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Relationship Between Yield Response to Applied Fertilizers and Laboratory Measures of Nitrogen and Phosphorus Availability¹

ABSTRACT

Surface soils (generally 0- to 7-inch depth) and subsoils (generally 7- to 21-inch depth) were collected from sites in 13 Western states. Total N, initial NO₃-N, NO₃-N present after incubation, and NaHCO₃-soluble P were determined on all samples. On 14 surface soils these laboratory measures were correlated with values of N and P availability obtained in a controlled light-temperature plant growth chamber. Growth response to applied N was highly correlated with nitrates in nonincubated soils, as well as with nitrates present following incubation. However, correlations were better with the sum of the NO₃-N produced during incubation and the NO₃-N present in nonincubated soils, than with only the NO₃-N produced. Correlation coefficients were higher when the soils were incubated 3 weeks, rather than 6 weeks, and also when no CaCO₃ was added to the soil prior to incubation. The following measurements were good predictors of increase in barley yields following N fertilization in growth chamber experiments: NO₃-N in nonincubated soils; NO₃-N present following incubation; total N in plants from non-N plots; "N" values of Munson and Stanford; and relative uptake of total N by plants in the P and NP plots.

The following measurements were good predictors of yield increases of barley when P was applied in the growth chamber: NaHCO₃-soluble P; A values of Fried and Dean; total P in plants from non-P plots; and relative uptake of total P by plants in the N and NP plots.

In 46 field trials, laboratory measures of available N in the soil were compared with yield response to applied N fertilizers. Similarly, in 44 field trials, yield response from applied P was compared with NaHCO₃-soluble soil P. When the laboratory estimate for available soil N or P was low, growth response to fertilization was generally obtained in the field studies. When the soil level of available P was high, there was generally no yield increase following the addition of P fertilizer. However, yield response sometimes occurred on sites having high soil test values for available N. Differences, in the ability of the N and P laboratory tests to predict response to fertilization in field trials, are attributed to the influence of climate. Except for the relatively small amounts of initial available N in the soil, soil moisture and temperature determine the N supply from mineralization. However, mineralization of organic P may not be very important for supplying P to plants on soils having fairly high initial NaHCO₃-soluble P levels.

¹ Contribution from the Soil and Water Conservation Research Division, ARS, USDA. Presented before Div. IV. Soil Science Society of America. Chicago, Ill., Dec. 6, 1960.

² Soil Scientists, USDA, Mandan, N. Dak. H. R. Haise is now at Ft. Collins, Colorado. F. Turner, Jr., is currently with the University of Arizona at Safford.

Received for publication January 21, 1963. Accepted for publication May 8, 1963.