14 A MANUAL OF WEEDS 
costing only about a cent a pound. As an herbicide it should be 
used as a spray, in a solution of about a hundred pounds to a barrel 
of water (52 gallons), which should be a sufficient amount to spread 
over about an acre of herbage. .A dust spray of this chemical has 
also been used, but is effective only when the plants are wet with 
‘dew. Iron sulfate is particularly useful as a grain-field herbicide, 
applied in dry, clear weather, when there is no likelihood that rain 
will wash off the plants before the chemical has done its work. 
Grains and grasses are very resistant to injury from the spray, 
partly, no doubt, because their growth is from the center and 
they quickly recover from such slight harm as may have been done 
to the outer leaves; also, they are smoother in texture than many 
of the grain-field pests, such as Corn Cockle, Charlock, and King- 
head, so that the spray does not cling so readily to their slender, 
blade-like leaves. The spray must be applied before the grain 
begins to “head” or the weeds to bloom, at a time when both are 
making the most rapid growth, for then the grain recovers so swiftly 
as scarcely to receive any check in its growth, and the weeds 
succumb most readily when they are most green and succulent. 
In the pea-field also this spray may be used to kill weeds without 
serious injury to the crop, but not with beans. Clover and alfalfa 
leaves are blackened, but recover rapidly if the solution has not 
been too strong. 
When successfully carried out, this method of cleaning a field of 
its undesirable plants pays the farmer very well; for returns from 
crops that have been relieved from competition with weeds for food 
and moisture and space to grow, are often half as large again as 
those from similar fields untreated, and are greatly improved in 
quality as well as in quantity. 
Bluestone, or blue vitriol (Copper sulfate). This well-known 
fungicide is also a most effective herbicide, if used when the weed 
foliage is young and tender. The formula for the solution is eight 
to twelve pounds of Copper sulfate to a barrel of water (52 gallons), 
using fifty to seventy-five gallons per acre. Professor Bolley found 
twelve pounds of Copper sulfate to be as effective as one hundred 
pounds of Iron sulfate. Like that chemical, it should be used in 
clear weather, when the plants are not likely to be rain-washed for 
at least twenty-four hours, as such a bath would render the work of 
