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better than the sod-mulch treatment, while it also gives much larger 

 crops. 



Grass in an apple orchard is evidently a detri- 



Why is grass ment, and it acts against the best interests of 



harmful? the trees in several ways. These ways have been 



so fully discussed in Bulletin No. 314 that it 



is only necessary to state them here. 



(1) The growing grass lowers the water supply, since every plant 

 uses and evaporates many score of times its own weight of water. 

 Under rare conditions this reduction of the water content of the soil 

 might be an advantage to the trees, but in ordinary seasons, on soils 

 neither very deep nor specially retentive of moisture, as in most New 

 York orchards, the trees need all the rain that falls during the 

 growing season, and the draft of the grass roots on the supply of 

 water left near the surface by showers is robbery that affects both 

 the crop of apples and the trees that bear them. 



(2) With the water there goes into the grass a certain amount 

 of plant food, which will become available to the tree roots only 

 after a considerable time and some of it probably never. The use 

 of fertilizers in certain portions of the Auchter orchard proved 

 this factor of plant food of less consequence than that of water; 

 yet the trees in sod responded promptly and profitably to applica- 

 tions of nitrate of soda. Trees under tillage, on the other hand, 

 seemed to have enough and to spare of nitrogen, as well as all the 

 other food elements they needed. 



(4) The growth of grass on a soil reduces its temperature. Whether 

 this is a serious disadvantage we cannot say, but most New York 

 apple soils are comparatively cold; so it would seem reasonable 

 to suppose any additional cooling influence harmful, as heat causes 

 the food substances in the soil to dissolve more rapidly, hastens 

 their diffusion through the soil water, aids soil ventilation, stimulates 

 the absorptive action of the roots, and helps to form nitrates in 

 the soil. Thermometer readings made over a considerable period 

 showed that the tilled soil in June and July is more than a degree 

 warmer than the sodded soil in the morning and more than two 

 degrees warmer at night. 



(5) The supply of air is less in a sodded soil than in a tilled soil; 

 and good soil ventilation is essential not only to the life of the plant 

 itself, but also to the activity of the bacteria which make certain 

 forms of plant food available. 



(6) Sod affects deleteriously the beneficial micro-organisms in the 

 soil. The experiment given supplies no definite data to support 

 this statement; but the lowering of the humus content of the soil, 

 restriction of the air supply, cooler temperature and smaller moisture 

 content of the soil under the sod are all factors unfavorable to the 

 development of those bacteria whose action in the soil we know 

 to be beneficial to plants. 



