66 SOILS 



Available plant food in the soil is small. Even in 

 very fertile soils a great deal of available plant food is 

 never present. Available plant food comes and goes ; 

 especially is this true with soils having poor texture and 

 poor physical conformation. There seems to be a certain 

 possible limit, depending on the condition and the treat- 

 ment a soil has been given previously. When this limit 

 has been reached, available plant food passes into some 

 insoluble form that loss may be ever kept within reason 

 an organized retreat, rather than an utter rout. 



Soils abundantly supplied with vegetable matter are 

 the least susceptible to this changing state. They hold 

 plant food better and longer, so long, in fact, as the 

 vegetable supply is kept replenished. These humus soils, 

 when favored originally with all needed mineral materials, 

 lead in the race of high production, other things being 

 equal, like water, heat, tillage, and correct management. 



How much plant food in the soil? Since nitrogen, 

 phosphorus and potassium are the elements, as a rule, 

 lacking in the soil, we need consider them only in esti- 

 mating the plant-food content of any soil. 



To illustrate this point just a bit of evidence will be 

 produced. The data below, arranged by Roberts, present 

 the case: Average analyses of 49 soils: Nitrogen, 3,053 

 pounds; phosphorus, 4,219 pounds; and potassium, 16,317 

 pounds. These quantities are present in each acre, the 

 depth being twelve inches. 



The not-immediately-available plant food. Passing 

 now to the second form in which plant food exists, we 

 have that which is unaffected by the dissolving effects 

 of soil water soil water, you know, secures the available 

 form at once or very quickly and only slightly by the 

 acids exuded by the roots. This not-immediately-avail- 

 able plant food, in so far as the present crop is to be fed, 



