THE IMPROVEMENT OF SOILS. 47 



Green Manures add no Minerals to the Soil. It 

 is observed, from the foregoing, that the use of green 

 manures can add no minerals to the soil; nevertheless, 

 its chemical qualities are improved. In the first place, 

 constituents existing in the soil, as well as in the subsoil, 

 have been collected by the roots and stored in the whole 

 plant, which, turned under, concentrate these constituents 

 in the surface soil ; and, secondly, the constituents con- 

 tained in combination with vegetable matter are readily 

 given up again, because of the tendency of such sub- 

 stances to decay. 



Improvement Due to Lime. The addition of lime 

 also improves the physical nature of soil. Upon sandy 

 soils its effect is to fill up the openings, which makes 

 them more adherent and more retentive of moisture, thus 

 absorbing less heat during the day, and retaining more 

 at night. On clay soils, the effect of lime is still more 

 important; the fine particles are separated, and the soil 

 made more open, porous, and friable ; air and water cir- 

 culate more freely ; the soil is warmer and easier to work. 



Tillage. Natural soils are further improved by til- 

 lage. Tillage includes the operations of plowing, culti- 

 vating, harrowing, rolling, etc., the result of which is to 

 destroy weeds and foreign growths ; to subject larger 

 portions of the soil to contact with air, thus increasing 

 the tendency to decay ; and to pulverize the surface soil, 

 and render it more absorptive and porous, and more favor- 

 able for the germination of seeds, and for the penetration 

 and activity of the fine roots. 



Methods of Plowing. -- The methods followed in 

 plowing vary with the conditions and character of the 



