58 BULLETIN OF THE BUSSEY INSTITUTION. 
of nitrogenized compounds. This fact explains at once the power of 
bearing trees which is possessed by much of the sandy and gravelly 
“ drift” of Eastern Massachusetts. As is well known, many of our drift 
hills and ridges have been devoted to the growth of hardwood inces- 
santly for a very long period. Every twenty or thirty years the trees 
are cut down, and, the sprouts which spring from their stumps being 
suffered to grow, a new crop of firewood is found ready for the axe at 
the end of another term of twenty or thirty years. This practice of 
continually growing and carrying off a single crop without respite, 
manure, or rotation has already been adduced by Professor Johnson 
as an excellent illustration of what farmers call working on the natu- 
ral strength of the land. But the experiments of Series C go to show 
that in some instances at least, and probably in most, where trees 
flourish upon drift, the elements of strength are contained in the very 
sand itself. So long as there is a deposit of humus on the surface of 
the land to supply the necessary nitrogen, there is no need of supposing 
that much of the food consumed by the trees must be brought to them 
from afar by the soil-water, for, as has just been shown, the sand in 
which the trees stand may be competent to supply potash, phosphoric 
acid, and the other ash ingredients. 
The experiment enforces the lesson that glacier sand (such as com- 
poses the New England drift) is ordinarily a very different thing 
from the sea-sand of the dunes upon the Atlantic coast. It is plain, 
for that matter, that a quantity of crushed rocks and minerals slowly 
undergoing decomposition and disintegration, through the agency of 
fresh water and the matters held in solution by such water, in heaps 
or hills such as glacial action has thrown up all over the Northern 
United States, must usually contain a much larger proportion of plant 
food than sand that has been formed upon sea-beaches from the same 
kinds of rocks. From the sand that forms the dunes almost every- 
thing but silica has doubtless been removed through long-continued 
grinding of the rocky materials upon the beach in a saline solution 
that was continually changed and renewed. 
Another series (D) of experiments was next undertaken in order to 
compare the pit-sand with coal-ashes under precisely similar condi- 
tions as to the time and season in which the crops grew, and also to 
compare the ashes and pit-sand both with a very poor sand and a 
very fertile one. Four sets of glass jars, each set containing sixteen 
