Measuring phosphorus fluxes through the root zone of a layered sandy soil: comparisons between lysimeter and suction cell solution

SUMMARY Chemical and physical processes govern the extent to which phosphorus dissolved from the plough layer is retained by or leached from the subsoil. The adsorption capacity of deeper layers, the velocity of solution flux and the amount of soil volume in contact with the leaching soil solution are important determining factors. Soil solution from 90‐cm depth of a layered sandy soil, sampled over a 3‐year period with teflon suction cells, was systematically lower in P content than soil solution sampled with free drainage lysimeters from the same depth. On average 4.6 times as much inorganic P and twice as much organic P was found in lysimeter solutions. Determination of apparent equilibrium solution concentrations of phosphate was performed by means of a series of batch experiments with soil and soil solution of varying initial P content. The results indicated that the suction‐cell solution was close to equilibrium with soil from the 90‐cm layer. Rhodamine dye was applied, and flow paths of soil solution observed. Preferential flow was initiated from layers of finer texture overlying layers of coarser texture. On the basis of these observations it is concluded that suction cells give the best representation of stagnant soil solution, whereas lysimeters give a better representation of mobile soil solution.