By their chemical composition, the colloids from various types of soils in Israel may be grouped into three classes. According to their mineralogical composition the colloids represent various associations, two of which are conspicuous.(a) Colloids with SiO,/R,O, ratio of 2.0-2.6. They are devoid of calcium carbonate, relatively poor in potassium and phosphorus, and content of organic matter is generally moderate. In the combination of clay minerals found, kaolin is predominant; in addition there are present illite, hematite, quartz, and, in some instances, montmorillonite. These colloids are characteristic of soils belonging to the Mediterranean red earths including terra rossas, brown Mediterranean soils, and brown-red sandy soils-leached soils of the humid and sub-humid regions, formed from hard limestone, dolomite, hard chalk, and kurkar.(b) Colloids with SiO,/R,O, ratio of 3'6-5.2. Rich in calcium carbonate, potassium, and phosphorus, relatively poor in organic matter. In the association of clay minerals, montmorillonite is predominant; in addition there are also present illite, kaolin, and quartz. I n some instances attapulgite was identified in appreciable quantities. T h e colloids also contain considerable quantities of calcite. T h e soils for which these colloids are characteristic were formed from soft chalk, marl, aeolian desert dust, and are found principally in the semi-arid and arid regions of Israel. They include rendzinas of the mountains and valleys, loess, aeolian sandy soils, and hamadas.(c) Colloids with SiO,/R,O, ratio of 3.4-3.8. Most of them contain calcium carbonate, their potassium content is moderate, and they are relatively poor in phosphorus and organic matter. By their mineralogical composition these colloids resemble the combination in which montmorillonite is the principal clay mineral. They are characteristic of alluvial soils formed from alluvium of the Mediterranean red earths and rendzinas. T h e soils were developed under subhumid and semi-arid climatic conditions. IN the course of the soil survey carried out on Israel soils, a study of the nature of the soil colloids was conducted in order to supplement and clarify views on the origin, formation, and properties of various soil types.Among the factors influencing the formation of the soils and determining their characteristics, the parent material and the climate are of prime importance. The variation in parent material, climate, and density of vegetation cover are great in the different regions of Israel.It may be noted that under identical humid and sub-humid climatic conditions on hard limestone or dolomite, terra rossa soils free of lime develop, and on chalk and marl rendzina soils rich in lime are formed. On the other hand under arid climatic conditions coarse desert calcareous soils develop from the same parent material which produces terra rossas in the humid and sub-humid regions.Changes occurring in the parent rocks during the course of the weathering process resulting in the formation of soils with very different p...