﻿12 BULLETIN 6, U. S. DEPARTMENT OF AGRICULTURE. 



enough to reach the permanently moist soil beneath the dry surface 

 layer, the 3 7 oung plants promptly die. It is safer, therefore, either 

 to sow the seed with a drill or to broadcast it during a heavy rain, 

 which will beat the seed into the ground and at the same time fur- 

 nish sufficient moisture to carry the young plants through the period 

 of danger from drought. 



The turning under of heavy leguminous crops on these sandy soils 

 restocks the land with humus and the humus decomposes to such a 

 stage that a condition of partial or temporary alkalinity appears at 

 times to have been reached, for good crops of even such nonacid 

 plants as wheat and timothy are sometimes secured from these natu- 

 rally acid lands after the treatment here described. 



BENEFICIAL EFFECTS OF SOIL ACIDITY. 



An actual beneficial effect from soil acidity is likely to be felt in 

 another direction hitherto insufficiently recognized, namely, the con- 

 trol of some of the fungous diseases of cultivated plants. Reference 

 has already been made (p. 8) to the fact that the fungus causing 

 the scab of the potato can not grow if the soil reaction is acid. 

 Another example is furnished by the root-rot of the tobacco plant, 

 caused by a fungus named Thielavia basicola. Briggs has shown 

 that this disease is prevalent in tobacco plantations that have re- 

 ceived excessive applications of lime or other alkaline fertilizers 

 an l that it is readily controlled by the use of acid fertilizers. 



In Porto Rico the extension of the pineapple industry has been 

 retarded by a disease known as chlorosis, the principal external 

 mark of which is the yellowing of the foliage and the consequent 

 poor nutrition of the plant. From* investigations by Gile and by 

 Loew it appears that the yellow color of the leaves and the accom- 

 panying weakness of the plant are due to the lack of iron, and that 

 where the soil contains an excess of lime the organic acids which are 

 needed to dissolve the iron of the soil are themselves neutralized 

 and the iron, although present, is not available for absorption by the 

 pineapple roots. 



In the upbuilding of the agriculture of the arid Western States 

 certain diseases of plants have appeared which are commonly called 

 by plant physiologists cases of " malnutrition." The causes of these 

 maladies are unknown. The maladies themselves, however, are asso- 

 ciated with pronounced alkalinity of the soil and they occur in 

 plants that were native in humid regions where the soil varies from 

 weak alkalinity to actual acidity. May it not be worth while for 

 investigators to ascertain whether some of these mysterious " mal- 

 nutrition " difficulties can not be remedied by an acid treatment of 

 the soil? 



