Application of K-bearing rock for fertilization of Cowpea, (Vigna unguiculata)

Abstract

The main concept of this research work is to apply a mineral source of potassium for supplying the soil by
this element in order to substitute the expensive commercial fertilization of some crops.The applied Kfeldspar
rock has about total potassium of 10.0%, in extractable form 395 ppm and in soluble form 125 ppm.
Spectrographic semi-quantitative analysis of this rock revealed that it has 70.40% SiO2, 15.48% Al2O3, 8.38%
K2O, 3.22% Na2O and traces of other elements such as Fe, Mg, P and Ti. A field experiment was conducted to
grow cowpea on poor virgin sandy soil. Four treatments were followed: potassium sulfate only, ½ potassium
sulfate + ½ potassium feldspar, ¼ potassium sulfate + ¾ potassium feldspar and potassium feldspar only.
There was a highly significant variation between the vegetative growth of the cowpea under the second
treatment, in which half K-requirement was added as K-sulfate and the other half as K-feldspar, and all other
treatments. The same trend was obtained with respect to the number of branches in the plant, number of
kernels and weight of seeds. In spite of the fertilization requirements of nitrogen, phosphorus and organic
compost have been added equally to the four treatments, there were some fluctuations in the content of
these elements of the yield of cowpea. The content of nitrogen, phosphorus and potassium in the four
treatments can be arranged as followed: 2nd>1st>3rd>4th treatment. The lowest vegetative and yield quality of
plants under the 4th treatment, in which potassium feldspar was added, can be explained by the fact that
these minerals are quite resistant to weathering and supply relatively small quantities of potassium during
the growing season. However, their cumulative release of potassium over several years is very important
acting as slow release fertilizer. The positive influence of the fine grains of the K mineral bearing rocks is
improving the poor structure of loose sandy soil, consequently the water and nutrient capacities of this soil
will be enhanced and increase their ability to plant uptake.It is worthy to refer that the cost of chemical
fertilizer (K-sulfate) is about 7000 LE /ton while the K-feldspar rock is ranging between 600-800 LE /ton.