decrease (Cook and Harris 1950; 
1956; and many others). 
2. Percentage of carotene decreases (Atke- 
son et al. 1937; Whitman et al. 1951). 
3. Percentages of nitrogen-free extract and 
crude fiber (Savage. and Heller 1947) or lignin 
(Gordon and Sampson 1939) increase. 
4. Digestibility of most plant constituents 
decreases (Cook et al. 1961; and others). 
5. Percentage of tannin increases in plants 
that contain this compound (Donnelly 1959). 
Oelberg 
Most of these changes result from plant ma- 
turity and are only indirectly affected by the 
decrease in soil moisture. However, early 
drying of soil under drought conditions makes 
plants dry or mature earlier, which, in turn, 
hastens the seasonal changes in chemical com- 
position. The decrease in protein as soil mois- 
ture becomes deficient is at least partly caused 
by a breakdown of protein occurring as leaves 
wilt (Thompson and Morris 1966). Late sum- 
mer or fall rains replenish soil moisture and 
cause green regrowth of plants, which is more 
nutritious to animals than the dry herbage 
(Atkeson et al. 1937; Mellin et al. 1962; Sav- 
age and Heller 1947; and Skovlin 1967). 
Soil moisture and stage of growth affect 
ether extract, ash, and calcium content in vari- 
ous ways, depending on species and location. In 
some species percentages of ether extract and 
ash decrease as soil moisture decreases and as 
plants mature (Cook and Harris 1950; and 
Savage and Heller 1947). However, the trend is 
less regular than for protein or phosphorus 
(McClean and Tisdale 1960), and no change 
occurs in some species (Cook and Harris 1950, 
1952). 
Calcium content of grasses has been re- 
ported to decrease during drought (Ferguson 
1931). However, Orr (1929) found that irriga- 
tion caused little change in calcium content. In- 
creases in calcium with increase in maturity of 
grasses were reported by Orr (1929), Daniel 
and Harper (1934), and Pritchard et al. 
(1964). Savage and Heller (1947) found no 
correlation between stage of growth, time of 
year, or precipitation and calcium content of 
grasses, but found that calcium increased in 
forbs as plants matured. Many other examples 
of varying reaction of calcium to moisture 
could be cited. 
Chemical composition of leaves of deciduous 
shrubs often follows the trends just described 
for grasses and forbs during the growing sea- 
son. However, chemical composition of some 
shrubs may not be strongly affected by summer 
drought or season because shrubs often have 
deeper root systems and thus are assured of 
more soil volume from which to draw moisture 
(Oelberg 1956). In Idaho, Blaisdell et al. 
(1952) found that protein and phosphorus in 
40 
the leaves of sagebrush and other shrubs de- 
creased only slightly during the summer and 
that the shrubs contained more of these nutri- 
ents than the grasses and forbs in the fall. 
Soil Depth 
Cook (1959) studied the chemical composi- 
tion of seeded grasses on deep, sandy loam 
soils and shallow, rocky, clay loam soils in 
Utah. Plants on the shallow soil (1) contained 
more protein and ash and less lignin and cellu- 
lose and (2) were more palatable to livestock 
than the plants on the deeper soil. However, 
soil depth was only indirectly responsible for 
this difference; plants on the shallow soil were 
more leafy and had smaller stems than those 
on the deep soils. The leafy characteristic 
probably explains the higher palatability and 
generally better nutritive quality of the plants 
on the shallow soils. Digestibility of the plants 
was not studied, but it would probably be 
higher on the shallow soil because of the lower 
lignin content. 
Stoddart (1941) compared the chemical com- 
position of Symphoricarpos rotundifolius on 
three soils of different depths in Utah. Plants 
on the deeper soils had more ash and phospho- 
rus than those on shallower soils, but about the 
same amounts of other nutrients. The contra- 
dictory results of this study and the one cited 
by Cook were probably due to site differences 
not measured, such as soil nutrients or mois- 
ture. 
Nutrient Content of the Soil 
Many studies show that the nutrient status 
of plants is directly affected by the nutrient 
content of the soils. This is most clearly shown 
in studies of the effect of fertilizer on chemical 
composition. This subject will be treated in an- 
other paper, so only a few references will be 
included here. 
Midgely (1937) concluded that an abundance 
of available plant nutrients in soil is reflected 
in the chemical composition of the plants. 
However, this relationship does not exist for 
all soils and all species, and the effect of the 
nutrient status of the soil may be changed or 
modified by other factors. For example, Cook 
and Harris (1950) found that plants growing 
in aspen stands contained more protein and 
phosphorus than those grown in sagebrush 
stands, even though many of the sagebrush 
soils contained more nitrogen and more phos- 
phorus than the aspen soils. Increased shade 
and higher soil moisture content in the aspen 
stands were believed responsible for the differ- 
ences. 
Daniel (19384) and Daniel and Harper 
(1934) found only a slight relation between 
