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Subirrigation and Plant Nutrition: II. Utilization of Phosphorus by Alfalfa from the Soil Surface to the Water Table¹

ABSTRACT

Utilization of phosphorus by subirrigated alfalfa from the soil surface to the water table has been studied. Total and available phosphorus content of soils from adjacent fields, one cropped intensively to alfalfa and the other having little or no alfalfa history, were compared.

Long time alfalfa culture resulted in substantial reductions of total and available phosphorus to a depth of 11 feet. Phosphorus utilization by subirrigated alfalfa has been correlated with available moisture in soils. Soils with shallow water tables (10 feet or less) have water at relatively low tension throughout their profiles. Root activity is concentrated mainly in the surface soil gradually decreasing with depth. Intense root activity in the surface 3 feet of soils having relatively deep water tables (10 to 20 feet) appears to be associated with moisture from precipitation which is usually available for only a part of the growing season. Less intense but more constant root activity is to be expected in the region of capillary moisture above the water table. Decreased phosphorus utilization between these two regions coincides with a relatively dry area with consequent low root activity.

Surface and subsoil depletion of phosphorus by subirrigated alfalfa indicates that phosphate fertilization will be needed in the future if continued high yields are to be maintained.

The availability of nutrients in the substratum of subirrigated soils must be evaluated before reasonable fertilizer recommendations can be made for legumes.

¹ Published with the approval of the Director as Paper No. 673, Journal Series, Nehraska Agr. Exp. Sta. Presented before Div. IV, Soil Science Society of America, St. Paul, Minn. Nov. 10, 1954. Received Oct. 19, 1954. This investigation was supported in part by a grant from the American Dehydrators Association.

² Instructor in Agronomy and Assistant Agronomist, respectively.