"Don't you get any fun out of doing a good day's work?" asked Jim.
A slide of earth from the top of the pit threatened to bury Newton in gravel, sand and good top soil. A sweet-clover plant growing rankly beside the pit, and thinking itself perfectly safe, came down with it, its dark green foliage anchored by the long roots which penetrated to a depth below the gravel pit's bottom. Jim Irwin pulled it loose from its anchorage, and after looking attentively at the roots, laid the whole plant on the bank for safety.
"What do you want of that weed?" asked Newton.
Jim picked it up and showed him the nodules on its roots--little white knobs, smaller than pinheads.
"Just white specks on the roots," replied Newton.
"The most wonderful specks in the world," said Jim. "Ever hear of the use of nitrates to enrich the soil?"
"Ain't that the stuff the old man used on the lawn last spring?"
"Yes," said Jim, "your father used some on his lawn. We don't put it on our fields in Iowa--not yet; but if it weren't for those white specks on the clover-roots, we should be obliged to do so--as they do back east."
"How do them white specks keep us from needin' nitrates?"
"It's a long story," said Jim. "You see, before there were any plants big enough to be visible--if there had been any one to see them--the world was full of little plants so small that there may be billions of them in one of these little white specks. They knew how to take the nitrates from the air----"
"Air!" ejaculated Newton. "Nitrates in the air! You're crazy!"
"No," said Jim. "There are tons of nitrogen in the air that press down on your head--but the big plants can't get it through their leaves, or their roots. They never had to learn, because when the little plants--bacteria--found that the big plants had roots with sap in them, they located on those roots and tapped them for the sap they needed. They began to get their board and lodgings off the big plants. And in payment for their hotel bills, the little plants took nitrogen out of the air for both themselves and their hosts."
"Their hotel-keepers--the big plants. And now the plants that have the hotel roots for the bacteria furnish nitrogen not only for themselves but for the crops that follow. Corn can't get nitrogen out of the air; but clover can--and that's why we ought to plow down clover before a crop of corn."