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SOIL & NUTRITION

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Soil Proflle and Landscape of Tea Gardens of South India

South Indian Tea Soils Tea soils are classified as latosols and extend along the Western Ghats and run in north to south direction parallel and close to the west coast of the peninsula. The south Indian tea districts vary in elevation from 300 to 2500 m above MSL and the annual rainfall varies from 900 to 7500 mm. In geological origin, the soils are mainly derived from gneissic rocks which had undergone considerable weathering as shown by their acidic nature and low calcium content while they are abundant in sesquioxides which results in strong P fixation.

Texture and clay minerals: Soils of High Ranges, the Anamallais, Central and South Travancore are light in texture, coarse and fine sand fractions comprising 62 to 67% of soil. The soils of Wayanad and the Nilgiris are of clay loam with the clay fraction prepondering. Tea soils contain predominantly kaolinite as the clay mineral and the soils have no fixation sites for potassium. Therefore, the leaching loss of potassium is considerable and it increases with the decrease in soil pH. Tea soils of south India contain large amounts of free sesquioxides and hence phosphorus fixation is very strong. Tea soils are basically poor in fertility and productivity depends on the organic matter content which builds the structure and sustains microbial activity. About 50% of the total cation exchange capacity of soils comes from the organic matter fraction. On an average about 150 kg N/ha will be released annually if organic matter is maintained at medium category appropriate to the elevation of the tea area of which 30 to 40% are available to the plants.

pH and K availability: South Indian tea soils have low calcium content and hence they are suitable for tea growing. In soils with pH >5.0, accumulation of calcium interferes with synthesis of carbonaceous materials and uptake of K. In soils with pH <4.0, the H+ and Al3+ ions interfere with K uptake. Nitrogen efficiency is maximum in soils between pH 4.5 and 5.0 and hence liming is recommended to maintain the soil pH around 4.8. Availability of water to the plants will be of longer duration in the heavy soils of Wayanad, the Nilgiris and Karnataka in comparison to lighter soils of other districts.

Kaolinite is the dominant clay mineral, which has a very weak bonding for potassium and low cation exchange capacity. Therefore, the leaching loss of potassium is considerable and it increases with decrease in pH of soils. Because of the base deficiency created by low CEC of soils in comparison to root CEC of tea plant, absence of fixation and decreased binding energies due to acidity, tea roots are placed in an unfavourable environment as far as cationic nutrition (ammonium and potassium in particular) is concerned. Hence, there is a need for regular and frequent application of nitrogen and potassium to even out the above effects. Thus splitting the application can increase the agronomic efficiency of nitrogen and potassium. As the tea soils have no fixation sites for potassium and the clay fraction is of non-expanding type, there is good percolation of water. The broadcasted fertiliser is therefore leached down to the root zone with ease. There is also a risk of leaching down of nitrogen and potassium below the root zone if the nitrogen and potassic fertilisers are placed in the subsoil. Hence nitrogenous and potassic fertilisers are recommended to be applied by broadcast method.

South Indian tea soils contain low calcium content which is preferable for tea growth. As kaolinite and sesquioxide colloids present in tea soils are of non-expanding type, the retentivity of moisture of the colloids are comparatively lower. The cation exchange capacity on an average varies between 4 and 10 meq% for the Anamallais, Central Travancore and High Range and 8 and 15 meq% for Wayanad,, Nilgiri- Wayanad, and Nilgiris. Based on the texture, the infiltration rate is slow (5-12.5 mm/hr) for Wayanad,, Nilgiri- Wayanad, and Nilgiris and moderate (15-25 mm/hr) for the Anamallais, Central Travancore and High Range. Major problem of tea soils of south India is severe soil erosion due to intense rain and steep slopes. Contour cultivation and bench terracing are found effective techniques against soil erosion.

Nutrition

An outstanding feature of life is the capability of living cells to take up substances from the environment and use them for synthesis of their own cellular components or as an energy source. The supply and absorption of chemical compounds needed for growth and metabolism is termed as nutrition and the chemical compounds required by an organism are termed as nutrients. Like any other crop, tea also requires N, P, K, S, Ca, Mg, Zn, Mn, B and Cl.

NITROGEN: Since nitrogen is responsible for vegetative growth, managing its availability in tea soils directly reflects in crop yield. Depending on the elevation, the organic matter is classified into three categories (Table 1). The N dose is decided based on the OM status and anticipated yield (Table 2). About 20% of annual N is supplied in the form of ammonium sulphate, 15% as calcium ammonium nitrate and the remaining 65% as urea. Nitrogen deficiency is identified by retarded shoot growth and yellowing of younger leaves.

Table 1. Classification of OM status

Table 2. Recommendation on N rate (kg/ha/yr)

POTASSIUM Apart from serving as a nutrient, it plays a vital role in the nitrogen metabolism. Muriate of potash (MOP) is the only potassium source used in south Indian tea fields. The K2O dose is decided on the basis of the source of nitrogen except for pruned year manuring. In the pruned year, 1:2 ratio of N:K2O manuring is followed at and below 45 cm pruning height. The ratio is 2:3 for < 60 cm pruning height and 1:1 for >60 cm height. In other years of pruning, the ratio is 4:3 when the source of nitrogen is urea (or) CAN. The ratio is 2:1 when ammonium sulphate is used as source of nitrogen. Potassium deficiency does not immediately result in visible symptoms. There is only reduction in growth rate at first followed by chlorosis and necrosis. Scorching due to chlorosis and necrosis occurring at the tip of the mature tea leaves and extends along the margin. Dominant purple/brown colour and reduced leaf size are the common symptoms.

PHOSPHORUS Results of long-term study indicate that all forms of P like water-soluble, citrate soluble or acid soluble are equally efficient. Rock phosphate is recommended for reasons of economy. Addition of citric acid along with rock phosphate has been found to increase the phosphorous releasing capacity. When the yield level is below 3,500 kg per hectare, it is suggested to apply rock phosphate by broadcasting at 30 kg P2O5 / ha / yr in two splits at six months interval along with 300 g citric acid / split application. When the yield level is above 3,500 kg per hectare rock phosphate is recommended at 40 kg P2O5 / ha / yr in two splits with 500 g citric acid / split application.

CALCIUM AND MAGNESIUM Dolomitic lime application once or twice in a cycle is a regular cultural operation for correcting soil pH, which takes care of Ca and Mg nutrition. However because of antagonistic nature of Mg with K, the fields supplied with higher K limits the availability of Mg. To overcome this antagonism, foliar application of 1.0% magnesium sulphate is recommended along with micronutrient mixture. Four to five rounds of foliar spray to deliver 8-10 kg of magnesium sulphate was found effective. Since Mg2+ is a mobile ion in the plant systems, the deficiency always begins in the older leaves. Yellowing of mature leaves, inter veinal chlorosis and premature leaf fall are the typical symptoms of Mg deficiency in tea. Magnesium deficiency is noticed in some of the south Indian tea fields. Hence in addition to foliar application, soil application of MgSO4 is also recommended as indicated in table.3.

Table 3. Recommendation on Soil application of Magnesium Sulphate

SULPHUR The sulphur content of tea shoots ranges between 0.08 and 0.30 % on dry matter basis and limits the productivity of tea. Its deficiency is described as “Tea Yellows” which is the “Net veining” in the younger leaves where the leaf blade takes a striking yellow colour and the veins down to the finest branching standout predominantly dark green. Because of recycling, sulphur deficiency is not commonly seen in south India. However, application of 20% of annual nitrogen in the form of ammonium sulphate will take care of S requirements of tea plants.

MICRONUTRIENTS Zinc deficiency is common in south Indian tea fields, since the applied /soil available Zn accumulates in the collar region and the mobility to the plant is limited. Zinc deficiency is identified by very short internodes, chlorotic and small sickle shaped leaves and stunted auxiliary shoots.

About 8 to 12 kg of zinc sulphate/ha/yr is given as foliar in four to six rounds. Application of 1% zinc sulphate in 200 litre of water is recommended along with 500 ppm Mn, 100 ppm of B and 10 ppm of NAA in each round.