Question: A Brisbane landscape supplier sold me soil for my raised vegetable beds. All my vegetables keep failing. I did a soil pH Test and the result was pH 9. Is there any hope I’ll be able to grow spring crops successfully?
Reply: It is rare to be able to buy genuine soil from a landscape supplier. All topsoil removed from development sites is kept within the industry. Unless you’re exceedingly lucky, what is sold is a combination of silt – dredged from rivers and creeks – mixed with fly ash (from coal-fired power stations), dune sand (mined from places like Stradbroke Island), composted bark fines and poultry manure.
Silty soils form a hydrophobic crust, making them is hard to moisten when dry, but these soils also become anaerobic – airless – when wet. I became very familiar with silty soil when I managed Sydney Botanic Gardens (1992 – 2003). Mulching doesn’t improve their workability. Silty soils are improved by regularly digging in organic matter, like compost.
Fly ash can be exceedingly alkaline. The quality of fly ash depends on the coal. It contains aluminium oxide, quicklime, silicon oxide and, depending on the source, may also contain traces of arsenic, cadmium, cobalt, lead, mercury, strontium and dioxins, none of which are desirable in food producing soils.
Poultry manure is also alkaline. Put poultry manure with fly ash and a raised soil pH of 9 is possible.
Soil pH 9 is 100 times more alkaline than pH 6.5 to 7, which is recommended for most plants, including vegetables. The reason why a slightly acidic to neutral soil pH is recommended is because at this level, most major and micronutrients (aka trace elements) become available for use by most plants. In extremely acidic and extremely alkaline soils, certain micronutrients may become liberated in high enough levels to become toxic to plants, while other micronutrients become unavailable to plants, leading to deficiencies, unhealthy growth – or even crop failure as you have discovered.
Zinc is a minor nutrient, essential for physiological processes within plant cells, however too much or too little can have equally dramatic results. Silverbeet and corn are also crops which are highly sensitive to zinc deficiency or toxicity in extreme soil pHs.
Soil pH 9 is radically high. For a quick fix, add 0.5 litres white vinegar to 4 litres water and apply to each 2 sq metres of soil.
A week later dissolve 1 tablespoonful of iron sulphate to 4 litres water and apply to every 2 sq metres. Wait a month and then retest the soil pH.
If the pH is still too high, a long term solution is raking in powdered sulphur. I have only once had to do this and it was in the succulent garden at Sydney Botanic Gardens where the freely draining artificial planting mix contained a high proportion of fly ash and the display plants were suffering from a sulphur deficiency – their growth was stunted and golden yellow.
Rake in 250g powdered sulphur per sq metre, and then water it in well. Agricultural suppliers, like Elders Stockman, are economical sources of this specific form of sulphur in bulk. One application should be all that is required. Powdered sulphur is the slowest acting of the three actions but it is the longest lasting. If a repeat application is necessary, wait three years.
I prefer repeating the application of iron sulphate seasonally – every three months – until the pH drops to 6.5-7.
Repeatedly mulching with pine needles or sheoak needles will, over time, help to lower soil alkalinity for acid-loving crops like strawberry.
Complete seasonal soil pH tests until you are satisfied with the progress.
You should also think about what you may have also added to the purchased soil since extra poultry manure may have compounded the problem. Sometimes cow and pig manure from non-certified organic sources can raise soil pH and add excess amounts of zinc (which is fed to livestock). Buy these manures only when certified organic.
Phoney ‘soil’ mixes purchased from a landscape supplier are a common problem. Substandard ingredients cannot replicate healthy topsoil. When I managed Sydney Botanic Gardens I had an agreement with a major landscape supplier so I could purchase agricultural loam, topsoil cleared from greenfield sites. Even then, I had soil samples tested before buying. It was this kind of genuine soil that allowed us to build the Herb Garden and Oriental Garden and to grow successful displays.
After successfully lowering the soil pH, it is wise to skip a season before cropping so that any minerals and micronutrients liberated by these changes have a chance to stabilise.
Routinely dig in compost into your soil; compost is the universal soil improver.
The Oriental Garden was developed using agricultural loam (2000).
14th June 2018