How to lower PH in a garden bed?

Question

I have made a 3m x 2m raised garden bed 30cm high.  I have filled it with garden soil from a landscape place. I have just tested the ph and it is reading about 8 to 8.5.  I need it more like 6 as I am planting a Acer Palmatum Elegans into it.  Is the Yates Liquid Sulfur the way to go?

JacobZ · Answer

Hi @Mapletree,

Essentially what Noelle is saying is if you only test an isolated section, and hit a pocket with high organic matter, it might not be accurate for the entire garden bed. You can get vastly different results if you hit a pocket of compost, versus a section of soil. Keep in mind that your plants root systems will spread out, so you need to understand the pH reading across the whole garden bed, not just in one small section.

I'd suggest giving the soil in the bed a really good mix first, then taking samples from several different spots and testing them individually. This should give you a much better idea of whether that pH of 8–8.5 is consistent throughout the bed or whether you're just seeing some variation because the soil is still relatively new and hasn't completely blended together.

If you're still getting inconsistent results and want to be certain before adding anything to alter the pH, having a representative sample tested by a laboratory could certainly help.

Jacob

Mapletree · Answer

Hi Jacob,

Thanks very much for your email. I have read that article and it was very informative.

It indicates that I could be getting a false alkaline reading because of high organic matter.  And to send a sample to a lab.

The soil has been in the bed for about 8 weeks.  If it's organic matter causing this reading, won't it still be too alkaline. Or should I just leave it alone and is there a way to get a true reading without sending off the sample to a lab.

Thanks

JacobZ · Answer

Hi @Mapletree,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

A pH of 8–8.5 means your soil is quite alkaline. At those higher levels, the availability of nutrients such as iron and manganese can be reduced, which is one of the reasons plants that prefer acidic soil can struggle. Your aim of getting the soil down around pH 6 for your Japanese Maple is therefore quite reasonable.

Sulphur is the standard product used to lower the pH of soil. The Manutec 1.5kg Sulphur Fine Granules would be a good option. It's elemental sulphur specifically intended for reducing the pH of alkaline soils.

The Yates Liquid Sulfur you've mentioned can certainly be used as a soil acidifier, and being a liquid makes it convenient to apply. However, considering you've got a fairly substantial 3 x 2m garden bed containing around 1.8m³ of soil, I'd be more inclined to use the granular sulphur. It is economical for treating a larger volume of soil and can be incorporated through the soil before planting.

Whichever product you use, I wouldn't try to force the pH from 8–8.5 down to 6 in one heavy application. Follow the manufacturer's application rates, water it in and give it time to work before testing again. Elemental sulphur relies on soil microorganisms to convert it into forms that acidify the soil, so the change occurs gradually rather than immediately.

I'd also suggest taking several pH samples from different locations and depths around the bed before you start. With that much newly imported soil, this will give you a much better indication of whether that 8–8.5 reading is representative of the whole bed and can guide you on how much sulphur needs to be added.

Have a read through @Noelle's article How to determine soil health. It gives some great information on soil pH and how to go about adjusting it.

https://www.workshop.bunnings.com.au/discussion/72226/how-to-determine-soil-health

Let me know if you have any further questions.

Jacob