Yield gap analysis in relation to soil properties in direct-seeded floodedrice

This article aims to identify and quantify the soil properties that limit r ice 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 pr operties were measured and their interrelations were evaluated. The most im portant soil properties for rice growth, were: (i) topsoil CEC, in strong a ssociation with clay and silt content and (ii) soil salinity. Soil salinity was positively correlated with the electrical conductivity of the groundwa ter and negatively related with yield. The groundwater was found to be of m arine origin and variable in its electrical conductivity (from 2 to 60 dS m (-1)). High groundwater tables did not have significant effects on rice yie ld, except where the water had a large salt content. Several statistical an d mathematical procedures were applied to compare differences in rice yield among fields. The simple correlation coefficient provided a comparison bet ween yields and soil variables. Stepwise regression enabled yields to be pr edicted 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 practic es, Several other factors could also have positive effects, even though the y did not emerge here. These statistical procedures, however, do not provid e mechanistic explanations for the plant-soil processes. Results obtained f or 1995 can be used to suggest field specific improvements of management al lowing a relatively high efficiency of natural resource-use also in years f or which no statistical analyses were made. (C) 1999 Elsevier Science B.V. AU rights reserved.