Combat common turf grass problems with improved soil depth

The key to success with lawns

Many common challenges facing lawns – including weed growth, wear, drying out and even plants’ ability to cope with pests and diseases – can be traced to problems with the physical condition of the soil.

Good soil physical conditions are required to support healthy root systems, the key to success with lawns.

A strong root system is the best defence a lawn can have against weeds, wear, drying out, pests and diseases. The roots will exploit available soil moisture and nutrients, and support dense top growth. The resulting lawn will then have the vigour required to keep weeds at bay and recover rapidly from wear, pests and diseases.

Soil physical conditions

When we talk about the physical condition of soil, we are referring to attributes such as its hardness, compaction layers, depth, ability to drain and structural composition (clay, sand or rock).

While the spotlight is often soil fertility issues such fertiliser requirements and pH, maintaining the physical condition of a lawn’s soil is often neglected.

Measuring soil depth and condition

A common, and often misunderstood, problem in lawns is shallow topsoils.

The softer surface layer of soil is often accompanied by a much harder (often clay) subsoil.

A quick method of gauging the depth of a topsoil is the ‘penetrometer test’. This method works best 24 hours after heavy rain or a good irrigation. The soil needs to be damp, but not saturated.

A makeshift penetrometer can be made out of a 40 cm length of 3 mm fencing wire. Bend the first 15 cm of wire into a loop to make a curved handle. Straighten the remaining wire and file notches at 2 cm intervals along the length from the tip.

Test for soil depth and compaction by applying a constant downward force, exerting a moderate pressure on the handle.

You will find that your penetrometer is easy to push through a good-quality topsoil, but becomes harder to push at the boundary between topsoil and substrate. Stop at that point and measure the depth of entry of the wire into the soil. If you hit a rock or tree root, try an adjacent spot.

By repeating this process in a regular pattern across your lawn, it is easy to quickly determine localised problem areas, as well more generalised problems.

Insert the penetrometer vertically into the soil

Lawns require a minimum root-entry depth of 100 mm. However, 200 mm, and preferably 300 mm, is ideal for conserving soil moisture and maintaining lawns between sparse falls of rain in coastal South East Queensland.

A spade is another valuable soil analysis tool. Dig into the soil to check on the volume of rock and presence of hard, compacted layers within the soil. Rocks and hard layers reduce the effective volume available for roots to exploit, which in turn reduces top growth.

With most new homes being constructed on a base that has been cut and filled, and with topsoil having been removed from some estates, most homes have at least some areas of soil with poor physical properties.

Problems with shallow soils
The harder and more impervious any layers are, the more important it is to increase the depth of your topsoil to support your lawn.

In addition to the impact on potential root mass, hard layers:

• make soil dry during low rainfall periods
• make soil prone to flooding, which drowns the turf, during heavy rain
• increase runoff and promote erosion, which reduces soil depth. When erosion occurs, sediment, fertilisers and applied chemicals are then carried into waterways, creating environmental problems
• expose the soil to temperature extremes which can kill off parts of the root system
• reduce vital soil oxygen supplies needed to sustain root systems
• reduce the potential of a soil to supply nutrients.

Lawns will not survive long dry periods if their root systems and growing soils are shallow. With more extreme weather predicted, soil’s physical condition will become even more important.

Shallow soils are particularly vulnerable to erosion during heavy rains

What depth?

Dr Rachel Poulter, from the Department of Employment, Economic Development and Innovation (DEEDI), showed that major water savings can be made by increasing the depth of topsoil from 100 mm to 200 mm.

Using rainfall data from Cleveland, Queensland, she calculated that the effective water saving, applied over 150 square metres of turf for a year, was 19,000 litres.

In addition, increasing the depth of topsoil from 100 mm to 300 mm resulted in an effective water saving applied over the same area of turf over a year of 29,300 litres.

She also found that topsoil depths of 50-100 mm were inadequate for efficient water storage for use by a lawn between periods of rain.

Dealing with shallow soils

Changing varieties or re-laying turf will rarely improve the outcomes of a lawn laid on a soil with physical problems, as the fundamental requirements for the growth of any new turf are still absent.

However, here are some things that can be done:

• Aim for a minimum topsoil depth of 200 mm.
• If possible, increase soil depth using available top soil already on site.
• Import quality new soil. Imported soil needs to have a minimum available water holding capacity of 15% by volume. Soil should meet the Australian Standard AS4419 ‘Soils for landscaping and gardening use’. Check whether your soil supplier meets these standards. Some manufactured soils are very high in organic matter which will break down over time. This causes slumping and is particularly troublesome in lawns.
• Have a soil analysis done and attempt to restructure clay soils by incorporating gypsum and organic matter. Cultivating the subsoil (e.g. with a rotary hoe) may help to mix amendments in while also improving the soil’s physical condition.

Further information

• Northern Rivers Soil Health Card, Department of Primary Industries, New South Wales, 2002