NITROGENOUS FERTILIZERS . 165 



the early spring. It is better adapted to sandy soils than to 

 heavy ones. 



216. Guano. — This was formerly a very important fer- 

 tilizing material, but there is comparatively little of it im- 

 ported into this country at present, because the world's 

 supply is nearly exhausted. It consists of the excrement and 

 carcasses of sea fowl. The composition of guano depends 

 on the climate of the region in which it is found. Guano from 

 an arid region contains much more nitrogen and potash than 

 that from a region of more rainfall, because these constituents 

 have been leached out of the latter. All of the plant-food 

 materials contained in guano are in a readily available con- 

 dition, and its fertilizing value is high. 



217. Effects of nitrogen on plant growth. — The all impor- 

 tant part that nitrogen plays in plant growth is that of an 

 indispensable constituent of protein, which is the basic sub- 

 stance in every cell of every plant. It is therefore concerned 

 in the formation of every part of the plant. If the supply 

 of nitrogen is inadequate, the effect is to decrease the yield 

 of the crop, especially the leaves, stems, stalks or straw, 

 while the quantity of grain produced is not curtailed to the 

 same extent. On the other hand, an excess of available nitro- 

 gen causes an abundant growth of the vegetative parts of the 

 plant rather than of the seed or grain. As a result, in 

 cereals the straw becomes so long and weak that the plants 

 fall down or " lodge." Grass crops are less likely to suffer 

 from an excess of nitrogen than are cereals, and nitrogen 

 is particularly beneficial to the grasses. Many vegetables 

 that are grown for their vegetative parts can utilize to good 

 advantage a large quantity of nitrogen. If nitrogen is not 

 present in sufficient quantity for cereals, the kernels are 

 shriveled and light. There can be no doubt that the lack of a 

 readily available supply of nitrogen at critical periods in the 

 growth of plants is a frequent cause of curtailed crop yields. 



