30 SOIL CONDITIONS AND PLANT GROWTH 



at all, because ripening may be so long delayed that frosts intervene 

 and damage the crop. 



Water supply and temperature are the two chief factors determin- 

 ing the distribution of crops. In the warm dry eastern counties crops 

 are grown for seed ; great quantities of wheat and barley are grown in 

 Norfolk, Suffolk, and the Isle of Thanet ; mangold seed and turnip 

 seed is produced in East Kent. Wetter districts are more favourable 

 for swedes and oats ; very wet districts for grass. The warm, moist 

 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 nutrition of plants is complicated by the fact that 

 plants synthesise their own food from various substances taken out of 

 the air and the soil. It is common in farmers' lectures to speak of 

 these as the actual foods, but the student must be perfectly clear in 

 his own mind that they are only the raw materials out of which the 

 food is made. We are here concerned only with the supply of raw 

 materials and not at all with the way in which the plant uses them. 

 These raw materials consist 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 Escombe (61) found that the amount varied at Kew from 2-43 

 to 3-60 l 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. 1898. 1899. 1900. 1901. 



CO 2 in 10,000 volumes of air . 2-83 2-88 2-86 3-11 



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 (62) 



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



