Chemical Analysis of Soils. ;U3 



(g) Lo.^s on Ignition. — This is the loss iu weight which occurs 

 when the dry soil is strongly heated. It represents chiefly organic 

 matter, water of constitution, and carbon dioxide derived from the 

 decomposition of certain carbonates when such are present. In the 

 absence of large quantities of carbonates the '' loss on ignition " is a 

 measure, though a rough one, of the amount of organic matter present 

 in the soil. Organic matter is largely derived from vegetable material 

 which has once grown on the soil, the residues of previous crops, 

 remains of native vegetation, and, if manuring has been practised, 

 the products of decay of the manure, all go to make up the organic 

 matter in soils. With the exception of certain peaty vlei soils and 

 one or two others, South African soils contain but little organic 

 matter as compared with the soils of more humid countries. This 

 is due to the rapidity with which decay takes place under our condi- 

 tions. The presence of organic matter largely influences the water 

 retaining capacity of soils. It is also a carrier of plant food con- 

 stituents. The organic matter is the store liouse in which combined 

 nitrogen is held in the soil and from which it slowly becomes available 

 to plants. Eoughly speaking, the greater the amount of organic 

 matter, the higher is the percentage of nitrogen. The organic matter 

 in the soil is the source from which the micro-organisms,, which are 

 present in all soils, derive their energy. These micro-organisms play 

 important parts in the 'changes which combined nitrogen undergoes 

 in the soil, in nitrogen fixation, and in the decay of non-nitrogenous 

 organic matter in the soil, liberating carbon dioxide. 



(</) Insoluble Matter. — When the soil sample is analysed a por- 

 tion is extracted, under certain definite conditions, with acid. That 

 port which does not yield to the treatment, and is not dissolved, is 

 termed "insoluble matter." The quantity is of no significance as 

 regards the fertility of the soil but gives some indication of the 

 texture of the soil and is a valuable factor in soil classification. 

 This insoluble matter still contains a certain amount of those elements, 

 calcium, magnesium, potassium, and phosphorus (though seldom much 

 of the last named) which are important as plant food constituents, 

 but these are supposed to be present in foims of combination "in whicli 

 they cannot be assimilated by plants.* 



(e) Iron Oxide and Alumina. — The percentage of iron oxide and 

 alumina in the acid extract of the soil has no practical significance. 

 The amount is usually highest in heaA^y clay soils and lowest in sandy 

 soils. Soils containing over 15 per cent, of iron oxide and alumina 

 are usually heavy. The percentages of iron oxide and alumina either 

 together or singly are of assistance in soil classification. 



(/) Lime. — Lime is one of the essential plant food constituents 

 without which a plant cannot thrive. There is usually sufficient lime 

 present in a soil to satisfy the needs of the crop. Many of our soils 

 contain very little lime of which much is of no value. The lime in a 

 soil which is of real value is that present as carbonate. The figure 

 given for '' calcium carbonate" represents the total carbonates in the 

 soil calculated as calcium carbonate. This is the form of lime which 



* Hopkins, C. G., Univ. HI. Agric. Exp. Stat., Bulletin 182, has, however, shown that 

 under suitable condition.^ plants are able to assimilate potash left in the "insoluble matter" 

 after extraction with ar-id. 



