BULK DENSITIES OF BRAZILIAN AMAZON SOILS RELATED TO OTHER SOIL PROPERTIES

The results of this study provide the first baseline for predicting D- b from soil properties for soils across the Amazon basin. Bulk density values are needed to convert nutrient content and organic carbon (OC) content to weight of nutrient and OC per unit area; unfortunately, co mmon field methods to measure D-b are limited with regard to reliable, complete, and uniform soil data. Much effort has been made in finding alternative solutions to predict D-b from soil properties. We hypothe sized that D-b could be reliably estimated by multiple regression of O C, soil textural properties, and some chemical properties. Using the d ata of 323 soil horizons from the Brazilian Amazon basin, a stepwise m ultiple regression (SMR) procedure was developed to predict D-b from O ther soil properties. Multiple regression relationships were obtained for all the data, which were also partitioned by layer and then by mai n soil order: Latossolos (Oxisols, 62 horizons) and Podzolicos (Alfiso ls and Ultisols, 212 horizons). The SMR on all the data showed that cl ay content is the best predictor of D-b, accounting for 37% of the var iation. Adding OC content increased the explained variance up to nearl y 50%. Predictions of the models were improved when the data were part itioned by order and by horizon type. In the case of Latossolos (Oxiso ls), the use of OC and day content as predictors increased the percent age of explained variation, reaching 71% using all layers and 79% for A horizons. The results of this study will provide a basis for estimat ing OC stocks in the Amazon basin.