42 
THIRTEENTH REPORT. 
general belief today that only three and at most four of these, nitrogen, 
phosphorus, potassium and calcium need demand the serious attention 
of the agriculturist. The amounts of phosphorus, potassium and calcium 
in virgin soils depends largely upon the composition of the rocks from 
which they originated. The amount of these elements in cultivated soils 
depends also upon the manner in which the soils have been handled. 
The nitrogen has practically all been introduced into soils since their 
formation and it is extremely fortunate that this most expensive form 
of plant-food may be, in a great measure, supplied to the soil by a due 
attention to the practice of crop rotation. It was originally supposed 
that only the inorganic forms of nitrogen, phosphorus and potassium 
could serve as plant-food and many attempts have been made to classify 
soils as good or poor on the basis of their content of these elements. 
Maercker of the Halle Station, Germany, has made the following classifi- 
cations : 
Grade of Soil. 
Potash. 
Phosphoric 
Acid. 
Lime. 
Total N. 
Clay Soil. 
Sandy Soil. 
Poor 
Below 0.05 
0.05—0.15 
0.15—0.25 
0 . 25—0 . 40 
Above 0 . 40 
Below 0.05 
0.05—0.10 
0.10—0.15 
0.15—0.25 
Above 0 . 25 
Below 0.10 
0 . 10—0 . 25 
0 . 25—0 . 50 
0.50—1.00 
Above 1 . 00 
Below 0.05 
0.05—0.15 
0.15—0.20 
0 . 20—0 . 30 
Above 0 . 30 
Below 0.05 
0.05—0.10 
0.10—0.15 
0.15—0.25 
•Above 0 . 25 
Medium 
Normal 
Good 
Rich 
Other investigators, notably Hilgard. have made similar attempts to 
classify soils but there is no unity of opinion on this point. It is not 
at all strange that there should be this wide difference when we con- 
sider that the amount of the so-called plant-food is only one of the many 
factors governing the productive capacity of the soil — other factors are 
drainage, cultivation, climatic conditions, seed selection, etc. 
Although it has long been recognized that organic matter is a great 
asset to the soil but little has been definitely learned in regard to its 
nature and composition. There is a tendency on the part of most soil 
experimenters to associate soil organic matter with the term humus. 
Many consider them to be synonymous, others consider humus to be com- 
posed of definite compounds produced by the decomposition of organic 
matter and that there might be organic matter present that had not yet 
reached the state of humus. At the present time the term humus is 
generally used to designate the product obtained by treating an alkaline 
extract of a soil with an acid. 
As early as ]$44 Mulder made an extensive study of the organic mat- 
ter of soils and claimed to have isolated seven distinct compounds to 
which he gave the following names: crenic acid, apocrenic acid, geic 
acid, humic acid and lmmin, ulmic acid and ulmin. He also gave to 
these bodies definite chemical formulae. A little later Grandeau de- 
veloped his method for separating humus which is essentially the one 
in use today. 
As the number of investigators on the subject of humus and organic 
matter increased, more or less controversy arose as to the composition 
of some of these supposed definite compounds. Different investigators 
