Active Phosphoric Acid and Pot Experiments. 
47 
equal quantities of phosphoric acid to plants on account of differences 
in the mineral phosphates present. 
We must, therefore, be cautious in comparing soils of widely dis- 
similar origin or nature. 
The phosphoric acid extracted from the soil is not the same as that 
dissolved, since the soil has the power to withdraw phosphoric acid 
even from acid solution. The fixing power of the soil is a matter 
which must be given careful consideration in applying the results of 
contain phosphates which are soluble in the acid, but inaccessible 
to plant roots. 
Carbonate of lime, etc., which are dissolved by the solvent may 
contain phosphates which are soluble in the acid, but inaccessible to 
plant roots. 
POT EXPERIMENTS. 
Pot experiments are more suitable for testing the needs of soils 
for plant food in comparison with chemical solvents, than field ex- 
periments. They are much less expensive and more easily conducted. 
Weather conditions which destroy or injure field tests are ren- 
dered of little effect. Insect pests may be excluded. The soil tested 
is prepared so that it is uniform, and placed under favorable physical 
condition, so that the condition which controls the growth of the 
plants is the plant food. Controlling conditions in the field are 
variable. 
' METHOD OF WORK. 
Five kilograms soil were placed in a Wagner pot which had pre- 
viously received 2 kilograms gravel. To one pot was added 2 1-2 
grams acid phosphate, 1 gram nitrate of soda, and 1 gram sulphate 
of potash. To another pot was added nitrate of soda and sulphate 
of postash. A comparison of the growth of the crop in these two 
pots shows the effect of phosphoric acid. Other pots were included 
in this work, but the description of them is foreign to the purpose 
of this article. Each pot received the same weight of seed, and equal 
amounts of water. The water content was maintained during the 
growth of the crop by adding a sufficient quantity three times a 
week to restore the loss of weight. The pots were kept in glass 
or canvas houses. At the end of the period, the crops were har- 
vested, dried, weighed, and in many cases, subjected to analysis. 
