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  • Rob Banks SoilFutures

Tree Planting: Don’t ignore soil preparation

Trees are planted for many reasons on farms and in urban landscapes across Australia. With increasing concerns about climate change and loss of biodiversity, dealing with dryland salinity or more traditional ideas of plantation timber on farms or just shade and shelter belts, there are plenty of reasons to plant trees. Most farms in Australia (where the rainfall is > 500 mm) can support up to 30% tree cover without a loss in net productivity. This is because the gains through slowing wind down actually reduce overall evaporative demands in open paddocks, shade ground, making the landscape cooler, as well creating more humidity around the trees (evaporative coolers if you like).

As I have worked across Australia for 30 years, watching people plant trees in rural landscapes, and planting a few hundred thousand myself, I have encountered lots of failures and lots of ideas for planting which seem logical, but don’t actually work all that well. I thought that it might be useful to share some of the ways to successful planting.

Australian Soil and its European History with Trees

We all know that that much of our country has been cleared or over cleared in most cases for agriculture since Europeans arrived. What most people don’t understand that what happened with clearing is that land was almost immediately heavily stocked, and that this cause massive erosion events. There are papers on the extent of these events that show that whether you were o the outskirts of Sydney, or right out at Broken Hill, the same thing happened. 1. Clearing. 2. Heavy stocking, 3. A large erosion period lasting 20 – 40 years before the landscape stabilised significantly. In many cases the erosion stopped because all of the erodible material was washed away. An incredible amount of soil gone from the landscape, leaving compacted topsoils or exposed subsoils which many of us think are the normal soils in the landscape today.

In many cases, the soil has changed so much that it has gone from wonderfully open and soft topsoils with high connection to clayey subsoils, to boggy topsoils, where water can hardly get into the subsoil. This can be seen in the New England tablelands at forest boundaries, where the soil is open and well structured in the forest, and pretty hard and awful just over a fence into grazing lands. In some cases the soil has changed so much that the native trees which once grew in these landscapes are almost impossible to grow, because it is no longer the same soil type. It is often easier to grow European trees in these landscapes.

Many people think that native vegetation is adapted to low nutrient status. This is partly true if you come from coastal sands, or sandstone or granite landscapes. Many of the soils on these parent materials are naturally very low in nutrients, particularly phosphorus, and the vegetation is adapted to these low levels of nutrients. These soils often have spectacular plant biodiversity such as the heaths of Western Australia and the NSW coastline.

Many of our soils in agricultural Australia are located on better quality soils, with moderate to high fertility. The native vegetation in these landscapes actually is adapted to a higher nutrient status, often resulting in incredibly tall trees in high rainfall areas, or in very thick bodied trees in the drier areas.

The fact is, if you live in an old farming or grazing area and you are not on sandy soil material, many of your original soils will be depleted in nutrients, particularly phosphorus, which soil organisms can’t make.

Take home message – A little DAP or a tree tablet per tree will go a long way to growing better trees.

Water and Trees – the biggest problem in Australia

As shown above, many of our soils have changed their water house hold (how and where they store water) since European settlement of many areas. On top of this, we all know that Australia is the land of Droughts and flooding rains. If we are going to plant trees, we need to know how much water they need. We also need to minimise any watering that we might have to do at planting and after planting. We want our trees to look after themselves as part of a healthy functioning system.

Consider this. 1 mm of rainfall over 1m2 equals 1 litre of water. I live near Gunnedah and our annual average rainfall is about 620 mm per year. This means each 1m2 of land received 620 l of water per year although in practice it is 250 l some years and 900 l some other years).

Consider a tree’s water use. If I look at the large and very old Bimble Box (Eucalyptus populnea) or the large White Box (E. albens) trees in my locality, they reach 20 - 30 m high and are widely spaced (> 20 m) with enormous canopies of leaves. The Bimble Box trees can use 20 – 30 000 l of water per year. That means they are using the rainfall from about 50 m2 around them unless they are near a creek or a river where they get extra water.

This means that tree plantings need careful consideration of spacing, but also really good ground preparation prior to planting.

Poor planting techniques

1. Rotary hole digger

The worst tree plantings I have seen are where someone uses a rotary hole digger to simply dig a hole, fill it with water, and plant a tree or where someone simply plants a tree in dry soil and waters it in. Unless these trees get supplementary water (sometimes for years) they are generally doomed because they can’t access soil water. The compaction resulting from digging the hole can be so great that roots can’t spread beyond the original hole. If you are going to go this way, scuff the inside of the hole to reveal the soil structure and be prepared to water regularly for up to a year. Using this method can work and councils such as Gunnedah do it all the time, but then a water truck visits each tree twice a week for 12 – 18 months. Very costly, very inefficient.

2. Dig a hole and water in

Whilst this might work in a garden where you have water or on some places with reliable rain after planting, it generally means the tree with die of thirst fairly quickly. To grow quickly and well, the tree needs good subsoil moisture to grow its roots into.

Good Preparation takes time

The most successful tree plantings I have seen in rural areas have success rates of around 90% because they are well planned, and sites are well prepared. A well prepared site for tree plantings can take 18 months or more before you plant a tree. Good site preparation generally gets rid of the need to water after planting. Good spacing and a bit fertiliser ensures that your trees won’t starve each other for water and will have the nutrients that they need.’

If you are a purist, and don’t like trees in lines, planning preparation with take a bit more work but generally people plant in lines because of the ease of preparation. You can do the same with individual patches of ground.

Steps to consider.

1. Ripping and possibly mounding a bit. Deep ripping to at least 60 cm will help open compacted soils in preparation for trees. It creates some space in soil for air and water to get into topsoil and subsoil alike. Make sure your rip lines are reasonably, spaced 4 m is good, keep in mind the water requirements of your adult trees.

2. Keeping your rip lines weed free! This might be problematic if you don’t like herbicides. My opinion is that a good once or twice application of herbicide to set up a system that will last hundreds or thousands of years is worthwhile. If you apply glyphosate or even a residual herbicide, to keep your rep lines weed free, then soil moisture will build up along the rip line and into deep subsoils. Make sure you have at least 2 – 3 m centred on your rip line kept weed free. This will create your water supply for the trees once planted.

3. Don’t rush. Keep your rip line weed free until you have had at least your annual rainfall. Even 15% weed cover will strip moisture from topsoils and subsoils in many cases. It takes as long as it takes. A safe (ish) period for this in the Gunnedah area is at least 18 months. The water stored in the soil will not run out the bottom, it will be held there by clayey subsoils until you plant.

4. Prior to Planting. You may need to do another shallow rip before planting if your rip lines are cloddy and don’t have a reasonable tilth (ie topsoils need to be friable and not made up of lumps.

5. Planting time. Plant trees when you have your trees ready and your rip lines have a moisture profile of at least 1 m, preferably 2 m plus. You can check this by digging by hand, or making up a moisture problem which only needs to be a piece of 10 – 15 mm steel rod with a handle that you can push into the soil. If it’s moist enough to grow a plant the probe will slide in under even a light person’s weight. If it’s dry, no amount of pushing will get the probe to go in.

6. Planting. When it comes to planting, think about your growing season, your rainy seasons, and your hot times. Some people plant in Autumn, Winter or Spring depending on their location. Don’t plant in summer unless you have a really good reason to or live in a heavenly location for weather (not Gunnedah). Planting is as easy as opening the soil with a spade, exposing the moist area, popping the tree in and pushing the soil back around it. Water with enough water to connect the soil around the seedling with the moist soil below (usually 10 l is enough). Tree guards are a great idea, and protect you seedlings. Some people mulch too, but this is costly and if preparation has been good, it shouldn’t be necessary.

7. Maintenance. Keep your rip lines weed free until your trees are growing well and above any surrounding grass. You can use a side cast mower to mow grassy areas between plant lines so that the cuttings form mulch over your rips if you were clever enough to design your planting that way.

8. Enjoy your trees! One of the best feelings I had recently was to go back to a farm which we used to own, and to see that our plantings were successful. The trees were 10 – 15 m tall, and shady, and exactly as we imagined them to be. When we bought the place it was like a desert! It’s a long term thing. A well established planting will last for generations to appreciate, and our land and production systems will be grateful for the effort.


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