MISSOURI BOTANICAL GARDEN BULLETIN 29 
tain others, while each is necessary in its specific way. In 
such a mixed solution, moreover, a deleterious agent like 
manganese exerts a toxic action only when it is present in 
much greater concentration than when associated with only 
one of the salts constituting the mixed solution. 
Group VIII. In asoil culture, the relation of plant growth 
to the various factors of the environment is even more com- 
plex than in a full nutrient solution culture, and in general 
a toxic agent is less injurious in a soil culture than in the 
usual solution culture. Manganese salts, for example, are 
injurious only when solutions are used twice as concentrated 
as in the previous group. Still, manganese is sometimes pres- 
ent in field soils to an injurious extent. The degree of its 
toxicity in a soil will depend upon a variety of factors of 
which the following are important: fineness of particles 
(more injurious in coarse sand than in fine), the relative 
abundance of antagonistic substances, the amount of organic 
matter present, and the acidity or alkalinity. There are 
probably very few soils which do not contain injurious sub- 
stances of some kind, but under such complicated conditions 
their effects are nearly always minimized. 
Group IX. A poor soil will, of course, encourage less 
rapid growth than a “fertile” one. Without attempting an 
analysis of all the conditions that constitute “fertility” or 
“infertility” of a soil, it may be said that plants show diverse 
capacities to tolerate acidity, and acidity of field soils is not 
infrequently a cause of apparent “infertility.” Lime (either 
as limestone or as air-slaked lime) is the most satisfactory 
corrective of this condition. It neutralizes the acid, and is 
said to “sweeten” the soil. In smoke-besieged cities, excessive 
acidity of lawn soils is common on account of the precipita- 
tion of a certain amount of sulphurous acid. Liming is 
therefore often required in the maintenance of satisfactory 
lawns. There are, however, soils which are naturally acid. 
The bog and certain woodland soils of the eastern states are 
notably acid, and under such conditions, the blueberry and 
other heaths, for example, grow luxuriantly. Recent work 
has also shown that the blue thrives in cultivation only 
when this requirement is observed. 
Group X. It might be supposed that under no conditions 
would our ordinary soil plants grow properly, even for a 
short time, if completely submersed. It is possible, however, 
to grow , wheat, and many other plants entirely sub- 
mersed for a few weeks, provided only that the water is 
streamed through the . This streaming of the water 
