PhD

An investigation was undertaken at the College of
Agriculture, Vellayani to establish soil test crop response correlation for
soil test based balanced fertilizer prescription for cassava var. M-4 in
laterite soils of Kerala. The field investigation consisted of fertility
gradient experiment, STCR experiment and technology verification trial.
The fertility gradient experiment was conducted during April-
May 1994 in the Instructional farm, Vellayani. The desired gradient in
soil fertility was created in one and the same field by applying graded
doses of N, P and K fertilizers and raising fodder maize var. African Tall.
The STCR experiment was conducted in the same field
during June '94-March '95 using the test crop, cassava var. M-4. The
treatments consisted of factorial combinations of four levels of N (0, 50,
100 and 150 kg ha-1), three levels of P (0, 50 and 100 kg P2O5 ha-1) and
five levels of K (0, 50, 100, 150 and 200 kg K20 ha-1) along with three
levels of farmyard manure (0, 6.25 and 12.50 t ha-1) fitted in a response
surface design.

Using multiple regression model, the fertilizer adjustment
equation for N at varying soil test values for available N for
maximum tuber yield (t ha-1) of cassava in laterite soil was derived as
FN = 136.6 - 0.2 SN where FN is fertilizer N (kg ha-1) and SN is soil
available N (kg ha-1). The equation becomes FN = 136.6 - 0.2 SN - R
for economic tuber production where R is the ratio of cost of one kg of
fertilizer N to price of one kg of tuber.
At varying soil test values for organic carbon% (OC) the
above equations become FN = 226.13 - 378.13 OC for maximum tuber
yield and FN = 226.13 - 378.13 OC - 1.25 R for economic tuber yield.
The behaviour of fertilizer P and K was found to produce
responses other than 'normal' and hence optimisation could not be done
for fertilizer P and K for maximum and economic tuber yield at varying
soil test values.
The nutrient requirements of cassava var. M-4 were estimated
to be 6.58, 2.37 and 6.28 kg N, P2O5 and K2O respectively to produce one
ton of tuber. In the laterite soil, the efficiencies of contribution of

nutrients from the soil for cassava were calculated as 40.17, 41. 3 3 and
48.60% N, P2O5 and K2O respectively. The fertilizer efficiencies were
worked out as 54.38, 47 and 52.650% N, P2O5 and K2O respectively. The
efficiencies of contribution of nutrients from farmyard manure were
calculated as 78.24, 57.33 and 69.66%) N, P2O5 and K2O respectively.
From the above basic data, fertilizer prescription equations
for specific yield targets of cassava var. M-4 in the laterite soil were
derived as given below.
Without FYM
FN = 12.10 T - 0.74 SN

FP2O5 - 5.04 T - 2.02 SP
FK2O = 11.93 T - 1.10 SK
With FYM
FN = 12.10 T - 0.74 SN - 1.44 ON
F P2O5 - 5.04 T - 2.02 SP - 2.79 OP
FK2O = 11.93 T - 1.10 SK - 1.58 OK
where FN, F P2O5 and FK2O are fertilizer N, P2O5 and K2O respectively
in kg ha-1, T is the target of tuber yield in t ha-1, SN, SP and SK are soil
available N, P and K in kg ha-1 respectively and ON, OP and OK are

quantities of N, P and K supplied through organic manure in kg ha-1
The technology verification trial was undertaken during
June '96-March. '97 in the laterite soil in three farmers' fields in
Thiruvananthapuram district and also in the Instructional Farm,
Vellayani. The treatments consisted of Package of Practices
recommendation for fertilizer, fertilizer recommendation by the Soil
Testing Laboratory and fertilizer doses for the yield targets of 15, 20 and
25 t ha-1 along with a uniform dose of 6.25 t ha-1 of farmyard manure for
all treatments.
The fertilizer doses based on targeted yield equations
recorded higher tuber yield and net income, benefit cost ratio and net
returns per rupee invested over the fertilizer recommendation by the Soil
Testing Laboratory and Package of Practices recommendation
emphasising the need for site specific recommendation based on soil
tests.
The yield targets of 15 and 20 t ha-1 recorded more than cent
per cent achievement and the yield target of 25 t ha-1 recorded about
90% achievement. Higher tuber yield, benefit cost ratio and net returns


per rupee invested could be achieved for the yield target of 20 t ha-1. The
fertilizer dose for the yield target of 15 t ha-1 recorded the highest returns
per rupee invested on fertilizers.
The study has revealed the superiority of fertilizer application
based on targeted yield approach over the semi-quantitative approach
employed in the soil testing laboratories and generalised state level
Package of Practices recommendation for the crop. In this approach, the
fertilizer dose can be adjusted in accordance with the specific objective
and available resources of the farmer.