• Many soils in Tasmania experience seasonal waterlogging in winter and spring
• Waterlogging results in poor and patchy plant growth, restricted paddock access, soil degradation, and plant and animal diseases
• Drainage can improve response to fertilisers, reduce environmental impacts and improve profit margins.

“The importance of understanding how much waterlogging really costs agriculture in Australia cannot be over-emphasised.” - Greg Gibson

These guidelines are concerned with agricultural drainage of waterlogged soils and the removal of excess surface or ground water from the land by man-made surface or sub-surface drains to improve crop growth and land access as well as improve soil health. Land drainage for agriculture is aimed at preventing surface water ponding and creating an unsaturated zone in the surface 40 cm of soil that increases aeration allowing for improved plant growth. Drainage associated with road or track construction into forested areas on farms should follow guidelines in the Forest Practices Code (Forest Practices Authority 2020).

An understanding of how and where water moves in the landscape and what is controlling it, is required in order to make decisions about the type and extent of the land drainage system needed as well as the placement and size of any drains. The correct diagnosis of waterlogging problems (Chapter 4) is necessary as part of the drainage planning process (Chapter 3).

Tasmania enjoys relatively high rainfall over much of the State which normally occurs with an excess of rainfall over evaporation in winter and spring with poor drainage limiting agricultural productivity in many areas of Tasmania. Waterlogging may also be a result of excessive application of water from irrigation, or a combination of irrigation that keeps the soil wetter over the summer and any subsequent rainfall that is in excess of the water holding capacity. Several soil orders experience parts of the year when they are saturated due to high regional water tables, low rates of water conductivity, perched water tables or seepage. Many soil types in Tasmania can experience periods of soil saturation expressed as imperfect to poor drainage including Dermosols, Hydrosols, Podosols, Kandosols, Kurosols, Sodosols and Vertosols.

Aims of drainage plus greenhouse gas production from waterlogged soils:

The history of land drainage is as old as agriculture itself. The earliest evidence for irrigation and drainage management is from Iran around 4000 BC. China, India, and the Americas also have extensive histories of drainage dating to around 3000 BC (Valipour et al. 2020). Many drains in England, built in Roman Times (AD 43 to AD 410) are still functioning today. The Aztec city of Tenochtitlan was located in a swampy area of Lake Texcoco where food was grown on raised beds that were built up with mud from canals dug in the late 1400s (Magdoff and Van Es 2021).

In Europe the first recorded tile drainage system was in France in 1620 and two centuries later a similar project was installed in England. The rate at which land was drained in England and Wales increased rapidly during World War II, as part of the drive to increase food production, and peaked during the 1960s to 1980s. Now over six million hectares of agricultural land in England and Wales has been drained with underground piped systems (Hill et al. 2018).

Waterlogged or poorly drained soil results in many consequences for plant growth and agricultural operations including:

• Poor root development due to shallow plant rooting depth or a lack of aeration due to water saturation (Figure 1). Oxygen is lost from soil water within one to two days of becoming saturated.
• Poor soil drainage may be limiting plant growth to the extent that no responses are gained from increased fertiliser use. No amount of fertiliser will fix a drainage problem! The full benefits of fertilisers cannot be obtained on poorly drained soils due to restricted root development, lack of aeration for root growth and leaching of nutrients.
• Wet soils take longer to heat up in the spring than dry soils and so waterlogged soils can result in delayed or failed seed germination that leads to a shorter growing season and delayed harvest dates. Spring growth in pastures can be delayed resulting in feed shortages at this critical time of year.
• Plants growing on poorly drained soils suffer more from drought as the plants have shallow rooting systems and consequently in dry periods, they are unable to obtain sufficient moisture to maintain full growth and they rapidly show drought symptoms. In pastures on well drained soils spring growth starts approximately two weeks earlier than on waterlogged soils and continues for two to three weeks longer in the autumn because the plants roots are deeper and able to access greater volumes of water stored in the soil in spring and well drained soils don’t wet up so early in the autumn.
• Uneven or patchy crop growth can often be the result of varying drainage conditions across a paddock. This commonly occurs in Tasmania due to variations in soil type within a single paddock or localised variations in topography that leads to surface water ponding in depressions.
• Soil compaction can result from soils being cultivated when they are too wet and plastic rather than friable. Waterlogged soils result in fewer days for timely cultivations and good seed bed preparation and so these operations have to be carried out at times when the machinery will cause serious damage to the soil structure.
• Whole crops of potatoes can be lost because of the impossibility of getting harvesters onto waterlogged paddocks or the crop can be devastated by pink rot.
• Making silage or hay can be delayed due to wet soil conditions.
• Waterlogging reduces the strength of the soil making it susceptible to pugging damage by livestock. Pugging can also seriously damage the pasture and its effects can last for a long time.
• Weeds and annual grasses thrive under poor drainage conditions competing out the desirable grasses and clovers. Soils waterlogged for long periods can also become infested with rushes.
• Crop diseases such as downy mildew and club root can attack crops weakened by poor drainage conditions and this can be a problem in crops such as poppies, beans, brassicas and peas.
• Animal diseases such as foot rot and liver fluke thrive in poorly drained areas. The liver fluke snail flourishes on wet land and the only sure way of preventing the disease is to destroy the snails’ habitat by draining the wet areas.
• Waterlogged soils often have soil bacteria that thrive under no oxygen conditions by being able to gain energy from sulfur compounds contained in decomposing organic matter. The bacteria release hydrogen sulfide as a gas that smells like rotten eggs. Unfortunately, hydrogen sulfide is toxic to plants roots and may even kill whole plants if waterlogged conditions persist into late spring when plant roots are actively growing.

Reducing the length of time soils remain waterlogged by the installation of appropriate drainage systems, results in greater ease of soil management, increased plant growth by improving aeration and soil temperature, plus control of plant and animal diseases. Improved drainage can also alleviate one of the causes of ‘grumpy farmers’! Farmers get sick of walking through pugged ground, working around ponded areas and getting machinery bogged. Improving the drainage results in the soil becoming friable rather than plastic, and less likely to be compacted or pugged. A more aerated soil encourages organisms which metabolise organic matter and stabilise soil aggregates. Improved drainage increases the depth of aerated soil allowing plant roots to explore a greater soil volume. This increases the pool of nutrients available, and with a greater volume of soil to draw on for water, plants can continue growing for longer during dry summer periods. Pasture growth and crop yields are increased as a consequence. Increased pasture growth during summer is often one of the unexpected benefits of improved drainage as pastures and crops on drained paddocks can develop deeper, more extensive root systems that draw on deeper soil water stores in dry weather.

Drainage can lessen the incidence of pink rot in potatoes, mildew in beans, peas and poppies, plus reduce fusarium and phytophthora root rots which can occur when plants are stressed by waterlogged conditions and poor aeration. Animal health problems are often reduced by improved drainage. These include mastitis, cracked teats, liver fluke and intestinal worms. Drainage is also an important way of improving the working conditions for farmers by removing the unpleasantness of muddy, wet conditions. Installing drains in waterlogged soils has the environmental benefit of reducing nitrogen losses through denitrification. Denitrification occurs in waterlogged soils releasing nitrous oxide, which is a potent greenhouse gas.

Waterlogging is resulting in the loss of considerable income to farmers through lost productivity. The cost of land has increased to such an extent that investing in drainage can be justified in order to improve potential crop growth and yield. It is cheaper to improve the land a farmer currently owns, rather than buy more land that is less productive. Drainage is a fundamental property investment, but the results are not guaranteed. There are multiple agricultural benefits to land drainage.

Having well drained soils allows farm machinery to operate in an efficient way and therefore reduces diesel consumption and reduces the likelihood of expensive rescue operations to retrieve stranded agricultural machinery.

Farmer: Rob Tole

Property: Greenvale

Location: Cressy

Soils: Canola black cracking clays (uniform), Brumby sandy loam (duplex), Panshanger sand (uniform), Eastfield sandy loam (duplex) on hills.

Crops and enterprises: Grass seed, clover, chicory, peas, beans, hemp, breeding sheep, trading lambs, drainage contracting

Precision technology used: RTK tractor guidance, variable rate irrigation, seasonal controlled traffic.

Why drainage?

Increased production started with getting more water onto the paddock via irrigation, but it wasn’t long before the realisation that it was just as important to get water off the paddock to reduce crop losses through waterlogging. “I am trying to get more out of the ground I manage.”

Drainage journey:

Started by using a Dennison rotary drainer in 1999 to install surface drains by eye and from a knowledge of where water lay in paddocks.

Began subsurface tile drainage in 2010, identifying locations by eye and using a laser to ensure even gradients for fall. “I started targeting small problem areas within paddocks and soon progressed to grid pattern subsurface drainage over entire paddocks.”

In 2014 started land planning ground used for clover production as needed level ground for wind rowing and harvest of seed. “I learnt that it is easy to create a levee bank beside shallow surface drains that inhibits flow into the drains”. Subsurface drains were installed after detailed elevation data was collected to map the paddock accurately.

In 2016 a T3RRA Cutta land plane was purchased for better control over waterway installation, particularly the side slopes to prevent forming a levee bank. There followed a few open field days to demonstrate the utility of the land plane which led to other farmers asking Rob to do work on their properties. This has led to the formation of a stand-alone drainage contracting business that has two T3RRA Cutta land planes, two Wolverine ditchers (Figure 3), a road grader, two tractors, utes and staff.

Key lessons learnt:

• Good accurate elevation data is critical in drainage planning.
• The drainage outfall must WORK.
• Accurate installation means that 0.1%grades (100 mm in 100 metres) will flow water. This is more critical for a subsurface drain than a surface drain, as you can always come back and make a surface drain deeper or bigger but not with an underground drain.

Farmer: Greg Gibson

Property: Mill Farm and Willow Vale Location: Hagley

Soils: Mill Farm: Cressy clay loam (gradational).

Willow Vale: Brumby loam (duplex), Cressy clay loam, Woodstock sandy loam (duplex), Kinburn clay (uniform).

Crops and enterprises: Wheat, onions, poppies, carrot seed, trade lambs

Precision technology used:

RTK tractor guidance, seasonal controlled traffic for sowing, spraying and fertiliser spreading, yield mapping, variable rate irrigation.

Why drainage?

Past yield losses of between 50-100 per cent in some crops due to inadequate draining, prompted Greg to investigate the causes of waterlogging. The yield mapping undertaken in wheat crops emphasised the variability in income due to waterlogging where the wettest areas were costing up to $440 /ha and the well-drained areas were generating up to $2410 /ha in profit (Figure 5). Drainage can control waterlogging and take the risk out of wasting the costs of cultivation, fertiliser, seed and spraying.

Drainage journey

1984 – installed 300 mm long clay tiles in wet parts of paddocks working with a local contractor, using a pull type Bruff chain trencher guided by laser.

In most years since then some drainage has been installed progressively but now plastic ag-pipe is used.

2013 – won a Nuffield scholarship that allowed Greg to travel to the US, Canada, the UK, and the Netherlands to research global best practice in combatting waterlogging and build on existing systems on his own farm.

2015 – purchased drain plough for installing underground drainage on his own property and to undertake contracting for other farmers.

In the last few years a mole drainage program has been undertaken to enlarge the footprint of the tile system.