Traiisiorniation of Pliosplioiiis 239 



plants ha\X' certain coiifcntratioiis ol carbon dioxide aronnd their 

 roots, because ol respiration of the niicroliiological popnhition of the 

 soil, especially in the rhizosphere. This resnlts in greater availability 

 of insoluble phosphates and may result in a transfer of some of the 

 phosphate thus made available to the roots of the crop. Soil condi- 

 tions, especialh' aeration, are of great importance in this connection. 



The methods no\\' in use for estimating the available phosphate in 

 the soil are based upon treatment of the soil with a suitable solvent, 

 such as citric or acetic acid or ammonium fluoride, and determina- 

 tion of the amount of phosphate in the extract. A number of bio- 

 logical methods are also in use. These include measuring the amount 

 of phosphate taken up by rye seedlings, or the amount of growth 

 made by phosphate-requiring bacteria, such as Azotobacter, or fungi, 

 such as Aspergillus, with soil as the only source of phosphorus. These 

 methods allow a proximate evaluation of the availability or shortage 

 of phosphate in the soil. No single method, however, is sufficient for 

 evaluating the potential responsiveness of soils to a given crop treated 

 with a certain amount of phosphate, since different crops vary 

 greath' in their ability to extract phosphate from the soil. 



The microbiological methods for evaluating the available phos- 

 phorus in the soil are based upon the fact that a certain parallelism 

 has been observed between microbial cell synthesis and phosphate 

 consumption. This observation has been used to advantage in de- 

 termining the available phosphate present in the soil. Various bac- 

 teria, notably Azotobacter cJiroococctim, and fungi, such as Asper- 

 gillus niger, Cimninghcniiella, Trichoderina, and A. oryzae, are util- 

 ized as test organisms. 



The method of analysis is usually carried out as follows: A certain 

 quantity of the soil in question is added to a sugar-salt solution free 

 from phosphorus and nitrogen and is inoculated with a suitable strain 

 of Azotobacter. After incubation for 14-30 days at 28"C, the amount 

 of nitrogen fixed is determined. The fixed nitrogen is a measure of 

 Azotobacter growth, which is controlled by the available phosphorus. 

 Since the ratio of cell nitrogen to cell phosphorus is 2:1, the amount 

 of phosphorus consumed can be calculated from the amount of 

 nitrogen fixed, thus giving the available phosphorus in the soil sample 

 added to the solution. 



This method has been \ariousl\' modified. In one of these modifi- 

 cations a carbohydrate ( sugar, starch, mannitol ) is added to the soil 

 in (iuestion. Some CaCO.-j is also added if the soil is acid. The soil 

 is then inoculated with Azotobacter and packed into small dishes. 



