DISEASED CONDITION OF PLANTS 49 
An excess of lime in the soil has also been shown to 
induce chlorosis of many plants. 
(c) Soil Population.—The micro-organisms that in- 
habit the soil bear an important relation to its fertility, 
By feeding upon the organic matter and chemical salts 
they produce important changes in the soil. These may 
be classified into two groups: (1) Down grade and (2) up 
ade. 
ia Down Grade Changes.—These are concerned with 
the decomposition of complex compounds and the 
production from them of simple compounds, such as 
carbon dioxide and ammonia. Among the different 
kinds of organisms which are instrumental in these 
changes the bacteria, fungi, protozoa, and alge are 
important. 
' Up Grade Changes.—These are concerned with the 
building up or synthesis of valuable plant foods from the 
simple compounds liberated by the down grade processes. 
The bacteria are mainly responsible for these changes, 
but in the light of recent knowledge it is probable that 
the algz also are concerned. In fertile soils the down 
grade and up grade changes are in equilibrium, but 
this is easily disturbed by a change in soil conditions, as 
these affect the growth and behaviour of soil micro- 
organisms much in the same way as they do those of 
plants. 
While much information has been obtained regarding 
the behaviour of micro-organisms present in agricultural 
lands, but little is known about the population of the 
soil under the abnormal conditions prevalent under glass. 
Investigations of these problems are very desirable and 
should elucidate many obscure problems connected with 
the infertility of glasshouse soils. 
Beside these organisms which assist in the main- 
tenance of soil fertility there are to be found in the soil 
organisms which are distinctly harmful, namely, those 
which cause plant diseases. While most soils are com- 
paratively free from these organisms, many become 
4 
