Yield gap analysis in relation to soil properties in direct-seeded flooded rice

This article aims to identify and quantify the soil properties that limit rice growth under fully irrigated direct-seeded conditions at field level in the Ebro Delta (Spain), with a temperate Mediterranean climate. A total of 50 fields spread along the area were selected where rice yield and soil properties were measured and their interrelations were evaluated. The most important soil properties for rice growth, were: (i) topsoil CEC, in strong association with clay and silt content and (ii) soil salinity. Soil salinity was positively correlated with the electrical conductivity of the groundwater and negatively related with yield. The groundwater was found to be of marine origin and variable in its electrical conductivity (from 2 to 60 dS m(-1)). High groundwater tables did not have significant effects on rice yield, except where the water had a large salt content. Several statistical and mathematical procedures were applied to compare differences in rice yield among fields. The simple correlation coefficient provided a comparison between yields and soil variables. Stepwise regression enabled yields to be predicted from soil variables. The law of the minimum of the limiting factors ('boundary line' method) identified the size of the yield-gap due to soil properties, ca. 3000 kg ha(-1). An average yield gap of 1000 kg ha(-1) was not identified and probably due to other factors such as management practices, Several other factors could also have positive effects, even though they did not emerge here. These statistical procedures, however, do not provide mechanistic explanations for the plant-soil processes. Results obtained for 1995 can be used to suggest field specific improvements of management allowing a relatively high efficiency of natural resource-use also in years for which no statistical analyses were made. (C) 1999 Elsevier Science B.V. AU rights reserved.