Water-Stability of Aggregates from Potato Plots as Affected by Different Rotation Systems Under Irrigation in Western Nebraska

A series of crop rotation plots under irrigation were established in 1912 on Tripp very fine sandy loam. From the series of rotations the following potato plots were selected for the study of water-stability of aggregates: (i) continuous cropping of potatoes, (ii) 3-year rotation (potatoes, oats or barley, sugar beets), and (iii) 6-year rotation (3 years of alfalfa, potatoes, oats or barley, sugar beets). Neither manure nor fertilizer was added to these irrigated plots during the 39 years of the experiment. The soil from the surface 2 inches of the potato plots was sampled and separated into 13 size fractions by means of rotary sieves. The resistance of aggregates in each size fraction to dispersion in water was obtained by shaking the aggregates in water for 2 and 2,000 minutes. The size distribution of aggregates in water was determined by means of sieves, elutriators, gravity and centrifugal sedimentation. The degree of aggregation of soil particles is expressed by the ratio: The size distribution of aggregates in water prior to any dispersion treatment is taken as that obtained by dry aggregate analysis, providing the aggregates are wetted either under partial vacuum or by capillarity. The ratios of geometric mean diameter for aggregates in water prior to shaking were 16, 10 and 31 for the continuous cropping, 3-year rotation, and 6-year rotation, respectively. Shaking the aggregates in water for 2 minutes produced a marked decrease in the ratios for the three plots: 1.79, 1.97, and 3.21. The marked decrease in the ratios in the 2-minute shaking period indicates the instability of large-diameter aggregates in water. The plot with alfalfa in the rotation showed a pronounced effect on the aggregation of soil particles as compared to the plots without alfalfa. Even after 2,000 minutes of shaking the aggregates in water, the plot with alfalfa showed a high degree of aggregation of soil particles. Highly significant regression coefficients were obtained with aggregation and clay content or total nitrogen content or apparent density of air-dried aggregate or 15-atmosphere tension values.