Climate, Soils and Rotation 95 



Drainage and soil air 



All crops require more or less air in the soil for health 

 and growth. The need of air in the soil is one reason 

 why potatoes do poorly on too heavy clay soils. One 

 reason for the ridge culture so much used on wet or heavy 

 soils like those of the Aroostook region and the volusia 

 soils region is the need of drainage and soil air. Air is 

 necessary to the work of bacteria in the soil and to the 

 chemical changes which make plant-food available in 

 it. Water is necessary to plant growth, but too much 

 in the soil is injurious. Tile-drainage removes the sur- 

 plus of water quickly after rains and so extends down- 

 ward the area in which roots can live. Western potato- 

 growers who irrigate have learned to apply the water in 

 furrows between the rows. The rows are ridged high 

 enough to allow the forming tubers and a large part of 

 the root system to remain above the level of the water in 

 the furrows. The potato is very sensitive to the presence 

 of too much water in the soil. This is shown by the 

 quickness with which potatoes die after being covered 

 by floods or by standing water after rains. The immer- 

 sion of the tops for a few days or even hours is fatal. 

 Too much water in the soil will kill part of the roots, and 

 greatly reduces the vitality of the plants. 



Critical period 



The potato is now very different from the wild plant 

 before its domestication. Then it produced few tubers, 

 and these were very small. The present varieties have 

 been produced from the original wild stock by hun- 

 dreds of years of breeding and selection to meet the de- 



