“It is true,” said Percy, “that soluble salts are brought to the surface in the rise of moisture by capillarity in times of partial drouth; and in the arid regions where the small amount of water that falls in rain or snow leaves the soil only by evaporation, because there is never enough to produce underdrainage, the salts tend to accumulate at the surface.  The alkali conditions in the arid or semiarid regions of the West are thus produced.  But in humid sections where more or less of the rainfall leaves the soil as underdrainage the regular loss by leaching is so much in excess of the rise by capillarity that soils which are not affected by erosion or overflow steadily decrease in fertility even under natural conditions, with no cultivation and no removal of crops.  Of course this applies at first only to the mineral plant foods, as phosphorus potassium, magnesium, and calcium.  While mineral supplies are abundant in the surface soil, there may be a large acumulation of organic matter and nitrogen, especially because of the growth of wild legumes, which are very numerous and in places very abundant, especially on some of the virgin prairies of the West.  However, as the process of leaching proceeds there comes a time when the growth of the native vegetation is limited because of a deficiency in some essential mineral plant food, such as phosphorus, or the limestone completely disappears and soil acidity develops which greatly lessens the growth of the legumes.

“Decomposition of organic matter begins almost as soon as any part of the plant ceases to live, and there is certain to come a time when the rate of decomposition and loss exceeds the rate of fixation and accumulation; and from that time on the organic matter and nitrogen as well as the mineral plant foods continue to decrease in the surface, until finally the natural barrens are developed, such as are found in different sections of the World and in some places even where the rainfall is sufficient for abundant crops.”

“Yes, Sir,” said Mr. West.  “I know that is true.  I have visited Tennessee and I know there are some extensive areas there of practically level upland which have always been considered too poor to justify putting under cultivation, and they are called the ’Barrens’.”

“I know about those barren lands, too,” said Percy.  “Our teacher of soil fertility in college told us that a farm is more than a piece of the earth’s surface.  He said if we only wanted to get a large level tract of upland where the climate is mild and the rainfall abundant and where all sorts of crops do well on good soil, including the wonderful cotton crop which brings a hundred dollars for a thousand pounds, while corn brings forty dollars for a hundred bushels,—­well, he said we could go to the Highland Rim of Tennessee where, according to analyses reported in 1897 by the Tennessee Experiment Station, the surface soil of the ‘Barrens’ contains eighty-seven pounds of phosphorus and the