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Two Approaches to Determination of the Fertility Status of Malheur County, Oregon, Soils¹

ABSTRACT

The fertility status of the irrigated soils of Malheur County was investigated through field experiments on sites selected on a non-random basis and by a survey of the nutrient status of the soil, using alfalfa as an indicator crop, on sites chosen at random.

The data from 66 field experiments, on 6 major crops on 40 farms, over a 4-year period led to the conclusions that although the level of soluble P was fairly low in several soils, under the prevailing conditions of farming only a few soils were deficient in P for the needs of the current crop, and that all soils are adequately supplied with K. Selection of sites for these field experiments was made on the basis of probable good cooperation of individual farmers. This method of selection could be biased towards the lands of the more progressive farmers, who may maintain higher than average fertility levels in their soils, in which case erroneous conclusions regarding the nutrient status of the soils of the area might be drawn. The conclusions from the field experiments were tested by making a survey of alfalfa yield and the P and K status of alfalfa and soil on approximately 150 sites chosen strictly at random.

Comparison of data from field experiments and survey reveal some bias in conclusions from field experiments as regards soil levels of P, which element has been in use in the area as a fertilizer for several years. Soils from field experiments were in general considerably higher in sodium bicarbonate-soluble P than soils from random sites. In the case of K, which has been used very little as a fertilizer in the area, experiments and survey led to similar conclusions.

Regression equations, calculated from the survey data, indicate that, in general, alfalfa yields in the area may be increased by increasing P and K in the soil. The equations indicate further that relatively large amounts of P and K are required to produce yield increases of the magnitude of the "least significant difference between treatment means" usually found in field experiments.

¹ Technical Paper No. 958, Oregon Agr. Exp. Sta. and Soil and Water Conservation Research Branch, A.R.S., U.S.D.A., Cooperating. Contribution of the Department of Soils, Oregon Agr. Exp. Sta., and the Western Section of Soil and Water Management of S.W.C.R.B.

² Senior Soil Scientist, O.A.E.S. and W.S.S.W.M., S.W.C.R.B., A.R.S., U.S.D.A., cooperating, Corvallis, Oregon; Research Assistant, Malheur Branch Exp. Sta., Ontario, Oregon (present address: So. Oregon Branch Exp. Sta., Medford, Oregon); and Assistant Soil Scientist, O.A.E.S., Corvallis, Oregon, respectively.

Received for publication March 8, 1956. Accepted for publication July 14, 1956.