It has now been established that the substances that a plant needs are suitable compounds of nitrogen, phosphorus, sulphur, potassium, calcium, magnesium, and iron. Small amounts of other substances are also found to be beneficial.
Since the protoplasm of which living things are composed is made up of carbon, oxygen, hydrogen, nitrogen, sulphur, and phosphorus, it follows that a constant supply of these elements must be always available for the plant’s requirements if any growth is to take place. The above experiments have shown that nitrogen, sulphur, and phosphorus are taken from the soil, and it is already known that carbon is obtained from the carbon dioxide of the atmosphere and hydrogen and oxygen from the water of the soil .
With the exception of nitrogen all the elements present in salts dissolved in soil-water have an inorganic origin – they have been produced in those disintegrating processes of the rocks which form soil.
Disintegration of rocks does not, however, yield nitrogen. The derivation of the nitrates of the soil is therefore a separate problem.
Nitrogen occurs in large quantity in the air, forming four-fifths of the atmosphere. It has already been seen that this is not a direct source of nitrogen to plants –r. in culture solutions that contain no nitrogen cannot grow, although they are surrounded by atmospheric nitrogen.
Certain facts have now been established with reference to the presence of nitrogen and nitrates, and some deductions can be drawn from them. (1) Plants will not live without nitrogen. (a) Plants take in nitrogen in the form of nitrates. (3) Nitrates cannot be formed by the breaking down of the rocks, since these contain no nitrogen. (4) Nitrogen is a constituent of all living things, and is therefore a constituent of the humus of the soil. (5) It may therefore be inferred that a plant gets its nitrogen from the humus.
Experiments can now be made to test the validity of the inference just stated.
Soil and subsoil are collected from the same place andset to grow in a pot of each. The seeds sown in the soil grow healthily, while those in the subsoil make little or no growth . Now, the difference between soil and subsoil is that the former contains organic remains, and it is thus seen that the capacity for sustaining growth which is possessed by the soil, but is lacking in the subsoil, is due to the humus contained in the former.
If the soil used in the last experiment is baked before the seeds are set to grow, the soil will behave in the same way as the sub- soil, and very little growth will result. This suggests at once that the conversion of humus into nitrate is brought about by living organisms which are destroyed when the soil is baked.
A further experiment bears this out. Two sets of seedlings are grown for some time in different. They are uprooted and a further set is grown immediately in one of the pots. ;The resulting yield is very poor and indicates that the available food has already been used up. The second pot of earth is left fallow in a damp place for a considerable time. It then supports a second crop of seedlings quite successfully. Evidently the food content of the soil has been renewed while the pot was left in the damp atmosphere. This renewal depends upon living organisms in the soil which are continually producing nitrates, provided the soil is kept damp.
Further corroboraticn of the existence of minute living organisms in soil is obtained by a very simple experiment. Such organisms, if they occur, must use up oxygen and produce carbon dioxide in respiration : a little damp garden soil is tied up in a muslin bag and suspended in a corked flask over lime-water. The lime-water soon turns milky . In a corked flask without soil the lime-water remains clear.
Thus it is proved that there are living organisms in soil, and that these need air to breathe. This is one of the chief reasons for keeping land well drained.
The amount of food substance present in soil at any given time is very small. If some soil is dried and weighed, cropped with something, then dried and weighed again, little, if any, difference in weight is detected.
To test for the presence of nitrates, soil is dried and put into a funnel with filter paper. A little water is then allowed to pass through it. To a few drops of this water three or four drops of diphenylamine sulphate are added, followed by three or four drops of strong sulphuric acid. A deep blue colour indicates the presence of nitrates.
The breaking down of the complex proteid substances of humus is brought about to some extent by fungi, but it depends chiefly on minute unicellular plants termed Bacteria, which live in the soil in vast numbers.
The following experiment demonstrates their presence : Into each of two flasks a little milk is put – the necks of the flasks are lightly plugged with cotton-wool and the flasks are then gently brought to the boil. When the milk is cool a small quantity of baked earth is put into one of them – a similar quantity of unbaked earth is put into the other. The following day the milk which contains the unbaked soil will be sour. The milk containing the baked soil will still be sweet. On examination of some of the sour milk under the microscope rod-shaped Bacteria are seen.
In order to carry on their work Bacteria need warmth, air, moisture, and organic matter, and it is the chief aim of cultivation to supply these needs.
The decaying animal and vegetable remains in the soil are continually being broken down from complex proteids to simpler substances. Firstly a large number of putrefying Bacteria set to work upon them – this is particularly the case with animal remains. After this, other Bacteria continue the breaking-down process until compounds of ammonia (NH3) are produced, together with carbon dioxide and water. At this stage a strong smell of ammonia can always be detected.
The Bacterium, Nitrosomonas, then acts on the ammonium salts and converts them into nitrites – after this another Bacterium, Nitrobacter, converts the nitrites into nitrates. The process of oxidation of ammonium compounds into nitrates is termed nitrification.
In this way nitrogen is brought into a form in which it can be absorbed by plants.
There are certain other Bacteria that are able to increase the nitrogen content of the soil by using free atmospheric nitrogen. These have the power of building up the proteid substances of their bodies by the absorption of nitrogen from the atmosphere. When they die, therefore, the soil is enriched and its nitrogen content is increased. Some of these Bacteria always live in association with plants belonging to the family Leguminosx in little-tubercles .
It follows, therefore, that a constant circulation of nitrogen is going on in nature. Green plants use the simple nitrate and build up complex proteid substances. Bacteria break down these proteid substances into simple nitrates. Both the building up and the breaking down processes are the work of plants. Animals limply absorb the proteids, incorporating them into their own bodies.
Increase in the actual amount of nitrogen in the soil can only be achieved by getting it in some way from the atmosphere, and thus the work of the Bacteria that are able to utilise free nitrogen is of the utmost importance and value.