Modelling the impacts of in-field soil and irrigation variability on onion yield

Abstract

Globally, onion (Allium cepa L.) represents an extremely important crop in terms of production, value and consumption. Similarly, in the UK onion production is considered to be one of the most important high-value field vegetables, with ca. 300,900 tonnes being produced from 8,448 ha (DEFRA 2010). However, a great variability in onion productivity (yield) has been identified due to a combination of environmental, genotypic, management and agronomic factors. The increasing demand for high quality vegetables and their supply year round is adding significant pressure on farming enterprises, which add to the challenges UK onion producers already face (e.g. crop management, irrigation and pest control decision-making). The aim of this research was to assess the impacts of in-field soil and irrigation variability on onion yield and quality. Therefore, the scientific evidence on the relationships between onion yield, crop water use, irrigation and crop quality were initially reviewed and the evidence corroborated with data from an industry survey. In order to evaluate the effects of soils and irrigation variability on yield, under different agroclimatic conditions, a crop growth model (AquaCrop) was calibrated and then validated using experimental field data. The scientific evidence in the literature and results from the industry survey were used to validate and calibrate the AquaCrop model for brown onion (cv Arthur). Statistical analyses were used to assess crop model goodness of fit. A series of scenario were then defined and the AquaCrop model used to assess the impacts of different onion cropping practices, production areas and typical and extreme climatic conditions on crop yield. The effects of irrigation non-uniformity (typical of a boom and linear move irrigation application system) on production were assessed under a series of agroclimatic conditions (five different years) and two contrasting soil types (sandy and sandy loam). The simulations showed that the lowest yield (8.6 t DM/ha) and greatest variability (standard deviation: 0.23 t DM/ha) occurred under the driest agroclimatic conditions. Production on sandy soils resulted in higher yield (in average 0.24t DM/ha) than on a sandy loam soil. The yield under hosereels fitted with booms were statistically significant (Kruskal-Wallis analysis) lower than for the linear move, although the difference was very small (average of 9.52 t DM/ha vs. 9.56 t DM/ha). Under ‘average dry’ conditions, the highest yield was produced on sandy soils (8.78 t DM/ha), contrary to ‘average’ agroclimatic conditions, where the highest yield was produced on sandy loam soils (9.55 t DM/ha). For the driest season, the effects of irrigation variability were only significant on sandy soils (8.80 t DM/ha and 8.73 t DM/ha for hosereel fitted with linear move and boom, respectively). The study of uniform versus non-uniform irrigation applications showed that onion yield was higher under uniform irrigation. The differences between yields produced under uniform and non-uniform irrigation increased with increasing climatic aridity (0.01-0.18 t DM/ha compared to average values). Differences were greater in cases of boom application systems. Onion yield generated by simulations of uniform conditions fell within the range found in the literature. The variability observed under non-uniform irrigation was the same (up to 30-40%) as the overall variation reported by growers.