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

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.