- CONTOUR RIDGE DESIGN
Bear in mind that a modern soil conservation system should measure up to three important criteria:
- It must be effective;
- It must be economical;
- It must fit into the farming and management systems practiced.
Should it not meet these three criteria, it would tend to be a burden rather than an asset.
The design must depend on an adequate knowledge of the soil balance (soil loss versus soil formation from bedrock) and storm run‐off quantities. Research into these‐subjects has yielded enough information to enable the recommendation of “standard” contour layouts for particular soil types and land slopes. Such recommendations are modified from time to time.
Although these standard contour designs are the ones that exist on most arable farms, because of their rigidity they do not necessarily comply with the three requirements mentioned above.
There is a place for the standard contour however for example, on lands where cropping is not mechanized, where the pegging team does not have the knowledge to put in more imaginative designs, where a conservation system is needed in a hurry etc. At least a standard contour, if properly made, will be fairly effective in cutting down water velocity and hence soil loss.
Diagram 1: The Standard Contour Ridge has the following dimensions:
The gradient of the channel must be 1 metre in every 200 metres. The maximum length of the contour ridge:
- 300 meters on sandveld
- 400 meters on red soil
Table 1: The vertical interval (V.I.) between standard contour ridges is given by the formula:
|V.I. = % Slope + Soil Factor x 1/3meters
|Where Soil Factors are – 4 for SANDS 5 for SANDY CLAY LOAMS 6 for CLAYS
For convenience table 1: is included showing both V.I. and Horizontal Interval (H.I.) for various slopes and soil types. Note that the table gives maximum permissible contour intervals. Preferably intervals should be kept below these figures.
Diagram 2: The Broad‐Based Contour Ridge has approximately the following dimensions:
Broad‐based contours should only be used on slopes of from 1% to 6%. Contour intervals should be as described above for standard contours. Their main advantage over standard contours is that they can be driven over by most (but not all) farm implements and therefore can be planted to crops which reduces the area “lost” to conservation works in arable lands.
Because of their size, broad based contours are much more expensive to build than standard ones. Often they are built up from standard contours. Ploughing must always be along the line of the broad‐based contour and in a direction so as to build‐up the ridge and improve the channel over the years (see page 3 on ploughing systems to maintain contours).
PARALLEL CONTOUR LAYOUTS
Obviously a contour layout in which the contours (or most of them) are parallel is a great advantage for mechanization purposes, especially where standard contours are used. A parallel design may not always be feasible. The more even the slope of the land, the easier it is to design a parallel layout. Where the slope changes considerably within the land, “wedges” have to be left out of cultivation, or else sections of the land may be non‐parallel in layout.
If a land is steeply sloping at the top and becomes gradually more gently towards the bottom (as in the diagram opposite), then in a parallel contour layout designed around a key contour about the middle of the land (AB) the upper contour ridges would be at a steeper gradient than those at the bottom of the land.
Thus, contours at the top and bottom of the land should be of different dimensions if they are to carry the same amount of water. Contours at the top would be steeper and therefore need not be as large as those at the bottom of the land.
Procedure for Design:
- Decide on key contour and mark on contour map.
- Calculate gradient of each of the other contours. (A suitable contour interval is chosen from Table 1, but this may vary at different points down the land.)
- Determine run‐off capacity needed at each contour, by multiplying hectares served by: 15 for well drained soil; 18 for moderate drainage; 22 for poor drainage. This gives run‐off in liters per second.
- Finally use table 2 to decide on width and depth needed for that gradient and required capacity.
REMEMBER THAT FOR ALL CONTOURS
- Too steep a gradient will result in scouring and soil loss.
- Too shallow a gradient will result in silting‐up and breaching.
- The outlet should always be constructed first.
- Storm drains and top contours must be completed first.
- Gradient in channel must be checked after construction.
2. PLOUGHING BETWEEN CONTOURS
The decision whether to plough over the contour banks or parallel to them is one which the individual farmer will have to make to suit each particular land and the shape and size of contour bank.
For the farmer who ploughs between his contour banks a useful method is discussed below which follows all the basic common sense rules which apply to this type of ploughing, namely:
- Avoid running tractors over contour banks. Apart from the damage caused to the bank itself by the tractor and plough travelling over it, it is a fact that a large proportion of lower link and plough breakages are the result of the implement’s swinging violently as the tractor negotiates a contour bank.
- Avoid trampling freshly ploughed ground. This is in line with normal good ploughing practice.
- A simple alternative system should be available so that direction of ploughing can be changed in order to maintain a flat land profile. Ploughing in the same direction every year with conventional ploughs will result in a terracing effect on part of the land (Fig1).
- Both the system and its alternative must be simple enough for the tractor driver to follow without difficulty, and should preferably be easily applicable to any shape or size of contoured land.
Figure 1: The effect on ploughing in the same direction
Many of the systems in common use today for ploughing contoured lands conform to some of the “rules” mentioned above but very few conform to them all.
A system which does uphold all these points is shown here in some detail. Although a little time must be spent on planning the system and marking out the land it will be found that for most irregularly shaped lands the advantages of this in terms of good ploughing far outweigh the frequently used methods which involve churning up freshly ploughed ground.
Step 1: Mark out, parallel to both contours, strips of land equal to half the narrowest width (or 7 meters, whichever is the narrower) and scratch these lines in with a shallow single furrow (Fig. 2).
This operation of inserting scratch marks can usefully be extended to marking in headland scratch marks at the same time. This ensures safe, clean headlands and also eliminates to a very large extent the stunted and uneven plants which are so often seen growing on the edge of the crop.
Figures 2 and 3: Shows how one should make the scratch marks on the land
Step 2: Then mark with sighting pegs the mean centre between these two scratch lines.
Step 3: Make an opening furrow on the centre line and proceed to “gather” around it (turning right handed), lifting the plough each time the scratched line is reached. In other words, plough the land between the scratched lines (Fig, 4)
If the curve of the land will allow a straight line opening, this, of course, will be simpler to do.
Figure 4: the middle protion of the marked lands should be ploughed
Step 4: What remains are two strips of land which can easily be ploughed out together by gathering. This system is very simple and leaves fewer margins for error than most other systems. No travelling over the contour banks is necessary, and there are also no finishing furrows in the land.
When next this land is ploughed it should be “cast” (turned left handed) starting along the contours throwing the soil outwards. Short furrows should be ploughed near the middle of the land so as to leave a parallel strip on which to finish.
This same principle of leaving parallel strips above and below the contours can be taken a step further for use on very wide, long contours. This time the parallel strips will be about 7 meters wide regardless of the width of the land.
The middle portion can then be marked out as for a flat land system using the eight yard strips as headlands (Fig. 5). The alternative; then, would be to plough the middle portion with openings on the old finishes, and the eight yard strips could be cast instead of gathered.
It would seem then that this system using parallel strips is most versatile, suiting with slight modifications, any shape or size of contour.
Figure 5: Opening of furrows for drainage
Using a reversible plough on contours is about as close to the ideal method as one can get. It is then possible to throw all the furrows uphill every year and so counteract the action of the rain. This
method is also simplicity itself, requiring very little marking out, in some cases, none at all. It maintains the land profile better than any other ploughing method (although some forms of minimum tillage may prove even better). Also, it demands the least headland travel, and it is not necessary to turn on contour banks or channels.
One method of using the reversible plough necessitates marking out a scratch line parallel to the bottom contour of a width equal to the narrowest width of the land. Starting along the bank, plough out this piece, or pieces, lifting the plough at the scratch to leave a parallel land and a finish at the channel.
Another method is to start along the top contour without previous marking out and plough until there is just room enough to turn the tractor at the narrowest point above the bottom contour
Then plough short furrows to leave a parallel strip on which to finish.
It is possible in many lands to make adjustments when pegging contours so as to make them parallel. Ploughing parallel contours with a reversible plough is the ideal system.
No matter which ploughing method is used on sloping land, owing to the fact that the washing effect of rain will be more acute near the bottom contour. There must eventually come a time when the land profile will change. It will be possible in some cases to rectify this by using the plough, and if corrective measures are taken soon enough, blending in short furrows, headland neatness, and so on, without wasting time.
In this connection, it has been shown without a doubt that to encourage a driver to enter for local ploughing competitions is an investment and leads to improvement in the ploughing. This is still the foundation of good crop husbandry.
MECHANICAL PROTECTION OF ARABLE LANDS
Table 2: Maximum acceptable Intervals for Standard Contour Ridges
VI = Vertical Interval HI = Horizontal Interval
Table 3: Contour Ridge Capacities at Upper and Lower Limits of Gradient
Table 4: Discharge in litres per second
|0,03 (with 0.03m Tolerance)
|0,1 (with 0.1m Tolerance)
|0,15 (with 0,15m Tolerance)
‐metricated from “Mechanical Protection of Arable Lands”
These tables are a rough guide only. Specialist advice should be obtained on parallel layouts.