128 Observations on Lt'ebig's " Organic Chemistrij.'" 



and that one plant may live in the matter rejected by another: 

 also the experiments of Macaire Prinsep, which show that 

 leguminous plants grown in water cause the water to acquire 

 a brown colour ; that plants of the same kind will not grow 

 in this water, while plants of corn grow vigorousl}'^ and clean 

 the watei". Some plants he found to return excrements of an 

 acrid resinous nature, which were poisonous ; others a mild 

 gummy excrement, which was nutritious. Excrements differ, 

 he says, from excretions, excrements may contain undigested 

 food ; excretions, the food being already extracted from it, cannot 

 give out food again, till it forms new combinations in the soil 

 by putrefaction and contact with the air. This putrefaction will 

 take place more quickly in a calcareous open soil, than in a 

 more dense and clayey one ; and the rotations may be more 

 quickly followed in the former than in the latter. In some 

 places, he says, clover will not thrive till the sixth, in others not 

 till the twelfth, year. The excrement, he states, must be com- 

 pletely transformed, before a new crop of the same article can 

 be grown. Flax, peas, clover, and even potatoes, are plants 

 the excrements of which require the longest time for their 

 reduction to humus. Other writers have represented the potato 

 as having very little if any excrement. The use of alkalies and 

 burnt lime, or wood ashes not lixiviated, will permit, by their 

 action, the crops to be renewed much sooner. In the soils 

 in the neighbourhood of the Rhine and Nile, which contain 

 much potash, and also organic matter, and in the irrigation of 

 meadows, which brings both substances, the fallowing of the 

 land is superseded by the inundation; the oxgyen of the water 

 also effects the more rapid putrefaction of the excrements. 



A fertile soil ought, he says, to afford all the inorganic as 

 well as organic hod'xes required for the plants. When the alka- 

 lies are in combination with mineral acids, as the silicic, or 

 silex, the ashes yield no carbonic acid ; when they are united to 

 organic acids, the ashes effervesce with mineral acids. The 

 silicic acid is the material from which wood takes its origin, as 

 salts in solution will crystallise round a grain of sand. Silicic 

 acid supplies the place of woody fibre in the £quisetaceae and 

 bamboo, as oxalate of lime does in the lichens. Some plants 

 will not thrive without phosphate of lime or magnesia ; others 

 will not do so without carbonate of lime. Wheat, which con- 

 sumes great part of the silicate of potash, should be followed 

 by such as require little potash, as beans, peas, turnips, po- 

 tatoes. At page lO^. the ashes of potato plants are stated as 

 1,500 compared to 83 of firs. The leaves and stems of potatoes 

 should be kept and returned to the soil, and the straw of 

 wheat. The same precautions must be observed in the rotations 

 of other plants, for phosphates, carbonates, &c. To supply 



