Onyango, GweyiNeumann, G.Romheld, V.2014-10-162014-10-162005African Crop Science Conference Proceedings, Vol. 7. pp. 1029-1032 20051023-070Xuri:http://www.acss.ws/Upload/XML/Research/651.pdfhttp://ir-library.ku.ac.ke/handle/123456789/11470The role of nitrogen forms on solubilisation and utilisation of rock phosphate was investigated on tomato (Lycopersicon esculentum) crops grown in minirhizotrons to assess root-induced chemical changes in the rhizosphere. Two tomato cultivars Cal-J and Moneymaker were supplied with rock phosphate as P source, ammonium (plus DIDIN-nitrification inhibitor) and nitrate as Nforms.NH4+ treatment significantly reduced rhizosphere pH while nitrate led to alkanization. Shoot P content increased with declining rhizosphere pH due to rock phosphate solubilisation. P nutrition was sufficient for NH4-N (intense rhizosphere acidification) but critical under NO3- supply. Shoot Ca contents were in the deficiency range under NH4+ supply, despite increased Ca availability due to rock phosphate solubilisation. Also Mg2+ and K+ uptake declined with decreasing rhizosphere pH. These findings suggest NH4 + and H+ competition with cation uptake, limiting shoot growth. Acid soils low in Ca and Mg, NH4 + in combination with NO3- may improve rock phosphate utilisationenAmmoniumnitraterock phosphatetomato cultivarsArticle