Root growth and phosphorus uptake in relation to the size and strength of soil aggregates. I. Experimental studies

Abstract

Cotton and sunflower were grown in pots containing aggregates of 6 different sizes in the range of 1–19 mm. The penetrometer pressures of individual aggregates from each pot were measured with a 1-mm diameter blunt (total cone angle 60°) probe, driven at 3 mm min −1. Root lengths within aggregates, in voids and along the surface of aggregates were measured after 15 days of growth. Root hair lengths were also measured. At this time, plant tops were harvested and phosphorus concentrations in the tops were determined. Maximum penetrometer pressure was significantly higher with large than with small aggregates. Total root length and the proportion of the total length growing within aggregates was greater with small aggregates, while mean root hair length was greater with large aggregates. Dry weight and phosphorus content of plant tops was higher in beds of small aggregates than in beds of large aggregates. The lower root growth and phosphorus uptake in beds of large compared with small aggregates is attributed to greater mechanical impedance to root penetration and lower availability of phosphorus.