; - 1 -^o 1 ; ,*>* ;'>. a 



v3^:??3-:-?*fi ?-? ? 



2 LAND DRAINAGE 



sition of a soil will insure a crop. A soil may be of good 

 character in these respects and yet fail absolutely to pro- 

 duce a good return. 



2. Physical condition of soil. The physical condition 

 of any soil bears a profound relation to its ability to pro- 

 duce a crop. It is undoubtedly safe to assert that soils 

 more frequently fail to produce large or even satisfactory 

 yields because of improper physical condition, than be- 

 cause of improper chemical or physical composition. 

 The physical conditions of soil upon which plant growth 

 most largely depends are : proper temperature ; proper 

 ventilation ; proper structure, or tilth ; proper moisture. 



TEMPERATURE 



There are three lines of activity within the soil having 

 to do with the welfare of the crop : (1) food prepara- 

 tion, (2) germination, (3) root activity ; to which may be 

 added (4) recuperation. 



3. Food requirements of plants. The chief food ele- 

 ments required by plants are : carbon, oxygen, hydro- 

 gen, nitrogen, potassium, phosphorus, calcium, magnesium, 

 sulfur, chlorine, and iron. Each of these is always com- 

 bined with one or more other elements before plants can 

 use it for food. The carbon is combined with oxygen, 

 forming carbon dioxide; the calcium is combined with 

 carbon and oxygen, forming calcium carbonate 1 ; the 

 nitrogen is combined first with hydrogen and oxygen, 

 forming nitric acid (HNO 3 ), and then exchanging the 

 hydrogen for some other element, such as calcium or 

 potassium, to form calcium nitrate (Ca(NO3) 2 ), or potas- 

 sium nitrate (KNO 3 ). 



1 Probably changed to nitrates before reaching the plant. 



