Dry seeding puts time pressure on moisture

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Drier autumns and the increasing adoption of dry seeding have led to a perceived increase in water repellency across Western Australian cropping systems

Photo of mouldboard plough

Disturbing dry non-wetting soils can increase water repellence by as much as twofold.

PHOTO: Evan Collis

Photo compilation showing a time course of water drop infiltration into disturbed and undisturbed dry water-repellent soil

Figure 1 Time course of water drop infiltration into dry water-repellent soil that has been disturbed dry compared with the same soil left undisturbed.

SOURCE: CSIRO

Graphic showing the water repellence of compacted and non-compacted water-repellent soil either disturbed dry or undisturbedGraphic showing the water repellence of compacted and non-compacted water-repellent soil either disturbed dry or undisturbed

Figure 2 Water repellence level of compacted and non-compacted water-repellent soil either disturbed dry or undisturbed.

SOURCE: Margaret Roper, CSIRO

A perceived increase in the occurrence of non-wetting soils in Western Australia is likely the result of:

  • drier autumns with less frequent and smaller rainfall events at the break of the season resulting in less opportunity for the repellent soil to wet up;
  • increased concentration of organic matter (the source of water repellence) at the soil surface from minimum tillage;
  • higher frequency of dry and early
  • seeding; and
  • widespread use of narrow knife-points, which can cause water-repellent topsoil to flow into the furrow and surround seed and fertiliser.

Water repellence increases with increasing organic carbon content, particularly on sandy soils where the surface area of soil particles is low.

Water repellence can be particularly severe in Mediterranean climates as new waxes become fused onto sand surfaces during hot and dry conditions over summer.

Drier autumns and larger cropping programs in WA over the past couple of decades have led to a steady increase in dry sowing, with about 40 per cent of growers in low-rainfall areas and 20 to 25 per cent in higher-rainfall areas sowing a proportion of their wheat crop dry each year.

However, despite its growing popularity the practice remains problematic, with many growers reporting that soil in dry-seeded rows remains dry while soil in the untilled inter-row wets up.

Photo of canola emergence from non-wetting sand seeded dry and following 25mm of rain

Figure 3 Canola emergence from non-wetting sand seeded dry (right) and following 25mm of rainfall (left).

SOURCE: CSIRO

CSIRO research shows disturbance (tillage) of dry non-wetting soils results in much slower water penetration than the same soils left undisturbed (Figure 1).

Disturbing dry non-wetting soils can lead to up to two-fold increases in repellence – as measured by the molarity of ethanol droplet (MED) method – (Figure 2).

Figure 3 illustrates the impact on crop emergence from seeding dry water-repellent soil. Canola emergence was severely impaired when the non-wetting soil was seeded dry (shown on right side of photo).

However, when the same soil was sown 12 hours later after 25 millimetres of rain, crop emergence significantly improved (shown on left side of photo).

The capacity for dry seeding is an important part of modern farming systems. However, further investigation is needed to fully understand the mechanisms at play in tillage of dry, water-repellent soil so mitigation strategies can be developed.

Wax-degrading bacteria

Many bacteria present in soil produce bio-surfactants capable of releasing the non-wetting coatings from sand surfaces in water-repellent soils. Inoculation of water-repellent soils with these bacteria under laboratory conditions significantly reduced repellence compared with the non-inoculated control (Figure 4).

However, inoculation of non-wetting soils under field conditions with the bacteria was less successful, most likely due to competition from other microorganisms. Enhancing populations of natural wax-degrading bacteria (for example, through liming) is likely to be the best way to lift their activity in repellent soils (see Tackle combined constraints for long-term benefits).

Figure showing water repellence over time

Figure 4 Water repellence over time of a non-wetting soil sample either inoculated with a wax-degrading bacterial population or left uninoculated.

SOURCE: CSIRO

More information:

Dr Margaret Roper, CSIRO Agriculture Flagship,
08 9333 6668,

margaret.roper@csiro.au;

Dr Phil Ward, CSIRO Agriculture Flagship,
08 9333 6616,
phil.ward@csiro.au;

Dr Stephen Davies, DAFWA,
0408 439 497,
stephen.davies@agric.wa.gov.au

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New initiative investigates non-wetting solutions

Photo showing bacteria and fungi living between sand particles

Bacteria (spirals and fine thread-like hyphae) and fungi (larger hypha, top left corner) living between sand particles. Many of the bacteria are capable of degrading the water-repellent waxy coating on sand particles provided sufficient moisture is present.

GRDC Project Code CSP00139, DAW00244, DAW00204

Region West