12 JAMESTOWN CONGRESS OF HORTICULTURE 



centration, and from the standpoint of plant functions they must be 

 profoundly so for organic life is almost inconceivably sensitive to 

 small quantities of matter. 



Our published results show, too, that we are able to recover from 

 the surface four feet of good soil as much water-soluble plant food 

 of both potassium and phosphorus as would be removed from a field 

 by nine 40-bushel crops of wheat, and from poorer soil as much as 

 would be removed by six such crops, and here again is what has 

 been thought a safe foundation for the contention that even in water- 

 soluble form the poorest soils contain plant food enough for good 

 yields. So there is, in absolute quantity but not in available quantity. 

 For example, a three-horse tread-power may be in such condition that 

 when one horse is put upon it no work is done ; adding a second 

 horse may yield only half an available horse-power, but when the 

 third horse is put in place its whole weight may yield effective power 

 so that the available work becomes three times what it was with 

 two horses. So it may be with soils. Plant food enough for per- 

 haps many crops must be present in order that enough for one may 

 become available. 



So far as we know, either from published data or on a priori 

 grounds, there is no foundation for the hope that the supply of plant 

 food in soils may be indefinitely maintained simply by good tillage 

 and suitable crop rotations which make positive additions only of 

 nitrogen to the soil. The only way Nature has ever produced crops, 

 and this is the way she has always maintained soil fertility, has been 

 to return to the field the whole crop, and working along this line 

 for a thousand years together she never did and never can bring 

 all her soils to an equality in productive capacity as should be the 

 case if all soils carry an abundance of plant food. 



A very simple calculation based on well established data will show 

 that an exhaustion of the plant food elements, large as these amounts 

 are, must necessarily follow any system of cropping which involves no 

 return to the soil other than nitrogen. The amounts of plant food 

 removed by certain crops are definitely known ; the absolute amounts of 

 plant food elements carried by good soils are known and, taking 20 tons 

 of potassium per acre-foot, which is about the amount carried, a quan- 

 tity equal to the whole of this would be removed in about 1,400 years 

 by wheat yields of 40 bushels per acre ; and the entire amount of phos- 

 phorus carried by the surface foot is equivalent to only about 400 such 

 crops. Careful records have shown that the Mississippi river carries out 

 to sea annually enough material to lower its entire drainage area one 

 foot each 4,000 to 6,000 years, which means that the surface foot of 

 soil may be completely removed and replaced by a corresponding layer 

 from below at the same rate ; but the rate of removal of potassium 

 by a 40-bushel crop of wheat is three to four times as rapid as this, 

 and the crop exhaustion for phosphorus is ten to fifteen times as rapid 

 as rock is being converted into new soil on the average, over the 

 Mississippi valley. Were Professor Whitney's contention true the 

 mean productive capacity of the soils of the Mississippi valley should 



