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 NO
3- additions to ammonium treatment
significantly improved biomass production in both soils. In both rhizobox
experiments, NO
3- 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.
