68 INDIAN CORN CULTURE. 
for this purpose, such as the clovers in the 
North and the cowpea in the South, increase 
the soil fertility by returning to it additional 
plant food secured from the atmosphere or soil. 
The clover plant, it has been demonstrated, 
adds materially to the fertility of the surface 
soil by securing nitrogen from the atmosphere 
and holding it, and also by absorbing and hold- 
ing nitric acid from below the cultivated sur- 
face, so that clover plowed under decidedly 
increases soil fertility. In the Southern States 
the cowpea plowed under is a recognized reno- 
vator of worn-out lands. 
According to Sir J. B. Lawes,* “the leguminous 
(clovers, peas, beans, etc.) are the only plants 
which ean be said distinctly to enrich the sur- 
face soil when plowed in, and 1 may mention 
that in a case where a crop of red clover was 
grown by us, and twice mown for hay, the in- 
crease of nitrogen in the surface soil was suffi- 
cient to be measurable by analysis when com- 
pared with another part of the field where the 
grain crop was grown.” 
Green manure is especially valuable on light 
soils or heavy impoverished clay land. The 
crop should be plowed in at about the time of 
well-advanced bloom, before seed formation. 
The plowing under of sod or stubble is ina 
measure a form of green manuring, for much 
* The Country Gentleman, March 12, 1885. 
