As I placed the white pomegranate tree next to the space I wanted to install it, I pushed the shovel into the ground. As expected, it went in a couple of inches then stopped. I hit hardpan and knew about 8-12 inches later, caliche awaited me.
I had soaked the ground two days before to soften the soil but it still wouldn’t give with my shovel or spade fork. After digging, using an augur and a pickaxe, I finally got the 20-inch deep by 3-foot wide hole I needed.
Hardpan is compacted soil. In the suburbs, this comes most often from the tractors and trucks that drove across the property as homes were built. It also happens along pathways where a large amount of foot traffic occurs.
Caliche is a natural “cement” created by calcium carbonate that binds the clay, gravel, sand, and silt in our soil. The only ways to get through it is with an augur, a pick axe, or jackhammer.
Welcome to our nutrient-rich low desert soil.
THE BAD NEWS
Our soil is heavy in construction-damaged clay, has a high pH of 7.5 (alkaline) or higher and drains very slowly. Most plants and trees like a neutral pH of 7 or below. Clay itself can be good soil, but the small molecules that form the clay often causes slow water penetration. Unlike sand that has large particles and therefore water flows freely through it, clay binds together. When you add water and it pools, the salt in the water often attaches to the roots of a plant and may kill it. A previous blog talks of the signs and results of “salt burn.” The soil also contains very little organic material.
THE GOOD NEWS
If we can lower the pH slightly and improve the movement of water, we have wonderful soil because clay is nutrient-rich.
Compost can bring down the pH slightly and add organic material to the soil. Compost IS NOT mulch, although it can be used that way. Mulch usually is wood chips, rock, straw, etc., while compost is completely decomposed organic material that looks like rich, black soil. Mulch should never be worked into the soil and rock shouldn’t be your mulching choice around trees or shrubs. It will be difficult to fertilize because you have to remove the rock each time and it can be burdensome.
Wood chips and straw still have material that needs to decompose. The process of decomposition requires nitrogen, so the mulch material will pull it from the soil when your plant needs the nitrogen to grow. Mulch is needed to prevent water evaporation at the surface but it doesn’t add nutrients to the soil until it decomposes.
For low desert soils, there are two schools of thought. Some of us believe that if a tree or shrub is not native or desert-adapted, you should mix some compost (as much as 50%) with the native soil before returning it to the hole. Others believe that the native soil should go back into the hole and you should only put the compost on top of it. Whether you mix it in or lay it on top, always give the tree or shrub some compost and then put mulch on top of that.
TEST FOR DRAINAGE
Drainage is one of our biggest problems with clay soil. The best way to know if you have a problem is to conduct a perk test.
1. Dig a hole at least 12 inches deep and 12 inches wide. Fill it with water and let it drain overnight.
2. Fill the hole with water again the next day and measure the depth of the water as soon as you fill it.
3. Every hour, measure the water depth.
If it is draining 1-3 inches per hour, the drainage is fine. If it is less than 1 inch, as it is in my yard, you will have to amend the soil or select a plant that tolerates wet feet continuously. My soil will often take several days before the hole drains.
TREAT POOR DRAINAGE
Both compost and gypsum are helpful if drainage is a problem. If you use gypsum, use the garden grade and not the construction grade. The easiest way to know is to only purchase gypsum from a nursery. The best way to use it is to sprinkle it on top of the soil around the tree until it looks like a light snowfall. Then work it into the soil without disturbing roots. I use a small hand rake. Water the gypsum down into the soil. The purpose of gypsum is to break up the clay’s tendency to bind together so that water moves freely through the soil to the roots.
You can also push the salts away from the roots through monthly deep watering. About once a month, give your trees and shrubs a longer soak than you usually do. This will wash away salts from the roots of your plants. Our high alkalinity (high in sodium, magnesium, and calcium) and salt in local water contributes to salts that attach to the plant roots. A good flush will help to control it.
My white pomegranate is now installed and happy. However, the day after I planted, my knee began to hurt. After three weeks without healing, I saw the doctor. The diagnosis: overuse of the knee that won’t heal without orthopedic intervention. Such are the battle wounds from our low desert soil.
I guess it would have been cheaper to hire the nursery to plant the tree instead of paying a doctor bill. Oh, well. Some lessons are never learned if you are a gardener and want to make sure plants are installed right.