I had read that generally edging (rock borders) on raised garden beds is a bad idea as such rocks can result in unnecessary chemical changes in the soil pH just for the sake of aesthetics.
However, I have also read that rock borders increase heat retention and improve water drainage in beds.
Based on reading about the negative aspects of edging for cosmetics, I decided to opt for simple trenches to demarcate my garden beds. But I'm confused by the possible benefits of rocks for beds also listed above.
What do y'all think? Is there a difference between the two? Are both effects plausible? Does it differ/depend on where the rocks are placed, what rocks are used, etc.?
Any suggestions or thoughts on this matter are greatly appreciated!
I'm going to have to vote for the increased heat retention and improved water drainage.
It makes me wince when I hear the phrase "soil pH", because pH is a property of aqueous solutions. Maybe thin runny mud has a pH you can measure, but it becomes a tenuous construct when you get to something as heterogeneous as soil. Sure, soils with a lot of humic acids will measure low on the pH scale, and alkaline soils in the west will measure high, but to say that adding rocks to soil will cause "unnecessary chemical changes in the soil" is putting the cart before the horse. Rocks are a component of soil, so you work with what you have. If limestone is what you have to edge your garden beds with, then realize that you won't be adding as much lime or other pH raising materials. You may, however, want to keep an eye on your plants and see if they want some acidic amendments.
Plants are adapted to a range of pH values, and adding a few rocks, especially the more inert ones (granite, quartz, basalt, etc.), is not going to seriously derange your garden soil. More likely your soil pH is dependent on the type of dirt under your living soil. If you are out west and have several feet of caliche under your garden, you have to learn to garden in a high pH environment and your acidic amendments will be swallowed up. Here in Georgia, the combination of rain and clay sucks out any alkaline amendments and so we look for acid loving plants to garden with and realize that adding lime is a periodic necessity.
Yes, both effects of edging with rocks are possible to occur, though pH changes might not be so much of a big deal. Rocks, like most everything on Earth (dark matter, dark energy, etc can be discussed elsewhere), is composed of elements, molecules, and compounds. Rock chemical composition will determine what is leached into your soil overtime, and the high thermal mass of rocks helps retain heat and carry it over to a later time (thermal inertia). A comparison of rocks at your site with the ones on the provided link may aid in identifying the composition of the rocks in your area, and this also helps determine how the rocks weather. Interestingly enough, one can smell rocks, too:
The pH of the soil will change depending on the amount of hydronium and hydroxide ions are introduced into the system. The more acidic, the more hydronium ions present, and the more basic, the more hydroxide ions are present. Here is a diagram explaining how pH affects absorption of some key macro- and micronutrients:
Rocks' thermal properties make it ideal for creating reptile (garden protectors) habitat and harvesting water with gabions or air wells. Drainage is altered depending on how far down and where the rocks are positioned; for example, a french drain works by the rock and gravel's non-water-retentive. However, most rocks are at least a little porous which is why they split apart from freeze-thaw weathering.
Overall, the effects of the rocks depends on their context within your site, your environment, their composition, size, placement, and quantity.
If you use native rocks to make your boarders and help with drainage and thermal mass the pH issue shouldn't be a problem, in my opinion. You are not going to dramatically change the chemical make up of your of your soil by doing so, on the other hand you could use the same properties to alter th pH (I.e. using limestone to increase pH), but if you use what you got it shouldn't be a problem.
One comment on pH which I just learned today, and want to share..
The American Community Gardening Association conference is meeting in Chicago this weekend, and I sat in on a "soils and the city" panel this morning. It was solid info presented by colleagues whom I greatly respect (Michael Webb, Dr. Shemuel Israel, and Mattie Wilson). When asked about soil pH, Michael Webb said he quit bothering about it years ago when he figured out that plants are more than capable of adjusting the surrounding pH, provided that they are already getting the right nutrients and that there is plenty of soil biology going on (read - organic matter and compost). He did mention it takes plants a few years to make this adjustment, but that it's not that big a deal.
As far as rocks go, I've placed them around berms in the fall where I had newly established trees rooting in, so that the roots would hold on to some warmth. In the spring I moved the rocks somewhere else. On a large scale, moving heavy rocks around isn't going to work out, but around particularly sensitive or valuable specimens it might be worth while. Thanks to @kctomato )tweet tweet) who first gave me the idea.
Location: North-Central Idaho, 4100 ft elev., 24 in precip
Here's a picture of a pretty tall rock edge I just finished up yesterday. Just thought I'd throw it in here. I used all native field stone from nearby fields, I don't worry one bit about altering pH by using them.