Replacing soil to change PH

[Jimmy H]

Hey guys,

My house has very highly alkaline soil (around an 8.5). I love little gem magnolia trees and intend to plant a couple despite the soil unsuitability. A lot of people in the area have them - they survive, but the leaves are pale green, and they don't seem to thrive. I'm considering two options. 1) bury a very large pot (I have found some 65 litre pots), filled with high quality soil of the correct pH. Or 2) dig a 3ftx3ft hole and replace the soil, then plant in the ground. I imagine if I go for option 2, the roots will extend beyond the slightly acidic soil into the highly alkaline soil. Will it still get the nutrients it needs in this scenario? And with option 1, there are all the problems associated with pot grown plants. Does anyone have experience with either option / thoughts or advice on which option would be the better rout? Labour is not a big consideration, I'm more concerned with results! Thanks in advance.

 

[Marck]

Greetings, welcome to the Forums.

A third option would be a raised bed. This would be partly like a container, though larger, for more root area. It would also be partly like a an in-ground bed not movable, except drainage would be improved and there would be less contact between the native soil and hydrology and the newly purchased soil, which might begin to affect the pH. You might correct that by periodically adding sulfur.
A raised bed improves drainage. What percentage of your soil is clay, silt, and sand?

A fourth option would be to just add sulfur periodically and lower the pH that way.

There are also intermediate states between in-ground and a raised bed, such as half raised half in-ground bed and also just mounding up instead of going down. The concern with a mound is erosion but mulch and underplanting of groundcover can largely alleviate this.

Of those four options, the container will limit the plants long-term growth the most, with it eventually becoming rootbound, though it could be a number of years before decline is seen. The in-ground hole with very different soil might still confine the roots to that hole, so you might want to go bigger than 3 feet. Out of curiosity I looked up Magnolia grandiflora 'Little Gem' boxed tree sizes and found 72" boxes available. I mention that only to show how much root area is allowed for commercially grown trees. Of course there may very well be a smaller space limitation on the size of any planting hole or raised bed, so that is the answer to what size right there.

What would I do? I'm leaning toward the half in-ground, half raised bed idea. Ideally I'd like to go to four or five feet wide if that is possible, but if it must be three then give that a shot. A depth of three feet sounds reasonable, but deeper in that direction can also have benefits. I would also incorporate 50% percent native soil back into the hole and use a more acidified introduced mix to compensate. The less sharp the demarcation between old and new soil the better. If pH continues to be an issue, sulfur application will still be necessary.

 

[Jimmy H]

Hi Marck,

Thanks for this excellent answer, this is really great stuff! I really like the idea of the half raised option. I like that this suits the root style of the magnolia -- reckon I'll give it a go

Yes I was wondering if the tree will eventually be compromised by the pot -- even in a big one.

I should check the soil composition on that part of the yard. It's all over the place, with some parts of the yard pure sand (we're a stone's throw from the beach) and other parts quite clay-like. I think drainage is pretty good in the whole (a bit too good in some spots!). I have been trying sulphur but it doesn't seem to be sticking - our area has a lot of limestone, so I think the pH might be hard to shift permanently. One other thing - do you have much experience with iron chelate spray? I was thinking I could supplement with this somewhat (especially as the roots begin to venture into the alkaline soil later down the track). Or are there other nutrients the magnolias will also miss out on?

Thanks again!

 

[Marck]

Magnolia do like somewhat acid soil though most do fine in neutral as well. For both sand and clay, mulch once or twice yearly with a nutrient-rich compost. That is the best way to improve most soils overtime. That should provide the needed nutrients. Still, if you do see evidence of iron deficiency, due to pH, an iron chelate product would certainly help matters.

Yes, sulfur is not a one-time-and-done fix. It does need repeated application. What ultimately determines soil pH is mineral composition and climate.

 

[DirtMechanic]

If you wish to use sulfur, please examine the idea that too much at once can be a bad thing, and more applications that spread less sulfur per acre on that basis would be the the right method. An acidic front or demarcation line can be created both with lime and sulfur. This can blunt growing root tips. There are garden calculators to help understand the spread from your current pH to your target pH and how much sulfur per 1000 feet it will take to get you there. Sulfur is somewhat slower than lime in its time to effect changes. Sulfur is antibacterial and antifungal to some degrees differently than lime and so its route to breakdown into useful forms is different than that of lime as a result of these natural processes. You definitely do not "go hard" in the application of sulfur, especially that first one.

 

[Jimmy H]

Thanks guys! Really appreciate the thoughts. Yeah I learnt about gradual application the hard way. I killed my old magnolia tree with too much sulphur too fast. Oops. I have also read that using sulphur when the parent material of the soil is largely composed of calcium carbonate is like using a bike pump on a tractor tire - compared to using sulphur on soil that is alkaline for other reasons (because of wet concrete having been hosed into it, say), which is like using a bike pump on a bike tire. In one case you're never going make a permanent change no matter how hard you pump it, in the other it'll do the trick.

 

[DirtMechanic]

Thanks guys! Really appreciate the thoughts. Yeah I learnt about gradual application the hard way. I killed my old magnolia tree with too much sulphur too fast. Oops. I have also read that using sulphur when the parent material of the soil is largely composed of calcium carbonate is like using a bike pump on a tractor tire - compared to using sulphur on soil that is alkaline for other reasons (because of wet concrete having been hosed into it, say), which is like using a bike pump on a bike tire. In one case you're never going make a permanent change no matter how hard you pump it, in the other it'll do the trick.

Good points and great analogy. Your description is one of the best descriptions of buffering capacity I have heard.