The pH of your soil is critical for good plant growth. When we buy in soil we expect it's suitable for growing plants in - but amazingly, this isn't always true. In fact, the issue of alkalinity of bought soil here in Canberra doesn't seem that uncommon - I've heard from several people of similar issues, so this post is relevant for anyone who's thinking of buying in some soil!
PH measures the level of acidity or alkalinity of your soil. Most of the plants we grow for food like to grow in a neutral to slightly acid soil (a pH range of about 6.5 to 7). Outside this band and you're likely to have problems.
So, here's our cautionary tale of pH problems... I'm pretty keen on wicking beds, because they are so low maintenance once they're established. Having grown up in Scotland, I still feel weird about having to water the garden, so anything that reduces the requirement to do so is a good thing by me!
In order to obtain a yield soon after we moved into our new place (a little over 2 years ago) we decided, before we planned out the rest of the garden, we could utilise an unused part of our driveway for growing plants in raised wicking beds made out of old apple crates.
After keenly making several beds, we needed to fill them. Having only been at the property a few months by that stage we certainly didn't have enough of our own compost to fill the beds (each needed about half a cubic metre). So I enthusiastically ordered some fantastic looking mediums to fill the beds.
First up was some delicious looking, well rotted horse manure. Rich and dark brown, it was light and crumbly and I had often admired the steaming piles on my way to garden centres. I figured it would be ok as the piles were hot enough to kill any seeds, plus horse manure isn't too strong and shouldn't burn plants as some manures can - and this was a mix from stables consisting of manure rotted down with sawdust making for a great texture.
The second was a product called veggie mix from a local supplier - a good mix for growing vegetables in I supposed... I had previously used some and it seemed to work ok. So I got a big pile of each and with some help from a group of most excellent friends, we dutifully filled each bed with each, in a 50/50 blend, and topped all the beds up with water, ready to go.
Waiting for a few weeks to ensure the soil wasn't hot (commercial composts can sometimes still be in the process of rotting down, so piles can heat up again) I enthusiastically planted up each bed with various veg and waited... and waited... and waited. Alas, the seedlings just sat there, looking sad, and didn't grow. Eventually some plant leaves turned purple and my seedlings began to die. What had gone wrong?
I bought a pH test kit from out local hardware store, and sure enough, the colour of the results told me that something was very wrong indeed! Instead of the nice green colour that the kit goes if the soil is neutral (neither acid or alkaline) my samples turned bright purple! Looking at the code, I realised my soil mix was extremely alkaline, a pH of 9 or above.
A pH of 9 and above makes it very hard for most plants to grow - the alkalinity of the soil means their roots can't take up any nutrients, and without nutrients and trace elements, plants don't grow. Leaves start to go purple and the poor things starve, even if the soil is rich in nutrients as mine was (it had loads of well rotted manure in it after all!)
But why was this soil so alkaline in the first place? And was it the vegie mix or the composted horse manure? Or both? I did some research and found out that it is a common practice to put lime (calcium carbonate) on stable floors to reduce odours, so whenever stables are cleaned out, there's a good chance that the mix of horse poo, urine, straw or wood shavings will have a fair bit of lime in it. Well, I hear you say, I put lime on my garden and it's supposed to be a good thing! The issue here is about quantity. Garden lime, which is alkaline, is sometimes sprinkled sparingly on garden beds at the beginning of each season to neutralise or 'sweeten' slightly acidic soils. But I guess a generous sprinkle of lime every time you clean out a stable results in an awful lot of lime in the resulting compost after a while! My mistake had been to use so much of the composted horse manure in the mix - it looked great, dark brown and crumbly, and just the right texture for a wicking bed (quite light, not likely to get waterlogged) but alas, way too alkaline to grow directly in.
Unfortunately, correcting alkaline soil is not particularly easy or quick. It takes treatment with elemental sulphur and many months. Bacteria in the soil gradually consume the sulphur and convert it into tiny amounts of sulphuric acid which then react with the lime and slowly neutralise it. The trouble is, you don't know how much to use, so you start with a little, then wait a few months, retest, and do it again. To say we were a bit disappointed that this had happened would be a bit of an understatement!
So I set to work, digging in the sulphur snd mixing it through in each bed, and gradually things began improve, plants started to grow (though still slowly) and over time and a few applications, the pH has almost got to neutral in the beds. But let's just say that our hopes of fabulous yields in our beds were thoroughly dashed that first year!
But the story doesn't end there... about a year later, after enjoying a pretty inspiring harvest of broad beans from our now working beds in the front garden, we decided to put a few more beds in the back garden. This time I wasn't going to risk any more horse manure compost, but I foolishly ordered some more 'veggie mix' from the same supplier as previously to fill up these beds. I'm sure the guy on the phone told me that it was neutral pH (as these products should be). At least this time I had enough sense to test the pile that arrived in our driveway before putting it into the beds.and what do you know - it was very alkaline too! In fact, that wasn't the only problem, as the pile also became very hot after a few days, showing it was not ready yet, and probably shouldn't have been sold as such.
After a bit more sleuthing, I suspect the reason for the alkalinity was due to the inclusion of some mushroom compost in the mix, which is known to be alkaline. The other likely contender is cow manure, which is another ingredient in veggie mix. I suspect that lime is probably added to keep smells down in cattle feedlots (likely to be the source of commercial manure), for the same reason it's used in stables. This is just my supposition - I haven't researched this a lot. But I once bought a bag of cow manure from our local hardware store and after testing this, found it to be pretty darned alkaline too. Note that composted manures should be pretty close to neutral unless something else has been added.
Back to our tale of woe - as you can imagine, we found this disappointing too. This time I mixed it with some regular garden soil/clay (in our garden this is slightly acidic) and some home made compost. But we still needed to treat the large amount of 'veggie mix' with sulphur as we put it in the beds. And this time I knew not to expect such great yields from anything planted straight away. From memory we managed a fairly minor harvest of tomatoes and some lemon balm seedlings I co-planted with them struggled for a while and then finally grew quite well. Interestingly, now the pH has mostly normalised, it's clear that the mix itself is not very fertile - a lot of it is composted woody matter, high carbon, very low nitrogen. Because it was still hot when we got it, I suspect a lot of the nitrogen in the mix was also used up as the woody matter broke down further. (In case you're wondering why, you need a good balance of nitrogen rich and carbon rich elements in your compost for effective composting - the nitrogen rich components like manure and kitchen scraps help high carbon ones like wood and paper break down. That's why it's not recommended to use woodchips as a mulch for vegetables - in case they 'rob' the soil of nitrogen as they start to break down, leaving not enough for the vegetables)
So what have we learned from all of this? Well, first and foremost, it's a good idea to pH test anything you are planning to buy and add to your garden! Most landscape supply companies have great big piles lying around, some will let you go up and take a look, so you can do a quick test. In places where they don't let you get too near their bulldozers, get them to bring you a sample you can test or provide assurances another way. As I discovered, even bagged products can be alkaline.
PH tests can be bought at any hardware store. There are two types: a hand held probe you put into the ground or a kit with indicator fluid, powder and a colour chart. Unless you're going to spend a significant amount of money on a high end probe, the cheap probes you can get in your local hardware store or garden centre are ineffective in my opinion. We tried a cheapie one and the tip of the probe broke off after a couple of uses. They're not known to be particularly reliable either.
The indicator fluid kits cost around $20 (see top picture) and they are a good option for back yard testing. I use these all the time and find them very helpful, and you can do a lot of tests with one kit. Handy hint, if you're testing dry soil, don't wet the sample with tap water, because Canberra's tap water is slightly alkaline and this will affect your reading! I often use a bit more of the fluid to wet my sample enough - or you could use any water you know is neutral (rainwater, filtered water). Once the liquid has soaked into the sample, dust with the supplied powder and it will take up the colour - then you check the colour of your sample against the supplied chart. Green means neutral, purple shades are alkaline, and yellows to red shades mean the soil is acid.
So, to summarise - don't assume what you buy is going to be a nice neutral medium for growing your vegetables! Check the information on the various landscaper websites - what do they say about each product? Consider spending money on a specialised potting mix (usually more expensive than veggie mix), or, much better... use your own compost! And if you don't have any, start making some!