a72 
and fats no other raw material is used, but in the manufacture of pro- 
tein a supply of nitrates, sulphates, and a small amount of phosphates 
are also used. Decaying organic matter, however, does greatly increase 
the fertility of soils through its favorable effect on the water content 
of the soil, ease of root penetration, soil aeration, and soil temperature, 
checking the leaching of minerals, and being a source of carbohydrate 
supply for some of the nitrogen-fixing bacteria and fungi in the soil. 
The decay of forest vegetation also adds humus to the soil, but not 
so rapidly nor in as great amounts as is added by the decay of prairie 
plants. The glacial drift that has been forested a considerable part 
of the time since the recession of the glaciers has much less humus 
accumulated in’ it than is found on the drift covered by a prairie 
vegetation. Soil surveys conducted by the Illinois Experiment Station - 
show that the forested soils have only from 25-50 per cent. as much 
organic matter in them as is found in the adjoining prairie soils. This 
is a result of the fact that most of the forest vegetation accumulates 
and mostly decays on the surface of the soil, while the numerous roots 
and root-stocks of the prairie vegetation accumulate and decay’ more 
slowly beneath the surface of the soil. This difference, of course, does 
not apply to flood-plain forests, where deposition during overflows is an 
important factor in soil fertility, nor to the occasional small forest areas 
that have developed from swamps in which the slowly decaying organic 
matter accumulates under water during the greater part of the year. 
The greater amount of humus accumulated in the prairie soils increases 
their fertility over that of the forest soils by increasing the check on 
the leaching of minerals, such as calcium, and by imcreasing the other 
favorable effects of humus noted above. This explains why farms for- ~ 
merly forested decrease in yield from year to year more rapidly than 
farms on adjacent prairie soils, both of which have had the same 
history except for the different kinds of native plants that have grown 
upon them. Where erosion follows deforestation the cleared lands be- 
come unprofitable for cultivation within a few years. Such areas should 
be kept in permanent forests. 
It is evident, therefore, that the variations in native fertility of 
the different soils of the state are a result, mainly, of the kinds of native 
plants that have grown upon them and the conditions under which these 
plants grew since glacial times. Prairie and forest, alike, developed 
on glacial till. Prairie vegetation has added more humus and brought 
about a bigger change in the surface soil than forest. Their subsoils 
are alike. It is obvious, therefore, that the cause of the original prairies 
or forests in Illinois is not to be found in the different ‘compositions of 
the soils of today: Aside from climatic conditions, the water content 
of the soil and not the kind of soil has: been the most important factor 
in determining what kind of native plants grew upon any given area 
of the ‘state. Under. the present conditions of drainage, whenever the 
prairie turf is broken by erosion or.other means a forest vegetation may. 
