57 6 VARIOUS FERTILITY FACTORS 



from a single variety of corn. Likewise, the sugar beet has been 

 developed by breeding under the most exact scientific control until 

 its sugar content has been changed from 4 per cent to more than 

 12 per cent, and until about one half of the world's supply of sugar 

 is now made from beets. But shall we practically ignore other 

 equally essential factors because of the importance of seed? 



Home of the plant. The home of the plant may vary from a stiff, 

 compact, almost impervious clay, offering a very shallow feeding 

 range for plant roots, to a porous, friable, easily penetrated, fine 

 sandy loam, affording a very deep feeding range. To markedly 

 modify the physical character of the soil is, as a rule, a difficult and 

 expensive problem. Thorough underdrainage and large use of 

 organic matter, including deep-rooting plants, which are the best 

 subsoilers, and sometimes heavy applications of marl or ground 

 limestone (10 tons or more per acre) will do much to improve the 

 clay soils, and heavy applications of " clay " (say one wagon load 

 to the square rod) will greatly improve some very light sand soils. 

 An old English saying runs: 



" Clay on sand is money in the hand; 

 Sand on clay is money thrown away." 



Temperature. A proper temperature is important in crop pro- 

 duction, although some plants grow well in cool weather, while 

 others do best under tropical conditions. Dark soils are warmer 

 than light-colored soils, a difference of several degrees being noted 

 during the forenoon in the early summer, and this difference extends 

 to a depth of several inches; but the largest control of soil tempera- 

 ture lies in the control of soil moisture. Improperly drained soils 

 are cold soils, because enormous quantities of heat are required 

 to remove surplus water from the soil by the process of evapora- 

 tion. Thus, to melt ice requires only 79 heat units, and to raise 

 water from the freezing point to the boiling point requires only 

 100 heat units, but to evaporate water requires 536.4 heat units. 

 If the surplus soil water can be removed by underdrainage, the 

 sun's energy may then be expended in warming the soil. 



Light. Light is an absolute essential in the most fundamental 

 process of plant growth, photosynthesis. One of the most damag- 



