diameter, were installed in augured holes, 28 mm in diameter and
1110 cm deep. These projected 50 mm above the soil and caps
were fitted over their tops with their base flush with the soil to
deflect rain and prevent differential flow down the sides of the
tubes. Tubes were installed 1.0 and 2.5 mfrom the southern side of
tree rows in Block 1 and northern side of rows in Block 3 between
Trees 10 and 11 in the middle of the rows. Two access tubes were
installed in the centre of the maize plots in the same blocks. The
profile probe was calibrated against gravimetric measurements of
soil moisture for samples collected from the same horizons
(Muthuri, 2004). Mean soil moisture content for the 0–60 cm soil
profile was calculated for all tree-based treatments at each
measurement date; the values were then grouped into 5% soil
moisture categories before calculating the corresponding mean
values for gas exchange parameters.