Over street and Dean 81 



Evans and Rost (/6) have shown the organic sulfur in many Min- 

 nesota soils to be an important part of the total soil sulfur. From 16 to 

 79 per cent of the total sulfur in these soils was found to be organic. 

 The carbon-organic sulfur ratio for Minnesota soils varied from 87 to 

 451. There have been no studies on the specific nature of the organic 

 sulfur compounds in soils. Little is known about the mineralization of 

 soil sulfur in relation to the total supply of available sulfate. 



Anions associated with the clay fraction of soils 



Phosphates, boron, and molybdenum tend to accumulate in soils 

 in association with the clay fraction. Analyses of clay separated from 

 soils quite generally show a large part of the soil phosphorus and 

 boron to be associated with this fraction. Analyses of soils have shown 

 heavy ones to contain greater quantities of boron and molybdenum 

 than those of light texture. What is known about the chemical nature 

 of the anions associated with clays has been arrived at by indirect 

 methods. Mineralogical and physical methods for the examination of 

 clays have failed to show the presence of crystalline compounds of 

 phosphorus or boron. Only through studies of the reactions between 

 clays and solutions containing phosphates or borates — that is, phos- 

 phorus or boron fixation studies — is it possible to arrive at many of the 

 properties of the phosphates and borates associated with the clay frac- 

 tion of soils. 



The availability of the anions associated with clays has not been 

 satisfactorily understood. The equilibrium between the clay surfaces 

 and the soil solution is considered by many to be a dominant factor 

 controlling the availability of phosphates in soils. It is not improbable 

 that the availability of borate and molybdate ions is similarly con- 

 trolled. 



Soil solution and soluble salts 



Studies of the water-soluble material in soils date back to very 

 early attempts to relate soil composition with nutrient uptake by plants. 

 The inadequacy of the soil solution theory is probably the key to our 

 present interest in nutrient availability. The amounts of soluble ma- 

 terials in soils range from very small amounts in the acid soils of the 

 humid regions to the saline conditions of soils in the arid regions. The 



