“Excepting none,” Percy replied.  “The legume plants, like clover, take nitrogen from the soil so far as they can secure it in available form, and in this respect clover is not different from corn.  The respect in which it is different is the power of clover to secure additional supplies of nitrogen from the air when the soil’s available supply becomes inadequate to meet the needs of the growing clover.  If the conditions are suitable for nitrogen-fixation, then the growth of the legume plants need not be limited by lack of nitrogen; whereas, nitrogen is probably the element that first limits the growth and yield of all other crops on your common soils.”

“Now, what do you think of that, Girls?  With millions of dollars’ worth of nitrogen in the air over every acre, our crops are poor just because we don’t use it.  I wish you would tell me something about the suitable conditions for nitrogen-fixation, Mr. Johnston.  You understand, Girls, that nitrogen-fixation is simply getting nitrogen from the inexhaustible supply in the air by means of little microscopic organisms called bacteria, which live in little balls called tubercles attached to the roots of certain plants called legumes, like cowpeas and clover.  Corn and wheat and such crops can’t get this nitrogen.  Now, Mr. Johnston is telling about nitrification, a process which is entirely different from nitrogen-fixation.  Excuse me, Mr. Johnston, but I wanted to make this plain to Mrs. Thornton and Miss Russell.”

“I am glad you did so,” Percy replied.  “As I was saying, nitrification has no connection whatever with the free nitrogen of the air.

“All plants take their food in solution; that is, the plant food taken from the soil must be dissolved in the soil water or moisture.  Of the essential elements of plant food, seven are taken from the soil through the roots into the plant.  These seven do not include those of which water itself is composed.  Now, these seven plant food elements exist in the soil almost exclusively in an insoluble form.  In that condition they are not available to the plant for plant food; and it is the business of the farmer to make this plant food available as fast as is needed by his growing crops.

“The nitrogen of the soil exists in the organic matter; that is, in such materials as plant roots, weeds, and stubble, that may have been plowed under, or any kind of vegetable maker incorporated with the soil, including all sorts of crop residues, green manures, and the common farm fertilizers from the stables.  When these organic materials are decomposed and disintegrated to such an extent that their structure is completely destroyed, the resulting mass of partially decayed black organic matter is called humus.  The nitrogen of the soil is one of the constituents of this humus or other organic matter.  It is not contained in the mineral particles of the soil.  On the other hand the other six elements of plant food are contained largely in the mineral part of the soil, as the clay, silt, and sand. thus the iron, calcium, magnesium, and potassium, all of which are called abundant elements, are contained in the mineral matter, and usually in considerable amounts, while they are found in the organic matter in very small proportion.  The phosphorus and sulfur are found in very limited quantities in most soils, but they are present in both organic and mineral form.