Sis SCIENCE OF AGRICULTURE. Part It 



Subsect. 3. Of discovering the Qualities of a Soil mechanically and empirically. 



2137. The physical properties of soils, and some of their most important constituents 

 relatively to the cultivator, may be ascertained to a certain extent by various and very 

 simple means. 



21:>8. The specific gravity of a soil, or the relation of its weight to that of water, may 

 be ascertained by introducing into a phial, which will contain a known quantity of water, 

 equal volumes of water and of soil, and this may be easily done by pouring in water till 

 it is half full, and then adding the soil till the fluid rises to the mouth ; the difference 

 between the weight of the soil and that of the water will give the result. Thus if the 

 bottle contains four hundred grains of water, and gains two hundred grains when half 

 filled with water and half with soil, the specific gravity of the soil will be 2, that is, it 

 will be twice as heavy as water, and if it gained one hundred and sixty-five grains, 

 its specific gravity would be 1825, water being 1000. 



2139. The presence of clay and sand in any soil is known, the first by its tenacity, the 

 oilier by its roughness to the touch, and by scratching glass when rubbed on it. 



2140. Tlir presence of calcareous matter in soil may be ascertained by simply pouring 

 any acid on it, and observing if it effervesces freely. Muriatic acid is the best for this 

 purpose. Calcareous soils, magnesian soils, and clays, are, for the most part, softer to 

 the touch than arenaceous soils. To ascertain the quantity of calcareous earth present, 

 dry soil thoroughly, and weigh 100 grains of it, which gradually add to one drachm of 

 muriatic acid diluted with two drachms of water in a phial poised in a balance : the loss 

 of weight will indicate the escape of carbonic acid, which will be 44 per cent of the 

 quantity of calcareous earth in the soil. 



2141. The presence of organised matter in any soil may be ascertained very satisfactorily 

 by weighing it after being thoroughly dried ; then subjecting it to a red heat and weighing 

 it again, the weight last found will be the proportion of organic matter and carbonic acid 

 gas, if there should have been any. The same object may also be attained by ascertaining 

 the specific gravity of the soil, but with less accuracy. 



2142. The presence of metallic oxides in a soil may generally be known by their colour. 

 Ferrugineous soils are red or yellow ; cupreous soils, interspersed with greenish 

 streaks, &c. Cupreous impregnations of soils are rare ; and the usual green matter in 

 such soils as the green sand of English geologists, appears to be coloured by iron, which 

 is almost the only metallic impregnation in considerable quantity in any soil. 



2143. The presence of salt, sulphur, coal, &c, may be known by the absence or 

 peculiarity of vegetation, as well as by colour, and the appearance of the water of such 

 soils. Saline soils may be distinguished by the taste ; sulphureous soils by their smell 

 when thrown on a hot iron ; and the presence of coal by its fragments, which will be 

 left after the soluble matters are removed by water and muriatic acid. 



2144. The capacity of a soil for retaining water may be thus ascertained. An equal 

 portion of two soils, perfectly dry, may be introduced into two tall glass cylindrical vessels 

 (Jig. 203.), in the middle of each of which a glass tube has been ^ 203 

 previously placed. The soils should be put into each in the 

 same manner, not compressed very hard, but so as to receive a 

 solidity approaching to that which they possessed when first ob- 

 tained for trial. If, after this preparation, a quantity of water 

 be poured into the glass tubes, it will subside ; and the capillaiy |||| 

 attraction of the soils will conduct it up the cylinders towards *- 

 the tops of the vessels. That which conducts it most rapidly, provided it does not risa 

 from the weight of the incumbent column of water in the tube, may be pronounced to 

 be the better soil. (Grisenlhivaite.) 



Sect. IV. Of the Uses of the Soil to Vegetables. 



2145. Soils afford to plants a fixed abode and medium of nourishment. Earths, exclu- 

 sively of organised matter and water, are allowed by most physiologists to be of no other 

 use to plants than that of supporting them, or furnishing a medium by which they may 

 fix themselves to the globe, lint earths and organic matter, that is, soils, afford at once 

 support and food. 



2140. The pure earths merely act as mechanical and indirect chemical agents in the soil 

 The earths all appear to be metallic bases united to oxygen : these oxides have not been 

 completely decomposed ; but there is no reason to suppose that their earthy bases are con- 

 vertible into the elements of organised compounds, that is, into carbon, hydrogen, and azote, 

 l'lants have been made to grow in given quantities of earth. They consume very small 

 portions only ; and what is lost may be accounted for by the quantities found in their ashes ; 

 that is to say, it has not been converted into any new products. The carbonic acid 

 united to lime or magnesia, if any stronger acid happens to be formed in the soil during 

 the fermentation of vegetable matter, which will disengage it from the earths, may be 



