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Bio drilling hardpan subsoil

I am interested in using plants that have a strong taproot to try to penetrate hardpan subsoil in Honduras. While I have received a variety of suggestions of plants with that ability, I have not received any firm confirmation of which plants could best do it. Therefore, I am going to test approximately 20 different plants to see which might be most effective in penetrating compacted subsoil. I’m curious to know if any of you have done this sort of testing. If so, I would appreciate hearing from you regarding your techniques. If there is interest, I am willing to post my testing plans here for others to observe, critique and offer suggestions in hopes of improving my planed testing.

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I have experience with this. I’ve been learning an approach to agroforestry called syntropic farming. It’s been practiced in Brazil for decades. They guys I’ve been learning from are routinely successful in breaking up compacted soils and hardpans using plants.

As a general rule, they use trees which will thrive in the existing soil conditions and then prune aggressively. This results in root penetration followed by some of the roots dying off, which leaves channels in the soil and substantially improves water absorbing. They also employ heavy mulching, which I’m sure is a key to soil transformation. Also an important key is planting in high density.

Some non-tree options that I’ve heard work good include: papaya, cassava and any root veggies, like potatoes.

If you want to get a little exposure to syntropic farming, this video is the best place to start. Best wishes with your project. That is definitely the way to break up hardpan.


Thanks Roger for sharing that URL and info. Very interesting. A little later, I’ll share some notes on how I plan to do the test.

One question. Is the main end goal of Syntropic to have trees? It seemed to me they are growing food crops in the short term but long term, the main crop is trees. That is the opposite of what we are doing. We are mostly growing food and feed crops but grow legumes trees and other plants with them for the benefits of diversification for the soil but we don’t allow trees to get large. We normally cut them before 6 feet tall so they will produce more young shoots and so their roots will release the N for the other crops to use.

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You might try driller daikon radish for deep penetration, also annual cereal rye which produces 5,000 miles of root hairs per plant in a single growing season.
Hope this helps.

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Thanks Michael for that suggestion. Here is the list of plants I plan to test.
I’d welcome other suggestions.

  1. Sunflower
  2. Vetiver grass
  3. Tree lucern (Echo seed)
  4. Daikon radish
  5. Pigeon pea
  6. Leucaena (Echo seed)
  7. Madreago
  8. Lablab bean
  9. Canavalia bean
  10. Mombassa grass
  11. Millet
  12. Pine
  13. Papaya
  14. Okra
  15. Sorghum-Sudan
  16. Chicory
  17. Barley
  18. Mexican sunflower
  19. Hemp
  20. Wattle (Echo seed)
  21. Samanea Saman
  22. White clover
  23. Red clover
  24. Stylo (Echo seed)

In a few days will post how I plan to do the test.

Here is my testing plan.

Rather than testing in the field, for the sake of uniformity, the test will be done in 15 inch long pieces of 2 inch diameter PVC pipe.

So as to ensure the soil does not fall out the bottom of each PVC pipe, I plan to attach a cloth bottom cut from feed sacks and use that to cover the bottom of each pipe. That way if pipes are lifted for inspection or moved for some reason, the soil will not be inclined to fall out. Because the soil will be compacted inside each of the pipes, I am assuming that it would not fall out the bottom but this is just to be sure. We will probably attach the cloth bottom using a wire “clamp” made with the soft wire that construction workers use for tying rebar, etc.

To try to ensure as much uniformity as possible within each PVC pipe, we will collect soil that lends itself well to compaction and then we will mix that soil throughly so there is uniformity before beginning to fill the PVC pipes.

We will collect two different types of soil so as to test each seed in two different types of compacted soil. A few miles from us they make baked clay bricks, floor tile and roof tile. We will use some of that unbaked clay in one set of our tests. The soil used with other pipes will be the light colored soil used for making adobe blocks. While the hard pan we are trying to penetrate in our fields is not like either of those two soils, I am assuming that if taproots can penetrate those two types of soil they would be able to penetrate the compacted subsoil in our area. If none of the plants pass the penetration test, in the follow up tests we will compact the soil less.

When adding soil to the pipes, we will use a tamping rod with a diameter slightly smaller than the PVC pipe. As soil is added, it will be compacted using that rod. We will add and compact the soil until there is 12 inches of compacted soil in each pipe. We will then add 2 1/2 inches of our normal field soil. The seeds will be planted in that field soil above the compacted subsoil somewhat simulating our field situation except our top soil is normally about 6 inches above the compacted subsoil but there is great variation within short distances.

I think we will measure the water put in each pipe so each pipe receives exactly the same amount of water each day. We will add enough water to keep the topsoil moist during germination and then allow it to get somewhat dry before adding more water so as to mimic real life conditions.

I will not begin this test until the end of June but posting the process now in case someone has suggestions.

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I’m assuming you are tamping by hand? It will be really difficult to hand tamp everything equally. Since that is the case, be sure to use an identical volume of soil for each PVC pipe by measuring it out using the same container. This is also assuming uniform moisture in soil samples as that will have an impact on how easily it will tamp. There will likely always be some air in the soil and using identical volumes may not be as accurate as using identical masses of soil but again, you’d have to completely dry the soil to eliminate all moisture/water. This could be done in the oven if absolutely necessary or a solar oven/dehumidifier…it kinda depends on how crazy you want to go.

Thanks Tyler for those good thoughts. While you are totally correct, I am not seeking that much precision in the testing. In other words, I am trying to somewhat replicate our situation and our hard pan is not uniform. It varies in thickness and texture and depth beneath the surface. So what I’m trying to do is compare the penetrating power of various different plants. I will replicate the test two or three times with each each variety of plant. That will give me a good indication of which plants have the greatest penetrating power and that’s all I really need. Every plant variety I am testing is one that is purported to have penetrating power into compacted soil. I want to see if some are considerably better than others. My assumption is trees will penetrate better than vegetables and small plants. We will see.

Some hardpans are very thick, and the chemical composition makes them difficult for plants to drill through. Have you considered manual digging prior to planting, to break the hardpan? I’m keen to learn more also, as we have the same issues in Zimbabwe. There’s actually a webinar happening tomorrow if you’re interested - a group in Malawi who is trialling Deep bed farming aka digging to break the hardpan prior to planting. Here are the details of the webinar if interested: Water School Africa: Tues 14th June at 12 noon (East Africa), 11am (Southern Africa), 10am Central Africa and 9am Most of West Africa (also UTC/GMT). Mr Isaac Monjo Chavula, an Executive Director of Tiyeni, Malawi, will introduce us to a pioneering method called Deep Bed Farming (DBF) which they have developed over several years of trial and error working with farmers and communities in Malawi . DBF involves breaking up hard compacted layers of earth several inches under the ground, which allows roots, water and air to penetrate more deeply underground, and helps the soil retain water and moisture long after the rains, instead of running off immediately after rainfall and causing devastating soil erosion. As always, we will have ample time for questions and discussion with Mr Isaac Monjo Chavula and his colleagues after the presentation.

The zoom link to join the gathering is:

Launch Meeting - Zoom

Meeting ID: 847 6836 9889 Passcode: 907626

Also, do you know what sort of hardpan it is, and how thick? E.g. Calcrete hardpans are predominantly calcium carbonate, but there are other minerals which form hardpans, and I’m guessing that will make a difference to the plants which are suitable.

Thanks for your notes Beth. You are helpful. There is no one type of hardpan or compacted soil we are dealing with because we are involved in various locations of Honduras. Some are the result of years of cattle grazing and I believe relief can be offered with the selection of plants with strong taproot while in other areas, nothing short of mechanical work will be able to penetrate.

Please post the results of your trials when you’re done, I’m interested. We mainly use trees here in our project in Tanzania, but we do plant pigeon peas, lablab vetiver (as fodder) and other stuff as well, so I would like to see how these plants improve soil conditions.

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Somehow I was never notified of your response. It looks like I need to manually configure this thread to get notifications. Based on all the responses, it seems like you got the info you need.

About syntropics, I have seen farmers do both, planting trees for the end goal to transition to a tree crop. But also planting trees and periodically pruning them back heavily with the end goal of continued veggie production. It seems like you already understand that both options are possible.

My intuition tells me that trees will out perform all veggies, but they require more patience. Anyways, best wishes with your test.

Can you please specify whether you mean annual rye or winter rye? The type I buy is a large seed, almost like a grain of wheat. The shop I buy from just calls it ‘annual rye’. Thank you.