122 Master Minds of Modern Science 



Fertilizers, though immensely important, are not the 

 only concern of the agricultural chemist. For instance, 

 the texture of the soil makes a deal of difference to the 

 growth of crops. Every gardener knows the value of a 

 fine tilth — that is to say, of breaking up and powdering 

 the soil properly before sowing his seeds. Sir Daniel and 

 others have tested various soils and shown that sandy 

 soil, which the farmer calls light, is actually heavier than 

 heavy clay. The latter weighs only just over sixty-six 

 pounds per cubic foot, while the sand weighs seventy-nine 

 pounds. The reason for the difference is that there is 

 more air space between the very small grains of the clay 

 than between the heavier grains of the sand. 



Again, the farmer talks of ' warm ' and ' cold ' soils. 

 Soil temperatures do vary greatly, for a well-drained loam 

 absorbs more heat than a wet, heavy clay or a pale chalk. 

 Experiments set forth by Sir Daniel in his book The Soil 

 show how land should be treated in order to gain the 

 greatest share of the sun's warmth, and the temperatures 

 required for the best growth of various plants. Wheat, 

 for instance, begins to grow at a temperature only eleven 

 degrees above freezing-point ; it makes its greatest growth 

 between eighty- three degrees and eighty-four degrees; 

 but if the temperature rises above 108*5 growth ceases 

 altogether. The melon refuses to start growing until a 

 temperature of 65*4 degrees has been reached, does best 

 at 91*4, and refuses to grow in a heat above 111 degrees. 



The farmer or fruit-grower has always known that 

 certain soils are better than others for certain crops. 

 For example, the Vale of Evesham grows the best plums 

 in England, while Kent has soil best fitted for cherry 

 orchards. The agricultural chemist has pointed out the 

 reasons for these peculiarities, and has helped the farmer 

 to find the best soils for such new crops as sugar-beet. 



Let us turn from the soil to the plant. In another 

 chapter of this book we have given some account of the 



