And phosphoric acid from the soil

And phosphoric acid from the soil

The sweet potato draws potash, nitrogen, and phosphoric acid from the soil, but in applying these as fertilizers the grower must study and know his own soil. If he does not he may waste both money and plant food by the addition of elements already present in sufficient quantity in the soil. The only way to come to reliable conclusions as to the needs of the soil is to try two or three different kinds of fertilizers on plats of the same soil, during the same season, and notice the resulting crop of potatoes.

Source: Agriculture for Beginners