Onion-forage cropping systems on a Vertic Argiudoll in Uruguay: Onion yield and soil organic matter, aggregation, porosity and permeability

Hostipastoral systems in small farming units that integrate horticulture crops, as main cash crop, with sheep/cattle raising on cultivated forage, in Uruguay, present physical restrains to crops because of high consistency and compaction of the vertic soils and unique tillage requirements. The objective of this study was to evaluate the effect of different hortipastoral systems on soil organic matter, aggregation, porosity, permeability and onion yield during a cropping year in onion-forage hortipastoral systems. A long-term experiment was installed in 1995 in the National Institute for Agricultural Research (INIA-Las Brujas) in Uruguay, on fine (mixed) smectitic thermic superactive Vertic Argiudolls. The soil was previously under native grasslands, and treatments consisted of cropping systems for onion (Allium cepa L.) production, maintained for three years and then rotated. Onion yield was evaluated during the period 1998–2006, and in the cropping year 2006–2007, corresponding to the end of a 3-year cropping cycle, we evaluated soil organic matter and physical properties and functioning in the cropping systems: alfalfa, fescue, forage consortium (birdfoot trefoil, alfalfa and white clover), onion after alfalfa, onion after fescue, and onion after consortium. At four times during the cropping year, aggregate stability and organic matter were determined for the 0–0.05 m soil layer, whereas soil bulk density, porosity, and hydraulic conductivity were determined in 0–0.10 and 0.10–0.20 m layers. The results showed high variability in onion yields from 1998 to 2006 (10.8–36.4Mgha⁻¹), mainly related to high rainfall (higher than 150 mm) that affected crop planting, and the low rainfall (lower than 30 mm) that affected growth and development. In two out of eight years, there were significant differences in onion yield among treatments, where alfalfa (1999) and fescue (2006) crops are more beneficial to onion yield than pasture consortium. To establish the onion crop in the vertic soil tillage is needed, but tilling the soil under forage crops reduces macro and increases microaggregation (reduces GMDw from 1.8 to 0.8 mm), increases macroporosity (close 0.07 to 0.25 m³ m⁻³) and functioning in terms of water and air permeability (Ka increase above 3 µm² after tillage) and decreases the degree of compaction (under forages decreased DC from 69-84% to 58–60% in surface layer). In conclusion, there is a significant potential of hortipastoral systems and proper management of soil organic matter to minimize adverse effects of climate, and improve productivity stability of intensive production systems. Single forage crops, either legume or grass, are more beneficial to onion yield than consortium of grass plus legumes.