32 THE WEED FACTOE IN THE CULTIVATION OF CORN. 



on a surface beneath which is a tangle of fine roots capable of absorb- 

 ing any moisture that might try to pass upward to the surface ? 



On the other hand, these roots would not interfere to any great 

 extent with the progress of rain water downward in the soil, for when 

 rainfall is great enough to saturate the surface soil there would be 

 more moisture present than the roots could absorb. Thus, while the 

 soil mulch is important on fallow soils as a means of holding moisture, 

 is it not possible that a soil well filled with living plant roots is not 

 in need of a mulch for this purpose ? This would at least be a plau- 

 sible explanation of the results reported in this bulletin. 



It is further suggested that these results may partly be due to the 

 fact that tillage mutilates the surface roots of the crop. Again, a 

 fall of rain too light to moisten the soil below the depth of stirring 

 would all be lost on the cultivated land because it would not reach 

 the roots of the crop, while on the weeded plat there would be enough 

 roots in the surface soil to absorb a considerable proportion of such 

 light rainfall before it had time to evaporate. 



While the suggestion that the presence of a maze of living roots in 

 the soil would itself, to a large extent, prevent loss of moisture by 

 evaporation at the surface, it is not proved that this is the case. 

 We must therefore consider other possibilities. Even with ordinary 

 methods of cultivation there would be some loss of moisture at the 

 surface, for tillage itself would expose a large amount of moist soil 

 to the atmosphere, so that in any case a growing crop would hardly 

 utilize all the rainfall that occurs during a growing season. If, how- 

 ever, the loss of moisture by evaporation is greater on the unculti- 

 vated plats than on the cultivated ones it is possible that this would 

 be compensated for by a correspondingly greater amount of nitrates 

 and other soluble salts being brought up from the deeper layers of 

 the soil to the surface, thus furnishing a greater quantity of plant 

 food for the corn roots. 



It should be clearly understood that the writers offer these remarks 

 merely as suggestions. The underlying causes for the phenomena 

 observed must be left for future determination. 



PEACTICAL SIGNIFICANCE OF THE RESULTS. 



An explanation of the results obtained in these experiments is of 

 secondary importance as compared with the interpretation we shall 

 give them from a practical standpoint. The results as a whole come 

 well within the limits of experimental error, showing no more differ- 

 ence in yield between the weeded plats and those receiving normal 

 cultivation than might be expected between two series of 125 plats 

 treated exactly alike. 



257 



