A comparison of the Niklas and Truog methods for the determination of available phosphorus in soils

When the results secured by the Niklas, Poschenrieder, and Trischler method and by the Truog method for measuring available phosphorus are compared in Figs. 1 and 2, it will be seen that the curves are very nearly parallel and the growth of A. niger as indicated by the weight of mycelial growth is apparently, therefore, directly correlated with the available phosphorus in the soil. It should be noted further that the A. niger nutrient medium contained 1.0% citric acid, giving the medium a reaction of pH 4.05. This citric acid was probably mainly responsible for making the phosphorus in the soil available for the growth of the mold. In fact it may be assumed that A. niger obtained no more phosphorus from the soil than that which was extracted by the citric acid. Hence, it appears that the amounts of phosphorus soluble in 0.002 N sulfuric acid and in 1.0% citric acid are relatively similar and both methods probably indicate the amounts of available phosphorus in the soil. When the results for the 6 weeks and the 12 weeks periods are compared, as shown in Figs. 1 and 2, it appears that at 12 weeks the relative amount of available phosphorus was greater than at 6 weeks, while the weight of the A. niger mycelial growth was slightly less at 12 weeks than at 6 weeks. Without further tests it would be quite impossible to give definite reasons for this. Possibly the difference was due to the age of the mold spores used in the preparation of the inoculating culture, which in both cases was 4 days. If, for any reason, the spores did not germinate as rapidly in one culture as in the other, there would undoubtedly be a great variation in the number of spores in the suspension used for the inoculation and this difference would probably affect the growth of mycelium. It will be noted from the data in Tables 1 and 2 that after 12 weeks the availability of the phosphorus in the soils receiving superphosphate or the equivalent application of rock phosphate, with or without lime, was not increased over that in the check soils nor were the increases as great in the soils treated with lime and the phosphates as with lime alone. Apparently, the phosphates stimulated biological action in the soil, and hence a part of the available phosphorus was utilized and was not extracted with 0.002 N sulfuric acid or 1.0% citric acid. Phosphorus absorption by the soil was also undoubtedly an important factor along with nitrification in reducing the availability of the phosphorus in the treated soils compared with that in the corresponding check soils. With the 1,000-pound applications of rock phosphate per acre, however, there were large increases in available phosphorus present, as might be expected. The applications of lime with the rock phosphate increased the availability of the phosphorus, the 3 tons of lime having as great an effect as the larger application. The influence of the lime was more definitely shown by the growth of A. niger than by the extraction of available phosphorus by the 0.002 N sulfuric acid.