PLANT GROWTH WITHOUT FERTILIZERS 7 



certain particular plants, especially those bred from wild 

 forms growing in districts near the sea. For many plants 

 the amount of salt spray blown in from the sea will be suffici- 

 ent, but in inland districts the amount of salt may need to 

 be increased before intensive plant production can take place. 

 It is only certain crops which require these substances as 

 fertilizers. Sodium has a wider range of usefulness than 

 chlorine (see p. 69). 



Availability of Plant Food in the Soil. Cultivation 

 and the activities of earth worms and bacteria render plant 

 food in the soil more easily available to the plant. Organic 

 manures and vegetable refuse form valuable materials for 

 supplying the necessary food for earth worms and bacteria 

 to live upon. Deep cultivation also assists in making the 

 plant food in the soil more easily available, because it admits 

 air which stimulates living things in the soil. Many methods 

 have been adopted for attempting to measure the amount of 

 available plant food in the soil. The amounts washed out 

 by water are so small as to be difficult to measure, and there- 

 fore weak acids have commonly been employed. The 

 method in use in this country is most generally Dr. Dyer's 

 citric acid method, in which the soil is shaken with a i % 

 solution of citric acid. The results of this method have been 

 carefully correlated at Rothamsted, and experience in many 

 parts of the world have shown the value of this method. 

 The results of other solutions which have been sometimes 

 employed in other countries have shown results possibly 

 as good, but certainly no better than Dyer's method. 



Soil Water. Variations in the carbon dioxide content 

 in the air of the soil will also cause variations in the concen- 

 tration of carbon dioxide in the water of the soil. As a result, 

 the concentration of hydrogen and carbonic acid ions will 

 be much increased in the soil if rapid oxidation of soil organic 

 matter is taking place. Like all the other acids that have 

 been used in the efforts to extract the available plant food 

 from the soil, carbonic acid gives soil solutions which increase 

 in strength more or less in proportion to the carbonic acid 

 supplied, and hence the concentration of soil solutions will 



