PHOSPHORUS IN MACROPORE WALLS OF A DANISH GLOSSUDALF

The distribution of iron, aluminium and phosphorus fractions in a pseu dogley soil (Fine-loamy, mixed, mesic Oxyaquic Glossudalf), including transects across albic macro-pores (tongues) in the glossic horizon ha ve been investigated by use of selective extraction techniques. In the glossic horizon a distinct redox-front is observed between albic ped faces adjacent to the macropore lumen and brown ped interiors. Samples , each representing a thickness of 2-3 mm in transects orthogonal to t he macropore and the redox-front, show up to five times more citrate-b icarbonate-dithionite extractable Fe and Al (Fe-d, Al-d) in brown ped interiors than in albic ped faces. Surprisingly, the content of total (P-tot) and inorganic phosphorus (P-i) have a nearly uniform distribut ion across the transects. In the albic ped faces the contents of citra te-bicarbonate-dithionite extractable (P-d) and oxalate extractable ph osphorus (P-ox) are both low whereas the amounts of P-i or P extractab le by 1 M HCl (P-HCl) are remarkably high. Calculations demonstrate th at the low content of P, is not due to phosphate saturation. The large P-HCl - P-d fraction in ped faces is attributed to occluded P, becaus e the P-HCl - P-d fraction is equivalent to the P-HCl - P-ox fraction and oxalate is found able to dissolve apatite. The occluded P is not p resent within Al and Fe oxides, but probably contained in silicates. I n the brown ped interiors most phosphate is bound to crystalline Al an d Fe oxides.