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 adversel y affect crop development. Conservation tillage systems are useful to contr ol soil degradation, but may lead to excessive soil compaction, negatively impacting crop growth. Physical measurements were made during 1994 on a Che rnozemic loam soil (Petrocalcic Paleudoll) with a petrocalcic horizon at a depth of 1.2 m in Balcarce (Buenos Aires, Argentina). The experiment starte d 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 p reviously 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 reduc ed growth of wheat under NT. Soil bulk density in the 3-8 and 15-20 cm laye rs was measured by the cylinder and the paraffin methods. There were no sig nificant differences between treatments (P less than or equal to 0.05). Mec hanical 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 a t 5-10 cm depth; NT: 1.6 MPa, CT: 1.0 MPa at 10-15 cm depth; and NT: 1.3 MP a, CT: 0.9 MPa at 15-20 cm depth. The function of pore size distribution de termined 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 conduct ivity determined by a constant head technique was significantly lower (P le ss 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 neutr on 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 sugg est 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.