When I first started my garden I had a blank slate to work with. I spread enormous amounts of autumn leaves, mushroom compost, manure, gypsum, and coarse sand, then spread around a bag full of field peas to grow as green manure. Once that had grown I hired someone to rotary hoe everything into the ground. That summer I did have quite a successful vege patch, but after the end of the next winter things didn't go so well.
The local gardening experts tell me that the winter rains make the subsoil alkalinity leach up to the surface, so that however much one improves the surface layer, it will revert to being quite alkaline inconveniently quickly.
Next I tried making raised beds for my veges. A friend gave me a whole bunch of old sleepers, and I used them to make three raised beds. Two of them were about 30cm high, and the other about 60cm. I bought garden loam for them and added lots of organic matter and manure. These worked very well at first, but after a couple of seasons my veges were again very disappointing, despite regular additions of manure and compost. Loose leaf lettuces, silverbeet, broad beans, peas and many herbs grew well enough, but things that are a bit fussy about soil pH, like tomatoes, cucumber, capsicum, eggplant, and numerous other things were dismal failures.
About a year ago I invested in a soil pH tester, and was shocked to see that the soil in my raised beds had become very nearly as alkaline as the base soil in my garden. One possible "solution" was to graciously go with the flow and simply grow veges that tolerate alkalinity, but I didn't like that idea much. I decided drastic action was required.
In my case, "drastic action" meant a major garden revamp involving an enormous amount of hard work and a lot of money. The areas of the garden where fruit trees, herbs, roses, and a few other ornamental things grow well have been left as is, but I've replaced the old raised beds and other ground-level patches with a number of new improved alkalinity-proof raised beds.
What I did...
My new raised beds are designed to stop the soil in the beds being affected by the alkalinity in the subsoil. I've constructed them in various ways, but essentially they all function like garden pots...garden pots that have enough area and depth to support small acid-loving fruit trees, kiwifruit vines, and berries, as well as having plenty of space for all sorts of veges.
I used 200uM builders plastic under the raised beds as a barrier to stop alkalinity leaching upwards, with the soil under the plastic contoured so that water drains out at the base on the sides (towards other fruit trees and plants). On top of the plastic I spread a layer of coarse gravel, then covered that with some old shadecloth. I then piled in a loose layer of coarse prunings, and then filled the beds with garden loam and mushroom compost. I also lined the walls of the raised beds with builders plastic to prevent the soil from drying out too quickly. Here is a very large L-shaped one - my main vege patch - with its winter crop in progress.
My chooks can free-range around all sides of this raised bed and intercept at least some garden pests before they reach my veges. This bed is 80cm high - hopefully deep enough for the mango and persimmon trees and the kiwifruit vines, planted in the central harder-to reach parts. I had hoped this would be too high for the chooks to get in but, despite their wings being clipped, I had to add the blue plastic trellis stuff to keep them out. This bed is constructed from 20 x10cm sleepers of various lengths, supported by 80mm permapine posts. I had to buy these materials and the gravel, soil, and mushroom compost, so this was a very expensive project (about $1,000 all up). However, already my veges are growing much better and quicker than ever before, I'm excited at the prospect of finally being able to pick some of my favourite types of fruit that don't like alkaline soil, and it is a sheer delight to be able to weed the vege patch without bending over! Already I think the hard work and expense were well worth it.
In the front garden, I replaced the old raised bed with a scattering of five 1.2m square planter boxes. I bought these in kit form, but the kit is for a 31cm high planter box, so I bought 3 kits for every 2 boxes, cut one in half vertically and added those halves on top of the other 2 boxes to make them deeper. These have the same layered treatment - builders plastic to protect from alkalinity, then gravel, shadecloth, prunings, and soil and compost. As seen in the photo below, a winter crop of potatoes is growing in these at the moment, but I plan to plant pumpkins and melons in these beds in Spring.
I've used the same type of planter boxes, with the addition of rope trellises for cane fruits, in my "indulgence section" of the back garden. In these I've recently planted raspberries, a marionberry, and a thornless blackberry under the trellises, and lots of strawberries at the sides. There are also leeks around the watering pots, but these are still too tiny to show up in the photo below. The leeks too are part of the indulgence. I really like cooking with them, but until that time they make very attractive foliage plants.
For the last of my alkalinity-proof raised beds, I reused the sleepers from the former ineffective raised beds. This bed is about 60cm high and currently has a green manure crop (broad beans and chickpeas) just starting to grow. This too is in the chooks free-range area, hence the blue plastic trellis seen on top of the walls in the photo below. I've planted a fig tree in the middle - partly because figs start fruiting sooner if there roots can't go down too far before hitting something (in this case the builders plastic), and partly because one can never have too many fig trees and I had nowhere else to put this one! In summer this one will also have pumpkins and melons.
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