158 CHEMISTRY OF FARM PRACTICE 



to stand as before, and filtered through the same filter, washed free 

 from chlorides with hot water, the filter burned and the precipitate 

 ignited in a blast until it ceases to lose weight, and weighed as calcium 

 oxide (CaO). This weight multiplied by the factor of calcium in 

 calcium oxide (7129) will give the weight of calcium from which the 

 percentage may be obtained by dividing by weight of the sample 

 taken for analysis. 



111. Field Tests. The best measure of the amount of 

 available plant food in a soil, and of food deficiencies, is 

 obtained by actual field tests extending over a number of 

 years to eliminate varying seasonal conditions and using 

 as many crops as possible to test out crop peculiarities. On 

 account of the expensive nature of such experiments they 

 must usually be left to the State Experiment Stations, and 

 those interested in soil chemistry will do well to study what 

 has been done by various States as published in State 

 Bulletins. It would be of great value if test farms could be 

 maintained on every large and distinct soil type. The crops 

 that are generally grown on this distinct type should be 

 tested in order that the information might be definite, and 

 this information should be available to the farmers living 

 within the area tested in order that they might apply the 

 knowledge thus gained in their own farming. 



The simplest effective form of field test consists in a 

 number of plots on which may be tested the value of 

 each single element to a given crop; then of every possible 

 combination of two elements; and, finally of all elements. 

 It is best to provide as many duplicate plots as there are 

 years in the rotation employed, and, in this way to pro- 

 duce every crop, every year, thus eliminating varying seasonal 

 conditions. These elementary tests may be enlarged to 

 include a comparison of the different sources of each ele- 

 ment of plant food, varying relative proportions of the three 

 most important elements, nitrogen, potassium and phos- 

 phorus, and different cultural methods used in connection 

 with fertilizing. A combination of two sources of nitrogen 

 is often most effective. 



