From the ancient sandy soils of Perth and south-eastern South Australia to the deeply cracked, dry clays of large parts of Queensland and western NSW – we dream of rich loam but end up working with sand castles or mud pies! Yet, continuing research has provided a number of new products as well as improved best practice to alter sandy or clay soils to create the best growing medium for the landscape design plant list.
Sand – Best Practice
When planting out large areas on sandy soil, practicality may need to prevail because establishing an acre of ‘English perennial borders’ will be like pushing sand uphill! Climate and soil are the biggest determinants as to what will grow where, so, choosing plants endemic to the location or from areas with similar growing conditions will always be best practice. That said, the ‘grass is always greener’ and clients and landscapers always want the unusual – flowering Magnolias, bountiful veggies – requiring soil amelioration.
The quick-fi x approach of adding top soil as a completely separate layer invariably leads to future drainage and growing problems. Creating a transitional zone by tilling 50-75mm of top soil into existing soil will assist water to drain down and percolate upwards. Tilling will also encourage roots to grow down rather than just remaining where life is good near the surface.
Adding and tilling in top soils with clay content provides a number of positive effects. Negatively charged clay particles have a higher CEC (Cation Exchange Capacity) attracting and holding positively charged nutrient ions in the soil. Clay has an external surface area around 1000 times greater than sand and when added to sand it improves water holding capacity from three to six-fold. Clay also acts as a binding agent between particles resisting the forces of gravity and evaporation.
One of the conundrums of sand is that, although it is free draining, its surface can become hydrophobic (water repellent). This occurs as organic matter breaks down coating soil particles over time. The rough and very low specifi c area of sand particles makes them easier to coat and, as sandy soils dry out more easily, these non-wetting organic acids dry out and attach to the particles. Interestingly, in some sandy soil areas in Australia, the adoption of modern approaches, such as no-till, farming has actually caused greater water repellency through the increased accumulation of organic matter and hence, waxy compounds at the soil surface.
Application of a water-soluble surfactant (soil wetter) remedies these situations by lowering the surface tension between solids and liquids. Conventional wetting agents are usually alcohol or petroleum distillates such as polyacrylamides.
With concerns around their interactions with pesticides as well as their breakdown and presence in waterways and the food chain, a number of new products are now available that contain organic-based surfactants and humectants (chemicals that retain or reduce moisture loss). Slowly being taken up by green keepers and the turf industry, many of these products also contain seaweed extracts (which as well as being a tonic, is itself, a natural wetting agent) plus bio-stimulants such as folic and amino acid.
Personally, I apply a wetting agent to the existing sand before adding and tilling in top soil to further improve the transitional zone between amelioration and existing.
The catch-cry of those residing on sand yet dreaming of exotic species and edible produce seems to always be: “add compost and then more compost!” For the landscaper, this usually results in the use of top soils with a high organic content or the tilling in of manures or compost (increasingly being produced from tub ground and windrow composted green waste).
Organic fertilisers are usually a good source of macro-nutrients such as nitrogen and phosphorus but can lack trace elements such as copper, silicon, zinc, calcium and manganese. Rock minerals, also known as ‘rock dust’ and ‘natural mineral fertilisers’ are usually made from crushing basalt or granite rock. They contain trace elements such as silicon and calcium that build strong plant cell walls assisting to guard against disease and drought while also increasing the availability of other nutrients. Importantly, for sandy soils, rock minerals break down slowly with the help of soil microbes and fungi slowly releasing trace elements without leeching out of the profile.
Clay – Best Practice
Again, like sand, if a top soil layer is required to increase available nutrients and improve soil structure in the vital fi rst 300mm – creating a transitional zone by tilling some of the new material into existing is desirable. With clay, the main problem lies in ensuring the surface hasn’t been compacted by construction activity. There have been some notable instances on large landscaping jobs where civil contractors have not broken up deeply compacted clay soils sufficiently before the arrival of the landscape crew, leading to plant growth and drainage problems well after the project has finished.
There are many great articles on the soil testing company, SESL’s website. One of Australia’s leading soil scientists, Simon Leake, sounds a warning concerning changing levels by adding top soil around existing trees. He states that, around 90 per cent of the tree’s root mass is in the top 200mm, while a tree will be most active to at least one canopy height away from the trunk (not just to the ‘drip line’). Changing depths will require the tree to change its root depth or usually die at an imperceptibly slow rate referred to as ‘reduced longevity’.
Drainage is a major problem in clay. We always have concerns over avoiding surface run-off that fi nishes up over paths or into buildings, yet, under the ground, continually water-logged soils will result in green life growing inadequate, shallow root systems to obtain enough oxygen. Installing sub-soil drainage is always good future-proofing.
Calcium-based products such as gypsum, dolomite and lime have traditionally been used in granular form as clay breakers. Calcium in these products seeks out and adheres to two clay particles, breaking the clay into smaller clods, making a crumbly, more friable soil. Dolomite assists in correcting acid soils as well as adding calcium and magnesium while lime has higher levels of calcium and a more dramatic effect on acidity. Gypsum, like the others, improves water penetration, drainage and aeration without altering pH.
There are now a range of clay breakers available in a concentrated liquid form (with one litre of product being equivalent to as much as 40kg of the powdered form). These new products are easier to apply via a pump and spray tank unit. Just as you can’t have junk food and have healthy people, you can’t have poor soils and healthy plants. The best landscapes – the ones where plants thrive above ground year after year – start with good soil prep. No more sand castles or mud pies, just… rich, friable loam!