ORCHARD MANAGEMENT 119 



certain kinds of diseases such as brown-rot and bitter-rot. 

 Water or vapor is more retentive of heat than is air. There- 

 fore, when the air is dry, it cools rapidly after sundown. 

 This is of great importance from a horticultural standpoint 

 for it affects the aggregate of heat units available during a 

 given period. It is also true that frost-injury is more common 

 in dry weather than in wet or cloudy periods. 



194. Winds. Differences in the temperatures of bodies of 

 air cause winds. Cold air, being heavier than warm, flows 

 in and displaces it, causing air circulation. Winds may be 

 helpful by keeping the air mixed so that frosts do not occur. 

 Winds may be harmful, especially during winter, because 

 when cold they have a drying influence. Winds sometimes 

 do harm by breaking trees, especially when they are laden 

 with snow and ice, by blowing fruit from the trees, and by 

 causing light soils to drift. 



195. Sunshine and light. As explained in Chapter II, 

 plants require light in order to manufacture food material. 

 Different plants require various intensities of light. Some, 

 such as orchard-grass, grow well in the shade, but most kinds 

 require direct sunlight, especially for the production of fruit. 

 Diffused sunlight, such as is present in cloudy weather, is not 

 sufficient for fruit production. This is one reason why trees 

 come into bearing quicker and ripen fruit faster in arid than 

 in humid regions. It is a matter of interest that experiments 

 have shown red light to be the best for plant growth, while 

 violet light seems to give the best blooming conditions. 



196. Soils. Fruit-trees differ much as to the type of soil 

 they require, but in general the soil should be well-drained, 

 deep, and fairly retentive of moisture. There should be no hard- 

 pan near the surface for this prevents the roots from penetrat- 

 ing the soil, lessens the feeding area, hinders the absorption 

 of rainfall, so that the surface soil may become water- 

 logged, and causes the trees to suffer in times of drought 

 on account of the lack of a deep water-holding area. 



