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| Wednesday, 22 September 2004 |
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Strategies and approaches to fertiliser use in plantations by Dr. N. Yogaratnam Fertiliser use has been considered as an important entity in the cultivation of plantation crops, although it is only to be expected that fertilizer prices and labour wages will increase over time.
Taking rubber plantations as a test case, based on a wide spectrum of the present state of knowledge on rubber nutrition, a thought provoking concept; possible new fertilizer use strategy and approach to meet these challenges, is discussed. Rubber plantation system, unlike tea, coconut and oil palm is a unique crop system yielding a unique crop, latex, which is a cytoplasm harvested from a highly specialised cell system, the laticiferous system, of a permanent organ, the trunk. Experimental evidence from stands well managed during immature phase indicates a general absence of yield response to manuring during the mature phase. Then, the question is, is fertiliser use in mature plantations indispensable? Nutrient bank The fertilizer applications during immature phase are reflected in leaf nutrient concentrations which are found to be higher with higher application levels, within physiological limits. There is simultaneous total nutrient build-up within the tree system, accompanied by increased biomass production i.e. the tree nutrient bank of all nutrients increases in magnitude with growth. The size of this nutrient bank within the rubber tree system can be quite large by the time of canopy closure and tapping and can form a significant proportion of the overall ecosystem nutrient bank. The nutrient bank existing within the mature rubber ecosystem in early years of maturity comprises mainly the nutrients locked up within the mature trees, nutrients within the interow cover system and the associated litter, nutrients in the decomposing rubber leaf litter and branch material arising from self-pruning and in shed reproductive parts including mature seed and fruit components, and the labile nutrients pool existing within the fertile top soil region. Nutrient outflow The nutrient outflow via the crop in rubber is known to be relatively small in comparison to outflow in other crops. This is true for outflow per unit weight of crop as well as for outflow per unit cropped area. The relationship between nutrient outflow via the crop and the tree nutrient bank for various nutrients i.e. nutrient harvest indices, also indicate that outflow is relatively small in rubber compared to the other major perennial tree crops. The tree nutrient bank is not a static system but is expected to be in equilibrium with the overall ecosystem. Besides the gradual process of net canopy thinning with age during the course of each year, the process of wintering or defoliation occurs and the thinning canopy is recycled.Prior to leaf fall, a significant proportion of the mobile nutrients in the senescing leaves (N, P and K) is mobilised and flows back to the storage tissues of green and brown twigs. A computation of the amount of backflow prior to defoliation indicated that approximately 35 kg of nitrogen and 5 kg of potassium per hectare per year are channelled back and retained within the tree system, in comparison to 60 kg of nitrogen and 10 kg of potassium lost in the newly defoliated litters. Such nutrient backflow and retention also occurs in the senescing branches and twigs prior to self-pruning. Nutrient recycling Associated with all these changes is the concomitant recycling of nutrients; nutrients lost from the tree in the shed plant parts subsequently become available again to the tree through decomposition. The tree appears to stringently retrieve the lost nutrients from the decomposing shed parts. Establishment of creeping legume cover such as Calopogonium caeruleum at the on set of planting is likely to contribute as much as 200 to 250 kg/ha of extra nitrogen per hectare into the ecosystem between the third and the fifth year of planting, with another reserve of about 100 kg/ha still in the cover. Within this ecosystem, there is also a continuous but relatively small outflow of mobile nutrients through leaching in the ground-water. The ecosystem is also expected to be replenished with various major nutrients through rainfall viz. via N-fixation by lightning (which can be very considerable) and other microbial agents in the air e.g. calcium and potassium from atmospheric dust. Possible new outlook In situations where adequate or even more than the normal manuring is carried out, in the immature phase, apart from improved growth during the immature phase, yields have also known to be markedly enhanced, at least during the initial phase of tapping. This manuring effect is true for a complete NPKMg schedule. This beneficial influence of immature phase manuring on the performance of mature rubber was reported in Sri Lanka as well as in Malaysia. It was reported that up to 50-60 per cent of the yield could be attributed to the influence of the immature phase manuring. It is therefore clear that the immature phase is more sensitive in terms of response to agronomic inputs such as fertiliser application and related factors. If the young trees are well nurtured nutritionally, then in their mature phase they can be expected to sustain themselves nutritionally without any yield depression at least for some five years. This proposition is supported by the knowledge on the insight into the overall ecosystem in relation to mineral nutrition and also by consideration of the unique nature of the rubber crop and the relatively low harvest index. Additionally, reviewing fertiliser experimentation on rubber, it is obvious that only about 40 to 50 per cent of the trials done on this subject receive scientific publication coverage. In particular those exhibiting positive responses are reported, while experiments which are slow to manifest responses or do not exhibit responses are prematurely terminated and 'cold stored' for reasons known only to the scientist concerned. Therefore, this phenomenon of publications bias in tree crops fertiliser research must be taken into account for a balanced insight into fertiliser responses. Closer scrutiny of all guidelines and recommendations that emerge, by competent scientists and industry personnel is therefore important before the adoption of such technologies. The extent to which experiment conducted at single site can be the basis for islandwide recommendation is being increasingly questioned. From the foregoing considerations, it appears that rubber fertiliser use policies in Sri Lanka demand a new strategy, emphasis to be directed more only to the critical immature phase, to be then followed by a NIL fertiliser use phase after commencement of tapping for a reasonable period during maturity say five years particularly during the economically lean years (unfavourable trading conditions for crop and/or fertilisers), and finally a phase of periodic tree monitoring, at three to five-year intervals by soil and leaf nutrient survey approach, a technique perfected in the late 70s for corrective and cost effective fertilisation as and when, if, required, until 5 to 6 years prior to replanting. During the critical immature phase it is expected that NPKMg fertilisers are applied regularly at the current normal rates or even may be in excess by 45 to 50 per cent of the currently used level, to meet immature growth patterns and tree nutrient needs and appropriate methods, time and frequency of application, are adopted to ensure efficient nutrient uptake, enlarged nutrient retention within the tree system and enhanced vigorous growth. A fertiliser use scheme should be developed to meet this objective, where application rates should be according to perceived growth stages during the immature phase as indicated in box. Adopting the system of fertiliser computation on the basis of the soil and foliar survey during the immature phase will lead to disastrous consequences and therefore should not be proposed. Non-discriminatory use of the discriminatory fertiliser use technique is undesirable, as it goes against the principles based on which the discriminatory site specific fertiliser technique was formulated. Growth stages during immature phase for fertiliser application Month after planting Immature growth pattern and stages 0 Planting hole. 0-6 Planting / establishment phase, sluggish vigour, low nutrient demand. 6-15 Moderate trunk girthing, spindly unbranched growth, moderate nutrient demand. 15-24 Active trunk girthing, branching and canopy development, increased nutrient demand. 24-36 Vigorous trunk girthing, dense canopy development, high nutrient demand. 36-54 Grand growth phase attaining zenith, high nutrient demand. It also appears that the limiting factors for rubber production are not nutritional. Therefore, it is likely that the mature yield and harvest index may be increased by other methods related to exploitation rather than manuring practices. In conclusion, based on the emerging needs of the plantation industry, improvements to the conventional and outdated systems of fertiliser use have been suggested with the singular purpose of enhancing the productivity and profitability of a vibrant and increasingly demanding industry. |
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Uncertain market conditions and weather pattern are also crucial issues
besides inconsistent policies on subsidies and other government
interventional non-monetary incentives. Given the current scenario,
continuation of the current conventional and outdated fertiliser policies
and practices in the plantation sector will be increasingly questioned.
