phosphoric acid

On page 24 you learned that phosphoric acid

On page 24 you learned that phosphoric acid, potash, and nitrogen are the foods most needed by plants. "Worn out," then, to put it in another way, usually means that a soil has been robbed of one of these plant necessities, or of two or of all three. To make the land once more fruitful it is necessary to restore the missing food or foods. How can this be done? Two of these plant foods, namely, phosphoric acid and potash, are minerals. If either of these is lacking, it can be supplied only by putting on the land some fertilizer containing the missing food. Fortunately, however, nitrogen, the most costly of the plant foods, can be readily and cheaply returned to poor land.

Fig. 230. Alfalfa ready for the Third Cutting

Source: Agriculture for Beginners