CONTRIBUTION OF THE COARSE SAND FRACTION TO SOIL GENESIS
ABSTRACT
The aim of the present paper is to emphasize the interest that microscopic study of the coarse sand fraction (2-0.2 mm) may have for soil genesis research.
The concentration and isolation of coarse sand grains from the remaining particles facilitates a quicker, easier and more complete analysis, than if such grains were examined in a microscopic preparation of undisturbed soil.
The mineralogical results from the study of 120 Spanish soils was used in genesis studies as follows:
1. Parent Rock
In the coarse sands, it was possible to find inherited grains which could furnish interesting data concerning the origin of the soil parent materials, e.g. rock fragments, monomineral grains indicating a particular petrological origin and minerals which in the parent rock would pass unnoticed, due to their low content but which are concentrated in this soil fraction.
2. Lithological Discontinuities and Buried Soil Horizons
The mineralogy of the coarse sands of soils which do not exhibit discontinuities was found to be either very homogeneous or to show gradual changes The values of the ratios between two minerals (or between two varieties of the same mineral) with regard to depth in those soils which present lithological discontinuities show a non-regular distribution.
The buried soil horizons can be manifested by the mineralogy of the coarse sand, although this is not always so.
3. Origin of the Minerals and Nature of the Alterations
In order to understand soil genesis, it is important to establish which minerals are inh erited from th e paren t rock and whic h are of pedolo gic al o rig in ( orth ic disorthic and allorthic). The degree and nature of alteration which they present is also of great interest.
4. Degree of soil Euolution and Weathering Indices
A comparison of the unstable/stable mineral ratios was of great help in the establishment of weathering indices for the different horizons or soils studied . It may be seen that most of the soils examined showed net increases in the weathering ratios of biotite/quartz (in 80% of the cases), plagioclase/quartz (71%) and Kfeldspars/quartz (74%).
This decrease in the percentage of unstable minerals as a function of evolution can be used to calculate weathering indices. In general, we observed good results working with the ratios of feldspars/quartz and biotite/quartz, and were able to calculate a simple weathering index on the basis of the value of this ratio in the lowest, and logically least altered, horizon and by dividing this value by the value of this ratio in each horizon.
Weathering ratios of two soil chronosequences have shown several interesting relationships between the mineralogy of the coarse sand fraction and the weathering These ratios give way to lower values with the age of the soil. Furthermore the weathering ratios also become progressively more pronounced with depth.