WASTES IN RELATION TO AGRICULTURE AND FORESTRY 



65 



soils" when about 10 to 15 percent of the base ex- 

 change capacity becomes saturated with sodium. 

 Thus, the base exchange characteristics become 

 an exceedingly important attribute in predicting 

 the manner in which waters varying in salt con- 

 tent will move into, through, and out of soils. The 

 behavior of soils with respect to adsorbed cations 

 also varies with the nature of the mineral pro- 

 viding the adsorption surface, that is, whether it is 

 kaolinite, montmorillonite, or vermiculite. 



"Slick spots" are a common problem in many 

 areas in the "Western States. These are areas in 

 fields that were at one time salt-affected soils — 

 specifically sodic soils. The soils became highly 

 dispersed and relatively impervious to water. 

 Hence, these "slick spots" are usually manifested 

 as barren spots in a cropped field. Understanding 

 the principles of base exchange phenomena afore- 

 mentioned lias been necessary in developing field 

 technology for reclaiming these salt-affected soils. 



Various amounts and kinds of soil amendments 

 have been used over the years as aids in the recla- 

 mation of salty soils. Research results have shown 

 that if the soil is afflicted with an accumulation of 

 neutral salts and has a low level of adsorbed so- 

 dium on the soil colloids, then leaching with good 

 water is all that is necessary for reclamation. Soils 

 containing appreciable levels of adsorbed sodium, 

 but also containing calcium and minerals such as 

 gypsum or calcium carbonate can be reclaimed by 

 adding sulfur, sulfuric acid, or other acidifying 

 amendments before leaching. If a soil contains 

 considerable adsorbed sodium but little or no cal- 

 cium, then it is necessary to add gypsum to the soil 

 before leaching. Understanding the principles of 

 base exchange reactions in soils is fundamental to 

 developing soil tests and predictive equations in 

 support of field management decisions on the use 

 of amendments for reclaiming salt-affected soils. 



It has long been held that weathering of soil 

 minerals is the main process in the production of 

 soil salts. Recent studies involving spectrographic 

 analysis have indicated that negligible amounts 

 of chloride and sulfate salts are released during 

 the weathering of primarily igneous materials. 

 Current evidence suggests that too little consider- 

 ation has been given to the role of accumulation 

 of fossil salt in soil as the origin of solubles. 



Soil Water 



The only way that salts can move into, through, 

 and out of soils is by the vehicle of water. Conse- 

 quently, studies on the physics of water retention 

 and movement in soil become fundamental to an 

 understanding of salt behavior. Soil colloids retain 

 water molecules tenaciously. Water molecules 

 closely attached to the surface of the clay may be 

 held with energy forces equivalent to thousands 

 of pounds per square inch. Research studies show 

 that when the water content of the soil drops to 

 the level called "permanent wilting percentage," 

 the water is held on the surface of the clay with 

 a force equivalent to about 200 pounds per square 

 inch. When soil is holding about all the water 

 it can against the force of gravity — a water con- 

 tent referred to as "field capacity" — the particle 

 surfaces are retaining the water with a force equiv- 

 alent to about 5 pounds per square inch. 



To understand water behavior in soils and its 

 role as a salt vehicle requires basic knowledge and 

 means of measuring the energy status of water in 

 soils. Attaining such information, as well as de- 

 signing apparatus that would make meaningful 

 measurements, has been a key objective of scien- 

 tists working on salt-affected soils for the past 30 

 years. The problem of measuring the energy of soil 

 moisture by an inserted instrument is illustrated 

 by the fact that the relative humidity of the soil 

 atmosphere in a soil at field capacity is about 100 

 percent, whereas the relative humidity in the soil 

 atmosphere at the permanent wilting percentage 

 is about 98 percent. Thus, an exceedingly narrow 

 range in relative humidity prevails over the range 

 of soil moisture content available for crop growth. 

 This situation is the basis for the tremendous 

 difficulties experienced in making psychrometric 

 measurements of the energy of soil water. Over the 

 last few years, rapid strides have been made in 

 perfecting such instrumentation for field work. 



Another fundamental need in determining salt 

 content of soils was instrumentation that would 

 measure salt content in situ without the tedium 

 of taking the soil into the laboratory to make 

 appropriate extractions and measurements. Dur- 

 ing the past few years rapid progress has been 

 made in developing a salt sensor that should be 

 practical for field use. 



