110 
IOWA ACADEMY OF SCIENCE 
weight of the soil, makes it more porous and increases its water holding 
capacity. It retards percolation and increases the water retaining power of 
the soil. It has practically no effect on capillarity and it hinders evaporation 
to a limited extent. Peat, when tested and observed, acts in a very similar 
manner except in regard to capillarity. 
Stable manure causes the soil to hold and retain more water, but does the 
soil give up this extra water to the growing crops? How does manure effect 
the drouth line? 
In reference to this point SnydeF says; “A soil rich in humus not only 
absorbs more water, but holds it more tenaciously in time of drouth than a 
soil poor in humus. As is well known, soils which are properly manured and 
thus supplied with abundant humus, retain more water and yield it up more 
slowly and evenly to growing crops than unmanured soils.” Prom some ex- 
periments by Sachs% it appears that plants cannot exhaust the retentive soils 
so completely of their water as they can th'fe soils which are non-retentive. 
His experiment is summarized in this table. 
EFFECT OF HUMUS ON CAPILLARITY. 
Soil 
Capacity for 
capillary 
water 
Wilting point for 
tobacco plant. 
i water 
T.nnm 
52 
8 
MiYfiiT-p nf Iniiinis and sanrl 
46 
12 
Coarse sand _ __ __ 
21 
1.5 
Experiments by Hellriegel have shown that any soil can supply plants with 
all the water they need, and as fast as they need it, so long as the moisture 
within the soil is not reduced below one-third of the whole amount that it 
can hold. 
These experiments suggest that soil treated with stable manure might retain 
its extra moisture in time of drouth to such an extent that plants would not 
be able to get it. However, the general impression is that this water is avail- 
able. In fact. Professor Spillman in Farmers’ Bulletin 245 says that this 
extra water is readily ayailable to crops, but gives no data or authority to 
support the statement. 
Three experiments were run to secure data on this point. The last one 
which was the most elaborate of the three, is here reported. The first two 
gave results similar to the last one. 
The effects of manure on the drouth limit was tried by ascertaining the 
per cent of moisture in some clover pots from a series which was finished. This 
per cent of moisture was found by noting the weight of the different pots at the 
different stages, as optimum water content, first wilt, etc., and then at the 
close of the experiment making a moisture determination of the total soil in 
the pots, finding the net weight of the soil, and then from this data calculating 
the per cent of the moisture held at the different stages. The pbts used had 
been planted to clover February 24th and this experiment began June 7th, 
therefore the clover was of gpod size. The data secured was as follows: 
lYear book, U. S. Dept. Agri., 1895, p. 138. 
^Storer’s Agriculture, Vol. 1, p. IIL 
