Nitrate Distribution and Variability in Irrigated Fields of Northeastern Colorado 1

Fertilizer N recommendations in the Great Plains and Western USA are commonly based on soil NO 3 − tests, although NO 3 − content within soil profiles is subject to rapid change. The purpose of this study was to evaluate NO 3 − distribution within diverse soil profiles and determine NO 3 − changes occurring overwinter. Twenty‐six irrigated farm fields were systematically sampled in 1974 and 1975 by dividing them into 61 × 61 m grids (0.37 ha) and sampling to a 120‐cm depth by 30‐cm increments. Eleven fields were re‐sampled for fall vs. spring comparisons. Nitrate content tended to be greatest in the surface 30‐cm and decrease with depth; only three fields deviated markedly from the general pattern. Nitrate contents in the 0 to 30‐cm and 0 to 60‐cm depths were both highly correlated at the 0.01 level with that in 0 to 120 cm (r 2 = 0.80 and 0.91, respectively), and actual NO 3 ‐N content (0 to 120 cm) was within 30 kg/ha of the regression lines for 25 of 26 fields. All fields significantly increased in NO 3 ,‐N from fall to the following spring, ranging from 8.1 to 68.9 kg/ha. Greatest increases occurred in the surface 30 cm. These increases were significantly related at the 0.05 level to decreases in NH 4 + (r 2 = 0.37), fall NO 3 − level (r 2 = 0.43), and fall NO 3 − plus NH 4 + level (r 2 = 0.42), but not to organic matter or total N. There was a highly significant relation at the 0.01 level between fall ‐ and spring NO 3 − content of each sampled depth, ranging from r 2 = 0.60 to 0.86. The above relationships should be useful in determining appropriate soil sampling depths for N fertilizer recommendations and developing calibrations based on sampling date.