Page:Popular Science Monthly Volume 84.djvu/59

Rh in the almost measureless deposits contains both calcium and magnesium in very suitable form both for plant food and for correcting or preventing soil acidity.

In the Illinois system of permanent fertility, ground natural limestone is applied, where needed, at the rate of about two tons per acre every four years. With the same price and purity, probably the dolomite is preferable to the high calcium stone, although both kinds have been used with very good results. Further data from investigations now in progress are expected to furnish definite information as to the relative value of these materials.

Phosphorus.—Attention was called to the fact that two million pounds of the average crust of the earth contains 50,000 pounds of potassium; but compared with this we find only 2,000 pounds of phosphorus. Likewise, the plowed soil of an acre of average Illinois land contains about 35,000 pounds of potassium, but less than 1,200 pounds of phosphorus. When grain is. sold from the farm, about equal amounts of phosphorus and potassium are carried away, while in independent systems of live-stock farming much more phosphorus than potassium leaves the farm.

At 3 cents a pound for phosphorus one can double the amount of that element contained in the plowed soil of our $200 land at a cost of $35 an acre,, while to double the potassium in the same stratum would cost more than $1,000 an acre.

Phosphorus can be purchased, delivered at the farmer's railroad station in Illinois, for about 3 cents a pound in the form of fine-ground natural rock phosphate, for 10 to 12 cents a pound in steamed bone meal, or for 12 to 15 cents in acid phosphate. It can be used with profit in any of these forms, but the data thus far secured in comparative experiments plainly indicate that, with equal amounts of money invested, the natural rock phosphate will give the greatest profit in rational permanent systems. At least 1,000 pounds per acre every four years should be applied, and for the first application even three or four tons per acre is not considered too much phosphate by those who best understand the need and value of phosphorus on normal land.

Nitrogen and Organic Matter.—There is a rather common opinion that the growing of clover enriches the soil in nitrogen, and many even believe that clover in crop rotation will maintain the fertility of the soil. These same people are likely to think that the application of limestone and phosphate involves much expense and work, and that the returns are much less certain than those from other labor and money investments.

Such opinions are largely erroneous. The mere growing of clover on normal land does not enrich it. Even the nitrogen is not increased unless the clover crop is returned to the soil either directly or in farm manure. Rotation with such crops as corn, oats and clover depletes the