30 SOIL CONDITIONS AND PLANT GROWTH 



south-west of Cornwall is very favourable for early vegetables, cabbage, 

 cauliflower, etc., whilst the cooler Lothians are well suited to potatoes. 

 It is possible by suitable operations to modify somewhat both the 

 temperature and the water content of the.soil, and so to make the soil 

 conditions rather more favourable for any particular crop. 



Food. — The food of plants, or rather the raw material out of which 

 food is synthesised, consists of carbon dioxide, water, oxygen, and 

 suitable compounds of nitrogen, phosphorus, sulphur, potassium, cal- 

 cium, magnesium, iron, and, apparently, manganese, silicon, sodium. 

 We have already considered the first three ; it remains only to be said 

 that the amount of carbon dioxide in the air is subject to slight varia- 

 tions which may be a factor of importance in crop production. Brown 

 and lEscombe (62) found that the amount varied at Kew from 2'43 

 to 3'6o ^ volumes per 10,000 volumes of air, the average being 2-94. 

 Taking the month of July as an example the following average values 

 were found : — 



Year. x8g8. xSgg, zgoo. igoi. 



CO2 in 10,000 volumes of air . 2"83 2'88 2-86 3'ii 



It is highly probable that the plant as a whole would respond 

 to variations of this order, making greater or less growth as the amount 

 of carbon dioxide rises or falls. 



Nitrogen. — Of all the nitrogen compounds yet investigated nitrates 

 are the best, and, in natural conditions, probably the only nitrogenous 

 food for non-leguminous plants. The seedling still drawing its sus- 

 tenance from the seed lives on other compounds : H. T. Brown (63) 

 found that asparagine was the most effective nutrient for the de- 

 tached embryo of barley, followed by other relatively simple sub- 

 stances like nitrates, glutamic and aspartic acids, ammonium sulphate, 

 etc., the more complex substances being less useful. The experimental 

 study of the nitrogen nutrition of adult plants is complicated by the diffi- 

 culty of growing plants under sterile conditions and thus obviating the 

 decompositions effected by bacteria ; much of the earlier work is vitiated 

 by this circumstance. Later work has satisfactorily shown that am- 

 monia is readily assimilated from solutions of ammonium sulphate, if 

 the concentration is not too high ; but even O"! per cent, was found 

 injurious by Maz6 (196). Krliger (157) concludes that ammonium 

 sulphate is less beneficial than sodium nitrate for mangolds, both 

 compounds are equally useful for oats, barley and mustard, while am- 

 monium sulphate is better for potatoes. Hutchinson and Miller (140) 



' Only on one occasion was so high a number obtained. 



