Root Distribution and Water Use Efficiency of Alfalfa as Influenced by Depth of Irrigation

Root distribution of several crop plants is known to be affected by different patterns of soil moisture. In alfalfa (Medicago sativa L.), however, such results have not been consistently obtained under field conditions. Our objective was to determine tbe effect of different irrigation depths on alfalfa water use efficiency, water extraction patterns, and root distribution under greenhouse conditions where examination of the entire root systems was possible. ‘Cimmaron’ alfalfa was planted in polyvinylchloride (PVC) containers (120 ✕ 30 cm) filled uniformly with a Leeper fine (fine, montmorillonitic, nonacid, thermic Vertic Haplaquept) sandy loam top soil. Irrigation treatments included: watering to saturate the soil one-third of the whole root depth, watering to saturate the soil two-thirds of the whole root depth, and no irrigation. Irrigation was applied when available soil moisture content within the root zone reached 35 % (−2.5 bars soil matric potential). The parameters studied included root distribution pattern, total root growth, water extraction pattern, and water use efficiency. The efficiency of water use was higher in deeply irrigated treatments than in shallow irrigated treatments during June and July (a period of high evapotranspirational demand), but it was not different during September to November. Depth of irrigation did not affect total root production, but it did affect root distribution at the irrigation zone. Deeply irrigated plants produced slightly more roots in the lower depths than shallow irrigated plants. Root production of nonirrigated alfalfa was low under the high evaporational demand conditions which occurred during June and July, but was highest among all treatments during the September to November growing period. Rookshoot ratios of 0.28, 0.22, and 0.62 observed in shallow, deep, and non-irrigated plants grown in the September to November period, respectively, indicate that the rate of root growth was increased in nonirrigated alfalfa as a result of limited water stress and under low evapotranspirational demand.