No. 6. DEPARTMENT OF AGRICULTURE. 1»5 



Pot experiments of the kind just mentioned are somewhat cum- 

 brous and extensive, and require an entire growing season for their 

 completion. 



The Bureau of Soils of the United States Department of Agricul- 

 ture has recently proposed another method in which baskets, three 

 inches in depth and diameter, made of paraffined wire gauze, are 

 used instead of large i)Ots. A single species of plants has been 

 selected for use, namely, a hardy variety of wheat, a half-dozen seed- 

 lings being employed for each pot. The plan involves the observa- 

 tion of the manner of growth during a period of live weeks, and a 

 determination of the green and dry weights of crop developed by the 

 end of that period. It is clear that where so small quantities of soil 

 are used that the difficulty of securing representative portions is 

 increased. This tendency to error is somewhat offset by the possi- 

 bility of conducting the same treatment upon a number of pots, say 

 five or six, and using the average result rather than that obtained 

 from the trial of a single pot culture. It is very certain also that 

 the results obtained during so small a portion of the cycle of growth 

 of a plant cannot accurately represent the results that would be ob- 

 tained in the whole cycle. To illustrate, much of the soil nitrogen 

 is not directly available to plants, but becomes available by the 

 action of nitrifying bacteria during the season of growth. The rate 

 of nitrification under field conditions is not uniform through the 

 growing season, nor is all the nitrogeneous material of the soil 

 equally susceptible to nitrification. These facts make it very im- 

 probable that even under field conditions the first six weeks' growth 

 of the wheat crop will accurately represent the soil changes and 

 plant gains through the whole season. The results thus far obtained 

 by the use of the wire-basket method indicate that it has consider- 

 able value as a means of determining the kind of fertilizer deficiency 

 in the soil, but that it docs not afford conclusive evidence respecting 

 the proportions in which the deficient constitiK^uts must bo supplied 

 in order that a maxinuun wheat crop may be secured, assuming that 

 the conditions of culture and season are most favorable. Since it is 

 clearly established that one crop may do fairly well whore another 

 species fails, it is clear that the wire-basket tost with wheat, or, in- 

 deed, a test of the soil by any method with wheat, cannot afford very 

 certain information as to the deficiencies of the soil for the crops 

 of other species, such, for example, as clover, potatoes or turnips. 



For these reasons, while the pot methods are to be regarded as 

 highly available means of soil examination to be conducted at cen- 

 tral stations, where continuous and skilled supervision can be given, 

 the need remains for tests also upon the soil in place. To reduce 

 the costs of such tests, and at the same time to make possible their 

 more accurate performance, experiments upon small plats rather 

 than upon large fields have been preferred. Another reason for 

 this preference is that by duplicating the series of plats receiving 

 the several treatments under comparison, it is possible to generally 

 reduce the errors due to local variations in the character of the 

 soil. 



The Experiment Station has not been able to enter largely into 

 the test of Pennsylvania soils by pot-culture methods. It has, how- 

 ever, recently inaugurated a series of soil tests with fertilizers fitted 

 for the growing of corn, wheat and potatoes. In 1906 I secured the 



