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The significance of subsoil moisture in alfalfa production

Kiesselbach, T.A., Russel, J.C., Anderson, A.
Agronomy journal 1929 v.21 no.3 pp. 241-268
Medicago sativa, crop production, subsoil, soil water, crop yield, irrigation, rain, Nebraska
1. The productivity of alfalfa meadows occupying the land for the first time on the Nebraska Agricultural Experiment Station farm at Lincoln has declined rather abruptly about four to five years after sowing, even though a good healthy stand remains. This has been found to be due primarily to depletion of the available subsoil moisture. Such results are regarded as rather typical of most uplands in eastern Nebraska and similar territory where the annual precipitation is not supplemented by either surface- or sub-irrigation. The natural restoration of the subsoil moisture supply is at so slow a rate under usual cropping conditions that far smaller yields may be expected from a second cropping to alfalfa than from the first. 2. The point of non-availability of moisture in these alfalfa meadows appears to be approximately 2% above the hygroscopic coefficient. 3. Alfalfa drew upon the subsoil moisture supply to a depth of 33 feet in a six-year-old meadow and 25 feet in a two-year-old meadow. At the end of six-, three-, and two-year periods of growth in three upland meadows the free-water content had been reduced to approximately 2.5% to depths of 25, 15, and 7 feet, respectively. The average moisture content between the fifth and fifteenth foot in the six-, three-, and two-year-old meadows has been reduced 11.0, 10.6, and 9.4%, respectively, below that in adjoining cultivated fields. 4. A six-year-old alfalfa meadow reached its peak of production in the third year, with 7.2 tons of cured forage per acre. Yields thereafter were curtailed by subsoil moisture depletion. The yields during the fifth and sixth years averaged only 2.0 tons per acre, even though a good healthy stand persisted. During the second to sixth years the forage yield of this alfalfa meadow was about twice the total yield of adjacent corn and wheat fields, whereas in the sixth year its yield was only 79 and 67% as large, respectively, as that of these two cereal crops. 5. Irrigation of a portion of this meadow in its sixth year increased the yield nearly three-fold, bringing it up to 6.72 tons for the season compared with 2.35 tons without irrigation. This is evidence that the shortage of production was due to moisture deficiency. 6. Because of its excessive water requirement ratio, alfalfa cannot be expected to be as productive as the common cereal crops after subsoil moisture has been exhausted and it has become reduced to dependence upon the annual rainfall. Computations based on a water requirement ratio of 858 indicate that approximately 27 acre inches of water were required annually to supply the transpiration needs of an alfalfa meadow during its second to sixth years. This is slightly in excess of the average annual precipitation during the period. During the second to fourth years water requirement considerably exceeded precipitation. This extra requirement was met by the moisture of the subsoil which appears to have lost during the six-year period a total of 32.9 inches. To maintain the high yield attained in the third year (12,206 pounds of moisture-free hay) would require 46 inches of transpirational water per year. 7. Under ordinary cropping conditions the natural restoration of subsoil moisture following alfalfa appears to be very slow. During 15 years of cropping to cereal crops following the breaking up of an established upland alfalfa meadow, very little moisture had accumulated beyond the seventh foot. The average increase in moisture content from the fifth to the thirty-fifth foot appears to have been 0.4%. At this rate approximately 225 years would be required to restore the subsoil moisture removed by six years of cropping to alfalfa. Subsoil moisture was replenished more rapidly on low-lying land that received runoff from higher ground. 8. Alfalfa is greatly handicapped on land that has at some time previously grown this crop. In one series of tests a meadow sown eight years after the previous alfalfa was broken up yielded as a three-year