In addition to sugars, oils and proteins are produced. Little is known concerning the chemical changes which are concerned in the formation of these substances. In the case of proteins, the early products x are caused to react with nitrogen to produce amino-acids, the nitrogen being obtained from nitrates absorbed by the plant. The greenof the plant are the organs where most of the protein synthesis occurs, the energy necessary being obtained from the sunlight. Elsewhere in the plant, energy for protein synthesis is obtained by respiration. The elements phosphorus and sulphur, which occur in certain proteins, are also obtained in the form of salts, I.e. as phosphates e.g. Phosphoglyceric acid. and sulphates. Apart from these, it has been found, largely as the result of the work of Sachs, that other mineral elements not immediately concerned in the formation of protein are necessary for the proper growth of plants.
The essential salts needed for plant growth are the sulphates, phosphates and nitrates of the metals, potassium, calcium, magnesium and iron. Actually these salts are absorbed in the form of their ions and not as molecules. In addition, the elements manganese, zinc, cobalt and boron are needed. These are known as ’trace ’elements, since they are required only in very small amounts. All these essential substances are found in the form of a very dilute solution which occurs as films of moisture around the soil particles. With the exception of the nitrates, these salts are derived from the mineral matter of which the soil is composed. The nitrates are derived from the humus in the soil. Certain nonessential elements, e.g. silicon, may be also absorbed by plants. Silicon is found particularly in grasses.
Since the essential mineral salts are obtained by all green land plants from the soil, it is appropriate that at this stage some consideration should be given to the latter.
This is the medium in which plantgrow. It consists of the following :—
Animals and unicellular plants and bacteria.
This is usually derived from the underlying subsoil which is composed of the debris of weathered rocks. The mineral matter consists of:—
Gravel and sand, often mixed with ferric oxide
Silt and clay.
Chalk or limestone.
The proportions of the above constituents vary in the different types of soil. In a clay soil the silt and clay predominate ; in a sandy soil, the gravel and sand ; while in a chalky soil, the chalk. Mixtures of all three constituents are known as loams, which are naturally the more fertile soils owing to their richer and more varied chemical content.
The particles of a soil vary in size, those of gravel and sand being coarser than those of silt and clay. This can be shown by shaking up a handful of soil with water in a 500 c.c. measuring jar and allowing the soil to settle out. The heavier, coarser gravel and sand sink most rapidly, followed by the silt and finally by the clay. The relative proportions of the various mineral constituents can therefore be estimated by reading the depth of each layer by means of the graduations on the side of the jar. This process is called sedimentation.
The texture in a soil affects :—
The amount of air in the soil.
The degree ofin the soil.
The amount of water retained by the soil.
The rise of water by capillarity in the soil.
If the following experiments are carried out with different types of soil, e.g. clay soil, sandy soil, loam, these effects will be illustrated.
Experiment 43—To find the Proportion of Air in a Soil
A small round tin is filled with tightly packed soil and levelled off at the top, or filled by pushing it into the soil.
Two hundred cubic centimetres of water are poured into a 500 c.c. measuring jar.
The earth is scraped out of the tin into the water and stirred until all air bubbles have escaped.
The combined volume of soil and water is read.
The internal volume of the tin is found by filling it with water and pouring the water into a measuring jar.
It will be found that the volume of air in a sandy soil is greater than that of the air in the same volume of clay soil. The finer particles of clay pack more tightly than those of the sand, leaving less room for air.
Experiment 44—To compare the Amounts of Water retained by Equal Weights of Dry Sandy and Clay Soils
Equal weights of dry sandy and dry clay soil are placed separately in filter funnels standing in the necks of 100 c.c. measuring jars. Fifty cubic centimetres of water are poured on to each and the water allowed to drain through. When no more water drips through, the volumes of the water in each of the jars is read.
Volume of water added.. =50 c.c.
Volume of water in the jar.. =x c.c.
Volume of water retained.. =50-A; c.c.
It will be found that considerably more water is retained by the clay than by the sand. This is due partly to the absorption of water by the colloidal clay particles and partly to the larger total area of the particles on which water can cling as films.
Experiment 45—To compare the Rate of Drainage of a Sandy and of a Clay Soil
The above experiment is repeated, using again the same funnels now filled with thoroughly wetted soil from the previous experiment. A small quantity of water is poured simultaneously on to each and the time taken forthrough the of the funnel to appear is noted.
It will be found that water drains much more quickly through the sandy soil owing to the wider spaces between the soil particles.
Experiment 46—To show the Rise of Water in a Soil by Capillarity
A long, wide glass tube is plugged at one end with glass wool and nearly filled with dry, tightly packed soil. The tube is stood vertically with the plugged end immersed in a beaker containing water. A few mustardare sown in the soil at the top. After a few days the mustard are seen to germinate, showing that water had reached them by capillarity, the water having risen by this means through the fine spaces between the soil particles.
By repeating the experiment with sandy soil, chalky soil, clay soil, it is found that water rises more rapidly in the latter owing to the finer spaces.
Capillarity may help to replenish the films of water around the soil particles when these are removed byhairs of plants, by causing more water to rise from damper regions of the soil lower down, though recent work has shown that this movement of water is limited to short distances.
Soils vary considerably in their content of water and humus. The water content will depend largely on the weather , situation, and proportions of clay and humus present. Both clay and humus aid water retention by the soil, but it must be noted that not all the water retained by these colloidal substances is available for plants. Some of it is held so firmly that thehairs of plants are unable to absorb it by osmosis. If plants are grown in of different types of soil in the laboratory and left unwatered, in time the plants will wilt, having absorbed all the available water. If at this stage the water content of the soil in each pot is determined, the water content at the wilting point in clay soil will be found to be considerably higher than that in sandy soil. This is why the effects of drought are often more pronounced on a clay soil than on a sandy soil.
Experiment 47—To determine the Percentage of Water and of Humus in a given Sample of Soil
A basin is weighed and then reweighed with a quantity of the soil in it. It is then dried in a steam oven to constant weight. The loss in weight is due to the water driven off by evaporation.
A few grams of the dried soil are then heated to redness in a weighed crucible. The soil is stirred gently with a wire and the heating continued until all the humus has been burnt away and no more smoke is seen. The crucible and its contents are then cooled and weighed. The loss in weight recorded is due to the ‘burnt off ’humus.
In both cases the results should be expressed as percentages of the original weight of soil taken.
This is a dark-brown substance consisting of the decaying remains of plants and animals. It will be seen dispersed through the water with the larger particles floating on top. It is from this humus that the nitrates needed by plants are formed.
The Nitrogen Cycle
The proteins in the humus undergo decomposition by putrefying bacteria, and are eventually broken down into simple substances such as carbon dioxide, water and ammonia. The ammonia is then changed by oxidation to nitrates by the nitrifying bacteria. There are two species of these : Nitrosomonas, which converts ammonia to nitrites ; Nitrobacter, which converts nitrites to nitrates. This conversion of ammonia to nitrates is called nitrification. It can only occur if there is sufficient lime and air in the soil and in the warmer months of the year. In sour soils, e.g. sandy heaths which are deficient in lime, or in water-logged soils, e.g. marshes poor in oxygen, nitrification is hindered.
Some of the nitrates may be reduced by denitrifying bacteria leading to loss of combined nitrogen in the soil, the nitrogen in the nitrates being liberated and escaping to the air. It does not occur to any extent except in badly aerated soils.
In addition to nitrification, which by itself does not lead to an increase in nitrates in the soil unless humus is added to replenish the nitrates faster than they are removed by the plants growing in the soil, another important process takes place. This is nitrogen fixation, by which combined nitrogen is added to the soil from the free gaseous nitrogen of the air. This is effected by the nitrogen-fixing bacteria which are able to construct proteins from carbohydrates and free nitrogen. Some of these live free in the soil, e.g. Azotobacter, obtaining their carbohydrates from the humus, while another species, Bacillus radicicola, lives in symbiosis in small nodules or swellings on theof plants belonging to the family Leguminosas, e.g. clover, beans, peas, sainfoin, vetches, and obtains carbohydrate from the plant. In return the plant is able to obtain nitrogenous compounds from the bacteria. In consequence these plants can be grown in poor sandy soils devoid of nitrates, provided other essential salts are added to the soil in the form of artificial fertilizers, e.g. superphosphate and kainite. The free living nitrogen-fixing bacteria help to increase the fertility of the soil. When they die their proteins formed with atmospheric nitrogen undergo decay and give rise to nitrates by nitrification. They are responsible for the building up of the accumulated fertility of virgin soils, e.g. prairies of Canada.
Also during thunderstorms, the high temperature in the neighbourhood of the lightning discharges causes a certain amount of the oxygen and nitrogen in the air to combine to form oxides of nitrogen. These dissolve in the rain water to form nitric acid which, when it reaches the soil, reacts with any carbonates present to form nitrates.
Urea bacteria convert urea excreted by animals into ammonia, carbon dioxide and water. Hence the smell of ammonia from a stable or manure heap.
In farming practice the fertility of the soil is maintained in the following ways :—
By the rotation of crops, e.g. the Norfolk four-course rotation.
This enables more humus to be added in the shape of farm manure, which is ploughed in before the planting of the ’root ’crop. Also the combined nitrogen in the soil is increased by the nitrogen-fixing bacteria in the root nodules of the clover plants, the residue of which, after being cut for hay, is ploughed in before the wheat crop is sown.
By the addition of ’artificial ’ fertilizers, e.g. ammonium sulphate, superphosphate of lime, potash salts, etc.