THE transformation of soil fertility into a crop is only possible by means of oxidation processes. The various soil organisms -- bacteria and fungi in particular -- as well as the active roots need a constant supply of oxygen. As soon as this was recognized, aeration became an important factor in the study of the soil. In this matter, however, practice has long preceded theory: many devices such as sub-soil drainage, sub-soiling, as well as mixed cropping -- all of which assist the ventilation of the soil -- have been in use for a long time.
The full significance of soil aeration in agriculture has only been recognized by investigators during the last quarter of a century. The reason is interesting. Till recent years most of the agricultural experiment stations were situated in humid regions where the rainfall is well distributed. Rain is a saturated solution of oxygen and is very effective in supplying this gas to the soil whenever percolation is possible. Hence in such regions crops are not likely to suffer from poor aeration to anything like the same extent as those grown in the arid regions of North-West India where the soils are silt-like and most of the moisture has to be supplied by irrigation water low in dissolved oxygen. Such soils lose their porosity with the greatest ease when flooded; the minute particles run together and form an impermeable surface crust. Only when the humus content is kept high can adequate permeability be maintained. Long before the advent of the modern canal, the cultivators of India had acted on this principle. The organic matter content of the areas commanded by wells has always been maintained at a high level. Irrigation engineers and Agricultural Departments have been slow to utilize this experience. Canal water has been provided, but no steps have been taken simultaneously to increase the humus content of the soil.
Rainfall, Temperature, Humidity, and Drainage, Pusa, 1922.
It follows from the constant demands of the soil for fresh air that any agency which interferes, even partially or temporarily, with aeration must be of supreme importance in agriculture. A number of factors occur which bring about every gradation between a restricted oxygen supply and complete asphyxiation. The former result in infertility, the latter in the death of the soil.
How does the plant respond to soil conditions in which oxygen becomes the limiting factor? Generally speaking there is an immediate reaction on the part of the root system. This is well seen in forest trees and in the undergrowth met with in woodlands. The roots adjust themselves to the new conditions; the trees establish themselves and at the same time improve the aeration and also add to the fertility of the soil; incidentally all other competitors are vanquished. Soil aeration cannot therefore be studied as if it were an isolated factor in soil science. It must be considered along with (1) the responses of the root system to deficient air, (2) the relation between root activity and soil conditions throughout the year, and (3) the competition between the roots of various species. In this way the full significance of this factor in agriculture and in the maintenance of soil fertility becomes apparent. This is the theme of the present chapter. An attempt will be made to explain soil aeration as it affects the plant in relation to the environment and to show how the plant itself can be used as a research agent.
The Soil Aeration Factor in Relation to Grass and Trees
Between the years 1914 and 1924 the factors involved in the competition between grass and trees were investigated by me at Pusa. Three main problems were kept in view, namely, (1) why grass can be so injurious to fruit trees, (2) the nature of the weapons by which forest trees vanquish grass, and (3) the reaction of the root system of trees to the aeration of the soil. An account of this study was published in the Proceedings of the Royal Society of London in 1925 (B, vol. xcvii, pp. 284-321). As the results support the view that in the investigation of the soil aeration factor the plant can always make an important contribution, a summary of the main results and a number of the original illustrations have been included in this chapter.
The climatic factors at Pusa are summed up in Plate III. It will be seen that after the break of the south-west monsoon in June, the humidity rises followed by a steady upward movement in the ground water-level till October when it falls again. In 1922 the total rise of the sub-soil water-level was 16.5 feet, a factor which is bound to interfere with the oxygen supply, as the soil air which is rich in carbon dioxide is slowly forced into the atmosphere by the ascending water-table.
The soil is a highly calcareous silt-like loam containing about 75 per cent. of fine sand and about 2 per cent. of clay. About 98 per cent. will pass through a sieve of 80 meshes to the linear inch. There is no line of demarcation between soil and sub-soil: the subsoil resembles the soil and consists of alternating layers of loam, clay, and fine sand down to the sub-soil water, which normally occurs about 20 feet from the surface. The percentage of calcium carbonate is often over 30, while the available phosphate is in the neighbourhood of 0.001 per cent. In spite of this low content of phosphate, the tract in which Pusa is situated is highly fertile, maintaining a population of over 1,200 to the square mile and exporting large quantities of seeds, tobacco, cattle, and surplus labour without the aid of any phosphatic manures. The facts relating to agricultural production in this tract flatly contradict one of the theories of agricultural science, namely, the need for phosphatic fertilizers in areas where soil analysis shows a marked deficiency in this element. Two other factors, however, limit crop production -- shortage of humus and loss of permeability during the late rains due to a colloidal condition of the soil; the pore spaces near the surface become water-logged; percolation stops and the soil is almost asphyxiated, a condition which is first indicated by the behaviour of the root system and then by restricted growth.
FIG. 2. Plan of Experimental Fruit Area, Pusa.
For the investigation of the soil aeration factor in relation to grass and trees at Pusa, eight species of fruit trees -- three deciduous and five evergreen -- were planted out in three acres of uniform land, each species being raised from a single parent. The plan (Fig. 2) gives further details and makes the arrangement clear. Two years after planting, when the trees were fully established and remarkably even, a strip including nine trees of each of the eight rows was laid down to grass. The two end plots, which were clean cultivated, served as controls. When the grass was well established and its injurious effect on the young trees was clearly marked, the three southern trees of the grass plot were provided with aeration trenches, 18 inches wide and 24 inches deep filled with broken bricks, these trenches being made midway between the lines of trees. To ascertain the effect of grass on established trees in full bearing, the southern strip of the northern control plot was grassed over in 1921. The general results of the experiment, as seen in 1923, are shown in Plate IV. The harmful effect of grass on fruit trees at Pusa is even more intense than on clay soils like those of Woburn in Great Britain. Several species were destroyed altogether within a few years.
The harmful effects of grass on fruit trees, Pusa, 1923.
As great differences in root development were observed between the trees under grass, under grass with aeration trenches, and under clean cultivation, the first step in investigating the cause of the harmful effect of grass appeared to be a systematic exploration of the root system under clean cultivation so as to establish the general facts of distribution, to ascertain the regions of root activity during the year and to correlate this information with the growth of the above ground portion of the trees. This was carried out in 1921 and the work was repeated in 1922 and again in 1923. The method adopted was direct: to expose the root system quickly and to use a fine waterjet for freeing the active roots from the soil particles. By using a fresh tree for each examination and by employing relays of labourers, it was possible to expose any desired portion of the root system down to 20 feet in a few hours and to make the observations before the roots could react to the new conditions.
The Root System of Deciduous Trees
The root systems of three deciduous trees -- the plum, the peach, and the custard apple -- were first studied. The results obtained in the three species were very similar, so it is only necessary to describe in detail one of them -- the plum.
The local variety of plum sheds its leaves in November and flowers profusely in February and March. The fruit ripens in early May, the hottest period of the year. The new shoots are produced during the hot weather and early rains.
The root system is extensive and appears at first to be entirely superficial and to consist of many large freely branching roots running more or less parallel to the surface in the upper 18 inches of soil. Further exploration disclosed a second root system. From the under side of the large surface roots, smaller members are given off which grow vertically downwards to about 16 feet from the surface. These break up into many branches in the deep layers of moist fine sand, just above the water-table. The Indian variety of plum therefore has two root systems (Plate V, Fig. I). The deep root system begins to develop soon after the young trees are planted out. In August 1923 the root systems of young custard apples, mangoes, guavas, limes, and loquats, planted in March 1922 were examined. The young vertical roots varied in length from 10 inches in the custard apple and lime to 1 foot in the mango, 1 foot 2.5 inches in the guava and 1 foot 8 inches in the loquat. Newly planted trees form the superficial system first of all, followed rapidly by the deep systems.
During the resting period (December to January) occasional absorbing roots are formed in the superficial system. When flowering begins, the formation of new rootless spreads from the surface to the deep soil layers. As the surface soil dries in March, the active roots on the superficial system turn brown and die and this portion passes into a dormant condition. From the middle of March to the break of the rains in June, root absorption is confined entirely to the deeper layers of soil. Thus on April 14th, 1921, when the trees were ripening their fruit and making new growth during a period of intense heat and dryness, most of the water, nitrogen, and minerals necessary for growth were absorbed from a layer of moist fine sand between 10 feet 6 inches and 15 feet below the surface. This state of affairs continues till the break of the rains in June when a sudden change takes place. The moistening of the surface soil rapidly brings the superficial root system into intense activity. These hitherto dormant roots literally break into new active rootlets in all directions, the process beginning about thirty hours after the first fall of rain. In the early monsoon therefore the trees use the whole of the root system, both superficial and deep. A change takes place during late July as the level of the ground water rises. In early August active roots are practically confined to the upper 2 feet of soil. Absorption is now restricted to the surface system. At this period the active roots react to the poor soil aeration due to the rise in the ground water-level by growing towards the atmosphere and even out of the soil into the air, particularly under the shade of the trees and where the soil is covered by a layer of dead leaves (Plate V, Fig. 3). This aerotropism continues till early October, when the growth above ground stops and the trees ripen their wood preparatory to leaf fall and the cold weather rest. During October, as the level of the ground water falls and air is drawn into the soil, there is some renewal of root activity near the surface and down to 3 feet.
One interesting exception to this periodicity in the root activity of the plum occurs. Falls of rain, nearly an inch in amount, sometimes occur during the hot season. The effect on the superficial root system of the plum of three of these storms was investigated. When the rainfall was 0.75 of an inch or more, the surface roots at once responded and produced a multitude of new absorbing roots. As the soil dried these ceased to function and died. In one case, where the rainfall was only 0.23 inches, no effect was produced. Irrigation during the hot weather acts in a similar manner to these sudden falls of rain. It maintains the surface root system in action during this period and explains why irrigation during the hot months is necessary on the alluvium if really good quality fruit is to be obtained. It is true that without artificial watering the trees ripen a crop at Pusa, but in size and quality the crop is greatly inferior to that obtained with the help of irrigation. Either root system will produce a plum. High quality is obtained only when the surface system functions; poor quality always results when the deep system only is in action.
Plum (Prunus communis, Huds.)
Fig. 1. Superficial and deep roots (April 25, 1921)
Fig. 2. The repair of the deep root-system (August 6, 1923)
Fig. 3. Superficial rootlets growing towards the surface (August 12, 1922)
Figs. 4 and 5. New wood under cultivation and grass (January 25, 1923)
Figs. 6 and 7. New shoots and leaves under clean cultivation (April 5, 1923)
Figs. 8 and 9. The corresponding growth under grass (April 5, 1923)
In the detailed examination of the active surface roots of the plum and of the seven other species in this experiment, fresh fungous mycelium was often observed running from the soil towards the growing roots. In the deeper soil layers this was never observed. In all probability this mycelium is connected with the mycorrhizal association so common in fruit trees. This matter was not carried further at the time. It is, however, more than probable that all the eight species of fruit trees in the Pusa Experiment are mycorrhiza-formers and that the fungus observed round the active roots was concerned with this association. The mycorrhizal relationship in the surface roots is probably involved in the production of high quality fruit. Plants with two root systems such as these are therefore admirably adapted for the future study of the relation between humus in the soil, the mycorrhizal association, and the development of quality. It would not be difficult to compare plants grown side by side on the sub-soil (to remove the humus occurring in the surface soil), the one manured with complete artificials, the other with freshly prepared humus. In the former there would be little or no mycorrhizal invasion; in the latter it would probably be considerable. If, as is most likely, the mycorrhizal association enables the tree to absorb nutrients in the organic form by the digestion of fungous mycelium, this would explain why quality only results when the surface roots are in action.
Support for the view of plant nutrition suggested in the preceding paragraph was supplied by the custard apple, the root development of which is similar to that of the plum and peach. In the custard apple new shoots are formed in the hot weather when the water, nitrogen, and nutrients are obtained from the deep soil layers only. After the break in the rains and the resumption of root activity on the surface, the leaves increase in size (from 5.8 x 2.6 cm. to 10.5 x 4.5 cm.), develop a deeper and healthier green, while the internodes lengthen. The custard apple records the results of these various factors in the size and colour of its leaves and in this way acts as its own soil analyst.
FIG. 3. Hot weather (below aa) and monsoon foliage (above aa) of the custard apple.
While this book was being printed specimens of the young active roots of the custard apple, mango, and lime were collected in Mr. Hiralal's orchard, Tukoganj, Indore, Central India, on November 11th, 1939, by Mr. Y. D. Wad. They were examined by Dr. Ida Levisohn on December 19th, 1939, who reported that all three species showed typical endotrophic mycorrhizal infection indicated macroscopically by the absence of root hairs, or great reduction in their number, and, in the mango particularly, by beading. The active hyphae in all three cases were of large diameter, with thin walls and granular contents, the digestion stages occurring in the inner cortex with clumping of mycelium, remains of hyphae and homogeneous granular masses. Absorption of the fungus appeared to be taking place with great rapidity. In the custard apple the same kind of mycelium was found outside the roots and connected with them.
The Root System of Evergreens
The most interesting root system of the five evergreens studied -- mango, guava, litchi, sour lime, and loquat -- was the guava.
The guava drops its foliage in early March, simultaneously producing new leaves. It proved an excellent plant for the study of the root system, as the reddish roots are strongly developed and easy to follow in a grey alluvial soil like that of Pusa. There is an abundant superficial system giving off numerous branches which grow downwards to the level of permanent water (Plate VI, Fig. 1). The whole of the root system, superficial and deep, was found to be active at the beginning of the hot weather (March 21st, 1921), the chief zone of activity occurring in a moist layer of fine sand 10 feet 4 inches to 14 feet 7 inches from the surface. As the hot weather became established, the absorbing roots of the guava near the surface dried up and root activity was confined to the deeper layers of soil. In 1922 the monsoon started on June 3rd. An exposure of the surface roots was made on June 5th, forty-eight hours after the rains started. From 1 foot 5 inches to 12 feet new roots were found in large numbers, the longest measuring I cm. As the soil became moistened by the early rains, the dormant zone produced new roots from above downwards till the whole root system became active. After July a change takes place as the ground water rises, the deep roots becoming dormant as immersion proceeds. On August 25th, 1922, root activity was mainly confined to the surface system in the upper 29 inches of soil, the last active root occurring at 40 inches. In the late rains the active roots escape asphyxiation by becoming strongly aerotropic (Plate VI, Fig. 4). An interesting change takes place after the level of the sub-soil water falls in October and the aeration of the lower soil layers is renewed. The deep root system again becomes active in November, the degree of activity depending on the monsoon rainfall (Plate VI, Fig. 5). In 1921, a year of short rainfall when the rise of the ground water was very small, the deep roots came into activity in November down to 15 feet 3 inches. The next year -- November 1922 -- when the monsoon and the rise of the ground water were both normal, root activity did not extend below 5 feet 7 inches.
Guava (Psidium Guyava, L)
Fig. 1. Superficial and deep roots (November 23, 1921)
Fig. 2. The influence of soil texture on the formation of the rootlets (March 29, 1921)
Fig. 3. The root-system under grass (April 21, 1921)
Fig. 4. Superficial rootlets growing to the surface (August 28, 1921)
Fig. 5. Formation of new rootlets in fine sand following the fall of the ground water (November 20, 1921)
Fig. 6. Reduction in the size of leaves after 20 months under grass (right).
Although the guava is able to make new growth during the hot season by means of its deep root system it is a decided advantage if the surface roots are maintained in action by means of irrigation. Surface watering in the hot weather of 1921 increased the size of the leaves from 9.1 x 4.0 cm. to 11.6 x 5.0 cm. and greatly improved their colour.
The root system and the development of active roots in the mango, litchi, lime, and loquat follow generally what has been described in the guava. All these species give off vertical roots from the surface system, but in the case of the litchi and the lime these did not penetrate to the deeper layers. The roots of all four species exhibit marked aerotropism in the late rains. The vertical roots of the lime were always unable to penetrate the deeper layers of clay.
Next: 9. Soil Aeration (cont.)
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