Assessing aluminum phytotoxicity in long-term phosphate fertilized pasture soils

A short-term bioassay was carried out to study the effect of continuous inputs of calcium (Ca), fluoride (F), and sulphate (SO4) from phosphate (P) fertilizers and changes in pH on aluminum (Al) phytotoxicity. Soil samples (40-100 mm) were collected from two adjacent pasture sites, one of which had no phosphate fertilizer applied for over 20 years (low phosphorus, LP) and the other which had high phosphorus input (375 kg ha-1 yr-1 single superphosphate) for 20 years (high phosphorus, HP). Soil samples were incubated at 80% field capacity for 6 weeks with either Ca(OH)2 or HCl to obtain varying pH levels. Short-term root growth bioassay was conducted using barley (Hordeum vulgare L.) seedlings. Decreasing soil pH markedly increased the soil solution Ca, total monomeric Al (Al(t)) and total soluble F in the HP soil compared with the LP soil. Aluminum3+ and Al(OH)2+ were the dominant monomeric inorganic Al species below pH 4.0, but Al-F complexes dominated at pH 4.0 to 5.5 in the HP soil. The inability of total monomeric Al, and 0.02M CaCl2- extractable Al (CaCl2 - Al) to explain the variation in root growth indicated that the relative proportions of the toxic Al were different for fertilized and unfertilized soils. These differences may be due to higher proportions of the less-toxic AlF complexes in the HP soil. Barley root growth was at 90% of maximum relative root length (RRL) at pH (H2O) of 4.8 and 5.5 and CaCl2-Al of 7 and 5 microgram g-1 in the HP and the LP soils respectively. RRL of 90% was observed at the Al(t) concentration of 35 micromolar and 6.5 micromolar and toxic Al (Al3 + Al(OH)2+ + Al(OH)2(+)) activity of 2.4 micromolar and 0.9 micromolar in the HP and the LP soil respectively. The Al3+/Ca2+ activity ratio associated with 90% RRL was 4.2 X 10(-4) and 4.5 X 10(-4) for the HP and the LP soils respectively. The study indicates that Al3-/Ca2+ activity ratio would be the most consistent predictor of Al toxicity in soils with phosphate fertilizer history.