Abstract
This study was carried out to investigate the changes that may occur to the soil water holding capacity under center pivot irrigation systems when grown with different crop patterns over a long period of time. The changes of water holding capacity were checked as affected by crop location and depth. The study was carried out in a dominantly sandy loam soil irrigation scheme, north of Saudi Arabia in 1999. The field capacity (FC), permanent wilting point (PWP) were determined by the pressure membrane method. The general trend of the results showed that crop sequences that were fallow or had cereal crops in their rotation before sampling period considerably affected FC and PWP. This is represented by crop sequences 6, 5 and 2 which produced the highest means of FC at 14.75, 12.79 and 12.68%, respectively. Moreover, the lowest means of FC were produced with alfalfa dominating the crop rotation prior to the sampling period. This is represented by crop sequences 7, 3 and 4 at FCs of 9.56, 9.74 and 10.28%, respectively. Crop sequences 1,4,5 and 6 gave the highest values of PWP, while the lowest one was in crop sequence 7 (4.79%). The highest means of available water (AW) were found in crop sequences 6, 2, 5 at 8.65, 6.87 and 6.35%, respectively. While the lowest value of AW was found in crop sequence 4 at 3.69%. The results showed that the soil samples collected from inside center pivots, for all crop sequences, produced higher means of FC at 12.87%, PWP at 6.53 and 6.34% compared with soil samples collected from outside the center pivot. The relative decrease of FC, PWP and AW due to increasing the soil sample depth from 0–15 to 60–90
cm was 21.55, 22.52 and 20.53%, respectively. The measured values of FC, PWP and AW showed that the highest values of standard deviation (S.D.) and coefficient of variation (CV), for FC, PWP, and AW were found in soil sample depth followed by soil sample location and crop sequence respectively. The interaction effect of crop sequences and soil sample depths on PWP and AW was significant.