Ever had to use a pick axe to dig in a plant but the hole retains water without draining? Or filled a plant hole with water only to have it disappear?
Hard earth and coarse sand both complicate desert watering since neither retains moisture in ways that benefit plants. But challenges never stop a gardener from planting.
A few technical terms to move us along:
| Dirt | Particles of rock, sand, silt, clay with no nutrients or microbes - “dead”. | |
| Soil | Dirt particles + minerals + microbes. Can sustain healthy plants - “living”. | |
| Soil Structure | Density + shape of soil, and its permeability. | |
| Soil Texture | Type of dirt + proportions of organic material. | |
| Soil microbes | Bacteria, fungi and protozoa that affect soil structure and texture, water retention, and plant vitality. Microbes optimize conditions for plants. They rely on plants to produce carbohydrates for them. The microbes colonize dead dirt and turn it to sliding soil, deliver minerals to roots in plant soluble form, attack disease-causing organisms, and form a living net that anchors soil to keep it stable. |
HARD EARTH
Caliche, clay, rock, and dirt compacted by development are so impermeable that water either runs right off the surface or sits in cavities without draining. Roots can’t penetrate a compressed soil structure. Because it lacks porosity there are no air or water pockets for microbial life, making it ‘dead’ dirt. Working with it sometimes requires a jackhammer.
To plant in this type of dirt, make an extra wide and deep hole so that water can percolate away from the rootball to keep the plant from drowning. The sides and bottom should be rough. Break your back-fill dirt into smallest chunks possible. It also helps to feather in topsoil and add 30% peat-free organic materials. Good wood mulch or small cut-up branches and leaves can go into the mix, but avoid yard debris from plants that suppress other types of plants such as creosote, eucalyptus, oleander and tamarisk. Once the plant is installed, water it much less than you would a plant in free-draining soil. Gradually the roots and organic material will work together to make tiny inroads into the dense dirt. Plant by plant, soil structure will improve.
Adding the organic material into back-fill creates small air gaps that interrupt the blocky soil structure. Microbes now have a place to set up shop. As they glue together small dirt particles, their activities create small voids that leave behind a porous texture. Air and water can start passing through, making the underground environment less hostile to plants. Over time the microbes continue to build soil by multiplying, making more pathways, and depositing organic material.
Two commercial treatments, sulphur and lime, are sometimes used for dense ground but both must be avoided. Dousing with sulfur results in sulfuric acid which kills most plants and causes groundwater nitrification. Lime raises soil pH even higher in our very alkaline desert soil. In addition to harming beneficial microbes, lime has a limiting effect on minerals and robs plants of their nutrient supply.
COARSE SAND
Now let’s look at dirt that’s too loose. This structure is so open that plant holes fill themselves back in as fast as you dig, water instantly disappears, flushing away minerals and soil life.
Just as with hard dirt, start by amending the back-fill of the plant hole. Add the same 30% organic mix as you would for compacted soil, and wet it down thoroughly. In this case the organic material is introducing absorbency and slowing down water flow, just like veggie scraps in a kitchen drain. Microbes now have surfaces to attach themselves to and they can start the business of massing dirt particles, webbing together soil, depositing organic bits, and synthesizing minerals for plants.
The process of changing problem dirt into plant-friendly soil can be sped up by using broken down (aged) compost as an amendment or mulch. But not all of us have aged compost piles standing by so a user-friendly option is to apply humic acid in diluted ‘tea’ form. This helps to inoculate the ground with microbes and buffer tap water to make it more like rainwater. Teas can be used for all parts of a garden as an occasional treat for plants.
Humic acid teas can be purchased in bottles or made at home using aged compost or packaged worm castings. Castings are the byproduct of compost that’s gone through the digestive system of worms, and it provides all the benefits of aged compost plus organic material from the worm processing. A little bit goes a long way and it helps all soil types. Check out our recipe!
For a deeper dive into soil, here’s a great workshop by Dr. Elaine Ingham that digs into the science behind building great soil.
Also check out our local expert Robin Kobaly’s The Desert Underground.
WHAT ELSE?
Ever had the soil in a plant container go really dry and actually wick water away when you give the plant a drink? Or the top may get damp but the soil below stays dry and water just runs out the bottom? That soil has become hydrophobic, or resistant to water. Try mixing some kelp (fertilizer in liquid or meal form) into the next watering. Kelp acts as a wetting agent to breakdown surface resistance between soil and water.
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Dry Potting Soil
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Try mixing kelp in liquid or meal form into the next watering
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