1.3. Tillage

Tillage is mechanical modification of soil structure. Tillage tools modify soil structure through a wide range of soil:tool interactions (e.g., cutting, milling, crushing, beating, rebound). The outcome of soil:tool interactions varies with respect to both the characteristics of the tillage operation (e.g., action, depth and width of disturbance, timing) and the characteristics of the soil that is being tilled (e.g., texture, structure, moisture, friability, plasticity).

Tillage has been used over the centuries to control weeds, to incorporate previous crop residues, and to prepare the seedbed. The optimum choice of tillage practice is determined by tillage economics and the amount and timing of precipitation, the topography of the field, soil type and crops being grown each year and in rotation. The economics of tillage selection is a balance between the additional costs of the tillage operations versus the additional revenue of the improved yield and quality of the crop. Unfortunately, tillage can also have negative environmental impacts related to increased soil erosion and surface water runoff, loss of soil organic matter, pollution of surface and ground water resources by crop chemicals and nutrients, and evolution of ammonia, greenhouse gases and odours. These negative environmental effects can in turn produce negative economic impacts, especially when large areas or long time periods are involved.

Tillage includes any physical manipulation of the soil, usually done in preparation for some aspect of crop production and management. The main contributions of tillage are improved capture and crop uptake of water and improved corn stand establishment. Since corn yield is very sensitive to the drought stress common in late May, through June and July, potential benefits in water use efficiency should be considered carefully. The crop also is population sensitive, so it is important to achieve the desired plant stand, with reasonably good uniformity of plant spacing.

Depending upon capabilities of the grain seeder being used, preseeder tillage can be very important in achieving a good plant population. In the case of conventional tillage, the ability of the planter to handle some soil clodiness and unlevelness may require more or less secondary tillage. However, unnecessary secondary tillage is costly, time consuming and frequently it is a major culprit in causing soil compaction and surface soil crusting. This undesirable effect of tillage contributes to erosion, water pollution and subsequent drought stress for the crop. In the case of conservation tillage, the planter may require some in-row preparation such as fluted or bubble coulters, row cleaners or in-row subsoiling - again, depending upon soil properties, residue conditions, and seeder capabilities.

Meaning – Although certain soils appear to have a natural tendency to produce grass it left uncultivated, no land that the British farmer handles would similarly bring forth wheat, turnips, or red clover. These and most other arable crops are artificial forms of vegetation, and prosper only when under man's protection and support In the first place, they are incapable of gaining possession of the ground if it is already occupied by weeds ; these competitors must first be suppressed by artificial processes In the second place, they attain satisfactory development only when the soil has been suitably prepared for their reception and is maintained throughout their growing period m such a state as is favourable to crop growth.

With a view to making the soil as favourable as possible for the crop at all stages of its growth, the husbandman subjects it to treatment of various kinds, including the addition of plant food for the better nourishment of the crop, lime to remove injurious substances, and, if necessary, drainage to take away excess moisture and admit air. These processes are all included in the term "cultivation/' and tillage is one branch of cultivation. The term “tillage" refers to the mechanical operations to which the soil is subjected in the regular course of crop production.

Objectives of tillage. Tillage has been part of most agricultural systems throughout history because tillage can be used to achieve many agronomic objectives:

• Soil conditioning (modification the physical conditions of soil structure to favor agronomic processes such as soil seed contact, root proliferation, water infiltration to regulate the soil moisture content, soil warming to modify soil temperatures, to provide proper conditions for the action of friendly bacteria, to aerate the soil, etc.)

• Weed/pest suppression (direct termination or disruption of weed/pest life cycles)

• Residue management (movement, orientation or sizing of residues to minimize negative effects of crop/cover crop residues and promote beneficial effects)

• Incorporation/mixing (placement or redistribution of humus-producing materials such as fertilizers, manures, seeds, residues, sometimes from a less favorable location to a more favorable spatial distribution);

• Segregation (consolidation of rocks, root crops, soil crumb sizes)

• Land Forming (changing the shape of the soil surface – simplest variant is probably leveling; ridging, roughening and furrowing are also examples, to prevent surface erosion;)

The objectives of tillage are to produce suitable soil physical conditions for the sowing operation and for seed germination, seeding emergence, plant establishment and crop growth by:

• loosing and sometimes re-compacting soil;

• controlling weeds;

• buying plant residues and disintegrating clods;

• preventing spread of some diseases;

• incorporating manure, fertilizers, soil amendments or chemicals.

In any specific situation, tillage may have one or more of these objectives. However, it also has some adverse effect, and no tillage operation should be carried out unnecessarily. The optimum method and number of operations vary greatly depending of factors such as soil type, climate, previous crop, amount of trash and weeds infestation.

Stated in general terms, the objects of tillage are to relieve the crop of competition with weeds and to bring about and maintain that physical state of the soil which experience has shown to favour the development of crop plants. This condition is known by the term "tilth." Each crop has its own special require­ments with regard to tilth, and in order that these requirements may be met. account has to be taken of the variable factors of soil, climate, and time of sowing. There are, however, certain common features of good tilths for different crops; a good tilth is clean, sweet, moist, fine, firm, and deep at the time of seeding. These qualities favour crop development, because they offer a soil that allows of the wide, deep, and rapid extension of root fibres, and furnishes air and moisture to the roots and to the germs engaged in the preparation of the nutriment sought by the roots; and the crop has not to compete with weeds for the supplies of food, moisture, air, and light Another factor of crop growth, warmth, cannot to any appreciable extent be controlled by ordinary tillage operations. Indirectly it is influenced by the control of moisture in the soil; but the control of temperature can hardly be included in the list of the regular objects of tillage.

Although tillage operations are mechanical processes, tilth is not merely the direct result of mechanical movement of the soil. The influence of the weather on the soil is an important factor in tilth formation, and the mechanical part of tillage is more or less subservient to weather considerations.

Aeration – Soil that has borne a crop such as potatoes, during the growth of which it is frequently stirred, may (after this crop has been removed) be immediately sown with another crop after only such tillage as may be desirable to enable the drill to work efficiently. On the other hand, if the land has been under a corn crop, during the growth of which it has lain undisturbed for several months, a preparation merely sufficient to enable the seed to be drilled would not ensure the development of as good a following crop as that soil might be capable of producing. Such a superficial tillage (apart from the questions of manure and weeds) suffices in the first case and does not in the second, chiefly because the soil has been well aerated in the one and badly aerated in the other. The fact that grass and clover seeds can be sown in the spring and continue to occupy the ground after the removal of the corn crop, without such aeration as above mentioned, is apparently an exception, attributable to the special character of these plants; the grasses and clovers are more adapted to settled con­ditions of soil than corn or green crops.

Aeration is undoubtedly one of the main objects of tillage. Oxygen is required for the germination of seeds, the respiration of the underground parts of the plant, and the activity of the beneficial germs in the soil. Some of the benefits of working the land at seeding time and during the growth of the crop are thus attributable to the ventilation of the soil. It cannot be said, however, that the fertilising effects of aeration are fully understood. Prolonged aeration has an effect which the farmer expresses by the term "sweetening" ; this is not necessarily connected with the supply of oxygen to seeds, roots, or germs, as the soil may be well sweetened at a time when life processes are dormant. Long and thorough exposure of the soil to the atmosphere certainly facilitates pulverisation, and thereby helps to ensure aeration; but even in soils with no tendency to form clods, aeration is beneficial. For instance, peaty soils, which do not require weathering to ensure complete pulverisation, benefit from winter ridging. Possibly the effect of prolonged exposure of the soil to the action of the air is to decompose organic compounds that are in some way injurious to plant growth.

To ensure aeration, the soil must be broken up out of its settled condition and left in such form as will facilitate the entry and circulation of air among its particles. If there is little time in autumn between the dates of breaking up and of sowing the next crop, the soil should be pulverised as soon as possible, and perhaps stirred again before sowing. On the other hand, if the soil is to lie through the winter, it should not be left with a level pulverised surface, otherwise it would be liable to run together, and in that condition it would hold moisture in the surface to the exclusion of the desired air. The better practice is to leave the soil in plough-seam or with some roughness of surface to facilitate percolation of rain and to avert the fine particles running together; this also exposes a greater surface to the air. Leaving the land in ridges and splitting them back during the winter is also good practice from the point of view of aeration. In any case, it is the under parts of the soil that are most in need of aeration; hence ploughing, which brings these to the top, is a particularly good form of tillage for autumn work. And subject to proper safeguards, which will be mentioned later, the deeper the soil is ploughed, the deeper the benefits of aeration will extend.

Moisture – Modern soil scientists all agree on the importance of moisture in crop production. Wood differ from those of their predecessors in the leadership of British agricultural science in the emphasis they lay on the need for storing and con­serving a good supply of moisture for the use of the crop during the active growing months. Crops require enormous quantities of water; a 30-tons crop of mangel, for instance, consumes about 1500 tons, and a 6-quarters crop of oats about 1200 tons. An inch of rain amounts to 101 tons per acre, so that the above yields correspond to about 15 and 12 inches of rainfall respectively.

If all the rain that fell on the soil during the most active growing months actually remained in the soil and subsoil for the use of crops, it would still be only in the wetter districts that good crops could be grown. But according to the Rothamsted records, only about one-third of the rain falling during April. May. June, July, and August percolates through the soil, the rest being lost by evaporation. Hence a normal fall of 2 to 3 inches per month from April to August would not suffice for yields of more than 6 to 10 tons per acre of roots or 12 to 20 bushels of oats. Summer rains, though beneficial, cannot therefore be relied upon to suffice for the production of good crops. The crop must draw upon reserves and supplies other than those coming from time to time in the form of rain during the growing period.

The six or eight inches of depth usually tilled cannot store and furnish the extra moisture required in the production of a good crop. A considerable part of the moisture present in the soil proper is often dried out in the operations necessary to the formation of a seed-bed. This loss must be made good by the rise of moisture from the subsoil, and further supplies of moisture must rise in the same manner to furnish the crop with the quantity necessary for satisfactory yields. As explained above, from a half to two-thirds of the moisture needed for a good crop must come from subsoil reserves. It is obvious, therefore, that the storing and conservation of subsoil moisture is an important factor in crop production, and, being greatly influenced by tillage operations, may be regarded as one of the main objects of tillage.

If the soil be shallow and near the solid rock—as is the case in many parts of the limestone districts—good yields are possible only when the seasonal rainfall is favour­able as there is no depth of subsoil to serve as a moisture reservoir. If the subsoil be of an open gravelly nature, as in the case of the pebble-bed formations, there is no subsoil reserve of moisture and the land is of little agricultural value. Again, when the soil rests on an impervious layer, or when a hard pan hinders the deep percolation of moisture, the subsoil cannot supply the moisture needed for good crops. Heavy soils under which a hard plough-pan has formed may be very misleading to the cultivator Owing to the slow percolation of moisture into the subsoil, they lie wet all winter and far into the spring. After the soil has been dried sufficiently to form a tilth, the crop makes only indifferent growth owing to deficient or tardy supply of moisture from below In this case an important improvement would be the breaking of the pan with the subsoiler to facilitate percolation. For the best results, however, a certain proportion of the moisture passing through the soil and subsoil should pass away into natural or artificial drains. The soil apparently needs a little cleansing to take away salts that otherwise accumulate in the soil and injure the crop.

The months during which a large proportion of the rainfall may be stored in the subsoil are October to March. In dry districts particularly, it is important that the land be ploughed or otherwise loosened so that the rains of October and November may readily enter the ground. If also the land be ploughed in such a fashion that the" rain can reach the subsoil quickly and without having to percolate through the whole of the soil, there will be less loss by evaporation and a greater proportion of the rain secured. Further, this early autumn tillage will, by its exposure of the sod to weathering influences, so facilitate the spring cultivations as to reduce the loss of moisture otherwise liable to be incurred in the preparation of the seed-bed. One of the greatest advantages of autumn cleaning is that it saves loss of moisture in spring. Having caught the autumn and winter rains, care must be exercised m the spring to avert the loss of these reserves. A timely cross-ploughing in February or March, when practicable, checks the escape of moisture from the subsoil and allows the upper layer to dry out for further working and refinement. After the requisite degree of pulverisation has been secured, the soil is re-firmed to restore the moisture connection with the subsoil and bring up supplies for the use of the seed and the young roots. The upward current must, however, be arrested before it reaches the surface, other­wise there would be loss by evaporation. With this object in view the surface is kept loose and dry as long as possible during the growth of the crop.

Fineness – The husbandman aims to obtain a certain degree of fineness in the soil composing his seed bed, the actual degree of refinement varying according to crop, soil, and time of sowing. Pulverisation entails looseness of texture; but both the fineness itself and the looseness that is associated therewith are only temporary requirements, being concerned with the aeration of the soil, the control of moisture, and the proper firming of the seed-bed after aeration, rather than with the immediate needs of the plant's roots.

Pulverisation is essential to the action of the air on every soil particle; the attain­ment of a fine tilth is, therefore, necessary for a proper aeration of the soil. Pulverisa­tion has, however, a leading part in the management of the moisture supplies in the soil; and on heavy land the moisture content of the soil has an important bearing on the attainment of a fine tilth. If heavy soil be tilled when too damp, not only will the particles fail to separate as desired, but on the contrary they will on drying cohere more firmly and form clods. On the other hand, if such land be allowed to dry through in lumps, these also will harden and form clods, unless they have previously been frozen through. With a view to securing the fullest advantage of the assistance of frost, heavy soils are preferably ploughed in autumn, and if prac­ticable reploughed sometime before the end of February, so that both sides of the furrow may be acted upon.

The loss of moisture that occurs in spring, when a soil is exposed to drying winds while in solid or lumpy condition, may seriously affect the prospects of the crop that is to be sown. The lumps themselves dry out, while, owing to the large air spaces about them, considerable quantities of moisture may escape from the lower layers of the soil by evaporation.

A superficial pulverisation that leaves the under part of the seed-bed cloddy is very unfavourable to crop growth. The clods bridge open spaces under the seed thus preventing the rise of moisture into the region of the seed and young roots and when the clods have crumbled down, as they ultimately do, the soil still lacks that firmness which is requisite for good growth. Surface tillage after the germination of a crop sown on a properly prepared seed-bed is a different matter. The object m this case is, although usually concerned with the destruction of weeds, to arrest the upward flow of moisture before it reaches the atmosphere, into which it would escape by evaporation. This loose surface layer also acts as a soil ventilator. In farm practice pulverisation has a number of functions other than those men­tioned above. Weeds cannot be drawn out of the soil until it has been so refined that it falls away from them. Fertilisers cannot be thoroughly mixed with the soil unless it be sufficiently refined. And seeds cannot be drilled regularly and at a uniform depth, nor covered with a suitable protective layer of soil, unless the seed-bed has been properly comminuted.

Firmness – Aeration and firmness are somewhat opposite characters. The soil cannot be well aerated unless its firmness be broken down and the particles be separated sufficiently for the circulation of air about or among them. The condition that is requisite for thorough aeration is not, however, suitable for the growth of crops After the soil has been sufficiently exposed to the sweetening influence of the air it must be restored to such a consistency as is favourable to root action – i.e. a suitable degree of firmness must be restored to the layers beneath the surface.

All farm crops require an aerated but somewhat firm rooting-ground. Some such as wheat, clover, and mangel, call for a firmer condition of soil than others such as barley and potatoes. Firmness is requisite for several reasons: - The plant must have a suitable anchorage. Then the feeding organs—the root hairs-must apply themselves closely against the soil particles, in order to draw upon the film of moisture surrounding them and to attack soil constituents not already dissolved in the moisture film. Further, as the moisture is used up or dried from one part of the soil, it is desirable that the loss be replaced from another part, usually the lower lavers The movement of moisture cannot take place in a loose spongy mass: the particles must be in sufficiently close contact. On the other hand, if the mass of particles be too loose and open, moisture will escape by evaporation. Lastly, a seed-bed that is loose and open at the time of sowing tends to settle upon itself when left to the actions of gravity and surface tension. The seed and rootlets, being lighter than the particles of sand and clay and forming a connected structure, do not subside with the bulk of the soil and are consequently left nearer the surface than may be desirable to escape injury from either frost or drought, as the case may be.

The soil particles, after having been separated or clotted by timely tillage, do not immediately settle down to form a solid mass. When the lower layers have regained their connection with the subsoil and moisture has begun to move upwards from particle to particle, the soil gradually assumes more cohesion, this property extending upwards. The particles do not, however, cohere so closely as in unfilled land, where the pore spaces between the particles may be so small as to restrict the spreading of root fibres and prevent the proper circulation of air in the soil.

Natural settlement before sowing is undoubtedly preferable to artificial con­solidation before or after sowing. In the one case the soil firms from below upwards; in the other the soil is made firmer at the surface than underneath. Skilful consolida­tion may reduce the difference. Gardeners like to prepare their onion beds some time in advance of the date of sowing, and the practice might with advantage be adopted in agriculture. On certain soils it is undesirable to refine the surface until near the time of sowing, otherwise there is danger of the surface running together under the influence of the rains; but in the case of corn crops sown in the autumn it is better to perform the necessary harrowing immediately after ploughing rather than delay it until just before drilling. In the case of roots sown after cleaning the land in May, a "plant" is often secured by ploughing up a little moist soil in which to drill. The plant comes, but its progress is poor until the soil has assumed the requisite degree of firmness and moistness to support proper growth. The better practice is to obtain the necessary firm moist condition by consolidation and time before sowing.