As the Ministry of Plantation Industries is promoting extension of rubber growing in the dry zone, particularly in the North and East of the country, it is important to discuss the subject of land evaluation and selection for rubber.
Mere selection of few trees for experiments and say that the entire North and East are suitable for rubber is not acceptable. People in those regions cannot afford to waste their valuable resources.
Climate forms the first major part of any land evaluation exercise and therefore the evaluation of climate should be used in combination with the evaluation of the soils and landscape features in the selection of land for any agricultural purposes.
Rubber (Hevea brasiliensis), a native of Amazon tropical forests, can grow in a wide range of climate and soils. Nevertheless, in order to achieve the best possible results the land is selected for, planting should conform to acceptable standards.
Physiography
Most of the world’s commercially-grown rubber occurs within 10” of the equator at altitudes of less than 500m.
With increasing distances to the North and South of the 10° latitude, prospects for successful rubber cultivation diminish. The Sri Lanka rubber plantations lie within 7° of the equator and is situated between 80°.09’ and 81°.04’ E longitudes and 6°.50’ and 7°.38’ N latitudes.
Rubber is normally grown in Sri Lanka on land varying from flat to very steep. Between these two extremes, most of the rubber are grown in Sri Lanka on conditions ranging from undulating to moderately steep land.
Rubber grows and yields reasonably well on these steep and usually stony slopes. Nevertheless, the period of immaturity can be shortened and yield capacity enhanced by providing contour terraces or individual tree platforms with linking paths. There are very few drainage problems on rubber plantations in Sri Lanka
Climate
There are three major climatic zones in Sri Lanka, viz. wet, dry and intermediate zones. Rubber is grown in almost the entire wet zone and certain regions of the intermediate zone. Climatic factors of importance for the successful establishment of rubber are rainfall, temperature, evaporation, relative humidity and wind.
Rainfall
The ideal annual rainfall for rubber should fall within the range of 1650 mm -3000 mm and be reasonably uniformly distributed throughout the year. Both foliage and panel diseases are favoured by high rainfall and growth and yield tend to be depressed if rainfall is low. The time required for low rainfall to influence tree growth rate and yield performance depends on the soil moisture retention properties, this may be less than one month in free draining sandy soils and upto two months in well structured clay soils which permit satisfactory root development to depth.
Although precise limits have not been established, it is generally accepted that tree performance will be severely affected if rainfall over a six month period is less than 500 mm, especially when it is not uniformly distributed throughout that period.
The distribution of rainfall also influences the quantity and quality of latex harvested by interfering with tapping operations and by contact with latex. Ideally, rainfall should occur in the late evening and should cease before 03.00 hours to permit taping from
dawn, trees yielding best during the cool of the morning. Unless special guards are provided, heavy rain after tapping will reduce total crop by preventing completion of tapping and by displacement of latex from cups. The tapping of wet trees is known to spread bark rot or black stripe, a phytophthora disease.
Hence, the main climatic factor affecting rubber cultivation in Sri Lanka is incidence of rainfall, with the wettest rubber growing districts receiving significantly more rain than the generally accepted annual 1650 – 3000 mm and two districts receiving less.
The wetter districts can benefit from both the south-west and the North – East monsoons whilst the two drier districts have to rely on the North – East monsoon, which can be irregular.
Temperature and altitude
The temperature conditions in different physiographic regions do not show much variation. The annual average temperature in the plantation surface of the lower peneplain is very similar in value, approximately 27.8°. In the transitional area, somewhat cooler conditions are found; with temperature lower by about one or two degree Celsius. The mean annual temperature in the wet zone is about 28°C } 2°C.
The temperature ranges from 20°C – 25°C in the night to 30°C – 35°C during the day in the months of December – February.
The ideal mean annual temperature for rubber should be within the range 23°C – 28°C and temperatures should not fall below 20°C for more than few weeks.
The temperature decreases at about 0.6°C per 100 m in height. In such conditions, Hevea will grow most rapidly below 200 m, and trees require approximately 3 -6 months longer to reach tappable size with each rise of 200 m above sea level, so that plantings above 600 m are not normally advisable.
Moreover, at lower temperatures potentially damaging diseases, such as phytophthora secondary leaf fall and oidium leaf disease, are more common. Very high temperatures, in excess of 30°C over a prolonged period, also adversely affect tree performance by increasing evapotranspiration to the extent that physiological processes are impaired.
Relative humidity
The relative humidity ranges are generally similar except in the high elevation area with the afternoon values being generally less than the morning values.
High relative humidity favour crop growth by allowing the crops to absorb moisture and also reduce the evapotranspiration and consumptive use of water. However, high incidence oi diseases can be a problem for rubber production under high humidity conditions.
Sunshine
The duration and intensity of sunshine should have a significant influence on latex sucrose levels. An increase in sunshine duration towards the end of the rainy season is often associated with an increase in.latex production. Also, lower latex
production during rainy season can be attributed to reduced sunshine hours. High radiation and its long duration from December to April cause scorching of bark in young rubber plants. The optimum sunshine requirement is about 2000 hours, at the rale of six hours per day in all months.
Wind
Rubber trees are susceptible to damage by strong winds and Clones differ in their susceptibility.
The most wind prone clones may suffer severe damage in gales
of over 40 knots, but even the most wind resistant clones suffer severe damage in windstorms of force exceeding 10 on the Beauport scale (upto 56 knots). The extent of wind damage depends on the timing of winds; trees devoid of foliage suffer least damage whilst trees bearing dense canopies of moisture laden foliage shortly following refoliation are at greatest risk.
Recognizing that climate forms the first major part of any land evaluation exercise, method to assess the suitability of climate should be followed.
The evaluation of climate should be used in combination with the evaluation of the soils and landscape features.
Soil type
Many soils of different origin as well as morphological characteristics can support a viable rubber plantation. The important criteria for selection are soil pH, soil depth, soil structure, soil water holding capacity, its permeability and drainage.
In Sri Lanka, those soils that satisfy the primary requirement of an acid soil fall into three major soil groups. The largest of these extents are in red yellow podzolic soils, followed by reddish brown latosolics soils and then the immature brown loams.
A land suitability evaluation system for rubber cultivation is made up of a framework of soil and physiographic features which are considered to be desirable for optimal growth of rubber.
On the other hand, the poorer soils have properties which limit the optimal growth and performance of rubber and these limitations are graded as minor, moderate or serious in their degree of limiting crop performance.