Quantifying the effects of land conditions on rice growth.

This thesis represents detailed research on the "rice-soils-weather" system of the Ebro Delta (Spain) providing knowledge on how temperature, radiation, soil properties and farm management determine rice growth. After an introductory chapter, the findings are developed step-by-step. (i) Chapter 2 is an overview of the conditions in the study area. (ii) Chapter 3 focuses on the study of the soils creating a soil geographic database of the northern part of the Ebro Delta. Four properties were used for defining soil units; soil development, drainage status, texture and salinity. (iii)Chapters 4 and 5 deal with rice growth under weather limited conditions. Potential productions of 13000 kg ha <sup>-1</SUP>were estimated, while maximum yields of 11000 kg ha <sup>-1</SUP>were recorded at field level. In Chapter 4, the phenological development, the daily dry matter production and the leaf area development of the rice crop were modelled. In Chapter 5, the use of remote sensing techniques at field level for monitoring the rice crop status were tested. (iv)Chapters 6 and 7 analyze rice growth in relation to soil properties and farm management. Two soil factors were found to dominate the effects on yield: one was topsoil CEC (in strong association with clay content) with a positive effect, and the other one was soil salinity with a negative effect. High groundwater tables did not have significant effects on rice yield, except when the water had a high salt content. Four main groups of causes within the cropping status limited rice growth: potassium and zinc shortage where a strong antagonism of either factor with sodium was observed, low plant establishment where a minimum number of 160-180 plants m <sup>-2</SUP>was necessary to maximize yield, and length of the growing season, especially the length of the pre-heading period in which the potential size of the crop was primarily determined.Potassium and zinc shortages in the plant were mainly induced by soil salinity. High K in saline soils did not increase K uptake. It remains to be determined if addition of Zn will increase Zn uptake. Information on environmental conditions of the study area and data on rice cropping is presented so that farmers, extension workers and decision-makers find themselves in a better position to adapt their management and policy making. This thesis intends to support development of decision systems and to increase cooperation between agricultural and environmental scientists focusing on concerns of society