A deep tillage and deep fertilization experiment having a split-plot design with depth of mixing of soil and fertilizer as major plots and fertilizer amounts as subplots with four replications was laid out in August 1955 on Weir silt loam, a claypan soil, at the Cooperative Agronomy Research Center at Carbondale, Illinois.
Four depths, 9, 18, 27, and 36 inches, of tillage or mixing and fertilizer placement were used. Four rates of treatment were established, the minimum being 9 tons of limestone, 3,630 pounds of 0-20-0, and 1,452 pounds of 0-0-60 per acre. Rates 2, 3, and 4 were 2, 3, and 4 times rate 1, respectively.
Particle size distribution, bulk density, and 1/3- and 15-atm. moisture determinations on the tilled soil zones indicated that relatively good mixing was obtained. Redistribution of the clay in the soil section by mixing the silty A horizon with the clayey B horizon did not alter the available moisture storage capacity as calculated between 1/3- and 15-atm. moisture percentages.
There were no significant differences in the average moisture content of the soil below a depth of about 24 inches during the 1956 growing season on any of the plots tilled to various depths. The average moisture content of the soil from 0 to 24 inches in depth was lower in the plots tilled 9 and 18 inches than in the plots tilled 27 and 36 inches, probably because less clay from the B horizon was incorporated in the upper 24 inches of the 9- and 18-inch tilled plots.
Depth of mixing of the soil did not increase corn yields irrespective of rate of fertilizer treatment. For the two years, 1956 and 1957, fertilizer including limestone at the minimum rate increased corn yields an average of 40 bushels per acre over no fertilizer or limestone. There were no significant increases for rates of fertilizer over the minimum rate. In general, the higher rates of fertilizer tended to depress yields, especially where the fertilizer was placed only in the upper 9 inches rather than at greater depths.
Tillage alone did not influence corn root penetration in 1956. Fertilization increased root penetration, especially where the fertilizer was placed throughout the entire tilled zone. Very high rates of fertilizer placed in the upper 9 inches reduced root penetration.