FALL AND SPRING SOIL SAMPLING FOR MINERAL N IN NORTH-CENTRAL ALBERTA

Abstract

The timing of soil sampling for mineral N was investigated by sampling in fall and in spring from the 0- to 30-cm depth in 100 field experiments. On the average of the 100 experiments, the NO3 – N in soil increased from 16 kg N∙ha−1 in fall to 34 kg N∙ha−1 in spring and the mineral N in soil increased from 28 kg N∙ha−1 in fall to 49 kg N∙ha−1 in spring. In 18 of the experiments, fall to spring increase in NO3 – N ranged from 31 to 90 kg N∙ha−1, and 16 of these experiments were among the 56 Black Chernozemic soils in the study. The correlation coefficient (r) between soil NO3 – N in fall and grain yield in check plots was 0.55, while the r value between soil NO3–N in spring and grain yield in check plots was 0.72. When mineral N was used instead of NO3 – N, the r values were similar. The relationship between fall NO3 – N (X) and spring NO3 – N (Y) was best described by the linear regression equation (Y = 14.84 + 1.22 X) with an r value of 0.66. Of the 100 experiments, 26 were sampled twice in the fall (early fall and late fall) and once in the spring. The increase in soil NO3 – N from early fall to spring was 33 kg N∙ha−1, but from late fall to spring was only 15 kg N∙ha−1. The linear regression equation to predict the spring NO3 – N values (Y) from early fall NO3 – N (X) was Y = 27.50 + 1.29 X (r = 0.63) and from late fall NO3 – N (X) was Y = 20.47 + 0.88 X (r = 0.76). The relationship of grain yield (or N uptake) with soil NO3 – N in late fall samples was much closer than with NO3 – N in early fall soil samples, and the correlation was similar to that obtained with soil NO3 – N in spring samples. Even though NO3 – N was substantially less with late fall sampling, as compared to spring sampling, the correlation with the two times were moderately close (r = 0.76). Soil test programs in the Prairie Provinces are based primarily on field experiments with only spring sampling, while farm sampling is mostly conducted in the fall. At least in north-central Alberta, fall sampling for soil test for NO3 – N apparently should be restricted to the late fall and probably an adjustment to N recommendations should be made for the smaller amounts of NO3 – N found in fall rather than spring. Key words: Early fall, late fall, mineral N, nitrate N, N recommendations, soil sampling, spring