PLANT-FOOD AND AGRICULTURE. 221 



exhaustion of plant-food going on, but the physical quality of the 

 soil tends to change, and the oxidation of the humus disturbs the 

 relations which have existed between the soil and the plant-food, 

 and affects the retaining power and preparation power of the soil 

 on the food to fit it for the plant. 



It is this disturbance of relation, the removal of plant-food 

 beyond the annual supply of the weathering of the soil, and from 

 atmospheric agencies, which constitutes the perils of agriculture. 

 It is in the recognition of this difference between the history of 

 the natural and the agricultural field that true agricultural science 

 has its foundation. It is in the application of the facts of plant 

 feeding and plant growth, as applicable to the artificial or man- 

 created pursuit of agriculture, that we must expect that science 

 applied to practice which shall develop profit for ourselves, without 

 robbing our grand-children. 



As concerning plant-food and agriculture, we have the following 

 to consider : plant-food, the soil, the plant, and man's interests 

 and relations. 



Our attention being thus called to plant-food, we first note that 

 we cannot consider it agriculturally as apart from its relations, and 

 that its most striking feature, as a subject for thought, is in its 

 changeability and variability ; its gains and its losses ; its Protean 

 facilities for change. It is now a chemical element in the soil, 

 inert and useless ; again a food to an agricultural plant ; soon 

 formed into growth-tissue, and affording sustenance to man or beast, 

 and again returned to the land. It now has one relation with the 

 plant, and soon again another. Its position in the soil changes ; 

 its form is continually changing ; its relative proportions, and then 

 again passing downward with the water, is is wasted, to be again 

 saved in part by physical actions in the soil, and by the agency of 

 plant-roots. 



Agriculturally we must define the capacity of our land by its 

 relations to plant-food, and as in mechanics we require the measure 

 of length, before we can give accurate expression to our ideas, so 

 here we must have a measure for the agricultural capacity of our 

 land ; yet from the varying nature of the case, we cannot expect 

 that exactness which is given by our foot-rule. The measure 

 which we shall use for our purpose is the natural fertility and the 

 maximum fertility of our land. We will therefore define these 

 terms, so that we may not only see what they mean, but also 

 realize their importance. 



