112 ELEMENTS OF AGRICULTURE 



the fundamental cause for decrease in crop yields. This 

 affects crops in many ways. It may result in an unfavor- 

 able physical condition of the soil that will limit the crop 

 when there is no shortage of food. The soil may "bake" 

 or it may lose its water-holding power. Since the humus 

 furnishes the nitrogen by its decomposition and encourages 

 the fixation of free nitrogen, the exhaustion of humus 

 will be accompanied by a shortage of nitrogen. Or because 

 of the lack of humus the mineral elements may not be 

 rapidly enough dissolved, although present in abundance. 

 In such a case the addition of phosphoric acid or potash 

 might increase the crop, but it would usually be wiser to 

 apply humus so as to render available the food that is 

 already in the soil. 



Many soils are losing their fertility in all of the ways 

 mentioned above. 



110. The Limiting Factor in Crop Growth. When all 

 plant-foods are supplied in abundance, if the temperature 

 is too low or too high, it becomes the limiting factor 

 and determines the yield. If all other conditions are favor- 

 able, but there is not enough sunshine, the crop will be 

 limited by this factor. A shortage of water or any other 

 plant-food may be the limiting factor. The other plant- 

 foods that often limit the crop are nitrogen, phosphoric 

 acid, potash and lime. These foods are rarely needed 

 to the same extent. If nitrogen is most deficient, the 

 addition of nitrogen will increase the crop; but we may 

 reach a point where a second element is necessary. Sup- 

 pose that the weather and all other conditions are such 

 as to allow a crop of 100 bushels of corn, but that the 

 phosphoric acid supply is so short as to limit it to 70 



