Chemical, organic sources to cater to nutrient need and sustain soil health in rubber sector
The Sri Lankan rubber industry is well over 135 years old. Most of the current cultivations in traditional rubber growing areas are about fourth generation rubber. Hence, the soils in rubber growing lands are degraded in all physical, chemical and biological properties. This can be attributed to disturbing of soil during uprooting of the old stand for replanting, poor adoption of soil and moisture conservation practices and extreme weather conditions that we experience from the recent past. Under such soil conditions the rubber plants respond positively to chemical fertiliser indicating that soils are deficient in all the major plant nutrients.
The Rubber Research Institute of Sri Lanka (RRI) has scientifically categorised soils in rubber growing areas into different groups on the basis of nutrient availability. Based on the plant requirement and the nutrient availability in soils fertiliser mixtures are developed with ideal nutrient ratios, i.e., nitrogen, phosphorous, potassium and magnesium. Immature rubber cultivations of both plantations and smallholders are fertilised on this basis. Another methodology adopted by the RRI is to perform a leaf analysis on mature rubber cultivations and give a site- specific fertiliser recommendation on a need basis.
Soil and moisture conservation
RRI strongly recommends soil and moisture conservation practices to the rubber growers to at least conserve the remaining much depleted top soil layer by preventing further losses. There are number of methods recommended and a re cat e g o r i z ed under - Agricultural (timing of agricultural activities, contour planting), Biological (legume ground covers, hedge rows) and Mechanical ( drains and terracing) methods. The correct adoption of these practices will sustain the soil health enabling a higher efficiency in the usage of chemical fertiliser. Among others some key indicators of soil health will be soil carbon (C) levels and cation exchange capacities.
These soil properties are responsible for preventing leaching of nutrients. Soil pH too is an important parameter.
Nutrient Use Efficiency (NUE)
Specific recommendations are given by the RRI in order to enhance nutrient use efficiency (NUE) when chemical fertiliser is used. In addition to contribution accrued from soil conservation practices, guidelines for frequency of fertiliser application, enhancing organic matter content in soil, timing and placement of fertiliser application also contributes to NUE. Increase in NUE take place through minimising losses and enhancing uptake. Nutrient losses mainly occur through erosion, leaching and volatilization whereas creating conducive conditions for root proliferation and root activity will enhance nutrient uptake.
During placement fertiliser soil has to be sufficiently moist for the nutrients to be made available to the plants. Fertiliser has to be placed into around 15 cm deep pits made using mamoty forks at four or more points around the base of the plant where the feeder root density is believed to be highest. Placing the fertiliser 15 cm deep would prevent loss of the nitrogen through volatilization.
Technology adoption
Though the RRI recommends a number of important recommendations to sustain the soil health and improve NUE in rubber plantations, their adoption levels are wanting. This together with high intensity rainfall the soil C levels are much lower than the requirement, i.e., less than 1 percent, with very little or no top soil. This scenario is especially true in rubber cultivations in steep terrains. Therefore, when chemical fertiliser application is done, its use efficiency is rather low. It’s likely that nutrients are lost through surface run off and also leaching. Further, nitrogen losses through volatilization are possible due to inadequacies in fertiliser placement method. Above scenarios would have contributed to current poor performance of the crop and highlights the importance of soil and moisture conservation and enhancing NUE. Drains and terracing are vital during the mature phase of the crop since it is apparent that ground covers are generally weak at this stage and are not adequate for protecting rubber soils.
Immediate conversion to organic fertiliser
In this context of poor soil health, the government policy of focusing on the use of organic fertiliser is a step taken in the right direction. The recent gazette notification appointing a Presidential Task Force states among many, development of a front- loaded action plan to transform the agricultural sector to an organic-based regime has triggered much attention and dialogue among all stakeholders. This is a healthy situation that could result in scientifically valid strategies to improve soil health whilst maintaining current crop performance and in anticipation of subsequent crop improvements.
However, immediate and complete conversion of plantation crops to organic fertiliser would not be scientific and logistically possible. We should not forget that current soils are poor in nutrient availability, organic matter and microflora. Also, the nutrient levels in organic fertilisers available to the industry right now are very low, maximum 2 percent nitrogen, compelling to use 20 to 25 times the quantity of chemical fertiliser as organic fertiliser. We are not sure about the nutrient use efficiency when organic fertiliser is used when compared to around 30 percent with the chemical fertiliser. If it is proven that the nutrient use efficiency with organic fertiliser is at a higher level, then the quantity of compost to match the nutrient taken up by the plant through chemical fertiliser will be lower than the 20 to 25 times stated above.
In this context immediate 100 percent conversion to organic fertiliser would lead to further depletion of nutrients available to plants due to lack of adequate quantities of quality compost. The outcome would be impairing of crop growth and yield leading to economic losses. National production levels will come down negatively impacting downstream activities of the industry and losing foreign markets established over a long period of time to other competing countries.
Way forward to organic fertiliser
A gradual transition up to most economic organic to chemical ratios could be a more practical as well as a more scientific approach. Making adoption of soil and moisture conservation and NUE practices mandatory to qualify the growers for an entitlement of subsidised organic fertiliser would be a step in right direction to compel the growers to sustain soil health for the benefit of future generations. During the initial phase of the transition to organic fertiliser around 25 percent of the RRI recommended nutrient requirement to the plant may be given using quality compost whilst the balance could be given through chemical fertiliser. Exact quantities of chemical fertiliser will need to be given to the growers to prevent using beyond recommendations driven by subsidised chemical fertiliser prices. Removal of subsidy on chemical fertiliser will also be a good strategy to compel it to be used only in recommended quantities.
In a chemical organic hybrid nutrient management programme if we assume that the N content of organic fertiliser is 1.5 percent and the use efficiencies of chemical and organic fertilisers are 30 and 50 percent respectively, the equivalent compost requirement will be around five times the chemical fertiliser. It would mean that for a plant recommended to be given 1,000 g of chemical fertiliser the hybrid requirement will be 750 percent of chemical fertiliser and 1,250g of organic fertiliser. It should be emphasised that with the organic fertiliser in the soil, the NUE of the chemical fertiliser component too will be enhanced. Based on the crop performance with the initial phase of the transition, changes will need to be done to the chemical fertiliser to organic manure ratio. The challenge however is the sourcing of the organic fertiliser requirement and also to produce organic fertiliser to match the nutrient ratio required by the plant, e. g., 12: 14: 14. The RRI will need to identify organic sources for this purpose.
With the use of organic fertiliser, the NUE could be further increased by modifying the methodology of placement. With a chemical organic hybrid system of nutrient supply to rubber cultivations placing organic fertiliser into suitably cut and placed pits around 4 to 6 weeks prior to chemical fertiliser application will encourage feeder root growth in to them. Application of the chemical fertiliser component to these pits containing new and active feeder roots is likely to further increase NUE. During phase one of the transition, the chemical fertiliser need for the sector could be reduced by around 25 percent with low probability of negative impacts of crop performance whilst laying the foundation for improving soil health.