DISCRIMINATRORY FERTILIZER RECOMMENDATION
Fertilizer recommendation based on soil and leaf analysis ensures site-specific fertilizer application for rubber and is the most ideal approach. It is desirable to assess the soil nutrient status before undertaking planting in nurseries or in main field, especially when the area is outside the rubber growing tract. Fertilizer recommendations based on initial soil analysis may be followed in the main field during the first four years, if the growth of plants is satisfactory. Discriminatory fertilizer usage based on soil and leaf analysis can be practised from fifth year onwards. In nurseries, the soil should be analysed once in three years.
Method of soil sampling
The samples collected should be a true representative of the area sampled. Collect samples two to three months after manuring. If there is uniformity in the nature of soil, lie of the land, manurial history, age of the rubber tree and growth of rubber and cover crop, one composite sample of soil and leaf would suffice for an area up to 20 ha. If soil and leaf samples are simultaneously collected, the suitable period is from August to October. But if soil sample alone is collected, the period between November and March is also suitable. Take composite soil samples at two depths, 0-30 and 30-60 cm.
Select at random 5 to 15 spots (depending on the total area) and dig 60 cm deep pits at these spots. As it is necessary to ascertain the effect of past manuring on the fertility of the soil, locate pits at the site of past manure application. Do not sample road margins, labour line sites, cattle sheds, areas recently fertilised, marshy spots, or other non-representative locations. After removing the surface litter and mulch, cut a thin vertical section of soil from top to a depth of 30 cm using a sharp edged tool such as chisel. Pool all the samples of 0-30 cm depth from the different pits and mix well. If the size of the composite sample is large, reduce by quartering. For this, spread the well-mixed soil into a thin layered square on polythene sheet or newspaper. Divide the square into four equal squares and discard the soil in the diagonally opposite squares. Repeat thisp until about 500 g sample of soil is obtained. Prepare composite sample from 30-60 cm depth also in similar manner. Dry the samples under shade and pack them in clean cloth bags and never in manure-contaminated bags. Label each sample giving details of block sampled, depth of sampling and date of collection, and put the label in the bag. (Write the label with pencil and never in ink).
Method of leaf sampling
Leaf samples are collected during August to October period at the age of 6-10 months. Depending on the extent of area, select 10 (up to 5 ha) to 30 (above 20 ha) trees at random. For areas between 5 and 20 ha, select proportionate number of trees. In the case of branched immature trees and trees under tapping, collect four basal leaves from the terminal whorl of lower branches in shade from cach of the selected trees. Four basal leaves from spur leaves (small off- shoots with only one whorl from the trunk or main branches) are also suitable for sampling mature rubber. Branches with new flushes and leaves infected by diseases are unsuitable for sampling. Leaves formed during the onset of south -west monsoon are also not mature enough for sampling. Do not select trees affected by tapping panel dryness or root disease. In the case of unbranched young plants, select plants without new flushes, and collect four basal leaves from the topmost whorl. If 30 trees are selected, collect only the middle leaflet from each leaf. For 15 trees, collect the two leaflets on either side and 10 trees, collect all the three leaflets, so that about 120 leaflets would be available in one composite sample. Place the leaves between sheets of newspaper, and label each composite sample. Send the samples of soil and leaf to the Director, Rubber Research Institute of India, Kottayam-9, Kerala or any of the regional soil testing laboratories of the Board as early as possible. If it is not possible to deliver the leaf samples within 24 hours after collection, the samples may be dried by pressing with an electric iron heated to the temperature used for pressing cotton clothes. Along with the sample, also send the case history of the field represented by each sample.
