“One other thing is also necessary.  That is to keep in mind the number of bonds, or hands, possessed by each atom.  The atom of hydrogen has only one hand, and the same is true of potassium.  Each atom of oxygen has two hands; so that one oxygen atom can hold two hydrogen atoms in the chemical compound called water (H-O-H or H20).  Other elements having two-handed atoms are magnesium and calcium.  Strange to say, the sulfur atom has six hands but sometimes uses only two, the others seemingly being clasped together in pairs.  I will write it out for you, thus: 

Hydrogen sulfid:  H-S-H or H2S

Sulfur dioxid:  O=S=0 or S02

“The carbon atom has four hands, and atoms of nitrogen and phosphorus have five hands, but sometimes use only three.  Thus, in the compound called ammonia, one atom of nitrogen always holds three atoms of hydrogen; so, if you buy seventeen pounds of ammonia you would get only fourteen pounds of nitrogen and three pounds of hydrogen.  This means that, if the two-eight-two fertilizer contains two per cent. of ammonia, it contains only one and two-thirds per cent. of the actual element nitrogen, and a ton of such fertilizer would contain thirty-three pounds of nitrogen.  In other words it would take six tons of such fertilizer to replace the nitrogen removed from one acre of land in four years if the crop yields were fifty bushels of corn and oats, twenty-five bushels of wheat, and two tons of cowpea hay.”

“Six tons!  Why, that would cost a hundred and fifty dollars!  Well, well, I thought I knew we couldn’t afford to keep up our land with commercial fertilizer; but I didn’t think it was that bad.  Almost forty dollars an acre a year!”

“It need not be quite that bad,” said Percy.  “You see this two-eight-two fertilizer contains eight per cent. of so-called ‘phosphoric acid’ and two per cent. of potash, and those constituents may be worth much more than the nitrogen; but, so far as nitrogen is concerned, the two hundred pounds would cost from thirty to forty dollars in the best nitrogen fertilizers in the market, such as dried blood or sodium nitrate.”

“Well, even that would be eight or ten dollars a year per acre, and that is as much as the land is worth, and this wouldn’t include any other plant food elements, such as ‘phosphoric acid’ and potash.”

“No, that much would be required for the nitrogen alone if bought in commercial form.  I understand that the farmers who use this common commercial fertilizer, apply about three hundred pounds of it to the acre perhaps twice in four years.  That would cost about eight dollars for the four years, and the total nitrogen applied in the two applications would amount to 10 pounds per acre.”

“It is not quite correct to call ‘phosphoric acid’ and potash plant food elements.  They are not elements but compounds.”

“Like ammonia, which is part nitrogen and part hydrogen?”

“The problem is somewhat similar, but not just the same,” Percy replied.  “These compounds contain oxygen and not hydrogen.”