THE TILLAGE OF STUBBLE LAND 151 



force the seed, or the surface soil is in such condition 

 that it does not cover the seed satisfactorily after seed- 

 ing. The surface soil can be made more mellow by 

 surface cultivation or by plowing; the moisture content 

 can be more or less controlled by the same means; the 

 stubble, if too long, may be either burned or plowed 

 under or left uncultivated and the condition of the sur- 

 face or seed bed can be improved by suitable tillage if 

 necessary. 



123. Importance of "Available" Plant Food. All of 

 the plant food in a soil cannot be drawn upon by the 

 growing crop. Since plants "drink" their food it is clear 

 that only that portion of the fertilizing constituents that 

 becomes soluble or available can be used by them. 



The agencies causing the breaking down of plant food 

 constituents in soil are more or less dormant during 

 the dry autumn and long winter with the result that the 

 amount of available plant food in stubble fields is 

 relatively small. By far the most important of these 

 agencies is moisture in the soil. In stubble fields the 

 amount of moisture is very low at best and the supply 

 cannot be materially increased ; but any form of tillage 

 that results in getting more moisture into the land or in 

 preventing weeds or evaporation drawing upon what is 

 already there, will tend to increase the total amount of 

 "available" plant food for the use of the stubble crop. 



124. The Stubble A Nuisance, Yet Important. The 

 stubble of cereal crops is made up of elements derived 

 by the plant from soil and air. If the stubble is burned 

 the most valuable fertilizing element secured from the 

 soil, viz., nitrogen, passes off into the air. The burning 

 of stubble also dissipates "organic matter," the con- 

 stituent that helps to keep soils from blowing, the one 



