2 Hall, Hoiv does the Pla?it obtain its ?nitriuient ? 



increase the yield. For example, a soil of less than 

 average fertility will contain 2,500 lb. of phosphoric acid 

 per acre in the surface layer down to a depth of nine 

 inches only, yet the crop of turnips will be very small 

 unless the soil is further supplied with 50 lb. per acre of 

 phosphoric acid in the shape of superphosphate, though 

 an ordinary full crop would contain no more than 30 lb. 

 of phosphoric acid per acre. In the light of these facts 

 Daubeny, as early as 1845, introduced a distinction 

 between the dormant and active plant food in the soil, 

 the latter being the nitrogen, phosphoric acid and potash 

 that are combined in such soluble forms as to be avail- 

 able for crop production. Nitrogen compounds in the soil 

 have to be resolved into ammonia and nitrates by bacterial 

 action before the plant can use them, and the rate at which 

 this change will take place depends upon a number of 

 factors, such as warmth, the amount of air and water 

 in the soil, the supply of lime, that are independent 

 of the total nitrogen supply in the soil. Daubeny even 

 attempted to discriminate between the dormant and 

 active phosphoric acid and potash by estimating the 

 amount of these constituents that would dissolve in a 

 very weak acid solution, but this method for determining 

 the proportion of active mineral plant food, though it 

 received a wide extension by the work of Dyer in 1894, 

 has failed to yield the information expected of it. 

 Speaking generally, analysis has failed to measure the 

 productivity of an unknown soil except in the most 

 general way, because it always reveals such an excess of 

 the essential nutriments over the amount the crop 

 can utilise. 



A fresh point of view was introduced in 1903 by 

 Whitney and Cameron, the investigators in charge 

 of the Bureau of Soils of the United States Depart- 



