Soil Aggregation as Influenced by Microbial Gums, Level of Fertility and Kind of Crop

A study was made of the effect on soil aggregation of microbial gums produced in vitro and in soil. Attention was also given to the effects of fertility treatments, kind of crop, and season on the aggregation of Spencer silt loam. The bacterial polysaccharide synthesized by Agrobacterium radiobacter in pure culture had a marked aggregating effect when added to Spencer and Miami silt loams. Addition of as little as 0.02 gm. of this gum to 100 gm. of soil caused a 50% increase in aggregates > 0.1 mm. in diameter. The gum content of numerous samples of Miami and Spencer silt loams correlated well with their respective levels of aggregation. The addition of extracted soil gum to a poorly aggregated soil increased significantly the degree of aggregation. Undecomposed pulverized plant residues added to soil increased the gum content to a maximum in six days. The maximum percentage of aggregates > 0.5 mm. in diameter was not reached until the twelfth day. In a four-year rotation of corn, oats, and two years of alfalfa-brome on plots which were variously limed and fertilized, increases in percentage of soil aggregates occurred at pH 6.0, 6.5, and 7.5 as compared with the aggregation level of the check plot (pH 5.5). High levels of available phosphorus and potassium as compared with low levels promoted aggregation. In general, structural deterioration began under corn and continued until the following crop of oats attained considerable growth. Thereafter improvement continued and reached a maximum during the second year of hay. Seasonal variations in soil aggregation occurred on all plots, irrespective of soil treatment or crop, and was characterized by a gradual increase in percentage of water-stable aggregates during the spring and summer followed by a sharp decline in September.