The first indication that soils contain organic phosphorus was obtained in 1844 by Mulder (20), in experiments wherein he could not obtain phosphorus-free preparations of certain soil organic matter fractions. Since the time of Mulder, a number of investigations have dealt specifically with the problem of soil organic phosphorus, and it is now generally recognized that a portion of the soil phosphorus occurs in organic forms.

The distribution of organic phosphorus in different soil types and groups has been investigated in only a preliminary way, but it has become apparent that the quantities of organic phosphorus are correlated with the quantities of soil organic matter. In Iowa and other North Central states, there are extensive areas of soils that contain relatively large quantities of both substances.