Impact of nitrogen and potassium fertiliser application
on quality 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). 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 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 ,
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.
