f 
234 Journal of A gricultural Research voi. v, no. 6 
Minimum requirements for maximum plant development have never 
been established for any of the essential elements. In addition, the de¬ 
mands for sulphur will be related to the character of the plant compounds 
elaborated by the different species of plants, even in the leafy portion. 
A cabbage crop that absorbs 100 pounds per acre of sulphur trioxid 
makes use of this material in a different way from a potato crop which 
absorbs but 11 pounds of sulphur trioxid. In the cabbage, sulphur com¬ 
pounds characteristic of the species are formed in abundance, thus 
creating a demand for a large sulphur supply. Alfalfa hay, constructed 
abundantly of protein compounds even in the stem and leaf, will demand 
and contain more sulphur than the low-nitrogen-containing residual 
straws of cereals. In either of the above cases used for illustration— 
namely, cabbage and alfalfa—it has been found that 30 to 50 per cent of 
the total sulphur may be present as sulphate sulphur. Nevertheless, 
this makes the total organic sulphur in an acre's growth of these crops 
very considerable—about 30 and 50 pounds of sulphur trioxid, respec¬ 
tively. In this connection let us again mention the fact that the annual 
rainfall will carry to an acre not more than 17 to 20 pounds of sulphur 
trioxid, while the loss by drainage may equal and even exceed this quan¬ 
tity. While we have no knowledge as to whether the excess of sulphates 
absorbed by the plant is of physiological importance, it is, nevertheless, 
clear that a supply of sulphur in this form in the plant indicates that the 
plant has not been limited in the elaboration of organic compounds for 
which sulphur is necessary. In fact, we suggest that information as to 
whether sulphur is a limiting factor for plant growth in any soil may 
probably be obtained by testing for the presence of sulphates in the 
plants grown on that soil. Their presence would indicate that there 
was a sufficient supply for all constructive purposes in which sulphur is 
involved. 
From the facts presented on crop demand and soil supply we seem 
perfectly justified in including sulphur with nitrogen and phosphorus 
in the first group of essential elements which are limited in quantity 
in our common soils and in constant and relatively large demand by 
crops. On the same basis, potassium, calcium, and magnesium fall into 
a second group, while iron, constituting the third group, represents 
an element usually in abundance in soils and utilized in but small 
quantities by farm crops. Consequently, on the basis of total analysis 
and mathematics, sulphur should be of equal importance with phos¬ 
phorus. Here, however, is where very probably total analysis and 
mathematics will not find complete justification for their use as the 
sole instruments in measuring permanent soil production. In collabora¬ 
tion with Prof. Fred (3), the senior author has pointed out the very great 
difference in the effect of phosphates and sulphates on important bio¬ 
chemical processes in the soil. In these studies it has been shown 
that soluble phosphates increase enormously the number of soil organ- 
