Differential benefits of rock phosphate (RP) by tomato (<i>Lycopersicon esculentum</i> mill.) plant as affected by nitrogen froms and soil types.

Abstract

Phosphorus is one of the critical elements that limit plant production, particularly in humid and semi-humid soils. For realization of African Green Revolution, use of rock phosphate (RP) by resource-poor farmers may be an alternative to more expensive water soluble phosphate (P). Utilization of RP was investigated in tomato (Lycopersicon esculentum Mill) var; Moneymaker in minirhizotrons at Hohenheim to assess root-induced chemical changes in the rhizosphere with two soil types: - Arenosol and C-horizon of Luvisol. Additionally, field experiments were conducted at
Kibwezi and Maseno (Luvisol and Ferralsol, respectively) in Kenya. All trials received RP and soluble P as source of P and nitrate and ammonium (stabilized with DMPP as nitrification inhibitor) as nitrogen sources. Ammonium treatment significantly reduced rhizosphere pH in minirhizotron treatment with Arenosol with corresponding increase in shoot P content (but with significant negative shoot biomass accumulation), while rhizoplane pH differed significantly from rhizosphere pH treatment with C-horizon of Luvisol and there was no RP benefit to plant. The buffer capacity of the Luvisol was quite high and the pH 2mm away from rhizoplane was similar to that of bulk soil. However, minimal NO₃ - additions to ammonium treatment significantly improved biomass production in both soils. In both rhizobox experiments, NO₃ - led to rhizosphere alkalinization. Both shoot and fruit biomass was enhanced by RP application at Maseno, while RP had a negative effect on tomato plant production at Kibwezi. Therefore, role of RP on improved tomato yield at Maseno may partly be attributed to secondary factors other than P, viz; alleviation of aluminium rhizotoxicity since the Al content was significantly reduced by RP treatment, while RP may have led to partial alkalinization at specific root/rhizoplane, leading to Zinc deficiency at Kibwezi site. The application of rock phosphate in addition to acidifying nitrogenous fertilizer with consideration to soil types has potential of improving crop production and phosphate capital of resource-poor farmers.

Key words: Nitrogen forms, r-phosphate, soil-types, tomato