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. 

 Earthworms 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 sj'stem of 

 dry-soil mulching there is, of course, no root grow 7 th 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, 







