Tobacco growth, yield and quality affected by soil constraints on steeplands

Abstract

Soil tillage/management systems with minimum tillage and mulch are predominant in agriculture of southern of Brazil, but little is known for tobacco crop under conservation agriculture. Objective of this study was to evaluate soil tillage systems for tobacco crop and possibilities of changing the traditional system of tobacco cropping to improve the soil physical conditions and maintain high yield. In a randomized compete block design with three replicates, six tillage/management systems were studied: inverting tillage (IT); minimum tillage after fallow (MTf); minimum tillage after oat as cover crop (MTo); no-tillage without ridge (NT); no-tillage with ridge (NTr); and no-tillage with consolidated ridge (NTrc). The experiment was installed in a small, hilly watershed, located in southern Brazil, on a soil classified as Leptosol. The main physical properties evaluated were soil moisture measured with a time domain reflectometer in 0.00–0.06, 0.06–0.12 and 0.12–0.18 m soil layers, and soil temperature at 0.05 m depth measured with termopars, both during 87 days in the growth cycle; total porosity, macroporosity, microporosity, bulk density and soil water retention curve (WRC), in 0.00–0.06, 0.06–0.12, 0.12–0.18 and 0.18–0.24 m soil layers, 30 days after transplanting (DAT) and on 135 DAT, except for WRC, in 0.00–0.06 and 0.06–0.12 m soil layers; soil mechanical resistance to penetration; root distribution; nutrient availability in root zone; crop parameters leaf area, plant height, number of leaves and yield. Soil water retention was similar among tillage/management systems and soil layers, and decreased, approximately 50%, from soil saturation up to 33 kPa water tension. Soil moisture was longest within the range of available water of no-tillage (NTrc) and minimum (MTf and MTo) systems. Soil air-space was above the critical value (0.15 m3 m−3) during most days throughout the tobacco cycle in the surface soil layer for all systems. NTrc, MTo, NT and NTr provided longest consecutive period (30, 20, 13 and 11 days, respectively) with airspace below the critical in deepest soil layer. Soil temperature amplitude during tobacco crop cycle was highest for MTo and lowest for NT. Soil temperature had greater variations in the first 55 DAT, while from 55 to 75 DAT, there was a reduction in thermal amplitude and daily temperature average for most systems, with the increase in soil moisture and crop shading. The tillage/management systems did not show differences in bulk density in the surface, but differed in subsurface soil layers, being higher in no-tillage systems, but not reach critical values. Macroporosity behaved similar to total porosity and inverse to bulk density. Soil penetration resistance increased with depth in no-tillage systems, down to approximately 0.20 m and reaching 3 MPa. In NT, soil PR of 3 MPa was reached before 0.15 m depth; in IT and MTf, soil RP was less than 2 MPa down to 0.15 m depth and close to 3 MPa in deeper depths. No-tillage systems had the shallowest root development, not exceeding the 0.18-m soil depth. Contents of K and P in the tobacco rhizosphere were high, but there was no clear relationship between plant size (small vs. large) with the nutrient content in the root zone. The MTo and no-tillage systems, except NTrc, had higher leaf area index compared with IT and MTf. At 63 DAT, NTrc had significantly lowest leaf area index. The systems with higher dry-mass (NTr) and stem length (MTo) also showed higher productivity. The greatest productivity per area was observed in tilled soil (IT, MTf and MTo), while lower productivity was observed for NT. Less tillage and more surface mulch contributed to soil physical quality improvement, while no-tillage system reached some critical physical values, particularly in the deepest soil layer. Furthermore, tillage and ridging for tobacco cropping in shallow soil reduced the effect of some soil physical stressors to tobacco growth and development. Consequently, soil management systems with winter cover crop, ridging and reduced soil tillage increased flue-cured tobacco yield, even low effect of tillage and ridging occurred for leaves qualitative classification, cropped on shallow soils on steeplands.