Land clearance operations include a set of works for: land clearing from tree and herbaceous vegetation, tussocks, stumps, moss, stones, and other objects; tillage; addition of sand; addition of clay; addition of earth; trenching and primary treatment of soil; and other land-clearing works.

Overgrowing of agricultural lands by bushes will bring to systematic reduction of their area and therefore is extremely undesirable.

On fields littered by stones, the quality of treatment of soil and crops degrades, and they have to harvest crops from higher stem cutoffs. Stones on earth surface will cause breakdown and fast wearing of the operating elements of agricultural machines and tractors.

Researches are to be carried out to use efficiently the lands needed clearance operations. According to their results, amelioration maps are drawn up, on which the outlines of the following technological properties of the surface are marked out: degree of forest coverage and bushing; species composition of trees and bushes; height and diameter of trees and bushes; age of stumps (new or rotted); degree of littering with boulders/field stones and their average sizes; soil cover; presence of tussocks and their height; etc. Based on the amelioration maps, plans for execution of the works, their performance techniques are determined, and their estimated costs are drawn up.

Basic land clearance operations include the following operations:

• removal of trees and bushes, wood, stones, etc. from earth surface;
• elimination of tussocks and moss;
• removal of stones;
• leveling and primary treatment of soils;
• deep tillage of soil, including leveling of earth hills and banks, filling up of pits and ditches.

Land clearance operations are carried out on a year-round basis. First, earth is cleared from trees. Then bushes and stumps are grubbed up with special stubbing machines-loaders fastened on caterpillar tractors. It is easy to grub up bushes and stumps and rake them together as banks in winter season, because then mechanical erosion of frozen soil is minimum. In spring and summer, collected banks are burnt down, big stones and boulders are dragged out, small and average stones are removed, and earth surface is leveled.

When trees, bushes and boulders are removed from earth surface and marshes are drained, it is necessary first to create topsoil, and then cultivate it.

For different conditions different methods of primary soil treatment would be effective. Treatment of bog virgin soils and selection of soil-cultivating implements depends on the type and moisture of soil, capacity of humus stratum and condition of turf, degree and nature of infestation of a particular site.

Milling is applied to treat heavily getting-hillock and turf-covered soils on drained marshes; milling means soil treatment with an attached cutter drum that provides intensive tillage and careful soil mixing. The operating element of the attached cutter drum, a cutter with knives, revolves in the direction of travel of the tractor carrying this soil-cultivating implement. The cutter cuts off wedge-shaped shavings of soil, loosens it and throws to the array of casing. So the cutter hoes one soil layer in one cut.

Irrigation land levelling

Leveling of irrigated lands means smoothing of the surface of an irrigated area horizontally or at a required slope gradient as well as removal of slight unevenness by shifting soils from hillocks to bottom lands.

The main purpose of leveling in agriculture is to remove roughness that impedes carrying out of irrigation and mechanized land treatment from the field surface. Smooth field surface ensures efficient use of irrigation water, its uniform distribution over the field and soil moistening, favors isogonic growth of crops due to seeding down at an equal depth when sowing and consequently furthers isogonic growth of plants and gaining of big crops.

Requirements for leveling of the irrigated field surface. Field surface leveling degree depends on irrigation method and technique, irrigated crops, and relief and soil conditions.

Irrigated crop to some degree predetermines the technique of irrigation. For example, rice is irrigated by flooding; tilled crops, vegetables and fruits are watered by furrow irrigation or sprinkling; close sowing crops (cereals, herbage) are watered by border ditch irrigation or sprinkling.

Different requirements for surface leveling are made depending on irrigation technique. For flooding irrigation of rice, check plot surface is to be leveled for horizontal plane.

Leveling for irrigation of tilled crops, close-growing crops, orchards and vineyards is accomplished on so-called topographic surface with maximum approximation of design surface to the current one and allowance for changes in slope at every piquet with blending.

Leveling for inclined plane is allowed on plots with flat slope (to 0.002) and provided that the scope of work and soil transportation range do not rise by more than 10% as compared with leveling for topographic surface.

Leveled surface should have only positive slopes in the direction of irrigation up to 0.02 (through an exception up to 0.03) and to 0.002 across-track direction. Transverse grade should be only unidirectional and may be equal to zero. The length of undeviating plots is allowed to be at most 20-40 m lengthways for the areas irrigated by furrows and border ditches and not limited for the areas irrigated by sprinkling.

The best conditions for uniform soil moistening at furrow and border ditch irrigation can be provided on inclined surface with inclination gradually diminishing endwise and without inclination crosswise.

Acceptable soil cutting at surface leveling. When leveling, they cut off topsoil, the most fertile soil layer, on some part of a given area. The higher the depth of soil cutoff, the lower its fertility.

On Central Asian sierozems, natural fertility is restored in a year or two after its leveling, and if organic and mineral fertilizers are introduced down to 60 cm at the points of cutting, cotton productivity restores almost completely as early as the first year.

On chernozems, chestnut, light chestnut and other soils, fertility on cuttings builds up very slowly. Therefore, cutoff sizes should be determined carefully.

For soils with shallow occurrence of pebble beds, the thickness of the top skeletonless layer after leveling is fixed as more than 40 cm. If this layer is less than 40 cm, leveling is inadmissible and sprinkling irrigation is applied.

Methods of leveling. Leveling is carried out by the conventional method and by preserving the topsoil.

When leveling by the conventional method, which is major, the soil from heightened places (cutoffs) is cut by continuous layer and transported to lower places (filling).

Leveling method with conserving the topsoil has four forms: coulisse leveling; leveling by border strips with twofold displacement of the topsoil; leveling by border strips with one-time displacement of the topsoil; clamping.

Types of leveling of irrigated lands

General (construction) land levelling is carried out during the period of development when constructing new irrigation systems by using heavy levelers, scrapers, graders, bulldozers and is repeated in several years as far as leveled soil surface breaks. During land leveling, the surface from higher places is cut off and filled up to lower places. In the process of leveling, slight slopes are made in case of necessity (for gravity irrigation methods by furrow, border strips, etc.). Thorough leveling will cause disturbance of soil cover and lower soil fertility on cuttings. Entire leveling by the cut-and-filling method has some limitations; the rate of cutting is determined depending on the thickness of fertile humus layer. As a rule, cutoff of humus horizon for more than a half of the layer is not allowed. If the thickness of humus horizon is small and the physical, biochemical and other properties of the topsoil and subsurface differ very much between each other, special types of leveling are to be done: coulisse leveling, with preliminary cutting and clamping of the humus horizon, then leveling of the surface and uniform distribution of the previously cut soil material over the surface.

When developing new lands with complex relief and big difference between high and low surfaces, significant thickness of the soil profile is affected at leveling and not only humus but also lower horizons are cut quite often. For some soils (e.g. sierozem on aeolian soil), such leveling has no disastrous effects: aeolian soil has good physical and other properties and is easily cultivated. With a lapse of time, fertility on the cuttings and fillings improves. In other cases, when stony or other barren soils outcropped at leveling, the lands levelled in such a manner were impossible to be used for irrigation; as a result, these lands were abandoned or expensive works were executed on them to reclaim their topsoil. Therefore, thorough leveling should be preceded by special surveys and planning.

Thorough leveling is carried out in the following order:

1. preparation of the surface of the irrigated plot;
2. performance of land measuring works;
3. planning of leveling works;
4. construction layout;
5. carrying out of leveling works.

The scope of the works for preparation of irrigated plot surface includes:

• cleaning of the plot area from rush, weeds, stubble remains, bushes, stumps, and stones;
• ploughing and harrowing of the plot surface with complex microrelief with subsequent passes (1-2) of a long-span blade leveller;
• smoothing of the surface of plots of old-arable lands for 1-2 passes of a heavy grader or long-span blade leveller without preliminary ploughing and harrowing.

Depending on relief, command (fall) conditions and irrigation method they distinguish between the thorough leveling for topographic (curvilinear) surface or inclined plane (for furrow and border ditch irrigation) and for horizontal plane for flooding irrigation (in rice cultivation systems).

To reduce the scope of leveling works, design is drawn up with maximum approximation to the actual relief, i.e. for topographic plane. Leveling for inclined plane is carried out under compound soil-reclamation conditions (need for leaching of lands, etc.). The surface of rice check plots (paddy fields) at flooding irrigation is levelled for horizontal plane.

The volume of thorough leveling works has a wide range from 50 to 600 m³ per hectare and over and depends on the microrelief of levelled lands. An irrigated plot (15-40 ha) bounded by the permanent canals of irrigation network is taken as an indivisible designed ground. Designing of on-farm irrigation network is as a rule combined with the designing of leveling works so that irrigation ditches may be cut over already levelled surface.

Designing is carried out on the basis of field survey on a scale 1:2000 with basic geodetic network in the form of squares 200x200 in size and network 100x100 m, the sides of which are respectively parallel and perpendicular to the direction of irrigation. By means of the basic network, the filling network of squares 20x20 or 40x40 m in size is set up, then levelled by plotting contour lines at an interval of 0.25 m on a plan. Such a plan serves as the basis for working drawings of thorough leveling. Working drawings for leveling works are plotted in the form of longitudinal profiles or topographic plans.

The working drawing of leveling of an irrigated plot made by profiles consists of the base line ranging as a rule perpendicularly to the main direction of irrigation, with layout on the stationing baseline at every 20 m. The baseline is located beyond the upper bound of the irrigated plot or at the bottom of the canal-side dam of an on-farm field ditch at a distance of 10 m from it or on the ridge of the dam.

Parallel cross-sections are set at every 20 m perpendicularly to the baseline, at the direction of irrigation, on which stationing is set up at every 20 m; the scale of the dam sites is 1:2000. Then earth surface elevation marks are traced out at every point orientating to the cross-sections, based on which longitudinal profiles (grade lines) aligned with the plan are drawn up on a height scale 1:100 or 1:50. Lines of the designed surface of the levelled plot are plotted on the longitudinal profiles allowing for balancing of the volumes of the cuttings and fillings on every border strip or adjacent border strips keeping away from banquettes across the border strips and especially adverse slopes. The volumes of the cuttings and fillings are balanced so that the volume of the cuttings exceed the volume of the fillings by 10-15%, because the cut topsoil is looser than the subsoil, the elevation of the cutting surface after ploughing of the field rise, and that of the filling surface subsides.

Operational (smoothing/floating) land levelling implies annual light treatment of earth surface providing for smoothing of relatively small roughness. Sometimes it is also called seasonal leveling. Big soil clods form during ploughing; they are to be crumbled up and then field roughness is to be smoothed. Routine leveling is carried out by long-span blade levellers or deep ploughing with slight smoothing effect.

A drawback of the leveling works executed by long-span or short-span blade levellers consists in the roughness occurring on the field surface, which exceeds the machine base length.

Operating (routine) leveling allows eliminating little roughness of earth surface occurring during soil treatment in the result of machine passes on the rest of temporary irrigation and drainage network. Therefore, operating leveling must be carried out on irrigated fields on annual basis even after thorough leveling. Field leveling is to be performed before sowing, cultivation, after land ploughing in autumn for spring sowing, and in other cases as needed. After the field is ploughed up, use of long-span blade levellers, graders and other levelers will be the most efficient.

Operating (routine) leveling is fulfilled as agrotechnical measure before sowing of crops for the purpose of preservation of the relief made by thorough leveling.

Operating leveling of field surface is arranged regularly before sowing of crops by using of different leveling implements and once every 2-3 years by means of long-span blade levellers. Small pits and rising grounds, dead furrows, back ridges, and other irregularities are eliminated by operating leveling of the field surface. The surface of the field loosened at a depth of 5-7 cm is levelled for 1-2 machine passes (the first pass is to be done at an angle to tillage direction).

Operating leveling is carried out annually to eliminate slight roughness of the field. Operating leveling implies ploughing up and leveling of the surface of an irrigated field. In autumn, it is implemented after the field has already been ploughed up for spring sowing, and in spring before sowing.