122 FUNDAMENTALS OF FRUIT PRODUCTION 



in places where there is danger from alkah, as the rise of the water table 

 attending poor drainage may result in bringing salts from lower soil 

 to the surface and thereby increase concentrations in the upper layers 

 as evaporation takes place. Moderate, as opposed to excessive, irrigation 

 is a preventive measure. Though there is not often a choice between 

 two or more sources of irrigation water, the irrigation fruit grower should 

 remember that certain water supplies are more or less saline and that 

 special precautions must be taken to neutralize the injurious effect when 

 such water alone is available. Much can be done to avoid the effects of 

 soil alkali through the choice of alkali-tolerant fruit crops and particularly 

 the selection of stocks having this characteristic, though the roots of the 

 cion may be susceptible. The importance of caution in the use of fertil- 

 izers, particularly in irrigated sections, has been mentioned. 



Finally, it should be pointed out that insufficient as well as excessive 

 concentrations may exist. That extremely low concentrations permit 

 growth has been emphasized but it is the insufficient concentration of 

 particular salts that renders the use of fertilizers necessary. 



Soil Toxicity. — The chemical composition of the soil solution must be 

 considered in its effect on absorption. In this connection the presence 

 of toxic substances is of great importance. The toxins may be organic 

 compounds formed by bacterial activity from dead plant tissue. They 

 are not, as a rule, excreted as such from plant roots, though this occurs 

 under exceptional conditions, as for example when the supply of oxygen 

 is deficient. Fragments of dead root hairs, roots and possibly aerial 

 portions of the plant washed down into the soil, constitute the material 

 acted upon by microorganisms to produce poisons. 



It must not be overlooked that bacterial activity may also produce 

 compounds beneficial to plant life in so far as the products of bacterial 

 action may serve as a source of food, as has been pointed out.^*^^ 



General and Specific Effects. — The general effects of toxins are shown 

 in decreased green weight and inhibited growth. The specific morpho- 

 logical effects vary considerably with different substances, some producing 

 more marked effects on the roots than on the green parts of the plant. 

 For instance, vanillin-affected plants show decreased growth of the top 

 and root growth is strongly inhibited. Dihydroxystearic acid affects 

 the tops but especially the roots, the root tips becoming darkened, their 

 growth stunted; the root ends are enlarged and often turned upward 

 like fishhooks and their oxidizing power is strongly inhibited. Pyridine 

 and picoline affect the green parts more than the roots. Cumarine- 

 affected plants have stunted tops and broad distorted leaves; quinone- 

 affected plants are tall and slender, with thin narrow leaves. Guanidine 

 has apparently no effect on the roots, but the green parts develop small 

 bleached spots which spread, the plant becomes weakened and the leaves 

 break at the stem, wilt and die.^^i''^* 



