IMPROVING FARM LAND 
165 
graded to have a very gradual slope of about three to 
eight inches to the hundred feet, as shown by the illus¬ 
tration. Drains are located where it is most convenient. 
These are shallow open ditches to carry the overflow. 
(Fig. 119.) 
Commercial Fertilizers are those concentrated forms 
of plant food which are prepared in factories. Waste 
products from animals, which have been slaughtered, in¬ 
cluding bones and other parts not eaten by people, are 
Fig. 122.—Implements for terracing. 
ground and dried with limestone or other material. Bones 
contain a certain amount of phosphorus, and the meat 
and other organic materials contain nitrogen. In some 
parts of the world stone containing phosphorus is mined, 
ground, and treated with acid to make what we know as 
acid phosphate. We also have potash, a mineral mined 
in Germany, though it is also obtained from Kelp ashes, 
a form of seaweed. Nitrogen is obtained from nitrates, 
mined in Chile. A mixture of these in proportion makes 
what is known as a complete fertilizer. This should con¬ 
tain about three per cent nitrogen, twelve per cent of acid 
