42 Mineral Nutrition of Plants 



into the subsoil where it may be precipitated as a hardpan. The deple- 

 tion of the surface soil of its colloidal material and the formation of a 

 hardpan have an unfavorable influence on soil fertility. 



Some soil-borne plant diseases are favored by an acid condition, while 

 others are not. Certain fungous diseases like "finger-and-toe" develop 

 to a harmful extent only in acid soils, whereas other plant diseases like 

 the potato scab are most serious in well-limed soils, and, hence, some 

 acidity (pH 5.2) is desirable under many conditions for potato grow- 

 ing. Aside from the cases known to have a direct relationship to plant 

 diseases, there are a few special plants like the cranberry, blackberry, 

 and watermelon for which a soil of at least slight acidity seems desira- 

 ble and in some cases necessary for the best growth. It is not known 

 just why these plants grow better on an acid soil, but it seems possible 

 that in some cases plant diseases or malnutrition due to a lack of iron 

 where the pH is too high may again be factors. 



The influence of soil reaction on the competitive powers of different 

 species of plants to establish themselves and crowd out others needs to 

 be recognized. For example, the common sorrel (Rumex acetosella) is 

 sometimes said to grow best on acid soils. However, when grown under 

 controlled conditions free of competition by other species of plants, it 

 grows better on neutral than acid soils. It is commonly found grow- 

 ing on strongly acid soils because there it usually meets with less com- 

 petition from other weeds as well as crops. It would be found growing 

 even better on the less acid soils were it not that here other plants grow 

 so well and vigorously that they crowd out the sorrel. 



Undoubtedly, competition working in conjunction with soil acidity 

 or alkalinity in the way just indicated has a marked influence on the 

 powers of different species of plants for establishing and maintaining 

 themselves under natural conditions, and thus has affected the charac- 

 ter of the native vegetation found in many regions. 



Crop plants as well as plants in general vary greatly as regards the 

 reaction of the nutrient medium in which they grow best. Some species 

 of plants grow best in an acid medium, some in an alkaline medium, 

 and others in a neutral or nearly neutral medium. The reasons for 

 these reaction preferences are due to the direct and indirect influences 

 of soil reaction just discussed. 



