Application of the ‘Pedogenic Index’ to soil fertility studies, Saskatchewan

Abstract

The ‘pedogenic index’, originally used to quantitatively express the changes which occurred in soils during their development, has been applied essentially in pedogenetic studies. This study proposes the application of the pedogenic index concept to the field of soil fertility by examining the soil potassium, soil phosphorus and cation exchange properties by this mass-balance approach which incorporates depth-bulk density parameters in measuring the composition of soil horizons. In a series of soils ranging from forested Luvisolic to grassland Chernozemic soils, the index demonstrates that the total K 2O loss from the Luvisols amounts to about 12% compared to 2% in a Black Chernozemic soil; the K 2O loss was 6% in a transitional Dark Gray Luvisol studied. Of K-bearing minerals, the K-feldspars are weathering to a greater extent than the micas in both Luvisols and Chernozems, with feldspar-K losses amounting to 15% and 8%, respectively. There is evidence that K released from feldspars is being fixed by micaceous minerals, particularly in the lower parts of the sola. In contrast, the Luvisols have lost about 40% of their original total P, with 15% and 23% losses recorded for the Dark Gray Luvisol and the Black Chernozem. 32 to 55% of the Ca-phosphates, which originally accounted for over 90% of the total P in the soils studied, have been lost from the sola of the soils studied. Some of the Ca-phosphate presumably has been converted to other forms, particularly organic P in the grassland soils where organic matter accumulations are the greatest. The cation exchange capacity values of the soils studied are low in surface horizons of Luvisols due to low organic matter contents and loss of clay through lessivage, whereas the Dark Gray Luvisol and the Orthic Black Chernozem have higher values reflecting the organic matter build-up under grassland vegetation. The observed trends in this study confirm the increased weathering and leaching losses which occur in soils ranging from grassland to forested soils, but the study also contributes a quantitative evaluation of changes resulting from such processes. The study also indicates how %-composition values for soil nutrients may be misleading and suggest accumulations in horizons which have actually undergone serious losses. The pedogenic index applied to soil nutrients or other properties related to soil fertility provides a long-needed quantitative dimension in the evaluation of the fertility status of soils and in nutrient cycling studies.