ABSTRACT
The solubility of CaCO3 in water is such that it hinders the growth of only some plants. But the actual calcium content in the soil solution is not always the maximum. It depends on the nature of the solid phase. A test has been developed for this. A measured excess of ammonium oxalate (NH4)2C2O4.H2O is added to a soil sample. This compound reacts with Ca++ to give calcium oxalate. The quantity of ammonium oxalate left behind is determined and the difference is the quantity consumed by the calcium. This is active lime. The response of cultivated plants has been correlated with the results from this method, which is sufficient to select good stocks for grafting in arboriculture or viticulture. We have noted on numerous occasions that the more the specific surface area of minerals, that is, the more divided they are, the more reactive they become. Excellent linear correlation has been found (Fig. 7.19) between the content of active lime in soil horizons and the CaCO3 content of the clay + fine silt (0-20 µm) fractions (Dupuis 1975).
