スリランカとトーゴー産のリン鉱石に5Mのクエン酸、シュウ酸、酒石酸溶液を1:2の割合で加え、3日間反応させた。シュウ酸と酒石酸で処理したリン鉱石は容易に乾燥し、ほとんど全てのリン酸が水溶性となった。クエン酸で処理したリン鉱石は乾燥しにくく、また水溶性リン酸の割合も全リン酸の50%程度であった。これら有機酸で処理したリン鉱石のリン源としての有効性を、イタリアンライグラスを供試作物とし、マサ土(花崗岩風化土壌)を用いた土耕ポット試験で過燐酸石灰と比較した。クエン酸処理したスリランカ産リン鉱石を除き、他の全ての有磯酸処理リン鉱石は播種後168日目のイタリアンライグラスの乾物収量を過燐酸石灰と同程度に増加させた。酒石酸で処理したトーゴー産リン鉱石におけるリンの回収率は58%で、過燐酸石灰(52%)や酒石酸処理スリランカ産リン鉱石(53%)より有意に高かった。有機酸処理したリン鉱石を与えた区の跡地土壌の可給態リン(Olsen P)濃度とpHは、過燐酸石灰施与区や無リン酸区(対照区)より有意に高かった。実験期間を通じてイタリアンライグラスの生育と養分吸収に及ぼす有機酸処理リン鉱石施与の悪影響は認められなかった。以上の結果から、酸性土壌に生育する作物に対し、少なくとも酒石酸処理したリン鉱石のリン源として有効性は過燐酸石灰より良好であると判断された。Two phosphate rocks (PRs) from Sri Lanka and Togo were acidulated with 5 M of citric, oxalic or tartaric acid at 2:1 acid:rock ratio for three days. The resulting materials treated with oxalic and tartaric acid were easily dried up and nearly all the P was in water soluble form. The PRs treated with citric acid were rather difficult to dry and contained about 50% of the total P in water soluble form. The effectiveness of the materials as phosphorus sources were compared with that of single superphosphate in acidic granitic regosol with Italian ryegrass as the test crop. All the acid treatments were as effective as single superphosphate in increasing the dry matter yield of Italian ryegrass at 168 days after planting except the citric-acid treated Sri Lanka PR. Phosphorus recovery by the plants from tartaric-acid treated Togo PR was 58%, which was significantly greater (P<0.05) than those from single superphosphate (52%) and tartaric-acid treated Sri Lanka PR (53%). Significantly higher levels of available P (OLSEN P) and pH were found in the cropped soils which received the organic-acid treated PRs than those of the soils which received single superphosphate and control at the end of the experiment. No adverse effect of the organic-acid treated PRs on the growth and nutrient uptake by Italian ryegrass was observed throughout the growth period. These results indicate that at least the tartaric-acid treated PRs could be better sources of P to crops growing on acid soils than single superphosphate