DISEASED CONDITION OF PLANTS 45 
only small quantities of available plant foods at a time 
as the result of bacterial and fungal action. 
(c) Aeration.—Plant roots require air for their success- 
ful development. Artificial aeration experiments con- 
ducted with tomatoes at Cheshunt, in which warmed 
moist air was blown through a system of underground 
pipes, indicated that increased aeration produces bene- 
ficial results as regards crop yield and resistance to 
disease. In India, Howard has also shown that artificial 
aeration induces an increased resistance to disease. 
(d) Anchorage.—This factor is important in certain 
tender crops where in loose soil there is danger of rubbing 
at the stem base, with the consequent bruising and 
wounding of the tissues and the possible entrance of 
disease organisms. 
(e) Degree of Compactness.—This is important as 
influencing root development and the diffusion of ferti- 
lizers and sterilizing agents. Preliminary experiments 
upon the effect of ramming the soil have indicated that 
root development is improved by moderate ramming. 
In loose soil and in very tight soil the roots are less 
abundant than in moderately tight soil. It was found 
that soft, sappy plants could be made to produce a 
harder and more resistant type of growth by ramming 
the soil round the roots. Also in soils containing large 
lumps it is difficult to ensure a thorough dispersion of 
fertilizers or sterilizing agents. 
THE CHEMICAL CONDITION OF THE SOIL 
Upon the chemical condition of a soil depends its 
acidity, neutrality, alkalinity, and the balance of plant 
foods, all of which may be limiting factors in plant 
growth. 
(a) Soil Reaction—Soils which deviate to an 
appreciable extent from the neutral point are generally 
unsuitable for the cultivation of crops, and many methods 
have been devised for their amelioration. Fortunately, 
