It is impossible to say here just what amounts or what kinds of fertilizer should be purchased, because no two farms are exactly alike as to soil, methods of cropping or methods of tillage.

There are certain factors, however, which will serve as a general guide and which should be considered in determining the kind and amount of fertilizer to buy.

These factors are: 

  The crop. 
  The soil. 
  The system of farming.

THE CROP

Crop roots differ in their powers of feeding, or their powers of securing plant foods.  Some roots can use very tough plant foods, while others require it in the most available form.  Some roots secure nitrogen from the air.  The cowpea roots, for example, can take nitrogen from the air and they can use such tough phosphoric acid and potash that it seldom pays to feed them directly with fertilizers.

A bale per acre crop of cotton requires for the building of roots, stems, leaves, bolls, lint and seed: 

103 pounds of Nitrogen.
41   "    "  Phosphoric Acid.
65   "    "  Potash.

and yet experiment and experience have proved that the best fertilizer for such a crop contains the following amounts of plant food: 

Nitrogen                            20 pounds
Phosphoric Acid                     70   "
Potash                              20   "

This means that cotton roots are fairly strong feeders of nitrogen and potash, but are weak on the phosphoric acid side.

The small grains, wheat, oats, barley and rye, can use tough phosphoric acid and potash, but are weak on nitrogen, and as they make the greater part of their growth in the cool spring before nitrification is rapid, they are benefitted by the application of nitrogen, particularly in the form of nitrate, which is quickly available.

Clover, peas, beans, etc., have the power of drawing nitrogen from the air, but draw from the soil lime, phosphoric acid and potash.  Hence the phosphates, potash manures and lime are desirable for these crops.

Root and tuber crops are unable to use the insoluble mineral elements in the soil, hence they require application of all the important plant foods in readily available form.  Nitrogen is especially beneficial to beets.  Turnips are benefitted by liberal applications of soluble phosphoric acid.  White and sweet potatoes require an abundance of potash.

If we are growing tender, succulent market garden crops, we need nitrogenous manures, which increase the growth of stem and foliage.

Fruit trees are slow growing plants and do not need quick acting fertilizers.

The small fruits, being more rapid in growth, require more of the soluble materials.

A dark, healthy green foliage indicates a good supply of nitrogen, while a pale yellowish green may indicate a need of nitrogen.

A well developed head of grain, seed pod or fruit indicates liberal supplies of phosphoric acid and potash.