76 The Sugar-Beet in America 
a green-manure. With this combination, the produc- 
tivity of the soil should not only be kept up but should 
actually be increased. 
HOW TO DETERMINE FERTILIZER NEEDS 
In order that there may be no waste of material, it is 
important to know just what are the fertilizer needs of 
the soil. This problem is not so simple as it might at 
first seem to be. Soon after the methods by which 
plants feed and the elements they require from the soil 
were discovered, it was thought that by making a chemical 
analysis of the soil, its fertilizer requirements could be 
determined at once. It soon was found, however, that so 
many factors entered into the problem that this method 
could not be relied on. For example, an analysis may 
show a soil to be rich in potassium and at the same time 
this soil may give a marked response to the addition of 
potash fertilizers. This is true for all plant-food ele- 
ments. In some cases, the elements shown by a chemical 
analysis to be lowest in the soil are the ones that give 
least returns when added as fertilizers. Numerous exper- 
iments have shown that an analysis of the soil is useful 
when taken with other tests, but that alone it is not 
sufficient. 
Field tests carried over long periods of time have been 
found necessary in making a thorough diagnosis of the 
needs of a soil. These may be supplemented by pot 
tests and by chemical analyses. A complete understand- 
ing of a soil cannot be obtained without a combination 
of field and laboratory tests. When all this information 
