(3) Water of composition can only be determined by ignition. As has been fully shown, this process not only eliminates the water, but also destroys organic matter, decomposes carbonates and sulfids, and, to some extent, chlorids. Subsequent repeated treatment with ammonium carbonate may restore the loss due to carbon dioxid, but in many cases not entirely. The water which comes from organic matter may be approximately calculated from the humus content of the sample, but as will be seen further on the methods of estimating humus itself are only approximate. Nevertheless, in distributing the losses on ignition properly to the several compounds of the soil there is no better method now known than that of taking into consideration the humus content and carbonates present. The principles of procedure established by Berthelot and André, and Von Bemmelén, are to be applied in all such cases, modified as circumstances may arise according to the judgment of the analyst. =304. Estimation of the Organic Matter of the Soil.=—The organic matter in the soil may be divided into two classes. First, the undecayed roots and other remains of plant and animal life, and the living organisms existing in the soil. The study of the organisms which are active in the condition of plant growth will be the subject of a special chapter. Second, the decayed or partially decayed remnants of organic matter in the soil known as humus. Such matter may be present in only minute traces, as in barren sand soils, or it may form the great mass of the soil under examination, as in the case of peat, muck, and vegetable mold. It is with the investigation of the second class of matter that the analyst has chiefly to do at present. The problems which are to be elucidated by the analytical study of such bodies are the following: (1) The total quantity of such matter in the soil. (2) The determination of the organic carbon and hydrogen therein. (3) The determination of total nitrogen. (4) The determination of the availability of the nitrogen for plant growth. (5) The estimation of the humic bodies (humus, humic acid, ulmic acid, etc.). The importance of humus in the promotion of plant growth is sufficient excuse for the somewhat extended study of the principles which underlie the analytical methods, and the methods themselves, which follow. =305. Total Quantity of Organic Matter.=—The total approximate quantity of organic matter in the soil can be determined by simple ignition, in the manner noted in paragraphs =294= and =295=. The proper correction for free and combined water being applied by the further copper oxid or lead chromate combustion of the sample, and for carbonates and volatile chlorids, the approximate total of the organic matter of all kinds is obtained. =306. Estimation of the Organic Carbon.=—To estimate the organic carbon in an earth the sample may be burned in a current of oxygen, or after mixing with lead chromate.