"I greatly enjoyed my microscopic work," said Percy, "and still more the work in the chemical laboratory where we finally learned to analyze soils, to take them apart and see what they contain,—how much nitrogen how much phosphorus, how much limestone, or how much soil acidity, which means that limestone is needed. Then I also enjoyed the work in the pot-culture laboratory, where we learned not to analyze but to synthesize; that is, to put different materials together to make a soil. Thus, we would make one soil and put in all of the essential plant food elements except nitrogen, and another with only phosphorus lacking, and still another with both nitrogen and phosphorus present, and all of the other essential elements provided, except potassium, or magnesium, or iron. These prepared soils were put in glass jars having a hole in the bottom for drainage, and then the same kind of seeds were planted in each jar or pot. Some students planted corn, others oats or wheat or any kind of farm seeds. I grew rape plants in one series of pots, and I have a photograph with me which shows very well that all of the plant food elements are essential.
"You see one pot contained no plant food and one was prepared with all of the ten essential elements provided. Then the other pots contained all but one of the necessary soil elements, as indicated in the photograph."
"Why, I never saw anything like that," said Mrs. Thornton.
"But I have many a time," said her husband, "right here on this old farm; I don't know what's lacking, of course, but some years I've thought most everything was lacking. But, according to this pot-culture test, you can't raise any crops if just one of these ten elements is lacking, no matter how much you have of the other nine; and it seems to make no difference which one is lacking, you don't get any crop. Is that the fact, Mr. Johnston?"
One pot with no plant food, and one with all the essential elements provided, and still others with but one element lacking. All planted the same day and cared for alike.
"Yes, Sir," Percy replied. "Where all of the elements are provided, a fine crop is produced, but in each case where a single element is omitted that is the only difference, and in some cases the result is worse than where no plant food is supplied. It seems to hurt the plant worse to throw its food supply completely out of balance than to leave it with nothing except what it draws from the meager store in the seed planted. Of course all the pots were planted with the same kind of seed at the same time, and they were all watered uniformly every day."
"Those results are very striking, indeed," said Miss Russell," but I suppose one would never see such marked differences under farm conditions?"
"Only under unusual or abnormal conditions," Percy replied, "but the fact is that as a very general rule our crop yields are limited chiefly because the supply of available plant food is limited. Sometimes the clover crop is a complete failure on untreated land, while it lives and produces a good crop if the soil is properly treated; and in such cases the difference developed in the field is just as marked as in the pot-cultures. In general we may set it down as an absolute fact that the productive power of normal land depends primarily upon the ability of the soil to feed the crop.
"I have here a photograph of a corn field on very abnormal soil. They had the negative at the Experiment Station and I secured a print from it, in part because I became interested in a story connected with this experiment field, which our professor of soil fertility reported to us.
"This shows a field of corn growing on peaty swamp land, of which there are several hundred thousand acres in the swamp regions of Illinois, Indiana, and Wisconsin. This peaty soil is extremely rich in humus and nitrogen, well supplied with phosphorus and other elements, except potassium; but in this element it is extremely deficient. This land was drained out at large expense, and produced two or three large crops because the fresh grass roots contained some readily available potassium; but after three or four years the corn crop became a complete failure, as you see from the untreated check plot on the right; while the land on the left, where potassium was applied, produced forty-five bushels per acre the year this photograph was taken, and with heavier treatment from sixty to seventy-five bushels are produced."