Solubility Test in Some Phosphate Rocks and their Potential for Direct Application in Soil

There are various methods to evaluating phosphate rock for direct application. The first approach is solubility test of Phosphate Rocks (PR) using chemical extractant. In this study to compare the effectiveness of direct application of PRs, two samples of a sedimentary unbeneficiated PR from Lar mine of Yasooj and two samples of concentrated PR of Asfordi igneous PR of Yazd in Iran along with one sample PR of Gafsa (Tunisia) were analyzed for the main nutrients and minerals with XRF and XRD. Calcite and quartz were the main ingredients of Yasooj PR which cause problems for direct application. Yazd PR in addition to apatite had some iron and magnesium minerals which are also undesirable for fertilizer producing. The reactivity of these PRs was determined using the common extractants (Formic Acid, Citric Acid and Neutral Ammonium Citrate) and classified on the basis of International Fertilizer Development Center (IFDC) proposed classification method for direct application. The reactivity of Iranian PRs was quite low compare to Gafsa PR. The long-term dissolution kinetics of these PRs were determined with Formic Acid (FA) and Citric Acid (CA) showed slow reaction initially (0-20 min.) because of the presence of free calcium carbonate but later on the reaction rate increased surpassing Gafsa PR. Therefore Yasooj PR could be recommended to use in soil after beneficiation and removal of lime. The kinetics of dissolution of Yazd PRs followed a zero order equation.