Farmnote 66/96

Crop establishment series

By Greg Hamilton, Dan Carter, Paul Findlater, Ron Jarvis and David Tennant

Summary

Presents stubble management recommendations to control wind erosion, water erosion and sandblasting, including trials on moisture conservation, soil organic matter and rotations:stubble interactions.

This Farmnote is one of a series on crop establishment practices which minimise soil disturbance and improve stubble retention. In partnership with primary producers, Agriculture Western Australia is focused on improved and profitable farming which conserves or improves the quality of the State's soil resources.

The retention of stubble is advocated to

• control wind and water erosion;
• protect emerging crops from sand blasting during wind erosion events;
• conserve soil moisture;
• maintain or increase soil organic matter in cropping phases; and
• assist disease control.

Retaining stubble presents some management difficulties, such as

• obtaining adequate stubble clearance in seeding operations;
• obtaining good seed to soil contact in freshly sown crops;
• reducing the impact of knockdown herbicides on weeds; and
• harbouring some diseases and insect pests.

This Farmnote presents options for managing stubble in crop establishment practices that seek to minimise soil disturbance. Other aspects of stubble retention, such as the interactions between retained stubble and weed, disease and insect pest control are discussed in other Farmnotes in this series.

Wind erosion control

Intensive studies of wind flow over stubble have established the following management recommendations to maximise control of wind erosion.

• At least 50 per cent of the ground needs to be covered by stubble (Figure 1).
• Standing stubble (anchored by its roots) is twice as effective in reducing wind erosion as prostrate stubble.
• The minimum stubble height required to prevent wind erosion is 10 cm, provided the 50 per cent ground cover criterion is met.

Figure 1. The effect of stubble cover on wind erosion

This latter requirement is particularly important for stubbles of field pea, lentil, chickpea and faba bean, which must remain ungrazed if wind erosion control is necessary.

Water erosion control

Although Western Australian data on the benefits of stubble in controlling water erosion are not available, there is ample information from research done in the eastern States which can be used in this State. The substantially greater erosivity of rainfall in the eastern States means that stubble effectiveness in controlling water erosion has been tested under conditions more extreme than found in Western Australia.

Research clearly shows that stubble reduces raindrop impact, maintains infiltration and reduces runoff and erosion. Standing stubble is the best way to slow overland flow and enhance the deposition of any soil transported in runoff.

The recommendations for managing stubble to control water erosion are:

• Cut the stubble as low as practicable, that is, to 15 cm but no shorter.
• Remove, by grazing or catching, or disperse by spreading, as much chaff as possible.
• Do not graze stubble so hard that most of it becomes prostrate. Retain most or all stubble in an erect, undisturbed anchored condition.
• If stubble is so heavy that seeding machinery will have difficulty clearing it, graze, rake or thin excessive stubble into windrows and burn them in late autumn as close to seeding as possible.

Sandblasting control

All crops are adversely affected by abrasion from wind blown soil particles. However, cereals are less affected than lupin, canola, faba bean, field pea, lentil and chickpea, which do not recover from severe sandblasting. Unlike cereals, this latter group have their growing tip of tender tissue exposed at the top of their stems, and most of the adverse effect of sandblasting on these crops results from damage to or loss of growing tip tissue.

However, sandblasting production losses in cereals can be substantial. Experiments at Badgingarra, Wongan Hills, Pingrup and Esperance have shown average reductions in yield of 9 per cent, 26 per cent and 54 per cent from crops exposed to simulated sandblasting on one, two and three occasions, respectively.

Stubble retained at the 50 per cent ground cover level to prevent wind erosion reduces sandblasting losses to zero.

Moisture conservation

Evaporation of soil moisture occurs in two stages. The first stage, when the soil surface is wet, proceeds at a rate determined by the evaporative power of the atmosphere.

The second stage, when the very surface of the soil is dry, proceeds at a much slower and decreasing rate, which is controlled by the water transmission properties of the soil. In this stage water is converted within the soil to vapour, which then must move to the surface before being taken up by the atmosphere.

Stubble helps conserve soil moisture by

• increasing the amount of rainfall that infiltrates; and
• reducing the rate of evaporation by slowing the flow of wind immediately above the soil surface.

The slower wind speed in the stubble reduces the evaporative power of the atmosphere close to the ground surface. However, the net effect will depend on the time taken for the surface soil to dry. Hence, stubble is most likely to conserve soil moisture in circumstances of high evaporation demand, such as occur in summer and autumn. Research data from Merredin for a March storm of 82 mm illustrate these points (Table 1).

Table 1. Effect of stubble on soil water storage 50 days after a 82 mm summer storm at Merredin

This research also showed that the effectiveness of stubble in conserving water was more noticeable in drier than normal growing seasons compared to wet seasons, and that in such seasons the increased amount of stored water can be converted into larger crop yields (Table 2).

Table 2. Yield responses to retained stubble in two dry and one wet growing seasons at Merredin

Soil organic matter

The retention of stubble retains organic matter on the soil surface, but whether this leads to increased soil organic matter and soil nitrogen has not been proved in Western Australian conditions.

Long term trials (1988-95) which have monitored soil organic matter trends under burnt and retained stubble and direct drilled and cultivation seeding treatments, have shown

• no trends for increasing or decreasing soil organic carbon in burnt or stubble retained treatments; and
• maintenance of pre-existing higher levels of organic carbon in the direct drilled treatment (Table 3).

Table 3. Eight year averages (1988-95) of soil organic carbon (%) in a range of crop establishment treatments

Significant early tie-up of soil nitrogen and long term increases in soil nitrogen from continued stubble retention are unlikely under Western Australian conditions. Farmers should not be concerned to change their nitrogen fertiliser application rates and timing when introducing crop establishment practices which reduce soil disturbance and retain stubble.

Rotations and stubble management

Rotations of crops and crops and pastures are an essential part of no-tillage practices because the lack of cultivation makes the control of some weeds, diseases and insect pests more difficult (see Farmnotes Nos 67/96, 68/96 and 69/96).

Legume crops or pastures also improve soil nitrogen levels.

The following guidelines indicate the amount of stubble to retain in various crops for wind and water erosion control.

Field pea, lentil, faba bean and chickpea

On heavy soils only, graze for a short period after harvest. Try to retain all stubble in position. A light scarifying or rotary harrowing after rain will help anchor this stubble, to prevent it blowing away.

Lupin stubble

To retain sufficient stubble for 50 per cent ground cover requires 2 t/ha. This is equivalent to all the stubble from a 0.8 t/ha grain crop.

Canola stubble

Three tonnes per hectare is required of this crop to achieve a 50 per cent ground over. This is equivalent to retaining all the stubble of a 1.0 t/ha crop.

Contributors

Greg Hamilton, Principal Officer, Soil Management, South Perth and Soil Management officers Dan Carter, Research Officer, Albany, Paul Findlater, Research Officer, Geraldton, Ron Jarvis Principal Research Officer and David Tennant, Research Officer, South Perth.

Further reading

• Farmnote No. 67/96 'The effect of no-tillage cropping and stubble on herbicide performance' (Agdex 110/682).
• Farmnote No. 68/96 'Crop disease and no-tillage farming' (Agdex 100/630).
• Farmnote No. 69/96 'Weed control in no-tillage cropping' (Agdex 100/640).
• Farmnote No. 77/96 'Field pea stubbles: Wind erosion control and grazing management' (Agdex 166/14).
• Farmnote No. 90/91 'Harvesting straw for paper pulp: Minimising wind erosion' (Agdex 110/571).
• Farmnote No. 35/96 'Preventing wind erosion' (Agdex 571).
• Farmnote no. 67/2002 'Amount of stubble needed to reduce wind erosion'.
• Bulletin 4271 'Managing for stubble retention' (Agdex 579).

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