It is wise to test the pH of compost (including homemade compost) before adding it to your topsoil because the pH of topsoil determines which nutrients are available to plants. Well-made aerobic compost has a pH of around 6.0 to 6.5. Unfortunately, I didn’t follow my own advice recently when, temporarily short of homemade compost, we purchased from our local Bunnings store some bags of Richgro ‘All Purpose Organic Compost’ registered as an allowed input by BFA.
I noticed that the transplanted seedlings where I had applied the compost were not making any growth although the rest of the vege patch was doing well. And, in the area where I had mixed extra compost through each planting hole, the seedlings had turned yellow – a clear sign that the soil pH was too high for the plants to absorb phosphorus for root growth, or iron. Iron (along with nitrogen and magnesium) is essential for chlorophyll (green colouring) formation in leaves that allows plants to produce carbohydrates for growth. Iron deficiency (iron chlorosis) starts in the tips of plants and the leaves of the whole plant turn yellow between the veins or, in extreme cases, white. In contrast, nitrogen chlorosis starts in the older leaves because, when nitrogen is in short supply, plants transfer the available nitrogen to the growing tips, and magnesium deficiency starts in the margins of older leaves.
I tested the topsoil of the affected areas and, finding it above 7.5, I then tested the Richgro compost and the reading was between 8.0 and 8.5. As the pH scale is expressed as negative logarithms, a reading of 8.0 means that the soil is ten times more alkaline than a neutral reading of 7.0. This week I received a question from a blog reader who is having difficulty reducing his soil pH. He had also used the same brand of compost, bought from Bunnings in WA.
Richgro’s response that their “internal records” of the batches in question show a pH of 7.1 and 7.2 and that the test kit I used “do not work well with high organic fraction soils or composts” blah, blah, is not consistent with the physical symptoms of the seedlings. I suspect the problem may have been caused by the type of timber used to produce biochar added to the product. Wood ash contains a very reactive form of calcium and, as the ‘Gardening with Biochar’ website states, “Raising soil pH is biochar’s most important contribution to influencing soil quality“.
* I must add that Bunnings were quite happy to accept return of the unopened bags of compost after I explained the problem.
To test pH
Add equal quantities of topsoil from the bed you are preparing and the compost you intend to use to a clean bucket. Mix thoroughly, then mix again. Test a sample of the mixture according to the instructions on your soil test kit. Ideally, for healthy growth of most plants, the pH reading should be between 6 and 7.2.
If soil pH is higher than this, please see: ‘Changing soil pH’
Also see: ‘What’s soil pH?’