Impact of nitrogen and potassium fertiliser application on quality of south Indian CTC teas

The south Indian contribution to the national tea production is about 24% and almost in the form of black tea manufactured by CTC (crush, tear and curl) processes. The profitability of the operation is governed by the quantity as well as quality of tea produced. Though many of the basic relations between quality and chemical composition of plant tissue remain unanswered, there were several attempts to study the effect of changing manurial practices on quality parameters. Polyphenols and their oxidised derivatives (mainly theaflavins and thearubigins) are important chemical constituents in black tea, which are mainly responsible for colour formation during infusion and the unique black tea aroma. The promoting effect of K fertilisers on polyphenols is well known. Since the main aim is to exploit the full potential of vegetative growth of tea, there is no other alternative for nitrogenous fertilisers if commercial levels of productions are to be achieved. However, the data available on combined effect of nitrogen and potassium on quality of made tea is not traceable and deserves a study since NK fertilisers are always applied together in south Indian tea gardens. The metabolic activity of applied nitrogen will vary depending on the synergism/antagonism between the nutrients. In this attempt influence of various ratios/doses of NK2O fertilisers on certain biochemical and quality parameters were studied with special reference to south Indian CTC teas.

The free amino acid and total polyphenol contents of flush shoots were estimated in four harvesting rounds immediately after manuring. The statistically analysed mean data are presented in Table 1. The amino acid content was highest in the treatment where NK2O fertilisers were applied at 1:1 ratio (at the rate of 300 and 450 kg/ha/yr). On perusal of data in Table 1, it was inferred that at the same level of nitrogen, when the potassium dose was increased there was a significant increase in polyphenols and free amino acid contents of flush shoots till the ratio 1:1 is reached beyond which there was a sharp decrease. The increase in amino acid contents could be because of improved nitrogen metabolism induced by nitrate reductase enzymes at optimum N:K2O (1:1) ratio. The treatments supplied with 0, 150, 300 and 450 kg of N/ha/yr with out K2O did not differ in their amino acid content and recorded comparatively lower amount of amino acids. This observation underlines the importance of nutrient K in tea production. The range of polyphenols in clone UPASI-9 under south Indian conditions is around 25-35% of its dry weight. But in this study, value as low as 21.4% was observed. This field trial was initiated in 1994 and continued till the date of sampling and hence, the prolonged exposure of some treatments to either a minimum quantity of fertilisers or no fertilisers would have resulted in the lower values of polyphenols. There is a direct increase of polyphenols to applied potash fertilisers.

The made tea manufactured from tea shoots harvested in four successive plucking rounds after manuring were analysed for their crude fibre content (CFC) and water soluble solids (WSS). The mean data are given in Table 1. The CFC of all the treatments was slightly above the normal level, since crude fibre extractor was not used to remove the crude fibre from black teas. In general the crude fibre content was higher when increased quantity of potash fertiliser was applied. The highest value was recorded in treatment, which received 300 kg of N and 450 kg of K2O fertilisers. The percentage of water-soluble solids was in contrast to that of crude fibre content. However, the teas were not deficient in water soluble solids and they are well above 32 %, the limiting value below which the teas become substandard according to PFA (1954).

Caffeine content was estimated in the sample, analysed statistically and furnished in Table 1. It was inferred that at lower nitrogen levels (N0 and N150) the caffeine content increased with the increase in K2O fertiliser dose. It was observed that when the N dose was stable at 300 kg/ha/yr, caffeine content increased, along with increase in K2O fertiliser. This trend continued until the ratio 1:1 was reached beyond which the increase lacks statistical significance. Also in the absence of K2O, increasing N increased the caffeine content proportionately up to 300 kg/ha beyond which, the caffeine content showed significant decrease. Similar results were recorded in various tea growing countries.

Mean theaflavins (TF) and thearubigins (TR) contents are provided in Table 1, which revealed that TF and TR were higher in the treatment N450:K2O450 followed by N300K2O300. The increase in TF content could be because of the increased synthesis of polyphenols at optimum N:K2O ratio. Because of the increase in amino acid content at 1:1 ratio, the pH of the fermenting dhool might have gone down. The formation of theaflavins was favoured when the pH tends towards 5.0 by virtue of enhanced steady state concentration of simple catechin quinones.

The increase in flavour index FI (Table 1) was observed in higher N:K2O levels, due to the higher concentration of group II compounds that produce desirable aroma. But there was no significant change in group I compounds. Among group II compounds, linalool, methyl salicylate and benzaldehyde, which impart sweet aroma to tea, were the main compounds showing increased concentration. The flavour index was higher in 1:1 ratio while lowest in N450: K2O0 (lower than the control), indicating the need for a balance between N and K fertilisers. Since teas are valued for their aroma, which has direct relationship with FI, application of large quantities of N fertiliser with out balancing with K2O should be avoided. Thus this study clearly depicts that there is an improvement in the quality at N K2O ratio 1:1.
The challenge towards productivity during the past decades got changed towards the production of good quality teas by the turn of the century. Even now there are instances wherein one or two fertiliser applications are being deferred at the cost of yield. Such a practice of continuous withholding of fertilizers could result in the ultimate collapse in the production and deterioration in the quality of the produce. At this situation with no light in the horizon for the revival of the tea industry it has become imperative on the part of the producers to give judicious application of fertilizers in the field, if so, a significant improvement in quality is quite possible.

(Table 1 available on request)


Authors - S. Venkatesan & MNK Ganapathy