258 HARMFUL AGENTS IN SOILS AFFECTING FERTILITY 



disease infested fields are remedies suggested to control these 

 diseases or to avoid injury from them. Sometimes plants may be 

 developed that are resistant to certain diseases. Cabbage resistant 

 to yellows is a good illustration (Fig. 165). 



Too Much Water Is Harmful. On lowlands the harmful 

 effect of too much water is commonly observed, but the lack of 

 proper drainage on some upland fields having good surface drain- 

 age is very often least suspected. Frequently certain fields or 

 portions of them are found to grow good crops of hay and grain 



FIG. 163. Diagram showing how ants foster the corn root-aphis. Upper right, the corn 

 root-aphis, enlarged. (U. S. D. A.) 



but fail in growing corn, in spite of the fact that manure and 

 fertilizers may have been added. In almost every case the cause 

 of the poor corn is due to seepage water from higher land, which 

 keeps the subsoil too wet and cold to favor the growth of corn. 

 Tile drainage is the remedy (Fig. 166). 



Alkali Is Injurious. In regions of deficient rainfall, alkali 

 in soils often proves injurious, and in some sections in humid 

 climates, alkali spots occur. Frequently the irrigation of land in 

 irrigated sections causes the accumulation of salts near the surface 

 (being brought up from the subsoil through capillary rise of applied 

 moisture) in sufficient amounts to cause the most fertile lands to 



