Effect of Locust Trees upon the Available Mineral Nutrients of the Soil 1

F many years the black locust (Robinia Pseudo-Acacia L.) has been used in the repair of soils ruined by erosion and mismanagement. Its ability to establish itself on barren subsoils is equaled by but few other plants. Shortly after it is established the site becomes improved to such an extent that other species, in themselves less tolerant to barrenness, are found to thrive. Kentucky bluegrass may be encouraged to grow voluntarily on bare subsoil in association with locust. Ferguson (~)3 called attention to the fact that in a catalpa grove adjacent to a locust grove the growth of the catalpas was much better close to the locusts and that it decreased gradually as the distance from the latter increased. He reported, and it was later confirmed by Maclntyre and Jeffries (2) that the nitrogen content of the soil close to the locust grove was higher and that it diminished as the distance increased. Chapman (3) has likewise shown that the growth of catalpa, white ash, tulip poplar, black oak, and chestnut decreased progressively as the distance from the locusts increased. He found that the total nitrogen in the locust grove was 3,900 pounds per acre and only x,8oo pounds outside the influence of the locusts. In a 25-year-old locust grove in Mason County, Illinois, Gustafson (4) noted in ~934 that the surface of the sand under the locusts was covered with the remains of the locust leaves and some small twigs which had accumulated, and that Kentucky bluegrass (Poa pratensis) was well established under the trees, but that it did not gain foothold a short distance away. He attributed the beneficial effect of the trees to the nitrogen and other nutrients, and to better moisture and temperature conditions. Apparently, quantitative evidence of the nitrogen-accumulating power of the locust has been demonstrated. Its ability to bring available mineral nutrient elements to the surface and to alter the pH value of the soil is not as well known, or at least quantitative data to substantiate this belief are wanting. Of course it is well known that all trees possess this tendency. A measure of the mineral-enrichening function of locusts is reported herewith. About seven years ago a steep barren road embankment, cut into what is known as the Morrison soil, was planted to locust saplings. This soil, although underlain at considerable depth by dolomitic sandstones is very acid throughout its A and B horizons and the greater part of its C horizon. The B horizon contains a ferruginous clay. Under natural conditions the surface soil grows no bluegrass