THE EFFECTS OF FLOW-RATE AND DISTANCE TRAVELED ON THE MOBILITY OF PHOSPHATE IN SOILS

Effects of flow rate and distance travelled on average mobilities of p hosphate in a soil are estimated from breakthrough curves of phosphate at the outlets of small columns of soil, following step increases in the concentration at the inlets. Experimental results are compared wit h results from a computer simulation model of leached columns of soil. Average mobilities of phosphate in columns of soil, following a step increase in the input concentration, decrease with decreasing rate of flow and with increasing distance travelled and appear to be linearly correlated on a logarithmic scale with both flow rate and distance tra velled. An empirical equation, describing these relationships, is fitt ed to data from leaching experiments at flow rates between 30 and 600 cm/day in approximate to 10 cm long columns of soil. Coefficients are obtained by curve fitting breakthrough curves, calculated with a numer ical computer simulation model, to experimental breakthrough curves. T he fitted equation enables extrapolation to flow rates and travel dist ances that are more relevant to a field situation.