Rapid Soil Tests Used on Plots of a Vegetable Soil Acidity Experiment

SummaryThe rapid tests, whether on soil extracts made with 0.1 per cent acetic acid or with the Dahlberg and Brown acetate buffer, indicated much greater availability of calcium, magnesium and phosphoric acid on plots of high pH than on plots of low pH. The reverse was true for iron and aluminum. Ammonia nitrogen was more abundant, for a period after fertilizer containing ammonia was added, on the more acid plots than on those nearer neutrality. Significant changes in pH and in quantities of all extracted ions took place during the season. Seasonal fluctuations in extracted phosphoric acid and iron and aluminum were not well correlated with seasonal fluctuations in acidity. Seasonal fluctuations in extracted calcium and magnesium were negatively correlated with seasonal fluctuations in pH. The tests showed little or no increase in phosphoric acid and potash following the application of fertilizer containing these materials. This finding indicates that, although increases would probably have been found if adjacent fertilized and unfertilized areas had been tested at the same date, such increases were too small to show up when superimposed on major seasonal fluctuations. Either extracting solution dissolved nearly all the phosphoric acid from superphosphate and potash from muriate of potash. It appears that the process of shaking up the soil with the extracting solution serves to dissolve and bring the recently added potash and phosphoric acid thoroughly into contact with adsorbing colloids and quickly to establish an equilibrium between adsorbed and dissolved phosphate and potassium ions. Probably most of the response to added fertilizer on this soil is obtained by virtue of its localization either in bands or in particles, if broadcast. The accuracy of the tests, that is, the possibility of obtaining satisfactorily similar results upon repetition of the extraction and test, seems good enough for practical purposes except in the case of phosphoric acid with the 0.1 per cent acetic extraction and, perhaps, that of potash with either extraction. The acetate buffer solution removed considerably larger quantities of cations than did the weak acetic acid and somewhat more phosphoric acid.