310 Bulletin 179. 



of available phosphoric acid, then the amount of crop grown is regu- 

 lated by the amount of available phosphoric acid present. If this 

 quantity is increased, the crop will be increased. To apply nitrogen 

 or potash to this soil would be unwise, for it may be taken for 

 granted that these plant-foods are already present in sufficient quan- 

 tity for at least a fair crop. Again, suppose that the essential con- 

 dition which is at a minimum is a lack of humus, or it may be an 

 unsuitable texture. Then the total crop produced will be governed 

 by the amount of humus or by the texture. If the amount of 

 humus in the soil is increased or if the texture is improved then this 

 soil becomes more productive. An application of commercial fer- 

 tilizers to such a field would have little or no beneficial effect. 



It sometimes happens that some essential condition is at such a 

 low grade, and is of such a character that the farmer can recognize 

 it at a glance ; for instance, a soil may be a very stiff, hard, com- 

 pact clay, similar to a brickyard. Such a soil must be loosened and 

 made porous and friable or mellow. Its texture must be improved 

 before one could expect to obtain profitable results from the use of 

 fertilizers. But if the farmer wants to make the most that lie can 

 out of his farm he will not stop here ; for just as soon as one low- 

 grade condition has been improved and larger crops are thereby 

 obtained, some other condition may be at a minimum and conse- 

 quently regulates the amount of crop produced. This becomes the 

 condition to be sought out and improved in order that the produc- 

 tivity of the soil may be raised to a higher standard. 



These essential conditions may be likened to a lot of jackscrews 

 and the productivity or fertility of the field to a large house. If 

 we wish to raise this house, we distribute the jackscrews in various 

 places under it. If the house is to be raised without damage, every 

 one of these jackscrews must be carefully watched and that one 

 which is the lowest must be screwed up ; presently some other screw 

 becomes the lowest and that one in turn must be attended to, and 

 so on with every one of the supports. 



If all the screws are carefully watched the house rises properly ; 

 if only a part are watched and no attention is paid to the others, 

 before long something is liable to be strained and the house may be 

 badly injured. So it is with the fertility of a soil ; each and every 



