The Analysis of the Soil by means of the Plant

Extract

One of the main problems placed before the agricultural chemist is the estimation of the requirements of a given soil for specific manures, or the interpretation, by means of data obtained in the laboratory, of the behaviour of the soil towards these manures, as seen in properly arranged field experiments. For various reasons the obvious method of determining the proportions of Nitrogen, Phosphoric Acid, and Potash in the soil fails in many cases to give the required information; even the more modern methods of measuring only the quantities of these materials which are attacked by weak acid solvents, and in consequence regarded as available to the plant, by no means always accord with the results of experience. Hence from time to time attempts have been made to attack the problem from another side and to use the living plant as an analytical agent. The scheme is to take a particular plant grown upon the soil in question, and determine in its ash the proportions of constituents like phosphoric acid and potash. Any deviations from the normal in these proportions may then be taken as indicating deficiency or excess of the same constituent in the soil and therefore the need or otherwise of specific manuring in that direction. The theory rests on two assumptions, first that each plant has a typical ash composition, constant when the plant is grown under similar conditions; secondly that the variations in the proportion of such a constituent as phosphoric acid will reflect the amount of that plant food available in the soil, as measured by the response of the crop to phosphatic manuring. From this point of view a number of investigations have already been made: Hellriegel discussed the relative variations of the proportion of potash in the ash of barley straw and of the soil in which it was grown; Heinrich analysed the roots of oats and fixed certain minima, below which the need for specific manuring was indicated.