76 PLANT PRODUCTS 



is one of the most important points, since the effective use 

 of most manures will be largely determined by its presence 

 in sufficient amount. More than I per cent, of calcium 

 carbonate is probably unnecessary, and less than I per cent, 

 is probably only suitable to parsimonious systems of farming. 



Nitric Acid. The nitrates in the soil are very 

 evanescent. The plant gradually sucks them up and is 

 quite prepared to store them in the stem if it has the good 

 luck to find more than a scanty supply. The bacteria in 

 the soil will readily steal the oxygen of the nitrates if there 

 is much undecomposed organic matter present. On the 

 other hand, nitrates are being incessantly produced by the 

 beneficial action of bacteria in the soil. The amount of 

 nitrate in a soil is rather an evidence of the vigour of life in 

 the soil than of anything else. Nitrates are washed out of 

 the soil with great ease and rapidity. 



The Organic Matter in the Soil. The ordinary process 

 of drying a soil in a water oven and then igniting gives a 

 figure which represents both the organic matter and water 

 of combination together. The latter figure is, of course, 

 not constant, and depends upon the amount of hydrated 

 silicates present. The figure for oiganic matter in a soil 

 will, therefore, be nearer the mark in a sandy soil than it 

 is in a clay soil. Much labour has been devoted to studying 

 the organic matter in the soil, but it is such a very difficult 

 problem that it is almost impossible to give any wide view 

 of the subject. The mere estimation of the carbon will not 

 give one much insight, whilst the efforts to extract so-called 

 humic acid only touch the fringe of the question. Some idea 

 of the amount of decomposed organic matter can certainly 

 be obtained by a modification of Grandeau's method, that 

 is, by first acidifying the soil, washing out all calcium 

 compounds, extracting with dilute ammonia, and comparing 

 the colours obtained. An estimation of nitrogen is certainly 

 valuable, and helps to give one some idea of the amount of 

 organic matter present. The ratio of carbon to nitrogen 

 was investigated by I^awes and Gilbert at Rothamsted, 

 who found that carbon was oxidized away from the soil 



