Farmnote 65/96

Crop establishment series

By Greg Hamilton, Principal Officer, Soil Management and Ron Jarvis, Principal Research Officer, Soil Management, South Perth

Summary

Discusses the relationship between soil structure, water erosion and crop yields; wind erosion and water repellence; tillage and compaction; and grazing and compaction.

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.

Careful soil management with the appropriate treatments employed at the optimum time can certainly control land degradation. However, seasonal conditions may overwhelm the best management, and successful farmers constantly observe the condition of their soils.

This Farmnote examines a range of soil management options to control or prevent land degradation whilst maintaining or improving crop production. More detail is given in the other Farmnotes in the Crop establishment series.

Soil structure, water erosion and crop yields

Studies conducted throughout Australia for the past 20 years on the effect of tillage on soil structure have produced remarkably consistent results across a wide variety of soils and climates. In general, the fewer the number and intensity of cultivations:

• the more stable and porous the soil structure (Figure 1);
• the greater the rainfall infiltration;
• the less the runoff and erosion (Table 1); and
• no difference or slightly improved yields.

Figure 1. Changes in water stable aggregates over time for four tillage treatments (TDD = triple disc drill ; SC/TDD = scarify and triple disc drill ; DDC = direct drill with combine ; DP = district practice - cultivate and seed with combine)

Table 1. Runoff and soil loss on a loamy sand in a 61 mm rainstorm, Geraldton

Yield is not substantially affected by improved soil structure (Table 2) because the ease with which soil water and air are available to plants is rarely a major limiting factor of crop production. Only when some waterlogging or droughting has occurred are strong relationships between soil structure stability and yield demonstrated (Figure 2).

Figure 2. Relationship between soil structure stability and crop yield as affected by tillage treatments (TDD = triple disc drill ; SC/TDD = scarify and triple disc drill ; DDC = direct drill with combine ; DP = district practice - cultivate and seed with combine

Table 2. Effects of tillage on yield (6-year averages)

Wind erosion and water repellence

Tillage can both decrease or increase wind erosion, depending on the degree of soil disturbance and the soil moisture content at the time. Dry cultivation of soils with clay contents above 10 per cent (that is, loams or more clayey soils) can create a rough surface cover of clods. This roughness slows wind speeds at the soil surface and controls wind erosion.

With soils of lesser clay content (that is, sands and loamy sands) cultivation should occur only when the soil is moist to moderately moist. Cultivation at lower moisture contents increases the amount of loose surface soil available for transportation by erosive winds (Figure 3).

Figure 3. Relationship between wind erodibility and texture and condition of surface soil

An alternative means of establishing crops on highly erodible sands in windy environments is to seed in the rain. Moist soil will not move in wind. However, once the rain ceases, sands dry very quickly, and even paddocks sown by no-tillage seeders which hardly disturb the soil will erode unless stubble is present to protect them.

Water repellent soils present a difficult challenge in windy environments. Research shows that furrow seeding is a very reliable seeding technique for these conditions. This may involve some soil disturbance to form and seed into furrows, which are installed to harvest water and ensure even wetting around the seed. The safest reliable option is to use single pass operations with narrow points or discs on seeders, no inter-row cultivation and press wheels to form the furrows. However, even this combination requires stubble and undisturbed root material in the inter-row spaces to minimise the risk of wind erosion.

A further option, which does not affect the recommendation for no-tillage furrow sowing, is to top dress the water repellent soil with clay. This effectively removes the water repellency characteristic, as well as reducing the soil's erodibility to wind (see Figure 3).

To be feasible and economic, clay has to be available near to the site and easily extracted, that is, present at a relatively shallow depth (less than 50 cm deep). For much of the wind erodible, water repellent sandy duplex soils along the south coast and north coast, clay at shallow depths is a common feature (see Farmnote 14/97 'Claying water repellent soils').

Tillage and compaction

The normal agricultural traffic of stock and machinery causes all soils to compact. Extensive trials have been undertaken in Western Australia to assess the benefits of deep ripping to loosen compacted soil well below the normal cultivation or seeding depth. The soils on which compaction pans have restricted crop production and on which the greatest responses to deep ripping have been obtained, are the loamy sands and sands. Importantly, these soils cover 48 per cent of the agricultural area.

The depth of a compacted layer relates mainly to the clay content. In Wongan Hills loamy sands and yellow earths (about 10 per cent clay content), the pan is found at 20 cm depth. In the Eradu sandplain yellow sands and earthy sands (about 6 per cent clay), it is 25 cm deep and in the white and grey sands (about 2 per cent clay), it is 30 cm deep.

Traffic pans restrict root growth and plant uptake of water and nutrients. For example, in trials at Wongan Hills, eight weeks after seeding, wheat roots had penetrated 30 cm on the unripped treatment and 90 cm on the ripped treatment.

The most reliable area for crop production responses to deep ripping extends north of Kellerberrin to north-east of Geraldton. The responsive soils in this area are yellow loamy sands. In 46 trials over nine years, the average wheat yield response to deep ripping was 650 kg/ha. However, the variation in response is considerable (Figure 4).

Figure 4. Distribution of wheat yield responses to deep ripping on yellow loamy sand soils

Research on the south coast on sand over clay duplex soils with A-horizons deeper than 30 cm has also shown similar yields. Ripping on these soils also substantially reduces the incidence of Rhizoctonia bare patch disease. However, farmer experience has not been able to reproduce the research results.

The beneficial effects of deep ripping can quickly disappear unless minimal or no-tillage crop establishment practices are adopted to reduce the number of traffic passes. Recompaction to densities greater than before deep ripping can occur within two to three years if traffic is not reduced.

By comparison, where no-tillage practices followed the ripping treatment, the looseness created by deep ripping was substantially retained over 11 years. The soil strength in this treatment levelled out at about 60 per cent of that before deep ripping.

Grazing and compaction

Grazing stock influence crop establishment by:

• reducing stubble and summer weeds between crops;
• necessitating the introduction of pastures into rotations, thus providing grazing pressure and pasture manipulation opportunities to control weeds and break the life cycle of some diseases, as well as adding nitrogen to the soil via legumes; and
• compacting surface soil during winter and loosening it during summer.

Poor grazing management (particularly grazing loamy soils when wet) causes a deterioration of soil structure which will adversely impact on subsequent crops. Experiments at Merredin on a sandy clay loam soil showed that grazing wet soil caused a reduction in infiltration, an increase in soil strength and reductions in direct drilled and no-tillage crops of established plants and dry matter at tillering and anthesis. Reductions in the last two factors are likely to limit crop yield if seasonal conditions are drier than normal.

Where paddocks with loamy textures are returning to crop after lengthy grazing periods, a pre-seeding cultivation may be beneficial. No-tillage crop establishment on soils compacted by grazing is more risky as the inter-row areas are compacted and their soil strength is quite high. Infiltration is less speedy and possibly less overall, and the strength of the undisturbed inter-row soil restricts crop root exploration.

The only time the soil strength in these areas will not restrict root exploration will be in wet years when the soil is moist and the strength low enough for roots to easily penetrate.

Further reading

• Farmnote No. 4/95 'No-tillage sowing minimises water erosion' (Agdex 572).
• 'Claying water repellent soils' .
• Journal of Agriculture, Western Australia 'Tillage systems and soil stability' Vol 29, No. 2 (1988) pp 54-57.

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