What acid soil friendly nitrogen fixers won't be affected by pine allelopathy in zones 6-7?

Question

Is there a nitrogen fixer which will do well in the high acidity low-density soil preferred by Blueberries? It needs to be able to resist pine allelopathy and be friendly to waters edge environments.

I'm going to try to guild it with Blueberries (Highbush - Vaccinium corymbosum and Lowbush - Vaccinium augustifolium), Japanese nut pines (Pinus pumilla) and maybe some White pines (Pinus strobus) along the edge of a small pond.

My location is southern Indiana, sitting between zone 6 and 7. Either a shrub or an herb will do fine. The soil will be modified with coffee grounds, sand and compost to provide a less clay soil.

Answer 1

There is an ongoing discussion in the permaculture community whether ph changes to the soil are sustainable. I am in the camp which says they are provided the changes are not huge (going from highly alkaline soil to very acidic) and so I assume your pines are part of this effort. I have been looking at trying to do something similar in a similar zone but haven't got the blueberries in yet.

There are some outstanding questions in my mind which include how some of these plants affect soil pH.....

A couple observations.... A major part of the allelopathy of conifers seems to be nitrogen depletion. Conifer needles fall when they are brown, they form a very loose mulch which composts slowly in the absence of a lot of nitrogen. This creates physical and nutritional barriers to other plants around the trees even before taking into account toxins. While this is not based so much on observing pines, I have a 100 yr old sitka spruce and this effect here is quite easily noticeable. Shallowly rooted plants can't get much of a foothold because of the very low density, low nitrogen, low water needle layer at the top. My thinking is you are going to have to be doing a lot of additional mulching and composting to make up for this effect. This is not going to be the ideal happy guild that folks go for and is going to be comparatively labor-intensive by permacultural standards. The composting and mulching will likely help with allelopathic toxins too but given the level of composting required I think nitrogen fixers will be relatively secondary.

So on to specific recommendations, I have noticed the following plants seeming to do ok around my sitka spruce that are nitrogen fixers: white clover and russian olive. A lot of deciduous trees however raise soil pH so I don't know what impact the russian olive might have there. Also my soil is not quite the same as yours (mildly alkaline in most places, but it varies quite a bit across my yard for various reasons including historic uses, but I am assuming it is mildly acidic where these plants are growing because of the spruce).

One more thing that occurs to me is that the research I have seen on trees and soil pH is that trees become less efficient at changing pH as they get older. Consequently a few young pines will be quite a bit more effective than an old pine.

Answer 2

I would suggest gorse or scotch broom. I see them both growing in my peaty acidic soil of about 4.5.

Answer 3

Even with a pH of 5-6, soil acidity thou important is not detrimental to nitrogen fixing bacteria to survive. As for berry bushes,

• Frankia a species of nitrogen fixing bacteria have symbiotic associations with non-Fabaceae (pea/bean) plants. Including various Berry producing shrubs.