312 ANNUAL. REPORT OF THE Off. Doc. 



possessing this same high potential fertility, may, under certain con- 

 ditions, be so actually barren of results to the farmer as to lead him 

 to believe it absolutely devoid of plant food. 



A soil at Kothamsted, England, which has been successively crop- 

 ped to grain for 50 years without the addition of manure, and which 

 consequently had become exhausted especially in available phos- 

 phoric acid, still contained a total of 2,880 pounds of phosphoric acid 

 per acre in the first foot of surface. 



Of this only 72 pounds per acre was soluble in a one per cent, 

 citric acid solution. In other words, a soil which contained enough 

 total phosphoric acid to support a wheat crop for 300 years, had, as a 

 result of 50 years successive cropping, its store of available phos- 

 phoric acid so reduced as to leave a supply sufficient to last only be- 

 tween seven and eight years. This case is typical of thousands of 

 others, and is illustrative of what is meant by soil exhaustion. It 

 consists in using up original supplies of available plant food at a 

 greater rate than they are being manufactured in the soil. Most of 

 the older lands of the Atlantic seaboard which have been regarded 

 as "worn-out" and exhausted are in much the same condition. Never- 

 theless they still contain large stores of unavailable plant food, which 

 it only requires the application of modern agricultural practice to 

 unlock. In other words, soils still potentially fertile must be made 

 actively so, and since soils potentially fertile are low and those ac- 

 tively fertile high in bacteria, it would appear that one of the primary 

 requisites of active fertility is to fulfil those conditions of the soil 

 which favor the best development of bacterial life therein. 



NuTubers of haoteria in soils thus liecome an index of active fer- 

 tility. 



IV. METHODS OF DETERMINING THE NUMBER OF BAC- 

 TERIA IN SOILS. 



1. Drawing the soil sample. The determination of the number of 

 bacteria in the soil of a given area involves an elaborate and careful 

 preparation of the sample. 



Studies at the Delaware Experiment Station have shown that the 

 numbers vary considerably within rather narrow horizontal ranges, 

 and thus to obtain an average sample, representative of an entire 

 field, implies the collection and mixing of a large number of smaller 

 samples. For most studies it will be sufficient to collect the first 

 nine or twelve inches of soil, and for this a wood auger one inch in 

 diameter is very satisfactory. 



To preserve the boring intact a device such as is shown in Fig. 



