Soil Water Potential and Water Content Profiles with Wheat under Low Spring and Summer Rainfall
1971
Papendick, R. I. | Cochran, V. L.
Soil water potentials and water contents in the wheat (Triticum aestivum L.) root zone of several soils were measured late in the growing season under Pacific Northwest, U. S. A. conditions of low spring and summer rainfall. All profiles exhibited low potentials in the surface 1 to 1.5 m of soil, followed by a region of steep potential gradient over a depth increment of 30 cm or more. below this region the energy values were relatively constant with depth. In the dry zone the soil water potential in some cases reached −45 bars at the 60- or 90-cm depth, and in one case, −40 bars at the 150-cm depth. With coarser-textured soils the potential gradient across the moist to dry zone was steeper as compared with a finertextured soil. Depth of root penetration appeared to coincide with the base of the steep water-potential gradient. Plant wilting was absent or not pronounced when potentials below the steep gradient zone were −2 bars or higher; permanent or severe wilting occurred when the potential was −10 to −8 bars. Soil water potencials in the surface 1.5 m of soil were 8 to 10 bars lower with 110 and 220 kg N/ha applications on wheat than with no N. In most cases the water content profiles of the root zone bore little resemblance to the corresponding potential profiles. These results indicate that in the field the lower limit of water extraction by wheat may reach −40 bars or lower. However, the mobility of water in the root zone or adjacent layers underlying the root zone of the field soil is probably more important to the concept of “water availability” for plants than the actual energy status per se.
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