734 PLAi\T GROWTH AND PLANT COMMUNITIES 



tems there is usually no variant equilibrium constant; the latter changes 

 very significantly with the ionic composition of the exchange complex, 

 as we have shown above. 



Evaluation of soils as nutrient media 



Certain basic determinations have long been used by soil scien- 

 tists to evaluate soils, and these still retain their importance, although 

 we should now amplify the information they provide. We can list four 

 criteria, the first two of which are traditional. 



1. Determination of the organic matter in the soil. Rapid wet- 

 combustion methods are widely used to give an approximate value. 

 This then aflFords a relative measure of the annual release of nitrogen, 

 phosphorus, etc., under stated climatic conditions and agricultural 

 practices. It also defines roughly the share of the total exchange capac- 

 ity that can be attributed to the organic matter. The exchange capacity 

 of soil organic matter is high compared with that of clay. Cations are 

 more extensively dissociated from soil organic matter than from clays 

 ( Marshall and Patniak, 1953 ) . There are measurable difiFerences in 

 different organic-matter fractions and in soil organic matter derived 

 under different climatic and vegetative environments, but the over-all 

 situation can be judged fairly well from the simple wet combustion. 



2. Determination of exchange capacity and of individual ex- 

 changeable cations, including hydrogen and aluminum. These results 

 determine reserves and are of great relative value where soils of simi- 

 lar exchange complexes are to be compared. 



3. Cationic activities and bonding energies. As we have seen, abso- 

 lute activities are sometimes controlling. In passing from one type of 

 exchange complex to another, the bonding of individual cations be- 

 comes extremely important. This can throw important light on soil- 

 forming processes, as well as on general agricultural characteristics. 

 ( Brydon and Marshall, 1958 ) . Combining 2 and 3, we obtain measures 

 of the total immediately available reserves and of their ease of release. 



4. Recent determinations of ionic ratios by the use of dilute salt 

 exchange. These values are essentially potentials— that is, intensity fac- 

 tors. They are likely to come into increasing use as a means of charac- 

 terizing soil-root systems under conditions of zero uptake. They are 

 likely to prove important in the comparison of different types of ex- 

 change complexes as initial media for plant growth. Their significance 

 probably will decrease as metabolism rates increase, with consequent 

 large-scale changes in the proportions of exchange cations near active 

 roots. It remains to be determined by field experimentation under 

 widely different cationic ratios how far one can go in using this char- 



