Another essential factor is the nature and quantity of organic materials in the soil, whether leaf-mould and decomposing vegetable remains, stable manures, or other animal matters, all of which affect different species very differently, and produce very different results in different soils. It is necessary to apprehend in this connection what has been stated above: that soil is not a mere dead structureless medium, and that the root-hairs of ordinary plants cannot deal with large quantities of putrefying organic matter: that a good soil must abound in useful bacteria and fungi to render such substances available--and in very various ways--and that it must be open and aerated, of proper temperature and suitably supplied with water, and so forth, or disaster will result. Here, again, then we are brought into close contact with all that is known of fermentation, nitrification, and the various biological changes going on in soil, and the application of such knowledge to the practice of manuring and tillage in all its forms. In view of the above remarks, the danger of "over-feeding," in this sense, has a real meaning for horticulturists, though it must not be forgotten that no substance is really a food until it is assimilable into the protoplasm: manures, etc., are food-materials, not food. The futility of mere chemical analyses to prove what a plant requires is now well known, and it is only on the basis of long and carefully conducted experiments that we can ever discover what a particular plant in a particular soil, situation, and climate requires for healthy development. Again, the quantity of water in soil may be too great or too small for given species, and this either on the average for the year, or during critical periods only; and it is obviously important whether the excess or deficiency is due to improper supplies of water, the depth or shallowness of the soil, its retentive powers, or the nature of the sub-soil and so on, again bringing the whole matter into connection with our understanding of the physical constitution and structure of soils, and the nature of soil-drainage. For instance, a common way of killing ferns is to keep the roots and soil wet and the air and fronds dry, whereas the natural habitats provide for wet and shaded fronds and well-drained soil. It may be noted here that in most cases where gardeners speak of plants being killed under the "drip" of trees--_e.g._ Beech, the injury is due, not to the effects of water but to the shade: the loss of light is so great that the shaded plants die of inanition because their leaves are not able to provide sufficient carbohydrates.