Clay pan under trees in a dry climate

Hey all,

We've just moved to a smallholding in the mountains of southern Spain and are trying to work out what we can do with the land we have. One problem in particular has us a bit stumped.

There are a few open patches of land that we're thinking of using for growing vegetables, but a lot of our terraces are planted with fruit and nut trees - primarily chestnuts and walnuts, with a few olives and a smattering of other fruits (mulberry, pear, cherry, plum, fig). The primary trees range from maybe 15-30 years old; in most places they don't form a closed canopy at the moment. At the moment the ground cover, such as it is, is just wild grasses and flowers.

We don't have a lot of water at the moment, partly due to a series of dry winters, and partly due to local water politics. The trees have been drip irrigated for quite some time as far as we can tell - probably not particularly generously. The soil underneath the trees has been heavily compacted by (unauthorised) cattle grazing by the local herdsman, and in while digging post holes the other day I observed that there's a marked clay pan 20-30cm below the surface. The pan is probably only 15-20cm thick.

We'd like to plant under/between the trees to a greater extent, partly to be able to plant beneficial plants for the trees, partly to improve water retention, and partly to make better use of the space. However we're not sure what to do about the clay pan. A lot of olive farmers round here plough between their trees aggressively every few years, but I'm really nervous about doing this even once - I'm worried about damaging the root systems of the trees, particularly since they've been drip  irrigated and may have higher roots than normal. On the other hand, if we're going to improve things here I think we're going to need to break the soil up somehow, both for the long term healthy of the trees, and to make it possible to plant things underneath. Plus we need to start storing a lot more water in the soil,  which I think means breaking through that clay pan to stop it all just pooling on the surface when it does rain.

We have access to a large supply of (mainly unrotted) horse manure (riding stable just down the road), and we're also making a certain amount of mulch from prunings of dead wood using our chipper, but neither source is anywhere enough to systematically mulch the whole area to a useful depth (we're probably talking about 3000m2). Solutions that will require us to irrigate substantially during the 6 months or so when it doesn't really rain here aren't going to be viable for the time being either. We're planning to invest heavily in more water storage here but even so it's probably never going to be enough to water all the trees as much as we'd like.

What would people suggest here? Should we be looking at a mechanical approach to breaking things up? Is there a plant-based solution that wouldn't require huge amounts of water?

Cheers,

Plant lots of cover crop with deep roots.  Let the plants do what they do, deep till and scavenge for nutrients and moisture.  Popular plants are comfrey, turnips, alfalfa, hemp, bluestem, etc...  Any root mass will combat the cattle compaction on the surface; but there are many plants that will drive deep roots through hard pan to start the process of opening up the sub soil.  I agree that you don't want to damage the tree roots.  However, a "once to get started" deep aeration may be in order.  Get familiar with how a subsoiler works.  It opens up a single channel, deep, but does not overturn the soil.  Make a determination on where the root mass is primarily concentrated and sub soil between the rows in the safe zone.  At least then you will have some aeration to the microbes in the orchard to start the process of decompaction.  The manure, if free from chemicals from the feed, will do a lot over the roots where a subsoiler is unadvised.  Wood chips would be better; but I imagine in short supply in your area.  

The trees are already through the hard pan.  They are not surviving, even with drip irrigation, on 8-12 inches of soil (20-30 cm), so they have pushed through where they need to.  The rest of your plantings, will they need more than 30cm of soil?  I ask for two reasons.  Getting water deep in the ground on a macro scale is a positive thing.  However, if you poke holes in the bottom of the bucket, how does the water make it back up to the plants above the hard pan when it is needed?  If the soil structure below the hard pan is not conducive to the capillary action that will wick the water back to the plant, it may be great for the water table far below, but your plants won't benefit unless you are pumping it back to the surface, something the local gov't may restrict.  Just a thought.  

The other reason is a foot of soil will hold a lot of moisture given enough organic matter.  For every 1% increase in organic matter in this zone the water carrying capicity increases by 16,500 gallons or roughly half an inch of rain on a per acre basis.  So if you are getting an inch of rain a month (per week?) an increase of 2% organic matter will hold all that in the top 30cm of soil without making it a pond.  Uncompacted humus will act as a sponge and absorb and expand as it hydrates rather than ponding or running off.  If you can make the top foot of soil a sponge and hold your rain event without having to drain it deep into the water table, your plants will be better off.  (for reference:  http://www.agriculture.com/farm-management/conservation/boosting-topsoil-quality_556-ar42674 )

Those are excellent suggestions.  I also recommend looking into simple earthworks to help hold rain water in the growing area.  Just putting little berms or even rows of rocks on contour can help in the long term.  If you have the physical energy, you can dig out basins between the trees which can hold run off and irrigation where plants and organic material can help build the soil.  I'm trying this in hard clay soil beneath trees.  Not much is growing this first year, but I trust it will improve over time.

Jack Edmondson wrote:Plant lots of cover crop with deep roots.  Let the plants do what they do, deep till and scavenge for nutrients and moisture.  Popular plants are comfrey, turnips, alfalfa, hemp, bluestem, etc...  Any root mass will combat the cattle compaction on the surface; but there are many plants that will drive deep roots through hard pan to start the process of opening up the sub soil.  I agree that you don't want to damage the tree roots.  However, a "once to get started" deep aeration may be in order.  Get familiar with how a subsoiler works.  It opens up a single channel, deep, but does not overturn the soil.  Make a determination on where the root mass is primarily concentrated and sub soil between the rows in the safe zone.  At least then you will have some aeration to the microbes in the orchard to start the process of decompaction.  The manure, if free from chemicals from the feed, will do a lot over the roots where a subsoiler is unadvised.  Wood chips would be better; but I imagine in short supply in your area.

This was the answer I was kind of expecting - the thing I'm worried about is whether we're going to be able to keep those sort of plants alive effectively through the summer with the water we have. I've seen comfrey die in the summer here on other people's farms. Are there plants that are good for breaking up soil that are good in dry climates? We're above the snow line so we can't grow through the winter like many places here in Andalucia, so there's a relatively short window where temperatures are clement AND water is readily available...

Your points about the clay pan are certainly food for thought. There are people with subsoilers that we could borrow/rent here, but even if there wasn't the risk of it being a bad idea to break the clay pan, the trees are planted in a fairly "random" fashion, not in rows, so some root damage is almost inevitable.

Overall it seems like getting as much organic matter + some deep-rooted plants that are drought tolerant is probably the way to go, rather than trying to plough stuff up.

Also, Tyler, yes, we're intending to put in some swales to improve water retention. The terraces are *reasonably* flat already, which helps.

Lucian,

I have not tried this plant, yet; although it is on my spring planting list to experiement.  http://petcherseeds.com/pigeon-pea/  According to this source, pigeon pea will make a crop on as little as 30cm of rain, once established.  If anything can make it through your summer, this would be it.  It has a deep taproot and is a legume.  Again, I can't yet personally attest to it efficacy; but may be worth more research for your situation.

Jack Edmondson wrote:Lucian,

I have not tried this plant, yet; although it is on my spring planting list to experiement.  http://petcherseeds.com/pigeon-pea/  According to this source, pigeon pea will make a crop on as little as 30cm of rain, once established.  If anything can make it through your summer, this would be it.  It has a deep taproot and is a legume.  Again, I can't yet personally attest to it efficacy; but may be worth more research for your situation.

Excellent - thanks for the tip. We may also be able to get away with other cover crops like alfalfa - we're not a desert and we do have *some* irrigation water - but it's good to have lots of options!

We've also got to work out how to sow whatever we're going to plant into our compacted soil so that it can compete with the grasses. Normally I'd be thinking of sheet mulching to knock back the grasses and then planting into a new layer of organic matter over the top of the existing soil, but on both counts I'm a bit worried about suffocating the tree roots/preventing water from penetrating to them.

I would add to that list another two leguems:  Lab Lab and Sunn Hemp.  Both are heat and drought tolerant, fix nitrogen, and sunn hemp has deep roots.  

Lab Lab:  http://wildlifeseeds.com/info/lablab.html

Sunn Hemp:  https://plants.usda.gov/plantguide/pdf/pg_crju.pdf

I am starting a pasture reclaimation project on an overgrazed dryland pasture that gets 28 inches (840cm) of rain, so am in a similar situation.  Keep us posted.  I would like to compare notes.