Response of Soil Chemical Properties to Rhizobium and Rock Phosphate Fertilizer Application under Green - Grams in Tharaka Nithi County, Kenya

Soil fertility decline is one of the major constraints in agricultural productivity. Biological nitrogen fixation (BNF) in legumes can offer a cost-effective and sustainable means towards soil fertility management. There is limited information on green-gram BNF enhancement through Rhizobium inoculation with rock phosphate fertilizer. The objective of this study was to determine the effects of Rhizobium and rock phosphate fertilizer application on soil chemical properties under green-gram varieties. The study was carried out at Chuka university horticultural research farm in two seasons (November 2019 - January 2020 and February - April 2020). A factorial experiment of 2 x 2 x 2 was laid out in a randomized complete block design (RCBD). There were three factors; varieties (N26 and KS20), rock phosphate (0 and 30 kg P ha-1) and Rhizobium inoculation (0 and 100 g ha-1) making a total of eight treatments which were replicated three times. Soil sampling and analyses were done for soil pH, fixed-N, total nitrogen (TN), exchangeable cations (EC), available Phosphorous (P), total organic carbon (TOC), and exchangeable potassium (K) before planting and after harvesting of green-grams for the two seasons. Data was analysed using GENSTAT 15th edition (P≤0.05). Results from both seasons indicated that combined application of rock phosphate at 30 kg P ha-1 and Rhizobium inoculation at 100 kg ha-1 showed significant (P≤0.05) higher increase in soil chemical properties over other treatments. However, treatment R1P1KS20 recorded significantly higher results in soil pH (7.54), TN (0.58%), TOC (3.45%), P (68.20 ppm) and EC (0.95 CmolKg-1), fixed-N (0.50%) and K (1.75 CmolKg-1). On the other hand treatment R0P0N26 recorded significant (P≤0.05) lower results in soil pH (5.23), TN (0.04%), TOC (1.86%), P (8.76 ppm), EC (0.21 CmolKg-1) and K (0.58 CmolKg-1). Therefore, Rhizobium inoculation in green-grams in combination with rock phosphate is an important legume nitrogen fixation enhancement method. This method is cost effective for farmers in sustainably supplementing nitrogen and phosphorous in their farms for improved soil fertility management. Based on the findings, combining Rhizobium 100g ha-1 and rock phosphate 30 kg P ha-1 with variety KS20 were recommended for a sustainable soil fertility management in Tharaka Nithi County.