Effect of no-tillage on some soil physical properties of a structural degraded Petrocalcic Paleudoll of the southern Pampa of Argentina

Soil structural deterioration from continuous cropping systems can adversely affect crop development. Conservation tillage systems are useful to control soil degradation, but may lead to excessive soil compaction, negatively impacting crop growth. Physical measurements were made during 1994 on a Chernozemic loam soil (Petrocalcic Paleudoll) with a petrocalcic horizon at a depth of 1.2 m in Balcarce (Buenos Aires, Argentina). The experiment started in 1992 with wheat (Triticum aestivum L.), followed in 1993 with soybean (Glycine max. (L.) Merr.) and in 1994 with wheat again. The soil had been previously cultivated for 25 years and presented structural degradation (40% of the optimum value). The aim of the study was to evaluate the effect of two tillage systems: conventional tillage (CT) and no-tillage (NT) on soil physical properties and to determine soil physical factors related to reduced growth of wheat under NT. Soil bulk density in the 3-8 and 15-20 cm layers was measured by the cylinder and the paraffin methods. There were no significant differences between treatments (P less than or equal to 0.05). Mechanical resistance measured by the cone penetrometer at emergence gave the following values (P less than or equal to 0.05): NT: 1.6 MPa, CT: 1.1 MPa at 5-10 cm depth; NT: 1.6 MPa, CT: 1.0 MPa at 10-15 cm depth; and NT: 1.3 MPa, CT: 0.9 MPa at 15-20 cm depth. The function of pore size distribution determined by the water desorption method was significantly different between tillage systems (P less than or equal to 0.05). The volume of pores with a diameter larger than 20 mu m was greater under CT than under NT (CT: 26.1%, NT: 16.8%). Structural stability as measured by both dry and wet sieving was not significantly different between treatments (P less than or equal to 0.05). Plots under CT and NT had low stability indexes (NT: 30%, CT: 26%), showing a deterioration of soil structure. The saturated hydraulic conductivity determined by a constant head technique was significantly lower (P less than or equal to 0.05) in NT than in CT plots (NT: 3.5x10(-7) m s(-1) CT: 10.9x10(-7) m s(-1)). Soil water content in the topsoil measured by neutron probe was higher for NT in the early in the growth season. From anthesis to physiological maturity no significant difference (P less than or equal to 0.05) in soil water content was found between tillage systems. Data suggest that increased soil mechanical resistance under NT can decrease growth of wheat roots and reduce dry matter accumulation and wheat yield. (C) 2000 Elsevier Science B.V. All rights reserved.