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Until the early 1960s, it was generally accepted that fertilisers were essential for immature rubber, whereas for mature rubber, fertiliser applications were given on an insurance basis. The large variability in yield responses was a major contributing factor in the reluctance to use fertilisers on mature rubber.
Subsequently, it was demonstrated that the pattern of responses varied according to soil types and soil nutrient status.

It was also accepted that the limited experimental data could be related to agronomic parameters such as soil, soil and leaf nutrient content, manuring history and existing ground conditions, as such data could then be interpolated to provide discriminatory fertilizer recommendations for mature rubber, which in all cases would give economic benefits.

Thus, from the late 1960s, with the above developments, fertiliser use in mature rubber came to be a regular practice. This entailed the need for refinements on investigation of soil/plant interactions, adequacy of rates of fertilisers used, a better understanding of factors affecting leaf nutrient levels, influence of exploitation systems and stock/scion on nutrient needs of rubber.

Storage of nutrients in the tree

It was known that large reserves of nutrients were stored in trees and indicated the adequacy or otherwise of the then existing fertiliser programme. Subsequently, it was noted that even assuming the same amount of nutrients immobilised as ascertained, the modern clones with much higher yields were being supplied with nutrients far short of their requirements; the extra nutrients being required just to compensate the nutrients drained in latex yield.

However, the nutrients immobilised in some clones are much larger than observed in the earlier study, thus implying that the newer clones are relatively more vigorous than older clones like PB 86. Additionally, the newer clones also drain more nutrients in the latex removed from the tree system. About 70 percent of the nutrients immobilised are locked up in the branches, green twigs and the bark. Though at any one time, the nutrients in these parts are considered as immobilised, these can be considered also as storage reserves.

At the time of onset of annual refoliation, the concentration of nutrients in the bark and wood drops to about 15 — 20 percent and thereafter, it re-stabilised at the initial higher level. The depletion in bark and wood tissue is followed concurrently with high concentration of nutrients in green shoots arid leaves, showing a movement of nutrients from the reserve to actively growing tissue.

Generally, the concentration of respective nutrients in bark or wood tissue is higher in well manured trees, compared to trees inadequately manured. The build-up of nutrients in bark and wood during leaf senescence and depletion at refoliation, confirmed that these tissues act as storage organs. This explains the time lag between fertiliser application and response in yield. Such time lag varies from a few months to over three years. The shorter time interval is normally the time interval to obtain responses to fertilisers in areas with poor nutrition and hence low storage.

In addition to this, a considerable amount of nutrients are also stored in the leaves. These are returned to the soil during the annual refoliation cycle but the rate of release was also relatively slow particularly for nitrogen and phosphorus. Thus, the current year’s supply through leaf fall may only be available in the following year.

Influence on latex concentration and properties

Major nutrients N, P, K and Mg have been shown to improve growth of bark anatomy yield and in some cases, bark renewal. However, their role in latex flow and properties has been generally overlooked. Early investigation showed that applications of ammonium sulphate, rock phosphate, potassium chloride and manganese sulphate increase, respectively the N, P, K and Mg contents of latex. The application of ammonium sulphate in addition to increasing the N content also increased the Mg content and reduced total solids and drc.

On the other hand, rock phosphate was shown to increase the P content and reduce the Mg content. It was also showed that ammonium sulphate increased VFA content and KOH number and thus reduced stability of concentrated latex. Simultaneous applications of rock phosphate reduced this adverse effect. It was thus considered that the adverse effect of ammonium sulphate could be due to the increase in Mg content and the beneficial effect of rock phosphate could be due to the increase in P and reduction in Mg contents of latex with a consequent reduction in the P and Mg ratio.
It was found that application of phosphate improved stability and they indicated that application of potassium also seemed to have an influence on improving stability of concentrated latex, while nitrogen fertiliser on its own tends to reduce stability. The positive effect of the P/K combination was described by them to be due to a more balanced P/Mg ratio in the latex.

It was also found that applications of potassium in addition to increasing K content in latex increased the P and reduced the Mg content. Thus, potassium has a direct influence on the P/Mg ratio. It was suggested that application of K-based fertilisers could be used for narrowing the Mg/P ratio but this could not be substantiated in field trials.

Effect on nutrient balance

Applications of ammonium sulphate generally increased the N, K and Mg contents and the Mg/P ratio in latex. Phosphate applied as rock phosphate, increased P and Ca contents and reduced the Mg/P ratio. The effect of concurrent applications of ammonium sulphate and rock phosphate was intermediate to their individual application.

Potassium applied as potassium chloride generally increased K and P contents but reduced Ca, Mg and consequently the P/Mg ratio. The applications of Mg on the other hand, resulted in increase in Mg and decrease in K content in latex, with the consequent decrease in the K/Mg ratio and an increase in the Mg/P ratio.

Effect on flow

Generally, when the nutrients were applied without causing any imbalance to the other major nutrient content in the leaf tissue, there was an increase in yield. However, where excessive amounts of P or Mg in particular were applied, they reduced yields. Potassium generally had the largest effect on latex flow both in terms of flow rate and yield. Nitrogen while increasing yield and time of flow, did not affect the average rate of flow. Where excessive rates of P or Mg were applied, the flow rates were also reduced.

However, as the cost of fertiliser is generally showing an increasing trend, it is of paramount importance to optimise the use of fertilisers. Such increased efficiency could be brought in by a better understanding of the effect of fertilisers on soil. Further, the use of legumes to fix atmospheric nitrogen and return to the soil has been known to be a method of obtaining nitrogen in an inexpensive manner.

Efficient use of fertilizers

Leaching losses: It is generally an accepted practice that fertilisers should be applied, to the root zone of the crop. For rubber during the initial stages after establishment, the roots are confined to a small circle around the plant .Thereafter the roots extend well into the mid-point of the interrow. The distribution of the active feeder roots in mature trees increases from about 60cm away from the tree to the peak at a point about 300cm from the tree and thereafter this declines. Thus, fertiliser applications have to be confined to this zone.

During the first year of field budding, the fertilisers are applied in a small circle 30 — 40cm in diameter. This amounts to about 270 kg/ha of NPKMg fertiliser. Thus, the effective zone of application is only 0006 ha and the effective rate amounts to 40 tonnes per ha. Thus, one dressing of 170g per tree would amount to 13 tonnes per ha containing about 5 tonnes of ammonium sulphate. A considerable proportion of this fertiliser at these rates could be lost by leaching.

More than twice as much nitrogen and potassium is lost from sandy to sandy loam soils than from clay to clay loam soils, when an average rainfall of about 1cm is experienced. However, if 2 - 5cm of rain per day is experienced, then even on the clay to clay loam soils, up to 50 percent or more of the fertilisers is lost by leaching within 10-15 days of fertiliser application. The split application of fertilisers at greater frequency than hitherto practised becomes of paramount importance.

Timing: In addition, correct timing of application is a pre-requisite. During the early stages of growth, the application of fertilisers should be at closer frequencies and as far as possible, being in relation to the active flushing of leaves which is continuously taking place. In mature rubber, uptake of nitrogen is active at the commencement of refoliation and the uptake diminishes after four to five months.

Slow release fertilisers: Ideally, a slow release fertiliser with controlled release of nutrients would satisfy both these requirements. However, the commercially available slow release fertilisers are either cost prohibitive or are not satisfactory in tropical soils. However, some work has shown that NR latex encapsulated fertilisers have considerable promise.

Conclusions

For maximising productivity of rubber, optimal nutrition is essential. The work reviewed shows that not only the yield is affected by proper nutrition but a balanced nutrition seems to enhance the quality of the latex, thus indicating the possibility for elimination of artificial additives during processing. This has been particularly found to be possible in as far as the PRI of auto-coagulated rubber is concerned and could have tremendous implication in the smallholder sector.

Additionally, more efficient utilisation of fertilisers could result in use of lower amounts of fertilisers or increased returns for the same investment in fertilisers. Continuous cropping with rubber and regular application of fertilisers results in a build-up of nutrients, which can be used for subsequent croppings. The knowledge of such build-up allows for manipulation and adjustment of fertiliser inputs whereby such applications could be eliminated or reduced during certain periods, particularly when prices of the fertilisers are high or prices of rubber are low.


(N. Yogaratnam can be contacted at [email protected])