A Shoi't Lesson on Soil 91 



is useful in rendering soil porous, but sodium sulphate becomes 

 detrimental if an excess accumulates. Evaporation is checked 

 in dry farming by keeping the surface soil pulverized, so that 

 the crystals accumulate on the surface less rapidly, or the soil 

 may be shaded by shallow rooted plants. Plants belonging to 

 the Beetroot family have been found useful for this purpose as 

 they are halophytes and can take up brak-forming salts to a 

 considerable extent. None of these methods seems to be as 

 effective as under-draining. 



Even in dry countries, soil drainage is as important as soil 

 irrigation. In the rainless districts of the Nile nearly as much 

 effort has been expended on drainage as has been in construct- 

 ing the immense dam at Assuan and the irrigating ditches. 



Calcium is of further use as a vehicle for supplying nitrogen 

 in a soluble form as calcic nitrate. It, also serves to neutralize 

 acid soils and oxalic acid which accumulates in plant tissues 

 as waste matter. Seedlings, in water cultures, without calcium, 

 seem less able to respond to the influence of light and gravity. 



Silica is usually mentioned as valuable in giving rigidity to 

 ste.ns such as the grasses where it is found in abundance. It 

 has been found, however, more abundant in leaves than in the 

 stems where it would be of advantage in supporting the heads 

 of grain. An interesting series of water cultures carried on in Dr. 

 Hahn's laboratory by Mr. I^undie showed that grains flourished 

 and came to perfection in the absence of any trace of silica in 

 the solutions. But the experiments suggested that its presence 

 might defend the plants against the entrance of the rust fungus. 



Plants may be grown in the laboratory in the following 

 nutrient solution : — 



gr- 



Ferric chloride should be added when the seedlings are put 

 into the solution, which should be diluted with distilled water 

 to one-sixth the strength of the stock solution, 



