FACTORS IN CROP PRODUCTION 577 



ing effects of weeds is that they shut off the light to a greater or 

 less extent from the agricultural plant. Nurse crops drilled north 

 and south permit the strong midday light to reach the young clover, 

 and thus insure a hardier clover plant than when the nurse crop is 

 sown broadcast or drilled in an east and west direction. In green- 

 house cultures light is very commonly the limiting factor in plant 

 growth. However, under ordinary field conditions, the light is 

 probably adequate for crop yields at least ten times the present 

 averages. 



Moisture. Moisture is perhaps the most variable factor in 

 crop production; and, in consequence, many seem to think that 

 if we have timely rains, we should always have good crops; although 

 on almost every farm, there are some patches of ground which 

 produce twice as much as others, even though the rainfall, seed, 

 preparation, cultivation, etc., are alike on both areas. 



It may safely be stated that when corn or other crops begin to 

 " fire " in time of partial drouth the real cause of the " firing " is 

 more commonly due to a lack of plant food than to a lack of mois- 

 ture for its own sake. To be sure, a more ideal rainfall, which we 

 cannot control, would help to render available a more nearly ade- 

 quate supply of plant food, even from a poor soil; but, on the other 

 hand, a liberal enrichment of the soil, which we can control, will 

 often render unnecessary additional rainfall. Almost every season 

 in some part of Illinois, we observe the " firing " of corn on unfer- 

 tilized land where the soil is incapable of producing more than 25 

 to 50 bushels per acre, while at the same time on adjoining properly 

 fertilized plots which yield 75 to 100 bushels, and where the crops 

 are actually drawing much more moisture from the soil, there is 

 little or no evidence of " firing." Even in the pot-culture lab- 

 oratory, where water is daily supplied in sufficient abundance, 

 plants " fire " with inadequate food supplies. In other words, the 

 lower leaves die, and much of the plant food which they contain is 

 translocated to the new, growing parts, in order that reproduction 

 may ensue if possible. 



The conservation of moisture in humid sections is a matter whose 

 importance is commonly greatly exaggerated. If the expense so 

 much advised for extra cultivation were devoted to a more liberal 

 use of manure, clover, limestone, and phosphorus, greater and 



