We recently bought a property in a Winter rainfall, Mediterranean climate (roughly 650mm rain per annum, mostly in winter).
Our soil is very thick/dense clay (the type they make clay bricks out of) with a very thin topsoil layer of nutrient-poor-low-humus-sand.
If we were to dig contour swales I fear that the water will just sit there in the clay and wait to evaporate (not soaking in at all)?
We had a soil specialist out to advise us and he said not to expose the subsoil clay to the top, because nothing will take root in it and it will prevent water from going down into the soil below it.
Good news is (i guess) that we could probably line a few ponds with the clay but how will I get any fruittrees to grow in such terrible soil?
I don't have experience working in clay like that, but: there are plants that will grow in pure clay. (I have a patch of it where day lilies thrive, for example.). So I think you could plant the swales and, over time, improve the soil's permeability.
I have similar conditions. Just built a swale and working on the second.
The swales will hold water and permeate SLOWLY. It is hard to start plants on the upturned soil on the berm.
Running a subsoiler/keyline plow along the bottom of the ditch and as close to the berm as you can without driving on it will help with infiltration.
You can amend the soil with a little compost and mulch to get a cover crop to establish. Daikon and a nitrogen fixer. Daikon build organic matter DOWN. But it doesn't like my pure clay so it takes a while.
Once the tree roots grow down to the water, they will take off and run. You may need to nurse them a little bit in the beginning.
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Pure Clay can seem like something insurmountable to improve but it can be with the right plantings and by using chop and drop methods along the way to creating a more fertile environment. I would go ahead and create your swales, and any other landscape modifiers you want. The following list will give you a variety of plants to use to break up the soil and add organic material to this soil type.
Aster, Golden Rod, Blackeyed Susan, Day lily, Yarrow, Little Blue Stem, Fountain Grass, Switch Grass, Ironweed, Canna, Bluestar, Baptisia, Coresopsis, Purple Cone Flower, Sea Holly, Perennial Geranium, False Sunflower, Cora Bells, Hosta, Blazing Star, Bee Balm, Sedum, Yucca, Miscanthus.
These plants can all be used, grow quite well in pure clays, and some are very ornamental while others are also Medicinal plants. They can and should be planted both on the swale mound and in the trench.
Hope this information gives you some ideas of how to proceed.
Similar to you and R Scott, I also have swales on a farm that has very dense clay underneath shallow topsoil. If you haven't built your swales yet, be sure you move your topsoil from your backfill so you can put it on top of your berm and not bury it in clay soil. But, my swales are clay in the ditch and so I did fill them with organic material to decompose. The water will leach some of the nutrients away downhill into your berm and below. That's actually what I hope for. But, I also chop and drop any grass or weeds that grow on the berm and in the ditch. There have been some forbs growing from inside the ditch, so I know things will grow in clay, as Mr. RedHawk suggests. And, of course, foxtail appearing on the swale berm. But, to get the water to percolate a bit quicker, I was going to do as R Scott suggests and run a subsoiler shank right down the middle of the swale ditch to give everything a chance to penetrate a bit deeper. I've attached a photo of one of our swales that we planted about 25 trees along. I know I'm losing water to evaporation while the trees are so small. So, I will be doing something to get the water to penetrate more quickly this spring.
I agree completely with using a subsoiler to rip deeper into any clay base. The whole idea of swales to me is that we want them to create a water plume down hill, not become mosquito breeding pits, that means the water needs to soak into the trench at a fair pace, not just be a mini lake. I have one area that is terracotta clay soil, I have not built the two swales there yet but when I do I will be trying to create them so that I can do two or three rips within the trench of the swales, these will be planted with appropriate grasses, the mounds will get the bee balm, cone flowers, yarrow and other flowers.
Really wouldn't worry to much about the standing water .
usually wont rain that much where you have this kind of conditions
you can plant stuff to help it along lambs quarters,amaranth ,sorghum,sun flowers,legumes,corns,maize,barley and Sudan most of your persistent weeds are good too .
stuff with good roots systems and lots of top growth that is drought tolerant.
we don't have a problem with lack of water we have a problem with mismanagement
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I have some pretty heavy clay as well. Our driveway holds water so well that every time it rains the ducks move from their handmade cement duck pond to the driveway. I planted 32 trees in July of this year and when I transplanted some grapes to be with them I was amazed at how well the root system of my trees was doing in my crappy soil. I think it has a lot to do with how nice and soft the berms were just being dug. So I'd say berm it!
PS. I worry about evaporation as well which is why I've tried to fill my swales with stuff as best I could. I've got cardboard, hay, straw, branches, etc. Lots of various materials. Seems to be working ok so far.
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