tailieunhanh - MAINTAINING THE SOIL FERTILITY

All plants when carefully burned leave a portion of ash, ranging widely in quantity, averaging about 5 per cent, and often exceeding 10 per cent of the dry weight of the plant. This plant ash represents inorganic substances taken from the soil by the roots. In addition, the nitrogen of plants, averaging about 2 per cent and often amounting to 4 per cent, which, in burning, passes off in gaseous form, is also usually taken from the soil by the plant roots. A comparatively large quantity of the plant is, therefore, drawn directly from the soil. Among the ash. | MAINTAINING THE SOIL FERTILITY All plants when carefully burned leave a portion of ash ranging widely in quantity averaging about 5 per cent and often exceeding 10 per cent of the dry weight of the plant. This plant ash represents inorganic substances taken from the soil by the roots. In addition the nitrogen of plants averaging about 2 per cent and often amounting to 4 per cent which in burning passes off in gaseous form is also usually taken from the soil by the plant roots. A comparatively large quantity of the plant is therefore drawn directly from the soil. Among the ash ingredients are many which are taken up by the plant simply because they are present in the soil others on the other hand as has been shown by numerous classical investigations are indispensable to plant growth. If any one of these indispensable ash ingredients be absent it is impossible for a plant to mature on such a soil. In fact it is pretty well established that providing the physical conditions and the water supply are satisfactory the fertility of a soil depends largely upon the amount of available ash ingredients or plant-food. A clear distinction must be made between the_ total _and -available _plant-food. The essential plant-foods often occur in insoluble combinations valueless to plants only the plant-foods that are soluble in the soil-water or in the juices of plant roots are of value to plants. It is true that practically all soils contain all the indispensable plant-foods it is also true however that in most soils they are present as available plant-foods in comparatively small quantities. When crops are removed from the land year after year without any return being made it naturally follows that under ordinary conditions the amount of available plant-food is diminished with a strong probability of a corresponding diminution in crop-producing power. In fact the soils of many of the older countries have been permanently injured by continuous cropping with nothing returned .