Roots And Root Nutrition
( Originally Published 1915 )
Absorption by roots and root pressure. Physical and chemical nature of soil. Manures and their importance. Bacteria in soil. Root tubercles of Leguminous Plants. Rotation of crops. Trenching and ridging.
We have seen that the sap is raised in plants in part by the transpiration of water vapour which takes place through minute pores which are found scattered over the surface, particularly the lower surface, of leaves. In addition to this suction exerted by the leaves there exists a definite upward pressure of the sap by the roots known as root pressure. This upward force is due to the fact that the roots of plants are covered in by a porous membrane, and that the cells of the root contain a cell sap of greater concentration than the water contained in the soil. When ever such a condition of things occurs a physical law determines that water passes through such a permeable membrane from the less dense to the denser liquid.
Water is thus attracted into the cells of the root and, causing them to swell, it forces the liquid up certain conducting channels of the root and into the stem.
If, during a period of active root absorbtion, we cut a plant above the surface of the soil we find that water will be forced out of the ,exposed wound and, fixing a glass tube tightly in the place of the stem which has been removed, we find that the liquid which exudes can be forced up to a considerable height.
Particularly in spring when root absorbtion is very active the pressure exerted by the roots may be very considerable and causes the " bleeding " of plants, as the active exudation of sap is called. This occurs sometimes when trees and shrubs are pruned too late in the spring after the sap has begun to rise. The active absorption of water takes place in the young roots a little way behind the root-tip, where the root can be seen to be covered by a mass of delicate hairs which, possessing very thin walls, offer great facility for the penetration of the water. It is important, therefore, in transplanting young plants from a seed bed or pan to injure as little as possible the delicate young roots, as by pulling instead of carefully digging up young plants the absorbtive part of the root anchored by its many hairs may be left behind in the seed bed.
Plants injured in this way will inevitably suffer a setback, as they will not be able to absorb nutriment efficiently until they have developed new roots to take the place of those which have been injured.
Let us now consider the nature of the soil in which the roots are growing. We have already seen that to be suitable for the growth of ordinary plants it must be well drained, whether we are dealing with a plot of ground or with pot plants. If the earth becomes water-logged the roots are deprived of the necessary air and soon die away. The physical condition of the soil is therefore as important as its chemical composition. In nature, soils are not all equally porous. Some therefore require the careful attention of the cultivator. Clay soils particularly are liable to retain too much water and need to be specially treated for successful cultivation. The retentive power of a soil depends largely on the size of the particles of which it is built up. Gravel or coarse sand allows the water to pass through it more rapidly than fine sand, and even the finest sand is composed of larger particles than those of clay. We can easily prove this by stirring up sand and clay in water. The particles of sand soon settle down and allow the water to clear while the water in which clay has been stirred up remains muddy for a long time. If, however, we add a little lime to this cloudy liquid we find that it clears rapidly. This is due to the " coagulation " of the very fine particles of clay which become collected into larger groups, and these being heavier than the single particles of clay, fall to the bottom of the water and allow it therefore to become char. It is upon this action of lime upon clay that the practice of "liming" heavy soils depends, as the admixture of lime causing the ultimate particles of the clay to become collected together, the ground becomes more coarsely grained and therefore more porous. It is then easier to work, being lighter and more open, and is also warmer. This physical change which is effected by lime is not the only benefit derived from 'its use. Near our large towns the soil is usually rendered acid by the fumes produced by the combustion of coal. In foggy weather this acidity of the atmosphere becomes very noticeable. It has been shown by experiment that such acid soil, and even ordinary soil, when watered with acid rain-water collected in towns, is very detrimental to plant growth. This acidity can be removed by adding lime to the soil, a practice therefore of the utmost importance to those who cultivate gardens or plots near our industrial towns.
Heavy soils can also, be lightened by the admixture of sand to the soil. Good natural learns, which are easily worked soils, consist of fine sand with some clay and a little lime.
Lastly, farmyard manure, quite apart from its important chemical effect upon the soil in enriching it with many valuable food substances, is of great value in improving the texture of all soils. To sands it gives great water-retaining power, while it renders clay soils more porous and friable.
Let us now examine some of the chemical needs of the roots of plants. By a chemical analysis of the ashes left after burning plants it has been ascertained that they are mainly built up of four chemical elements, namely, hydrogen, oxygen, carbon, and nitrogen, to which must be added smaller quantities of lime (calcium), magnesium, potash, sulphur, phosphorus, and iron. Of the four essential substances, the first two, hydrogen and oxygen, combined in the form of water are present in all soils in that form. Carbon, though present in the. soil largely in the form of lime or calcium carbonate, is mainly Obtained by the plant through its leaves from the atmosphere, where it occurs as carbonic acid. Nitrogen, on the other hand, of which there is a ,large supply in the air, cannot be made use of by the plant in this free form, but is mainly absorbed in the combined form as nitrates by the roots. In comparison to the other substances dissolved in the water, the nitrates are the most important salts in the soil as far as the nutrition of plants is concerned. We can realise therefore the usefulness of nitrate of soda as a fertilizer. In nature the nitrogen for plant food are largely obtained from the humus, or decaying vegetable matter, which is present in. all soils. In cultivation, where plants are removed when they die down or where they are taken up to be used as vegetable, it becomes necessary to replace the humus, which would naturally form, by leaf mould or other decaying matter such as manure. The nitrogen contained in rotting manure or in humus is, however, not mainly in the form of nitrates. It is contained in highly complex organic compounds, while in manure it occurs largely in urea, a compound of ammonia. These organic compounds require to be acted upon by bacteria, which are found in the soil and in dung before they are available as plant food. On the other hand we find in the soil micro-organisms of another kind which have the power of combining the free nitrogen of the air with the oxygen and ultimately build up the nitrates so important to plants. These nitrifying bacteria are constantly at work, and when a field lies fallow the soil will be found at the end of the fallowing to be richer in nitrogen compounds than it was at the beginning. In addition to these organisms referred to above, there are special forms which are always associated with the roots of plants belonging to the pea family. If we pull up a pea or bean we find that the roots, bear curious swellings called root tubercles, which are found when examined to contain numerous minute bacteria. The roots must not be considered diseased, though they may look like it.
They are in a normal condition, and the bacteria inhabiting these nodules do not injure the plants but enrich them with nitrogenous material which they obtain from the air found in the interstices of the soil. It is therefore particularly important to keep the ground around peas and beans open by hoeing, so that the roots may not only get the necessary oxygen for breathing purposes, but also the nitrogen they require for their nutrition. It is by the activity of these nitrifying bacteria that leguminous plants, as those belonging td the pea family are called, can grow in very poor soil, that is to say, in soil in which there are very few nitrogen compounds, and yet produce seeds which are very rich in nitrogen, and therefore vary important as food for mankind. Of further interest in connection with these plants is the fact that when they have yielded their crop they have not exhausted the soil of its nitrogen compounds, but will be found to have actually enriched it. This renders leguminous plants a valuable crop to alternate with other crops which deplete the soil of nitrogenous material. In agriculture it is often found expedient in the case of poor or exhausted fields to dig in a leguminous crop such as lupins. or clover, which are often grown for the purpose of enrichng the soil.
With a view to increasing the number of the nitrifying organisms in the soil attempts have been made in this and other countries to introduce more of these bacteria, particularly into pots or beds in which sweet peas or other members of that family are to be grown. In England preparations of these bacteria were distributed some years ago as " nitrobacterine," but the use of this preparation was not found profitable in all cases, probably as in many soils there is already a sufficient supply of these bacteria. Professor Bottomley has, however, now discovered a better way of cultivating these bacteria in peat, with which they can be easily distributed. Peat, which represents the partially decayed vegetable remains, differs from leaf mould or humus in undergoing decay under very wet conditions. As a consequence it remains permeated with certain substances which render it acid and unsuited to, the growth of most plants, though heaths, azaleas, rhododendrons, and other members of the heather family grow well in peaty soil. When rendered alcaline, however, peat has been found to, favour the development of roots, and therefore the whole growth of plants, and containing as it does a large amount of organic material, much of which is rendered soluble when alcaline, it has a high manurial value. This is said to be still further increased by inoculating the peat with nitrifying bacteria, which grow very vigorously in alcaline peat and thus increase the available plant foods: " Bacterised peat," as it is called, is not yet on the market, and has therefore not been extensively tried, but experiments which have been conducted at Kew Gardens with pot plants, and on a farm near Norwich, are of considerable promise.
It should not be forgotten that there are numerous other micro-organisms in the soil, many of which are not only useless to plant life, but may be actually injurious particularly by preventing the full development of the nitrifying bacteria. Dr. E. J. Russell, of the Rothamstead Experimental Station, has discovered that if soil is partially sterilised, either by steam or by volatile disinfectants, some at any rate of the harmful organisms are destroyed, and the subsequent activity of the useful bacteria is greatly increased. The effect of such partial sterilisation has been found particularly beneficial in the case of richly cultivated soil, such as that in use in greenhouses and market gardens, and sterilisation has been successfully adopted in many instances. The chemical antiseptic used by Dr. Russell was toluene, one of the coal tar products. Its action as well as that of steam is generally believed to destroy the numerous protozoa, which are microscopic animal organisms. Some of these are known to feed on bacteria, and are therefore very likely destructive of the nitrifying bacteria.
Before leaving the subject of the nitrogen nutrition the chief function of the roots of plants, it should be mentioned that excessive use of nitrogenous manures has been found to render many plants more liable to disease, particularly such as are produced by various fungi. It is well, therefore, to practice moderation.
As regards other substances than nitrogen which it has been found useful to add to the soil as fertilizers, the most important are potash and phosphoric acid. the latter in the form of various phosphates, including bone meal.
These plant foods must be used in differing proportions according to the richness of the soil, and also according to the requirements of particular crops. Speaking generally, gardens and allotments which receive an abundance of stable manure do not require much in the way of other fertilizers : but in heavy soils basic slag used occasionally will be found a useful way of adding the necessary phosphorus, while nitrate of soda is often valuable to push on early crops, such as lettuces, peas, etc. In agriculture it is usual to practice a rotation of crops whereby plants with different requirements as regards mineral salts succeed each other, and with the periodic introduction of a leguminous crop prevent the exhaustion of the soil. Though this is not so necessary in horticultural practice where the soil is generally richer, yet it is equally useful here to vary the crops in different portions of the plot, firstly because as shown above peas and beans enrich the soil in nitrogen, and secondly because such alternation often puts a stop to certain plant diseases which can only propagate themselves when the same plant is grown year after year on the same plot. This is, the case with such diseases as "club root," of turnips and cabbages, and the "wart disease" of potatoes.
Another feature of some importance must be touched upon. Probably every gardener has noticed that the deeper soil of his garden differs very considerably from the surface soil. On heavy land the subsoil, as it is called, consists partly of solid clay, and is not so porous and crumbly as the surface soil. It is also of less nutritive value for it contains little or no decaying organic matter.
Both physically and chemically, therefore, it is less suited to the nutrition of plants than the surface soil. It is important, therefore, when cultivating deep-rooted plants to improve the subsoil. This is done by trenching, a process which may be regarded as the cultivation of the subsoil. The surface soil is lifted off by removing one spit of soil, and then the subsoil may be improved both chemically and physically by digging in farmyard manure, or other forms of decaying organic matter. In the case of clayey subsoil, the addition of lime should not be forgotten. If the clay of the subsoil is too solid it may be necessary to replace it by loam, or it may be improved by burning it, whereby it is rendered more friable. By thus improving the subsoil, a much better nutrition of deep-rooted crops will be secured. In the case of lighter soils, where there is less difference, at any rate physically, between the surface soil and the subsoil, it is a common and beneficial practice when trenching to put the surface soil at the bottom of the trench and cover it with the subsoil, to which manure has been added.
Another useful process for the amelioration of soil is that of ridging, this is particularly beneficial in the case of heavy and tenacious soils. By digging up the soil and heaping it in parallel ridges a larger surface is exposed to the weathering, particularly to the action of frost during the winter. The effect of frost is to disintegrate the large masses of soil, and so to render the earth more permeable both to water and to air. By exposure to the atmosphere moreover the decay of organic matter in the soil is promoted, and thereby valuable nutritive matter is rendered soluble and available for plant nutrition. It is, therefore, sometimes recommended, before ridging, to spread any stable manure which is to be incorporated with the soil evenly ,over the plot. A trough is then dug- two spades wide, one spadeful being deposited to the right and the other to the left of the trough. The soil of the latter can also be lifted, and turned, or merely forked over.
This operation of ridging should be commenced in the autumn, and can of course only be undertaken on ground that is to remain empty during the winter.