MICHIGAN ACADEMY OF SCIENCE. 
41 
governing its productivity, other factors are organic matter, moisture, 
drainage, cultivation, climatic conditions, etc. Each of these factors is 
as important as the amount of plant-food, because each has a deter- 
mining influence upon the crop production and also upon the availa- 
bility of the plant-food in the soil. 
Availability is a convenient term which the soil investigator has used 
very largely in the past and is using today for describing a condition 
of things about which we know scarcely anything. We say that the 
plant-food must be available before it can be taken up by the plants. 
The truth of the statement will in all probability never be questioned. 
But the vital point is, what constitutes availability or in other words 
in what form must the plant-food be before it can be taken up by plants? 
The one requirement is, that it must be in solution in the soil water. 
The soil solution, however, is not a simple one, but, on the other hand 
is very complex and is influenced by many factors. These many factors 
are so closely inter-related that any slight change in any one of them 
is sure to change this relationship and it is impossible to predict, at 
least with our present knowledge, wliat influence the changing of any 
of these factors may have upon the composition of the soil solution. 
Many attempts have been made to extract the soil solution as it exists 
in the soil but the attempts have not been generally successful owing 
to mechanical difficulties. It is probably true, however, that the com- 
position of the solution would not be the same in any two soils in respect 
to all conditions and it is also quite certain that the composition of the 
solution in any given soil changes from day to day within certain narrow 
limits. 
Many methods have been proposed; for determining the availability 
of the inorganic plant-food in the soil. These methods have mostly been 
based upon the relationship between the amount of the inorganic plant- 
food constituents extracted from the soil by the solvent medium and 
that taken from the soil by the crop. Maxwell proposed a 1% solution 
of aspartic acid on the ground that the organic acids of the soil are 
amino acids and that they influence the availability of the inorganic 
constituents. Dyer recommended the use of 1% citric acid because he 
supposed it bore a near resemblance to the methods of solution of plant- 
food in contact with the rootlets of plants. Moore of the Bureau of 
Chemistry after several years of experimentation proposed a solution of ' 
HC1 of N/200 strength. 
Many other methods, too numerous to mention, have been proposed. 
None of them, however, had any sound scientific basis for their accept- 
ance, but were proposed because those who stood sponsor for them 
observed that with one particular crop grown upon one particular soil 
there was a relationship between the amount of plant-food extracted by 
the particular solvent medium and that removed by the crop. 
All attempts to adapt these methods for universal application have 
resulted in failure and when we consider the variation in soils and the 
great difference in the feeding powers of different crops, the variation 
in climatic conditions, seasons, drainage, cultivation, etc., it is extremely 
doubtful if any arbitrary method will ever be devised that may be relied 
upon to determine the available plant food in all soils. 
Although a large part of all the known elements are found in soils, 
only about 14 are vitally connected with the living plant and it is the 
