Tillage and crop residue management affect Vertisol properties and grain sorghum growth over seven years in the semi-arid sub-tropics. 2. Changes in soil properties

Abstract

Six treatments, disc (D), blade (B) or zero (Z) tillage, each with stubble (crop residue) retained (+) or removed (−), were imposed during fallow periods between annual grain sorghum crops from June 1978 to June 1985 on a grey Vertisol in the semi-arid sub-tropics of central Queensland. Plots were neither irrigated nor fertilized. Soil profiles for chemical analysis were sampled post-harvest and pre-plant after fallow. For surface soil (0–0.1 m and sometimes also 0.1–0.2 m) during the 7 years, net decreases were measured for organic and total carbon, total nitrogen, total bicarbonate extractable and calcium chloride extractable phosphorus, total sulphur, total and exchangeable potassium. Net increases were measured for exchangeable sodium, calcium and magnesium. No net changes were found for dispersion ratios and cation exchange capacity. A net decrease in nitrate at 0.6–1.6 m began after 3 years. At 0–1.6 m, changes in pH, electrical conductivity, chloride and ammonium were negligible.

General means at 0–0.1 m decreased annually from June 1978 by 3.9% for organic carbon, 3.1% for total nitrogen, 7.5% for bicarbonate extractable phosphorus and 10.0% for calcium chloride extractable phosphorus. Decreases in organic carbon and total nitrogen had similar trends for each tillage treatment, being greater with stubble removed than with stubble retained. Decreases were least for Z+. After five years the increase in exchangeable sodium was highest for Z−. The pattern for each tillage treatment was for higher exchangeable calcium and magnesium at 0−0.1 m and higer exchangeable potassium at 0–0.02 m with stubble retained than with stubble removed. After 7 years the silt + clay dispersion ratio was lower for Z+ and Z− than for the other treatments.

There was an average net gain of 30 kg ha⁻¹ of nitrate-N at 0–0.6 m during fallow periods followed by a similar loss during cropping periods. Nitrate at the end of the fallow was equally distributed at three depths: 0–0.1, 0.1–0.2 and 0.2–0.6 m. On two occasions after a crop, the pattern was less nitrate (0–0.6 m) with stubble retained than with stubble removed, the difference for each tillage treatment increasing in the order D<B<Z. At the end of fallow periods, nitrate at 0–0.6 m was generally less with zero than with blade and disc tillage: more nitrate with stubble removed than with stubble retained was evident in the first years of the experiment. Net nitrate decreases at 0.6–1.6 m during 7 years (from an initial general mean of 156 kg N ha⁻¹) ranged from 62 kg N ha⁻¹ for Z− to 128 kg N ha⁻¹ for Z+. The decrease for all tillage treatments was greater where stubble was retained than where it was removed. Decreases where stubble was retained were in the order D<B<Z.