A fertilizer is a material added to the soil for the purpose of supplying food for plants.

An amendment is a substance or material that modifies the physical, mechanical and chemical nature of the soil.

Stable manure is both fertilizer and amendment. Lime is used mostly as an amendment, since it is not often necessary to supply it for the plant-food that it contains. On sandy soils it may be needed as a fertilizer.

The extent of the fertilizer industry is indicated by the following figures of complete fertilizers manufactured in the United States in two given years : —

 

1900

1905

Increase

Per Cent of Increase

Quantity in tons of 2000 lb.....

1,478,826

1,603,847

125,021

8.5

Value.................

$26,318,995

$31,305,057

$4,986,062

18.9

Fertilizer discussions are concerned mostly with nitrogen, phosphorus, and potassium (always in combination with other elements, never used in their elemental form), since these are the elements most likely to be deficient in the soil. To be economically usable as a fertilizer, a material must not only contain some one or more of these three elements in available form, but it must be relatively low in price and obtainable in large quantities. Nitrate of potash (saltpetre) is a good fertilizer, but it is impossible to use it because of the cost. Many of the fertilizer materials, — as bone-black, blood, ashes, — are waste products or by-products.