Acidity, exchangeable fertilizer

Another type of reaction that responds to WD cycles is the fixation of K and NH4 ions by smectite (3-7). The fixation of K in smectite has been studied extensively by soil scientists because of its effect on the availability of plant nutrients. The reaction also decreases smectite s ability to swell, decreases its cation exchange capacity (CEC), and modifies its BrjSnsted acidity. Therefore, an understanding of this phenomenon is applicable to many fields of study that are concerned with swelling clays, fields such as soil fertility, soil mechanics, waste disposal, clay catalysis, and the geochemistry of ground and surface waters. 

The acidity of the soil may conveniently be characterized by the content of metal ions, relative to the total cation exchange capacity of the soil. In the acid types of soil considered here, this base saturation degree is usually below 10%. The degree of base saturation will be reduced when (1) the roots take up exchangeable cations from the soil, and (2) when accumulation of dead plant material increases the amount of humus, and thereby the cation exchange capacity. To a certain degree, both of these processes are reversible, but if plant products are removed from the area without application of fertilizers, manure or lime, this represents an acidification by reducing the available supply of cations. 

Aluminium in soils is closely connected to soil acidity and is also discussed in the chapters on acid soils and ion-water reactions. The acidity of acid soils is due to the reactions of water with exchangeable Al3+ on the surface of soil particles. The strong Al-water reaction repels H+ from the water molecules iuto the soil solution. This can create soil acidities as low as pH 4.5. Stronger acidity means other H+-yielding reactions—organic acids from soil organic matter decay, sulfur and sulfide oxidation, phosphate fertilizers, ammonia oxidation, acid rain, and Fe- and Mn-water reactions—are active. 

The simplest method for producing sodium phosphates involves the neutralization of phosphoric acid with either sodium hydroxide or carbonate, but, unless some waste source of alkali is found, it may not be the most economical. For this reason other methods of producing sodium phosphates were investigated. The two most promising methods involve conversion of monocalcium phosphate (or normal or triple superphosphate fertilizer) to sodium phosphate by ion exchange, and neutralization of phosphoric acid with dilute sodium hydroxide produced electrolytically from brine. 

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