PhD

Among the tropical tuber crops, cassava is a faithful friend to more than 500 million
people world wide who depend on its starchy root for food, feed and cash. The highest
biological efficiency coupled with drought tolerance and ecological adaptation to soils of
marginal fertility makes this crop important in the food economy. It is used as an animal feed
and raw material for industry particularly starch production other than human consumption.
Cassava though will grow on relatively infertile soils which are unsuitable for other crops, it
responds well to fertilization. With selected HYV and proper fertilization yields of25 to30 t ha! are
common and this can exceed 50 t ha:' with appropriate soil test based balanced fertilization
and manuring.
With a view to realize the maximum yield potential in cassava by optimizing the
nutritional status of the soil, a series of experiments were undertaken under the present
study entitled "Yield maximization in cassava (Manihot esculenta Crantz) through Hunter's
'systematic approach' in fertilizer use". This involved preliminary soil analysis, sorption study
and green house nutrient survey to demarcate the nutrient constraints and also to optimize the
soil nutrient status. The critical level of P and K for the particular soil series was also done
since the basic concept of this approach is based on the critical level of nutrients in soil.
Pot culture studies were undertaken to find out the critical level of P and K of
Vellayani soil series Typic Kandiustult using maize as indicator crop. The level was
fixed as 8.20 and 8.23 J.l g KI P and 43.5 and 45.5 J.l g g' K by the scatter diagram model
and discontinuous regression model respectively. The preliminary analy~is of the soil of the
experimental site indicated the overall fertility status of the soil as "poor" with acidic soil
reaction, low availaibility ofN and K, medium in P and organic carbon, very low in Ca and
Mg but satisfactory in micronutrient levels.

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Sorption study was performed for P,K,Cu,Zn,Mn,S and B to find out if any ofthe
applied plant nutrients fixed or complexed abnormally with the soil. From the sorption curve
fitted Cu, Zn and Mn were seen as nonlimiting where as P,K,S and B as limiting based on the
results of critical levels and preliminary soil analysis. The optimum quantities oflimiting nutrients
were calculated from these basic experiments as 136 J.1. g ml:' P, 0.338 meq 100 ml:' K,
20 J.1. g rn1~1 S and 6.025 J.1. g ml' B.
The green house or screen house nutrient survey was conducted to arrive at the
optimum treatment and. also to check whether the optimum fixed is the real optimum
treatment. For formulating the optimum treatment, N was applied at the rate of 50 J.1. g ml:' , P,
K and S and B were added at the above concentration. Ca and Mg were applied @
0.270g CaC03 and 0.092 g MgC03 to maintain Mg/K and CalMg ratio in the specified
range as 1.6to 14 and 1.2 to 6.2. Fe was not limiting, but Mo @2 J.1. g ml' was also included
since the pH is 4.8.
Hence there were altogether 14 treatments with optimium (1) comprising of
nutrients viz., N,P,K,Ca,Mg,S,B and Mo, these elements one by one were excluded
from the optimum (8) and the non limiting elements Fe,Mn,Cu and Zn were included
one by one to the optimum (4) and an absolute control (1). This experiment conducted
with Sorghum (CO-5)as indicator crop and the relative yield calculated from the biomass
yield recorded at 45th day of planting revealed N ,P,K and Ca only as limiting for the particular
soil.
These nutrients were selected' for the field experiment with cassava variety Sree
Vijaya, a short duration line released from Central Tuber Crops Research Institute,
Thiruvananthapurarn. From the earlier three basic studies the optimum levels ofN ,P and K
were fixed as 100:300:300 kg ha-'. Four levels of these nutrients viz; zero, sub optimal,
optimal and super optimal levels were taken. The treatments include one of the four levels of

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one nutrient along with optimal levels of the other two nutrients. Moreover, Ca along with
optimum also was included. These were compared with POP. These treatments along with
FYM @ 12.5 t ha:' also constitute another set of treatments and there was an absolute
control also. In addition, four more treatments were included to study the effect of substitution
of cornrnonsalt for muriate of potash, altogether forming 29 treatments replicated twice in
RBD.
The effect of these treatments on growth characters, yield and yield components,
total plant drymatter production and nutrient uptake revealed the significant effect of
higher levels as optimal and super optimal levels in effecting higher production and
hence on the interrelated parameters. Optimum treatment along with FYM produced a
tuber yield of 40.75 t ha" and 43.41 t ha' respectively during the first and second year.
Moreover, the super optimal level of one of the nutrients along with optimal levels of the other
two nutrients were equally effective in achieving such a higher production.
It was obvious that increasing levels ofN and diminishing rates ofK substantially
increased cyanogen content and slightly reduced the starch per cent of cassava tubers.
The chemical properties viz., organic carbon, available N,P and K were substantially
affected" by higher levels of fertilizers. The nutrient use efficiency computed indicated
the highest agronomic efficiency of Nil' and K associated with optimum + FYM. A
reduction in N,P,K use efficiency at increasing levels of the respective nutrients where
as it was modified at increasing rates of app1ciation of other nutrients. Studies on nutrient
balance sheet showed a deficit balance after two years of experimentation and application of
higher levels of one nutrient led to reduction in the loss of other nutrient.

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The optimwn quantity ofN,P and K calculated based on quadratic polynomial response
surface function was III to 115 kg ha" and 95 to 116 kg ha" N, 228 to 408 kg ha' and 299
to 345 kg ha-I P and 317 to 445 kg ha'! and 289 to 392 kg ha" K without and with FYM
respectively.
Economic analysis of the use of higher levels offertilizers indicated optimwn + FYM as
the most appropriate in terms of highest gross return (Rs. 1,31,225 ha'), net return
(Rs. 83,150 ha"), added return (Rs. 70,178 ha"), added profit (Rs. 56,142 ha") and BeR
(2.73).
The effect of substitution of common salt for muriate of potash on tuber yield
indicated partial and full substitution both alone and along with FYM are similar in
effecting tuber production but yielded substantially lower than no substitution. However,
partial substitution along with FYM was equally well to no substitution without FYM.
Hence the present investigation to realize the maximum yield potential in cassava
through 'systematic approach' in fertilizer use by taking into account the nutritional
constraints of the soil, manipulating these limiting factors and optimising the nutrient
status could achieve a potential yield of 43.41 tha' over the normal average yield of
20-25 tha' under prevailing package of practices recommendations. So it is worth mentioning
that the fertilizer management strategy developed by this approach CaR be recommended
after field level confirmation for large scale cultivation of cassava in the acid laterite soils of
Keralal as well as for industrial use by starch factories particularly in the Salem, Erode,
Namakkal and Dharmapuri districts ofTamilnadu, Samalkot and Peddapuram areas East
Godavari district of AndhraPradesh. Hence it is contemplated that with little more technological
back ups and market support these crops may asswne the role of a commercial crop and gain
importance as a raw material for animal feed and starch based industry in the coming decades.