Effects of 30 years of cropping and tillage systems on surface soil test changes

A common belief is that no-till systems with adequate fertility will improve soil quality over other tillage systems. The objectives of this study were to determine if crop phase, tillage systems, and n rate in a long-term rotation affected soil chemical analyses in the surface 15 cm of soil and to evaluate the trend in chemical analyses. To test this hypothesis, surface soil samples were taken from a long-term (30-year) cropping and tillage study. This study was initiated in 1965 on a Harney silt loam soil in Central Kansas with every phase of the wheat-sorghum-fallow (WDF) rotation included each year. Tillage systems included clear-till (CT), reduced-till (RT), and no-till (NT). In 1975, four nitrogen (N) rates (0.22, 45, 67 kg N ha(-1)) were incorporated by subdividing the tillage plots. Topdressed N, as ammonium nitrate, was the only fertilizer added throughout the duration of the study. Soil samples were taken at depths of 0 to 7.5 and 7.5 to 15 cm in all plots in 1965 and in 1995. In 1998, soils on 1997 sorghum plots were samples in 2.5-cm increments to 15 cm. Samples from all dates were analyzed for pH, available phosphorus (AP), and organic matter (OM), and deviations from the controls from 1965 to 1995 were assessed by subtracting 1995 results from 1965 results. The change in soil pH showed a crop phase by sample depth interaction. In the wheat phase, pH in the top 7.5 cm increased by 0.19 and increased by 0.28 in the 7.5-15 cm layer. In the fallow phase, pH increased by 0.04 and 0.35 in the top 7.5 cm and 7.5-15 cm layers, respectively. The pH change for sorghum was intermediate for both depths. The increase in overall pH from 1965 to 1995 was unexpected and contrary to normal expectations of a decrease over time. Soil OM was not changed significantly over the 30 years of the study, suggesting that OM buildup or depletion is very slow under this cropping system on a nearly level soil with minimal soil erosion. Increasing the rate of N application significantly reduced pH in the upper increment samples, but had little effect on pH below 10 cm. The NT system had the lowest surface increment in pH, but differences among tillage systems were minimal below 7.6 cm. The AP was highest for NT in the surface increment, but for CT at deeper depths. Likewise, OM was highest for NT in the 2.5 cm increment and the CT at deeper increments. Under the present N management, pH may reach levels where herbicide effectiveness and phosphorus availability could be affected adversely. Deep tillage by one-way or mold-board plowing might be an interim solution to raise the pH before liming is implemented or P fertilizer is added to maintain adequate AP throughout the top 15 cm. Nitrogen management may need to be changed to some form of band-type placement to reduce the total N applied. Under the conditions of this study (WSF, reduced tillage, and 57 cm annual precipitation), soil OM increased very slowly.