Growing the Desert Garden

Welcome to the Desert Garden, with garden coach Tyler Storey, where we talk about everything having to do with gardening and landscaping in the Desert Southwest. From composting to Cercidium and agaves to arugula — we'll cover everything you want to know to grow your own beautiful Desert Garden.

Wednesday, July 9, 2008

From the Inbox: Fruit Trees and Caliche

From a Correspondent:

My wife and I are fairly new to growing vegetables here in the desert. I've read nearly every book I can get my hands on and noticed a couple things that got me wondering.
1. Is foliar feeding really effective for plants? Seems to me that with the heat so much of what I spray on the plant would get evaporated before the plant has a chance to soak it up. Some of the information I've been able to dig up on the Internet suggests that the plant can get many times more nutrients from it's leaves than it can from the ground.
2. How can I tell if I have caliche without digging down the 4-5 feet recommended by some of the desert gardening books? We plan on planting some fruit trees in our yard this winter and want to give them the best chance for survival, but don't want to just dig down 5 feet unless I really need to.
Chandler, Arizona

Good morning, Tom,

These are both great questions. The answer to both is: it's all about the soil. But since they're from different perspectives, I've answered your first question in a previous post, and we'll tackle the second question here.

When I get a question from a client or a correspondent that includes "fruit tree," "caliche," and "dig down five feet," I know exactly what book they've been reading. It's a great book and very useful for learning about growing fruits and vegetables in the Desert Garden — except in this one instance, where it goes terribly astray. Don't do it!

Caliche is a hardened layer of calcium carbonate that sometimes forms a subsurface water-impermeable layer in some areas of the desert. Where it's present it can be a concern because it can interfere with drainage and can, if shallow enough, prevent tree roots from anchoring deeply in the soil.

As a side note, other soil issues can be mistaken for caliche. In newer developments, we sometime find a layer of clay soil that's been compacted by heavy construction equipment: this is called hardpan. In houses built on former agricultural lands, we sometimes find a layer of impervious compacted soil caused by years of plowing at the same level: this is called plowpan. Either can restrict root and water movement in a manner similar to caliche, but they're not the same thing.

To your question: not only do you not need to dig down five feet deep, but you really shouldn't dig down five feet. It's a very bad idea.

While I'm tempted to just answer your question and tell you what to do instead of a five-foot hole, I'm first going to explain the whys and wherefores. I can't help it.

There was a time when the recommendation for planting fruit and other trees in the Desert was to dig a hole five feet deep and five feet across, and then plant the tree in the hole, replacing a huge portion of the removed soil with organic material such as compost or rotted manure, or wood chips. The idea was that you could get through the caliche layer (if there was one) and also provide the tree with good soil in which to grow.

More recent research has demonstrated that there are two problems with this advice. First, it simply isn't possible to dig a hole large enough to provide "good" soil for the mature root growth of a tree. Consider that an apricot tree, for example, will grow a canopy 15 feet wide and have a mature root area somewhat larger than the canopy. To accommodate the mature root area, you would have to dig a hole 25 feet wide and five feet deep, in the process moving approximately 2500 cubic feet of soil. If you decide to try that, let me know: I want to come watch.

If instead you decide to improve the soil in a smaller area — let's say a five by five hole — one of two things will happen: either the tree will grow roots only into the improved area and thus have a small root area with poor anchoring and consequently be subject to poor development and wind-throw, or the tree will happily send roots out into the surrounding, unimproved, native soil, in which case it was a waste of time to improve the soil in the first place.

The second problem with improving the soil for a tree with organic amendments is that organic amendments break down. All that manure and compost and wood chip will decay and in the process get smaller. And the tree that you've planted on top of all that lovely organic matter will sink. It will sink a lot. It will become — to belabor the point — a sunken tree.

Sunken trees aren't good. Sunken trees collect water in a very small area near the trunk and consequently develop small root systems and become susceptible to rots at the base of the trunk.

A sunken tree also means that your tree is, as above, pretty much growing in native soil. Which means all your amendment was a waste.

So, here's what you do instead of digging a five-foot hole: dig a one-foot-deep hole where you want to plant your tree, about a foot or two wide. Fill the hole with water. If the water drains out completely within about a half a day, then you're all good to go. Let the soil dry a bit (never dig wet soil), then dig out the hole only as deep as the root ball of the tree you're planting, and about three times as wide. Remove the tree from its container, place it in the hole, and back-fill with only native soil, keeping the top of the root ball at ground level (no mounds and no "bowls"). Water it well and them mulch heavily and widely with an organic mulch such as fine chipped bark, keeping the bark slightly away from the trunk.

If the water doesn't drain within half a day, take a soil probe (a piece of rebar or a really long screwdriver) and push it through the bottom of the hole. As deep as it goes is how deep your obstruction is. At that point you can either dig down to try to break through the obstructing layer, or you can try the water test in another part of the yard. If you're fairly certain that the obstruction is at least three feet deep, go ahead and plant the fruit tree anyway; three feet of soil is going to be good for almost any fruit tree. In either case, dig only as deep as you must, and never back-fill the hole with anything other than native soil.

At this point you may be wondering why yesterday I told you to add lots of organic material to your garden soil, and today I'm telling you to not add any organic material to your tree soil. That's because organic material always breaks down, and so it takes multiple additions over a long period of time to make real changes to your soil. In a vegetable garden we can add more material — and new plants — several times during the year and eventually make a lasting change, while in planting a tree it's a one-shot deal. Trees eventually will grow into native soil, and they will be stronger and healthier if they just get used to it from the beginning.

That gardening book you've been reading is full of great information from a much-respected gardening expert, and you can learn a lot from it. Except those pages on caliche and the five-foot hole; just skip over those and we'll pretend it never happened.

I hope this helps,


1 comment:

ellis_family said...

Thank you so much. This is *exactly* the info I was looking for. I was being told to put in several truckloads of compost, plant my 10 fruit trees, and then mulch heavily. This will save hours of extra digging!
~Michelle in central TX