施用酸性物質降低鹼性或石灰質土壤的pH，將使被固定的磷較易釋出，因而提升土壤磷的有效性，但也可能增加磷損失到環境中的風險。本研究目的在探討降低pH值對鹼性土壤中磷有效性及可溶性的影響，故選用台灣地區的四種鹼性耕地土壤 (將軍系、鹿港系、太康系和大排沙系)，其pH值皆介於7-8的範圍，先分別施以不同等級添加量 (0、25、125、250和500 mg/kg) 的磷，於田間容水量下孵育90天，接著再以鹽酸將土壤樣本pH值調降至兩個標的範圍 (6-7和5-6)，於田間容水量下穩定兩週後，分別測定Bray No. 1、Olsen和Mehlich 3三種有效性磷指標及去離子水和CaCl2可抽出磷。由三種有效性磷指標的測定結果顯示，對於碳酸鈣當量和Mehlich 3可抽出鈣較高但草酸可抽出鐵偏低的太康系和大排沙系土壤，降低pH能顯著增加有效性磷；對草
酸可抽出鐵含量最高的鹿港系土壤，降低pH可能促使產生鐵-磷形式的固定反應，對增加有效性磷沒有顯著影響；而對於草酸可抽出鐵也相對偏高的將軍系土壤，則在磷添加量較高的處理下，降低pH所能提升之磷有效性的幅度也會變小。再者，由供試土壤的去離子水可抽出磷顯示，各土系的可溶性磷皆有隨降低pH而顯著減少的趨勢，僅大排沙系在pH範圍降至5-6，可能因鈣-磷大量釋出，造成可溶性磷顯著增加。因此，對於大排沙系、太康系和將軍系土壤，降低pH範圍至6-7，確實能提升磷的有效性並減少其可溶性，但大排沙系調整至pH < 6，則有提升可溶性磷的趨勢；對於鹿港系而言，降低pH雖能顯著減少可溶性磷，卻無法提升磷的有效性。由本研究結果可歸納得知，鈣含量高但鐵鋁氧化物低的鹼性土壤，降低pH可增加有效性磷並減低磷的可溶性；而鈣含量及鐵鋁氧化物皆高的鹼性土壤，降低pH會減少可溶性磷但無法提升磷的有效性。

As pH decreased by amendment of acidifying materials in alkaline or calcareous soil, soil P becomes more releasable, thus increasing P availability and also P loss into the environment. The objective of this study was to investigate the effects of decreasing pH on P availability and solubility in alkaline soils. Four agricultural soils (Chengchung, Lukang, Taikang, and Tapais) of Taiwan, whose pH range from 7 to 8, were used for investrgation. The soils were amended with five levels of P (0, 25, 125, 250 and 500 mg/kg) and incubated for 90 days. Then their pHs were adjusted to two target ranges (5-6 and 6-7) using HCl followed by a stabilization period for 2 weeks. The available P indices, Bray No. 1, Olsen, and Mehlich 3 methods, and the soluble P indices, water and CaCl2 extractions, for each soil sample were measured. In the results, pH decreasing to 5-6 and 6-7 significantly resulted in raise of available P in Taikang and Tapais soils which contain relatively high calcium carbonate equivalent and Mehlich 3 extractable Ca but low oxalate extractable Fe. However, for Lukang soil which contains extremely high oxalate extractable Fe, there was no significant raise of available
P with decreasing pH. Because Chengchung soil contains relatively high oxalate extractable Fe, there was less enhancement of P availability due to decrease of pH, when high levels of P was applied. Moreover, the water
extractable P in the four soils showed soluble P significantly reduced with decreasing pH. When the Tapais soil was adjusted to pH 5-6, the soluble P increased probably due to a large amount of Ca-P precipitates under this condition. T herefore, decreasing pH to the value of 6 to 7 for Tapais, Taikang, and Chungcheng soils could simultaneously increased P availability and reduced P solubility; nevertheless, decreasing pH to lower than 6 for Tapais soil could increase soluble P. Decreasing pH for Lukang soil could reduce soluble P but could not increase P availability. In summary, decreasing pH will increase P availability and reduce soluble P in the alkaline soils which contain relatively high Ca content but low Fe and Al oxides. However, decreasing pH for the alkaline soils with high Ca content and Fe and Al oxides can only reduce soluble P but not raise availability of P.