18 BULLETIN 499, U. S. DEPARTMENT OF AGRICULTURE. 
equivalent crust has been encountered in basins where an adequate 
organic mulch has been maintained. 
The application of an organic mulch to the basins is followed in 
some districts by a remarkable increase in the number of earthworms. 
Karthworms are not often seen in orange or lemon groves under 
clean cultivation, but in the basins in the Riverside district the soil 
teems with them; a double handful of soil would often be found to 
contain a dozen or more worms, and the soil is so riddled with their 
burrows that when the basin around a tree is filled with water a 
sea of bubbles appears on the water surface, due to the rapid escape © 
of air from the burrows. The air entrapped below a wet soil surface 
of fine texture presents a serious obstacle to the further penetration 
of water unless the soil contains cracks or openings, such as the 
burrows of earthworms, that are too large to be sealed by the capil- 
lary water films. The worms thus appear to have some effect in 
promoting aeration, and they surely facilitate to a marked degree 
the penetration of water into the soil. The beneficial effect of the 
presence of earthworms in the soil has been emphasized by Darwin. 
The vigorous root growth taking place in the surface soil under 
_ the mulch is very marked in the mulched basins. In the system of 
dry-soil mulching there is, of course, no root growth in the mulch, 
and quite often there is very little root growth even to a depth of 
8 to 10 inches, owing to the presence of plowsole. Under the mulch 
in the basins the ground is full of rootlets. The roots have not been 
observed to penetrate the mulch and are usually not found even in 
the very rich surface inch of soil. It can readily be seen, however, 
that any fertilizer applied in these basins soon comes in contact with 
the root system through the downward movement of the irrigation 
water and without the necessity of any mechanical incorporation of 
this material with the soil by cultivation. 
Since the mass of fine feeding roots is so near the surface of the 
soil in the basins, it is absolutely necessary to maintain a mulch that 
will thoroughly protect this root system, especially during the hot 
season of the year. A crusting of the surface soil in the basins due to 
the lack of sufficient mulching material would be very detrimental 
to the root system. 
FRUIT SETTING AND TREE GROWTH AS INFLUENCED BY THE 
MULCHING MATERIAL. 
SUNNY MOUNTAIN TRACT. 
There was no appreciable difference in the number of fruits picked 
in the spring of 1914 from the basined trees and the check trees in the 
Sunny Mountain grove. This shows the uniformity of the two plats, 
