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The Permaculture Information Net is a new feature of this web site intended to encourage visitors to contribute to the information available to the permaculture community. We are not soliciting opinions. Rather, we are encouraging people to share documented facts. You may also share your perplexities. You are invited to submit them to us for consideration for posting on this site. These should be questions not readily answerable by commonly available publications, related to permaculture design, and of general application. For example, do not submit a question such as How do I figure the right amount of ventilation for a solar greenhouse. The answer is more or less common knowledge. Contributors who submit documented facts will receive a copy of all the replies that we regard as useful. If we resolve a question, the material will also be used in an article in our journal and/or as supplemental data on our web page. We will post relevant replies on this site, as well, with credit. I reserve the right to edit them. At some point, we will expand the acceptable topics to design strategies as well as facts. For Mother Earth, Dan Hemenway
PINpoint Agroforestry Fertility Dynamics (August 2008)
A recent article in the LEISA (Low External Input Sustainable Agriculture) Journal cited two claims that I have often heard and read, but that I have never seen documented.
The article was about an agroforestry strategy on a New York State sheep farm: planting trees to improve pasture. The multiple benefits from such a strategy are well known. However, the following assertions gave me pause:
o The author asserted that trees in that ecosystem salvage nutrients leached deep into the soil and bring them to the surface. The same claim is often made for trees in highly leached tropical conditions. One often also hears of comfrey, which may have a larger percentage of deep roots than most trees, achieving the same thing. Yet I've never read of a study supporting this supposition. Therefore I suspect that it may be a case where something 'makes sense', so people think it is true. Plants don't care about what makes sense to people, however.
What I have read is that trees and perhaps most terrestrial plants grow most of their feeder roots near the surface, suggesting that they get most of their nutrients from the topsoil. Anchorage roots would appear to impart wind resistance as their primary function.
So if you know of any studies about whether plants, particularly trees, remove nutrients fairly deep into the subsoil, please let us know.
Here at Barking Frogs Permaculture Center, I have observed that peach trees planted near to the canal seem to resist severe drought. I speculate that their anchorage roots therefore also tap the relatively high water table during these times. So it is possible that they bring up nutrients with the water. I wouldn't want to guess about how significant this may be in the tree's overall nutrient budget. It is a good bet that the trees are very efficient at catching nutrients BEFORE they leach into the soil.
The action of mycorrhizal fungi add a further level of complexity. These 'root fungi' work in symbiotic relationship with many plants, including may tree species, to improve nutrient capture, among other benefits. I suspect that a healthy mycorrhizal population, which are unlikely to work deep in the subsoil, are more important to the capture of nutrients and prevention of their loss to leaching, than any deep roots.
I've already fired off an email to Robert Kourik, author of the recently published book, Roots Demystified, and his thoughtful reply reinforced my doubt that these claims have a basis in observed fact. I highly recommend this book, by the way.
o The same LEISA article reports planting honeylocusts in the pasture, claiming that as legumes they fix nitrogen. While I've not kept up on this topic, the last I heard, honeylocusts had not been observed to fix nitrogen. For unrelated reasons, they are an excellent choice in a sheep pasture.
If you know of studies on the potential ability of honeylocust (Gleditsia triacanthos) to fix nitrogen (or not), please share this data. We would also be interested in any documentation of the claim that all legumes fix nitrogen or evidence that this claim is false. I've known people to get rather emotional about the belief all legumes fix nitrogen. We are not interested in articles of faith, here but data.
Of course, besides nitrogen fixation, some plants have been show to be important in nutrient cycling. Comfrey has been shown to salvage and concentrate potassium, for example, and may be an example of a case of deep roots recovering a leached mineral. Potassium is especially vulnerable to leaching. Temperate white clover (Trifolium repens) and the tropical ice cream bean (Inga edulis) have been shown to salvage and concentrate phosphorous. Both are legumes, suggesting the interesting possibility that some other legumes may share this ability. Therefore, phosphorous recovery can be imagined to take place in the deep roots of leguminous honeylocusts, possibly a good master's thesis study for someone.
Robert Kourik, mentioned above, looks at some known cases of 'accumulator plants' in another excellent book, Designing and Maintaining Your Edible Landscape, Naturally. It seems that at least many accumulator plants can release nutrients that are chemically unavailable to most other species. I've not heard of any inference that they harvest deeply leached nutrients.