40 Mineral Nutrition of Plants 



ence due to its effects on the availability of the plant nutrients. Strong 

 acidity is, in general, unfavorable, while very slight acidity (about pH 

 6.5) is favorable. Figure 3 presents the subject diagramatically. It will 

 be noted that at strong acidity, there is a marked drop in the supplies 

 of available nutrients, which may exist as exchangeable cations. This is, 

 of course, to be expected, because with increasing acidity, increasing 

 amounts of these nutrient elements which might exist as exchangeable 

 cations become replaced by hydrogen. In certain ranges on the alkaline 

 side, some of the nutrient elements become much less soluble and avail- 

 able. A full discussion of this subject is beyond the scope of this paper 

 and may be found elsewhere (//, 12). 



The indirect influence of soil reaction on plant growth because of its 

 effects on the activity of soil microorganisms is extremely important, 

 particularly as related to the availability and fixation of nitrogen. In 

 general, the more desirable types of biological activity in soils are pro- 

 moted by a soil reaction that is neutral or nearly so. 



Increasing acidity favors the solubility of aluminum and manganese, 

 and also copper and zinc and other heavy metals should they happen 

 to be present in undue amounts. When soil acidity becomes more in- 

 tense than pH 5.0, concentrations of soluble aluminum and manganese 

 which are toxic to certain plants may occur in some soils. Unless built 

 up through the extensive use of spray materials or the addition in other 

 ways, toxic concentrations of copper and other heavy metals seldom if 

 ever occur. 



The physical condition of heavy soils, especially, is generally known 

 to be affected unfavorably by strong acidity which gives rise to a cal- 

 cium bicarbonate supply in the soil solution which is insufficient to keep 

 the clay well flocculated — a condition that is necessary for full promo- 

 tion of the desirable granular or crumb structure. In sandy soils, a high 

 level of available lime (low acidity or absence of it) may help to coun- 

 teract excessive looseness by acting as a binding agent. Improvement of 

 the physical condition of soils in the ways indicated helps to promote a 

 satisfactory regulation of the air and moisture supply of soils; this, in 

 turn, promotes the more desirable types of biological activity and chem- 

 ical reactions. A highly acid or alkaline condition, by inducing defloc- 

 culation, causes a movement of colloidal material from the surface soil 



