FERTILIZERS AND PLANT FOOD. 219 



3,600 pounds. These cases are sufficient to show that soils have 

 considerable producing power when left unraanured. This is 

 accounted for in the following way : All soils fit for agricultural pur- 

 poses contain a considerable amount of plant food ; for example, oa 

 the plots above mentioned, where wheat had been produced for 

 forty 3'ears and no manure had been applied, there was in the first 

 nine inches of the soil 2,000 pounds of nitrogen per acre. Plots 

 adjoining these and cropped the same, but which had received annual 

 dressings, showed much more. One plot which had received four- 

 teen tons of manure annually had in its top nine inches 4,000 pounds 

 of nitrogen. A soil analysed b\- the Department of Agriculture 

 showed 4,957 pounds of nitrogen 1,567 pounds of phosphoric acid, 

 17,429 pounds of potash per acre. If we compare this large quantity 

 with the comparatively small quantit}- taken up by a good crop we shall 

 see that there is enough for a great many crops, but plant food may 

 be present in vast quantities, and yet not be available to the plant. 

 This brings us to a new classification of plant food, namely, available,, 

 that which plant roots can pick up and use for the growth of the 

 plant, and unavailable, or that which is insoluble, so that the roots 

 fail to gather in any part of it, and the fact is that a very large 

 proportion of the 2,000 pounds or more of phosphoric acid, or pot- 

 ash, or nitrogen, that is in the soil, is unavailable. But so far as 

 the soil produces crops, even if small, plant food is furnished. The 

 average supply- of plant food taken up by the 13^ bushels of wheat 

 and 1,125 pounds of straw, in Lawes and Gilbert's experiments, 

 would amount to 20 pouuds of nitrogen, 17 pounds of potash, and 

 10 pounds of phosphoric acid, it is evident that this soil is capable 

 of supplying these quantities annually, else the crops would not 

 grow, and this small but necessary allowance comes from the change 

 of unavailable plant food into available by the action of air and 

 water containing carbonic acid, as well as other more complex influ- 

 ences, which render soluble a limited amount of plant food each 

 year, and this amount is a measure of the natural capacity of any 

 given soil. In raising crops, therefore, we need not be at the 

 expense of supplying all of the deficient plant food contained in the 

 crop we raise, but only the excess which the crop contains over and 

 above that which the soil is capable of supplying, year in and year 

 out, indefinitely. It must be remembered, however, that not all of 

 the plant food supplies will be received by the crop. Thus only 



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