Investigations of phosphorus leaching from sandy soil

Investigations on phosphorus leaching were conducted at a field site with sandy soil. From soil sample analyses a linear relationship was found between total phosphorus (P-t) and the amount of water-soluble phosphorus (P-w). Regardless of soil depth and sampling point location only approximately 5% of P-t is water soluble. At a small test site at the border of the field, Soil solution was extracted using ceramic- and nylon suction cups. The soil solution analyses showed that the concentration of total dissolved phosphorus (TDP) decreased significantly with depth. An average concentration of around 0.8 mg l(-1) was found at a depth of 30 cm, whereas at 180 cm about 0.1 mg l(-1) was observed. Phosphorus input to the groundwater is in the range of 0.1 kg ha(-1) year(-1) At that site organic P forms are more important for phosphorus leaching than inorganic fractions. Since the start of investigations no more fertilizer has been applied at the small test site which has led to decreasing ionic strength in the topsoil (30 cm). In contrast, at a depth of 30 cm a continuous increase in the concentration of soluble phosphorus was observed. It is shown that this increase in the concentration of soluble P should be attributed to declining ionic strength and especially to declining Ca concentration. As a consequence changing land use from cultivation to fallow land may lead to enhanced mobilization of previously accumulated phosphorus.