A reader has added too much mushroom compost when preparing his vege bed. I am including his question and my answer here as over–adjusting soil pH can to be a problem for some gardeners.
I would really like your advice. I purchased some “premium garden soil” and added a lot of straw, lucern, manure and bagged mushroom compost thinking I was making fantastic vege garden soil. I since tested it and found it to be very high ph of 9-10. I think I added too much bagged mushroom compost. I added agricultural sulphur about 2mths ago but it doesn’t seem to be working or is very slow. After reading some posts above it seems it might be too difficult to reduce the ph to a normal level as the ph is way too high and all the organic materials may be acting as a buffer against the sulphur too. Do you think the soil will gradually come down or should I get rid of it and start again? Should I add more manure and just plant in it using a potting mix and hope that it comes down? Please help. Thanks for your advice.
You are correct in thinking too much mushroom compost is the problem because mushroom compost often has lime added to it before sale. For future reference, it is always a good idea to test the pH before deciding how much to add to garden beds, as too much can be a problem, as a high pH means that phosphorus is not available to plants and this makes it difficult to get your veges growing. Adding sulphur will help correct moderately alkaline topsoil but clay soils require 4 times as much sulphur as sandy soils, and it is almost impossible to bring alkaline soils back to neutral with one application.
You could try adding some cow manure (which is quite acidic) to the garden bed. This will reduce the pH over a couple of months because organic matter releases hydrogen ions into the soil as it breaks down. These displace the calcium ions from clay and compost particles and the pH gradually drops. If you don’t need all the bed in the next couple of months, you could grow a green manure grain in part of the bed area, slash it when knee high and dig it into the topsoil. As the green manure breaks down it will also help to lower the pH as well as providing a habitat and food for the microorganisms that keep soil healthy. Don’t worry, a little practice at preparing beds and you will be an expert.
If you want to get a few things growing before the pH of the bed has adjusted, you can add a rockmelon-sized amount of certified-organic compost to each seedling planting hole before planting the seedlings, or add some to a furrow where you will be sowing seed. One of the marvelous things that mature compost does is buffer roots from an unsuitable pH in surrounding soil. Hopefully, by the time the seedling roots grow into the other soil the topsoil will have a more suitable pH. Amgrow Organix is a good brand of compost to use – it has a pH of around 6.5 – perfect for seedlings.