MSc

The study entitled “Nutrient optimization for grain cowpea (Vigna unguiculata
(L.) Walp.) in high phosphorus soils” was conducted at College of Agriculture,
Vellayani, Thiruvananthapuram, Kerala during 2016 to 2018. The objectives were to
optimise the levels of phosphorus, potassium and zinc for grain cowpea in high P
soils, to study the interaction effect of the nutrients and to assess the residual effect on
the succeeding amaranthus crop. The field experiment was conducted in farmer’s field
at Palappur, Kalliyoor Panchayath, Thiruvananthapuram from January to May 2018.
Grain cowpea variety Shubra, was raised during January to March 2018, followed by
the second crop of amaranthus, variety Arun (April to May 2018). The treatments
consisted of two levels of P (p0 - 0 kg ha-1 and p1 - 7.5 kg ha-1), three levels of Zn ( z0-
0 kg ha-1, z1- 2.5 kg ha-1 as ZnSO4 and z2-0.025% ZnSO4 as foliar spray at branching
and flowering) and two levels of K (k0-10 kg ha-1 and k1- 20 kg ha-1). The 2 x 3 x 2
factorial experiment was laid out in randomized block design with three replications.
Lime, FYM and nitrogen were applied as per package of practices recommendations
(KAU, 2016). After the harvest of cowpea, the residues were incorporated and ten
days later amaranthus seeds were sown in the respective plots. Amaranthus was raised
without addition of any external inputs and harvested by uprooting at 35 days after
sowing (DAS).
The results revealed that K2O @ 20 kg ha-1 resulted in the significantly highest
number of leaves plant-1(21) at 2 months after sowing (MAS). Earliness in flowering
(40 days) was observed with foliar application of zinc (z2) and K2O application @ 20
kg ha-1 (k1- 44 days). The nodule weight at vegetative and flowering stages were
significantly higher for 7.5 kg P2O5 ha-1(p1), 2.5 kg Zn ha-1(z1) and 20 kg K2O ha-1
(k1) and p1z1k1 interaction.
Leaf Area Index (LAI) at vegetative and flowering stages and crop growth rate
(CGR) at vegetative stage were superior in p0z2k0. Soil application of Zn (z1) and 20
kg K2O ha-1(k1) registered significantly higher Relative Growth Rate (RGR) at 30-45
DAS and Net Assimilation Rate (NAR) at flowering, respectively.
Application of K2O @ 10 kg ha-1 resulted in significantly greater number of
pods plant-1 (11.7), pod yield (1276 kg ha-1), grain yield (939 kg ha-1) and harvest
index (0.19). Among different Zn levels, foliar application recorded significantly
higher average pod weight (1.50 g) and pod yield (1302 kg ha-1). The pod and grain
yields were maximum (1676 kg ha-1 and 1288 kg ha-1, respectively) for p0z2k0.
The treatment combination p0z2k0 recorded significantly higher N and Zn
uptake at flowering and harvest. Available N, P and Zn status of soil increased while
soil organic C and available K contents were found to decrease after cowpea harvest.
Among the treatments, p0 and higher dose of K (k1) registered significantly higher
available N in soil after cowpea. Phosphorus, K and Zn status in soil were found to
significantly increase with the application of the respective nutrients.
The yield of amaranthus did not record significant variations with the
treatments imposed in cowpea. Nutrient uptake (P and K) in amaranthus varied
significantly with the nutrient interactions, uptake being higher with p0z2k0 given to
cowpea. The changes in soil chemical properties after the residual crop of amaranthus
harvest were not significant.
Net income (₹ 42,214 ha-1) and benefit cost ratio (1.49) of the cropping
sequence were the highest for the treatment p0z2k0 given to cowpea.
The results of the study revealed that in high phosphorus soil, the combination
of foliar application of ZnSO4 @ 0.025 per cent (at branching and flowering) and K2O
@ 10 kg ha-1( p0z2k0), along with recommended dose of N and FYM recorded higher
yields, net income and BCR in grain cowpea. Based on grain yields, it is could be
interpreted that a negative interaction existed between P and Zn and alsobetween Zn
and K. The interaction between P and K was not significant. The residual effect of
cowpea did not vary with the treatments.