soil. For if all the plant-foods are equally essential, it is evi- 

 dent that if in any soil there should be enough of all but one to 

 produce 40 bushels of wheat per acre, yet of that one enough to 

 supply only 15 bushels, then 15 bushels would be the largest 

 crop possible. Hence it may be that the soil which grows sor- 

 rel and daisies has sufficient potash, phosphoric acid and other 

 mineral plant-foods to produce a good yield of grasses, but it may 

 be lacking in nitrogen, or ma}^ not have available moisture. 



5. Plants require that plant-food be available. — Not every soil 

 that contains an abundance of the different plant-foods is a fer- 

 tile soil. The plant-food must be in such condition that the 

 plant can use it. Gravel-stones might contain all the necessary 

 mineral constituents, and muck has nitrogen ; yet a mixture of 

 muck and gravel-stones would not be a fertile soil . Let the stones, 

 through weathering, crumble and decay and become intimately 

 mixed with the material of the muck and soil, considerable avail- 

 able plant-food might be made. For plant-food to be available, 

 it must be in a condition to be dissolved by the soil water. Roots 

 absorb the soil water and obtain their food by using that which 

 is in solution. They never take their food in solid particles or 

 chunks. 



6. The soil water can dissolve ojily that paj't of the soil with which 

 it comes in contact. — It comes in contact only with the outside of 

 the ultimate particles of soil, and, obviously, the more numerous 

 the particles the greater chance there is for the plant-food con- 

 tained in these particles to be dissolved. It is in this light that 

 we are to look upon tillage as making plant-food available 

 by improving the texture of the soil. 



7. The proportions of the different plajit-foods in the soil are 

 variable. — Nature has supplied a superabundance of most of the 

 essential plant-foods ; but often some of them are either lacking 

 or are in an unavailable condition from which the plant finds 

 great difficulty in extracting them. Of the fourteen elements of 

 plant-food that are essential to plants, only four, and more often 

 three, are in this condition. These are nitrogen, phosphoric acid, 

 potash and lime. 



The term plant-food as ordinarily used b}" farmers includes the 

 first three of these substances only ; not that they are any more 



