Tankage contains 4 to 8 per cent. of nitrogen and 7 to 20 per cent. of phosphoric acid.  Slaughter-house waste, such as meat and bone scrap, are boiled or steamed to extract the fat.  The settlings are dried and ground and sold as tankage.  It is much slower in its action than dried blood and supplies the crop with both nitrogen and phosphoric acid.

Dried Fish Scrap is a by-product of the fish oil factories and the fish canning factories.  It contains 7 to 9 per cent. of nitrogen and 6 to 8 per cent. of phosphoric acid.  It undergoes nitrification readily and is a quick acting organic source of nitrogen and phosphoric acid.

Cotton-seed Meal contains 7 per cent. of nitrogen, about 2.5 phosphoric acid, and 1.5 per cent. of potash.  It is a product of the cotton oil factories and is obtained by grinding the cotton seed cake from which the oil has been pressed.  It is a most valuable source of nitrogen for the South.

The nitrogen in the dried blood, tankage, fish scrap and cotton-seed meal, being organic nitrogen, must be changed by the process of nitrification to nitric acid or nitrate before it is available.  They are therefore better materials to use for a more gradual and continuous feeding of crops than the nitrate of soda or sulphate of ammonia.

Scrap leather, wool waste, horn and hoof shavings are rich in nitrogen but they decay so slowly that they make poor fertilizers.  They are used by fertilizer manufacturers in making cheap mixed fertilizers.

SOURCES OF PHOSPHORIC ACID

The principal commercial sources of phosphoric acid are: 

  Phosphate Rocks. 
  Bones. 
  Fish scrap. 
  Phosphate slag.

The Phosphate Rocks are found in shallow mines in North and South Carolina, Georgia, Florida and Tennessee, and also as pebbles in the river beds.  They are the fossil remains of animals.  After being dug from the mines the rock is kiln dried and then ground to a very fine powder called “floats” which is used on the soil.  The phosphoric acid in the floats is insoluble and becomes available only as the phosphate decays.  This is too slow for most plants so it is treated with oil of vitriol or sulphuric acid to make it available.  The phosphoric acid in the ground rock is combined with lime, forming a phosphate of lime which is insoluble.  When treated with the oil of vitriol or sulphuric acid, the sulphuric acid takes lime from the phosphate and forms sulphate of lime or gypsum.  The phosphoric acid is left combined with the smallest possible amount of lime and is soluble in water.  It is then called soluble or water soluble phosphoric acid.

Now if this soluble form remains unused it begins to take on lime again and turns back toward its original insoluble form.  After a time it gets to such a state that it is no longer soluble in water but is soluble in weak acids.  It is then said to be reverted phosphoric acid.  Reverted phosphoric acid is also called citrate soluble phosphoric acid, because in testing fertilizers the chemists use ammonium citrate to determine the amount of reverted phosphoric acid.