Until 1942 cultivation of pepper P. nigrum
L. in Sarawak produced relatively small but regular yields. High demands after 1945 and restricted use of "burnt earth" compelled farmers to abandon the application of this traditional fertilizer. Instead, "fool proof" manufactured fertilizers of mainly organic origin were successfully applied in massive volumes and large yields were obtained. In 1954 and following years a sharp fall in price induced a decline in production, in part owing to much reduced applications of fertilizer. As a result the economic crop cycle of some 16 years was limited to some 1-3 years; the bulk of the total yield was obtained in the first year of production.
An initial survey on leaf symptoms and die-back indicated unbalanced and inadequate mineral nutrition of the crop. The current system induced severe deterioration of plant appearance in the period of monsoon rains, which coincide with berry expansion. During this time no fertilizers are applied as a rule.
A method has been worked out to diagnose the nutritional demands of N, P, K, Ca and Mg by chemical foliar analysis. Major aspects involved in the establishment of a sampling procedure were systematically studied. The effect on chemical concentrations was investigated of the portion of the vine, the presence of fruit, the branch, the age of the leaf, the presence of the petiole, leaf size, leaf thickness, sunshine, different stems on a plant, location of fertilizer dressings and the number of leaves to be sampled per vine. Similarly, the influence of the mode of cleaning of the leaves, of the drying temperature on loss of N and the effect of length of storage on the content of N were studied.
Studies on the error of bulk sampling and the effect of appropriate stratification of the different leaves in the canopy on the reduction of this error showed that suitable division into strata reduced the number of vines to be included in a bulk sample up to 16 times as compared with random sampling, assuming the same degree of precision. Furthermore the inclusion Of 4 appropriately stratified leaves from each of 60-70 vines in each bulk sample obtained from homogeneous areas, would represent chemical concentrations with a precision of 10% of the population mean (P = 0.05) irrespective of physiological condition.
Data on seasonal variation indicated declining levels of N and K within the monsoon; this was accompanied by gradually rising levels of leaf P and leaf Ca. Leaf Mg tended towards constancy. Bivariate ratios showed, on most occasions, a regular relationship with time, but constant values were observed only occasionally.
A sand experiment on deficiencies of nutrients showed characteristic discolourations due to nutrient shortage. Concurring foliar concentrations and ratios associated with full nutrients, partial or complete deficiency of a single element allowed tentative registration of normal, fair, critical and deficient threshold values in the leaves for each element (table 18). Single and multiple deficiencies could be recognized by using an appropriate grouping of concentrations and their ratios. Application of these values to random field data showed a satisfactory power of discrimination. The ratio values gave a good indication of the order of importance in the case of multiple deficiencies.
The influence was studied of dressings of NPK fertilizers on leaf concentrations and ratios. It was observed that increasing dressings of N, P, and K were directly reflected in risising concentrations of leaf N, leaf Pand leaf K, respectively. Simultaneously, N- P "antagonism" and "antagonism" between bases was also operative. Apparently, considered balancing of applications of different fertilizers is essential. The ratios between elements gave some indication of priorities of different dressings. It was also observed that under the influence of the very heavy, early applications leaf concentrations of N, K and Mg fall to deficiency levels. This indicates that the distribution in time also requires adjustment.
The development of physiological exhaustion could be attributed to an inadequate net supply of N, K and Mg to the leaves during translocation processes of nutrients in the period of fruit development. Stability of yield at a high level of production can be maintained by preventing development of nutrient shortages and ensuring fair to normal concentrations of N, P, K, Ca and Mg in the leaves throughout the year. Threshold values, independently obtained from field vines corroborate the tentative values for the normal levels found in the pot experiment. The latter may therefore be considered as fundamentally correct. Rather complicated interactions of "antagonisms" and "synergisms" may become operative if foliar levels fall below these normal levels. The data have also shown convincingly that the values for log N/P can be used as a satisfactory control for abundance of flowers at a specific potential for flowering.
Finally, the agricultural implications of these findings are discussed in Ch. 10. The data allowed a plausible interpretation of crop behaviour and crop performance under the traditional system of cultivation before 1942 and that in the period after 1945. By integrating the leaf data of this work with information concerning factors affecting the supply of nutrients to the plant, it was shown that foliar diagnosis furnished a suitable foundation to devise an appropriate fertilizer policy for pepper.