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| Soil Management for Growing Potatoes
Agdex 262/510Number 345
ISSN 0159-2971 Protection of Paddocks from Soil Erosion
Bed Preparation
Harvesting Operations
Conclusion
Potatoes are grown on a range of soils in Tasmania, from the red clay barns of the north-west and north-east to sandy barns of the northern Midlands and the south. The character of these soils and their management needs must be understood before they are cropped with potatoes.
The goal of good soil management is to maximise productivity and sustain it in the long term. Soil management is more than just cultivation; it includes all operations involving the soil. Erosion control, residue management, irrigation management, sowing, fertilising and harvesting procedures, rotations, green manure crops, grazing, salinity control, surface and subsurface drainage and trafficking are all part of soil management.
Some soils are more prone to erosion than others and some crops expose soil more to erosion than others. Potato crops establish quickly to give a protective vegetative cover to the soil, rendering it less susceptible to erosion in the growing phase than with some other crops, such as onions. Paddocks should be examined at least 12 months before planting to evaluate the risks of water and wind erosion. Time spent in reconnaissance is never wasted!
Water erosion can be prevented by controlling and directing run-off water into safe zones and moving it to the bottom of the slope with minimal loss of soil. The use of grassed waterways is an excellent way of safely disposing of water.
If it is likely that water will run on to paddocks from roads, culverts, tracks, yards, drainage lines or paddocks higher up the slope, then construct cut-off drains or intercept banks, install underground drains or consider leaving in pasture the part of the paddock at risk. If the paddock is long and steep, use graded drains or banks to shorten the length of the slope. Installing shaped and grassed irrigator pathways is a sound management strategy that provides safe disposal zones for run-off from rain and irrigation. Any necessary earthworks and sowing with a grass/oat/fertiliser mix should be done in the autumn before planting the crop, to ensure these areas are quickly and effectively grassed and stabilised.
Light textured soils are more fragile and prone to erosion than the red clay barns. The major risk of erosion on the light textured soils of the northern Midlands is from wind. This can be reduced when soil is left lumpy and surface residues are present. Therefore, plant potatoes into light textured soils after minimum cultivation and in paddocks where there is abundant trash and residues, such as after a cereal crop.
Immediately after harvest, potato paddocks are at risk from water and wind erosion. Harvested zones are bare, have a fine tilth and are heavily compacted. These areas should be protected by ripping across the slope and sowing a cereal crop into the rough seedbed. A simple alternative technique used successfully in Canada has been to broadcast cereal seed a few days before harvest, then cover the seed with soil during the lifting operation. This operation should be considered as part of the harvesting schedule. A stitch in time saves nine! 
Potatoes can be grown satisfactorily with minimum cultivation. Potato plants grow from relatively big "seeds" and do not require a fine tilth in the root bed for maximum yield. However, yields will be lowered if root exploration for water and nutrients is limited by wheel compaction in furrows and/or significant levels of clods in the ridges. The three main reasons for soil cultivation of potato beds are to:- control and incorporate weeds, crop residues and green manure crops;
- maximise soil volume for root exploration; and
- minimise cloddiness at harvest.
Mulching or pulverising residues or green manures prior to the primary cultivation will assist with incorporation and planter operation.
Before every cultivation ask: Is it necessary; or could the crop grow and be harvested as well or perhaps better without the extra cultivation? Recent work in Europe has demonstrated that on a number of soils there was little difference in the rootbed tilth after a primary cultivation with either a mouldboard or chisel plough.
Practical research in Scotland over several years has proved that secondary cultivation by spring tyne implements, spike tyne rotary cultivators, and power harrows (such as Lely Roterra) are generally more effective and efficient in breaking down clods than the L-blade rotary cultivators. Local experiments support this finding. The appearance of the soil surface after secondary cultivation can be quite misleading. Rotary cultivators tend to bury unbroken clods beneath a veneer of the fine material whereas power harrows and, to a lesser extent, spring tyne cultivators tend to do the opposite, leaving unbroken clods at the top and the fine material below the surface.
Cultivation down to 300 mm may be necessary before planting to remove deep compaction and to give sufficient depth of loosened soil for the planter to be effectively pulled through the soil. It is an energy hungry operation needing drawbar power of at least 7.5 kW or 10 hp/tyne. If deep cultivation is necessary it should be the last operation before planting to gain maximum effectiveness at minimum cost. Responses to deep cultivation usually only occur in situations where crops are water stressed.
Soil structure is degraded whenever wet soil is cultivated. The ideal moisture content for cultivation is called "moisture of structure formation". In this condition the soil crumbles rather than smears when worked.
Harvesting is the most devastating of all potato growing operations on soil structure. The soil is sieved through the web of the harvester and then over-compacted by the harvester’s wheels, often when the soil is too wet.
To minimise structural degradation at harvest, adhere to the following principles:- Use a paddock layout that minimises harvesting traffic.
- Restrict traffic, particularly trucks and trailers, to designated tracks and paths.
- Harvest in autumn those areas subject to wetness in winter.
- Store potatoes in sheds or clamps rather than in the ground.
- Keep trucks off productive paddocks and off-load harvesters on designated sacrifice areas.
- Keep off wet paddocks. The soil is seriously damaged if there is significant rutting or wheel slip.
- Use lighter machinery as it causes shallower compaction than heavy machinery. Deep compaction is expensive to remove.
- Use wheel and tyre technology, which minimises compaction.
- Remove compaction with a rigid tyne implement as soon as possible after harvesting and sow a cereal cover crop.
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