Maize for Silage
Technical
Insight 315

CULTIVATING DEVELOPING PEAT SOILS

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INTRODUCTION

Raw peat has a low level of natural fertility and a low pH (high acidity). Its ability to hold some major nutrients is poor. To cultivate peat soils and achieve success with crop or pasture establishment, there are a number of steps that must be undertaken. This bulletin aims to provide details on the steps in a cultivation program and why they are necessary.

RUN-OUT PASTURES ON PEAT

Pastures that are established on peat areas tend to run-out. Depending on the standard of previous work, this will normally occur after an interval of 5 - 10 years. The visible signs of a run-out peat pasture are: premature drought stress, the invasion of pastures by species such as browntop, sorrel and yarrow, significant reduction in clover vigour and a decrease in the productivity of the total sward. The reason for this is that bacteria in the soil break down the peat humus and reduce water holding capacity of the surface peat. Large volumes of material are lost as carbon gas, and the top layer shrinks. This shrinkage drastically reduces the plant root zone.

LIME

Peats become increasingly acid as you move downwards from the surface of the ground. In general, from ground level to a depth of 300 mm, each 75 mm horizon of soil is considerably more acid than the one preceding it. Table 1 gives an example of the soil profile you would expect in peat soils. You will need to do a layered soil test to establish actual acidity levels in your peat.

Table 1: Example Peat Soil Profile

Depth

pH (acidity)

0 - 75 mm

5.5

75 - 150 mm

4.8

150 - 225 mm

4.0

225 - 300 mm

3.8

Studies have shown that surface applied lime moves extremely slowly down a peat profile. Incorporation of lime during cultivation will give the greatest response. Considerable volumes of lime will be required to reduce the acidity of these lower horizons of peat. In a soil profile similar to that shown above, 12.5 to 15 tonnes of quality lime well incorporated to the indicated depth, would be required per hectare. 

FERTILISER

On developing peat areas it may be necessary to apply a large capital or base dressing of fertiliser. Commonly this will be 750 - 1,000 kg/ha of 15% or 30% Potassic Superphosphate. Check with adjoining farmers or the fertiliser representative in your area to establish minor element requirements. Any minor element deficiency (e.g. copper, zinc and/or molybdenum) should be corrected at the time of Superphosphate application. Copper is deficient in many peats. If the peat you are cultivating is copper deficient 10 - 15 kg/ha of copper sulphate should be included with the Superphosphate. N.B. Herbage test molybdenum levels will often lift with pH correction after lime application.

DRAINAGE

The physical properties of the peat are very different to mineral soils. Peats require special drainage techniques to control ground water levels and shaping to deal with surface water. Water does not move readily through peat either vertically or horizontally. Excessive drainage can lead to the rapid shrinkage of the peat, over drying with subsequent crop or pasture failure, plus an increased fire risk. Correct placement of drains is critical. In developing peat areas main drains (up to 1 m depth) should run parallel every 100 metres. Minor drains (up to 0.5 m deep) should be placed every 50 metres. For major drain placement consult your local Regional Council.

CULTIVATION

There is a high machinery requirement. Tractors and fertiliser spreading equipment should be fitted with extra tyres to enable them to negotiate the soft surface without breaking through. Basic steps are outlined below:

  1. Examine the peat: It is a good idea to have a close look at the peat before commencing work. Peat will be found in its natural fibrous form at a pH of 4.7 or less. The reason for this is that soil bacteria are not active in low pH (highly acidic) peat. Peat that is above a pH level of 4.7 will be "mineralised" or converted to topsoil to some extent by the soil bacteria. Using a spade, remove a spit of soil. Take a note as to the depth at which the true fibrous peat was first apparent. This will give an indication as to how much of the 300 mm profile is acid peat. Once you have determined how much of the peat is acid, it will be easier to determine how much lime needs to be added to bring the soil to a satisfactory pH. Repeat this procedure in areas where there is a variation in contour.
  2. Spray out: Any existing vegetation should be sprayed out with a desiccant (e.g. glyphosate).
  3. Timber: If large volumes of timber are present in the peat, a digger fitted with a special claw may be required to rake the area lifting out logs and stumps prior to any cultivation being undertaken. Collection of smaller timber can have a significant manual labour requirement. Alternatively, stump chipper machines can be used in some circumstances. 
  4. Sulphate of Ammonia: Apply 100 – 150 kg/ha of Sulphate of Ammonia. This will provide nitrogen to speed up the bacterial breakdown of any surface vegetation, clods and fibrous raw peat. It will also provide extra sulphur. 
  5. Rotary hoe: Use a rotary hoe for an initial pass, chip hoeing to 75 – 100 mm depth. Use a low ground speed to obtain small clod size.
  6. Drainage and contouring: Where drainage and contouring are required, diggers and large grader blades will be required. Any contouring should be carried out prior to lime application. Run the chisel plough through any areas where the soil has to be removed for contour work.
  7. Lime application: Apply the lime to the surface of the ground and incorporate it to 300 mm using a chisel plough. Generally three passes in different directions with the chisel plough will be necessary to achieve adequate incorporation of the lime and a high standard of cultivation. Remember to apply extra lime to areas where soil has been removed for contour correction (e.g. drain banks).
  8. Rota-till or Power-harrow: This will decrease the clod size. One or two passes are required.
  9. Base fertiliser: Carry out any minor contour corrections and then apply the base fertiliser dressing.
  10. Seed bed preparation: Rota-till or Power-harrow to a depth of 100 mm for seed bed preparation and incorporation of the base fertiliser. Make as many passes as are necessary to achieve an adequate seed bed. If seed bed is very soft and fluffy, consolidate with roller to retain moisture prior to planting.  
  11. Planting:
  • Maize silage. A high yielding maize silage crop can be grown as part of a peat pasture renewal programme. See the Pioneer Guide to Growing a Maize Silage Crop for further details on planting. It is recommended that a starter fertiliser containing N, P, & K is used on peat soils. Sidedress at about six weeks post planting (just prior to crop row cover) with 250 kg/ha of urea.
  • Pasture.  If the area is going directly back to pasture, roll with a Cambridge roller at least four times. Use freshly coated inoculated clover seed. Broadcast the pasture seed mix and re-roll the area twice.

 



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Revised: June 2015
Expires: June 2018