Relative importance of protons and solution calcium concentration in phosphate rock dissolution by organic acids

Abstract

A series of experiments was conducted to quantify the relative contribution of protons and other mechanisms to the dissolution of phosphate rocks (PRs) from six countries in solutions of low-molecular-weight-aliphatic organic acids. The amounts of P and Ca released after 3 d of incubation at 28"C were determined in all the experiments. In the first experiment the solubility of the PRs « 500 .urn particle size) in 100 M (25 mL g-l PR) oxalic, tartaric, and citric acids was compared with that in three mineral acids and four chelating compounds. There were no differences in the amount of P released by the mineral acids, but the organic acids released more P than could be accounted for by protonation. The chelating compounds were the least effective. In the second experiment, 1 g each of Sri Lanka and Togo PRs was incubated with 2.5 mmol of the organic acids using three acid concentration (mM) : acid volume ratios (250:10, 100:25, and 25:100). The amount of P dissolved from the PRs and the amount of acid remaining in the filtrate were about the same for all three treatments. A mixture of the organic acids and the respective Na-salts was prepared at six different acid: salt ratios in the third experiment. Total acid concentration in all the solutions was 100 mM. Higher proportion of the Na-salt in the mixture decreased PR dissolution, while the amount of the acid remaining in the filtrate increased. The Ca concentration in the filtrate showed an opposite trend. The results indicated that PR dissolution by the organic acids was initially dependent on protonation, which accounted for only 13-38% of the amount of P dissolved. Most of the amount of P dissolved could therefore be attributed to other mechanisms. Removal of dissolved Ca from the solution appears to be the major factor controlling the subsequent dissolution of the PRs. The ability of tartaric and oxalic acids to dissolve effectively the PRs was attributed to the formation of an insoluble calcium compound which was precipitated from the solution. For the use of organic acids as a potential amendment for improving the P availability of PRs, the Ca binding power of the organic acids should also be considered.

Plant- and microbial-based mechanisms to improve the agronomic effectiveness of phosphate rock: a review

“…The amount of P solubilized by P. Fuscum was found to be directly related to pH, while P. Bilaii solubilized more PR than could be accounted for by the pH change, indicating that another mechanism, in addition to H + production, was contributing to PR solubilization. Other authors have found no correlation between the amount of PR solubilized and the pH, supporting the fact that an alternative mechanism of solubilization exists (Cerezine et al 1988, Sperber 1958a, Whitelaw et al 1999, Sagoe et al 1998b). …”

“…Tricarboxylic acids are not always superior to dicarboxylic acids in PR solubilizing ability; Sagoe et al (1998b) found oxalic and tartaric acids, which are dicarboxylic acids, release more P into solution than citric acid. This exception was attributed to the fact that oxalic and tartaric acids form poorly soluble precipitates with Ca 2+ , effectively lowering the solution saturation point (Sagoe et al 1998b). …”

“…In addition, Aspergillus niger, used in the industrial production of citric acid, has been recognized as one of the most effective organisms for PR solubilization (Abd-Alla and Omar 2001, Agnihotri 1970, Omar 1998, Sperber 1958b. Tricarboxylic acids are not always superior to dicarboxylic acids in PR solubilizing ability; Sagoe et al (1998b) found oxalic and tartaric acids, which are dicarboxylic acids, release more P into solution than citric acid. This exception was attributed to the fact that oxalic and tartaric acids form poorly soluble precipitates with Ca 2+ , effectively lowering the solution saturation point (Sagoe et al 1998b).…”

Selection and evaluation of phosphate-solubilizing bacteria from grapevine rhizospheres for use as biofertilizers

“…P-solubilizing activity is related to the microbial production of organic acids, which chelate the cation bound to phosphate, thereby converting it to a soluble form (Sagoe et al, 1998;Rashid et al, 2004;Lugtenberg & Kamilova, 2009). In the present study, a significant negative relationship was observed between the amount of soluble P and pH in the culture medium (R 2 =0.953).…”

Phosphorous and Phosphate Solubilising Bacteria and their Role in Plant Nutrition

“…Acidification of the surroundings of microbial cell releases proton through the production of organic acids (Villegas and Fortin, 2002). These metabolite organic acids as a result of anion exchange of P by an acid anion can either directly dissolve the mineral P or can chelate Al 3+, Fe 3+ , Ca 2+ ions associated with inorganic P. The carboxyl and hydroxyl groups chelate the cation bound to inorganic P and convert it into soluble forms (Sagoe, 1998;Seema et al, 2013) with the net result of an enhanced availability of it to the plant. As the soil pH increases the divalent and trivalent forms of inorganic P, HPO 4 -2 and HPO 4 -3 occur in the soil.…”