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F6hse et al. (1988) have shown that P influx per unit root length in seven plant species growing in a low-P soil varied from 0.6 × 10 -14 to 4.8 × 10 -14 mol cm-ls -1. The objective of this work was to investigate the reasons for these differences. No correlation was found between P influx and root radius, root hairs, cation-anion balance and Ca uptake. However, when root hairs were included in mathematical model calculations, the differences of P influx could be accounted for. These calculations have shown that in soils low in available P, contribution to P uptake by root hairs was up to 90% of total uptake.The large contribution of root hairs to P uptake was partly due to their surface area, which was similar to that of the root cylinder. However, the main reason for the high P uptake efficiency of root hairs was their small radius (approx. 5 × 10 -4 c m ) and their perpendicular growth into the soil from the root axis. Because of the small radius compared to root axes, P concentration at root hair surfaces decreased at a slower pace and therefore P influx remained higher. Under these conditions higher Ima x (maximum influx) or smaller K m values (Michaelis constant) increased P influx. The main reasons for differences found in P influx among species were the size of Ima x and the number and length of root hairs. In a soil low in available P, plant species having more root hairs were able to satisfy a higher proportion of their P demand required for maximum growth.

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