Influence of soil properties on the aggregation of some Mediterranean soils and the use of aggregate size and stability as land degradation indicators

Soil aggregation in relation to other soil properties was studied along a c limatological transect in the Southeast of Spain. Three sites were selected along this transect ranging from semiarid to subhumid climatological condi tions. The aggregate size distribution, the macro and microaggregate stabil ity of the superficial soil horizon and their relations with other soil pro perties were analysed. Large aggregates(> 10, 10-5, 5-2 mm) were present in highest proportions in the most arid of the studied areas. These large agg regates were associated with high values of water-stable microaggregates; h owever, they did not improve soil structure and are related to high bulk de nsities and low water retention. Aggregates 1-0.105 mm were positively corr elated to medium, fine, very fine sand and silt fractions and to organic ma tter. Aggregates < 0.105 mm were positively correlated to organic matter an d clay content. Small aggregate sizes (1-0.105 and < 0.105 mm) had a positi ve influence on soil water retention and they seem a good indicator of soil degradation. Water stability of microaggregates showed a positive correlation with clay content while the stability of the macroaggregates depended on the organic matter content when the organic matter content was greater than 5 or 6%. Be low that threshold the carbonate content was strongly correlated with aggre gate stability. A general conceptual scheme of associations between aggrega te size distribution, water-stable microaggregation and textural characteri stics for the area was developed.