Soil by Peter van Velzen

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 On our property on the upper slopes (300m ASL) of Mt Tamborine, we are blessed with great views, minimum temperatures of only plus 8 (no frost) and a northerly aspect on sloping land that traps a lot of heat and sun. The soils are rich, black and volcanic in nature with a major presence of highly weathered basalt, overlaid by some clay and alluvial soil. Drainage is good once roots get below the thin clay layer. While this all sounds perfect, rainfall has not been perfect by any stretch of the imagination and this has been exacerbated by the northerly exposure with strong winds. Keeping soil wet is a major challenge, especially for newly planted trees. We have planted over 500 trees over the last 8 months, and even with a bore and plenty of water, we still ran into problems. The main issue is water repellancy of the soil once it has dried out. All the books I have read talk about the wonders of compost, sheet mulching with paper to suppress weeds, the joys of deep layers of mulch and many other topics related to the soil nutrition and pH. I am yet to see an article on soil that has dried out and the resulting water repellence problem. So here is my experience and my solution as applies to planting new trees in the sub tropics.

The soil it would seem gets much hotter and much drier than the average garden in Melbourne and Sydney. As a result, some of the practices suitable for conditions down south appear less suitable here in SEQ. The rainfall in SEQ tends to be more concentrated in the summer months with a considerable period of little soaking rain during the dry. This situation was worsened this year (2001/2002) with very little summer rain to date. As a result soils have dried out and irrigation does not alleviate the problem.

When we planted most of our trees, we planted the trees into the best commercially available “organic soil” into a shallow hole. The soil had poultry manure applied and was then covered with wet layers of paper to suppress the weeds with an area around the trunk of the tree to allow water to penetrate. The whole area was covered by a thick layer of forest mulch to depth of 20 cm at least. Liberal quantities of water were applied before the paper was laid and after the mulch layer. We watered religiously for the first 4 weeks and then a sprinkler irrigation system once a fortnight. It was at this time that we notices trees were becoming stressed. The problem became obvious after a simple examination of the root area of the trees. The problems with the system we used are listed below:

  • The ‘organic soil’ used did not hold water very well because the organic content was too low;
  • The soil applied to the roots was in many cases already water repellent and all the water we applied was shed and thus did not solve the problem;
  • The paper had dried out and become an impenetrable water repellent barrier, so all water applied over the mulch was shed. The paper didn’t break down, although in direct contact with the manure, because it was not wet;
  • In many cases, the trees were planted above the level of the surrounding ground for extra drainage and this did not allow the water to pool so that the soil, paper and mulch could not soak up the water;
  • The sprinkler irrigation system allowed water to penetrate into the soil around the plant, but the “organic soil” did not wick it up because it was water repellent.

 We have now changed our system of planting to overcome the problems listed above:

  • The soil used for planting (all soil really) should be kept moist and under a tarpaulin until used;
  • The organic content of commercially available soils needs to be increased with compost or other organic matter. We buy soils by the truck load, specify that enriched compost be added and mixed in before delivery;
  • Water crystals are always added to the soil at planting time. We apply a wetting agent to the soil, in addition to the water crystals during planting and no paper is used over the planting hole;
  • We hand water for 8 weeks after planting and never, never let the soil dry out during this period;
  • We plant in spring and summer now so that we catch the increased rainfall and also allows more cold sensitive plants to become established before the colder months.

If you are in the unfortunate position of having water repellent soils around your plants the only solution is to physically agitate the soil with a jet of water. Simply sprinkling the surface of the soil will NOT work no matter how long you water. The water jet method works well, but you do need to be careful that you do not damage the roots of the plant. Applying a wetting agent before you break the water repellency does not work either. Wetting agents can be applied after you have solved the problem, then the wetting agent helps to keep the problem under control.       Happy Gardening