CHEMICAL DETERMINATIONS CONCERNING THE SOIL. 129 



All plants may be regarded as requiring twelve different 

 kinds of food, out of which they are able to form their various 

 tissues. These foods are oxygen, hydrogen, nitrogen, carbon, 

 sulphur, phosphorus, potash, iron, lime, soda, magnesia, and 

 chlorine. The gardener need not consider some of these at all, 

 as he has not to supply them. For instance, oxygen and 

 hydrogen are provided for him in the form of water. Carbon is 

 always furnished by the processes of combustion and decay, and 

 by the breath of animals, including human beings, who give it 

 off from their lungs, as an effete product in the form of carbon- 

 dioxide, which is a combination of carbon and oxygen. This gas 

 when taken in by the leaves of plants is divided into two parts ; 

 the carbon is retained by the plant, and the oxygen is given off 

 into the air. This is a popular view of the process, and for the 

 moment sufficient, though not strictly accurate. 



Amongst the constituents needing careful attention from the 

 cultivator, the most important — with the exception of nitrogen, 

 to be considered in detail presently— are potash and phosphorus. 

 These are met with in only small quantities in most soils, but 

 their presence in sufficiency is essential to the plants, if a high 

 standard of fertility is to be attained. 



Lime in some form will also need to be considered. It is 

 present in nearly all soils ; but the amount is frequently below 

 that which is desirable for fruit and leguminous crops ; besides 

 its chemical and mechanical action on many clays make its use 

 as a manure often very beneficial. Tillage (cultivation) quickens 

 ■a, chemical change naturally produced in the soil, which is called 

 nitrification, and the nitric acid thus formed dissolves lime as 

 readily as hot tea dissolves sugar, and it is then washed out by 

 the rain ; and, therefore, unless re-supplied, a diseased condition 

 is rapidly produced, and the plant3 are generally stunted and 

 weak. 



Chlorine needs far less attention, yet it must not be wholly 

 disregarded. For instance, an acre of Mangel-wurzel will take 

 up 5G lbs. of it, while Wheat will only absorb h lb. 



Iron is sometimes wanting in a soluble condition, and plants 

 are frequently improved in colour of flower and fruit, and in 

 weight of yield by its manurial use. The remaining chemical 

 requirements for healthy plant food are, except in very rare cases, 

 present in an amount exceeding the demand. We have then 



K 



