recommendation based on soil testing now practised in the state is not suitable for coconut which takes into account the nutrient status of the top 15 cm layer only. Comparing the radial distance L, established significant relationship only with N while L2 established significant relation with N and K. Since> 75 per cent of the roots are residing inside the basin, sampling from outside the basin alone (L3) is also misleading. Hence mixed composite sampling (L2) including both inside and outside basin soils is more appropriate. Based on the above observations the most suitable sampling technique for coconut is composite sampling containing both inside and outside basin soils from a depth of 0-30 cm from the surface (L2D2).

In situation A organic carbon manifested maximum relationship to yield of nuts in the second season and potassium content in the third season whereas organic carbon content of the first season was related with yield in high yield situation. This would suggest that time of sampling is also important which differ between yield group and nutrient to be analysed . . Results also indicated that organic carbon content was related to soil fractions in both situations though, the relationship was in opposite direction. In situation A, finer fractions showed positive and coarser fractions showed negative correlations with organic carbon. The reverse was the case in situation B where coarser fraction had positive and finer fraction had manifested negative relations. It has also been noticed that organic carbon content was linked with nut production in both situations. Varying interaction of organic carbon with different fractions of soil on one side simultaneously with its direct bearing on productivity would point out to the necessity of evolving texture linked indices to get reliable information about nutrient availability status of soil. Therefore, some correction factor should be added to soil test values to account the anomalies caused by the coarser soil fraction which is not having a direct role in production and productivity.