223 PLANT-FOOD AND AGRICULTURE. 



When this fertilizer is applied to our land, and the rains fall, it 

 is washed into the soil, but only to a certain distance. The soil 

 has a retaining power on this element, and holds it firmly against 

 washing. So far as the phosphoric acid extends through the soil, 

 it is diffused : that is, it is evenly distributed, and the soil holds 

 it in saturation, but in a quantity for a given area dependent 

 on the nature of the soil. What follows from this? If we apply 

 a given quantity to the surface of our land, a quantity sufficient if 

 applied to our plant to raise a maximum crop, so far as this con- 

 stituent is concerned, the rains wash it down, and it is retained, 

 we will suppose, by the first inch of our soil. Now, as it is 

 retained by the soil in the position, despite the rainfalls, it is 

 evident that in this supposed case, only the root fibers which enter 

 this upper inch of earth can feed on it. If a double quantity be 

 applied then it will extend down two inches, and the same facts 

 about the plant feeding holds for this two inches and so on. It is 

 evident that it can only be by applying phosphoric acid in large 

 quantites, or repeatedly during a series of seasons, that we can so 

 saturate our land with this element, under these assumed conditions, 

 as to iurnish the whole root-area with plant-food. 



These considerations would lead us into the practice of looking 

 for the full efl'ect of this fertile element from repeated applications, 

 rather than from a single trial. They would also lead us to apply 

 this element within reach of the principal plant fibers in all cases. 

 They show us that it is safe to apply a superphosphate at any 

 period of the year, so far as loss by drainage is concerned. la 

 applying on sod for corn, we like to apply this element of our 

 fertilizer on the surface, so that it may be washed in by the rains 

 before ploughing. Then when the sod is inverted, shallow, as 

 is our custom, the fertile strata of the earth is in the right position 

 for the roots. We also have an action of the acids of our com- 

 bined fertilizers in rendering more soluble the phosphoric acid 

 which has reverted in the soil, but space will not admit of further 

 considerations. 



Potash, as supplied in commerce, occurs in three forms, — the 

 carbonate, sulphate, and muriate. The first form occurs in ashes, 

 and we will suppose a bushel of ashes to weigh 48 lbs. to the 

 bushel, as nas been determined as the average for house ashes as 

 they usually occur. Now ashes contain about 6 per cent, of 

 potash, and about 2 per cent, of phosporic acid, in a reduced or 

 insoluble form. Ilence a bushel of ashes would be held as of the 



