Gardening in Western Soil

A geologic overview of the formation of Western soils, and how to make it healthy.

Asclepias tuberosa
Item # 23202 		Asclepias tuberosa
Western Gold Mix

each $8.99
3 to 6 plants $8.79
7 or more $8.59
  • Topic: Soil
  • Keywords: soil, clay, humus, drainage, mulch, Soil, gardens
  • Date: November 2001

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The soils in New Mexico and much of the southwest are the result of three different ancient geological forces. Volcanic activity produced porous bedrock, saline seas provided limestone and sandstone, and violent mountain formation resulted in heavy granite. All of these have settled down through the ages into our subsoil, and the topsoil above is a composite. In foothill locations you find gravelly soils; in old flood plains and along arroyos the soil will be more of a sandy loam; but the most common soil type in New Mexico is primarily clay in composition, which can be one of the toughest soils in which to plant. What’s left out of all of these soil types is organic matter—it makes sense to be thinking of how to add this when planning and planting your garden.

Many people who have moved here from the east or Midwest start out believing that they’ll have the same green thumb here as there—the “just add water” syndrome. They soon discover that in New Mexico, creating a healthy garden starts by creating a healthy, fertile soil. And while it may be a labor of love, it is possible to bring to your garden soil just what it needs to nourish all sorts of plant life—which, in turn, nourishes you.

Here are three key elements of a healthy soil:

1.) Loft. Plant roots require oxygen just as much as water. When amending your soil you will need to keep in mind that good loft will enable the roots of your plants to absorb the oxygen that exists in between soil particles. A lofty soil is desirable precisely because of the soil oxygen level it creates. But loft is hard to come by here because of the dearth of organic matter in our topsoil. Additionally, any activity requiring heavy equipment will further compact the soil, often for several feet directly underneath.

If you’ve just built a house, chances are that the area right around it—i.e., where you’d most like to garden, will have been particularly affected by the construction equipment. So the first order of business is amending the soil with an organic compost. This can be from your compost pile, from manure (make sure it’s heavily broken down) or from a variety of products we sell in bag or bulk. As a starter, we recommend the Soil Mender Blend, a mix of composted cotton burr hulls and composted steer manure. If you mix this with the dirt in your future garden (it’s not to be used to plant in directly!) you’ll be creating a healthy soil, filled with that lofty organic matter.

2.) Drainage. This is closely related to loft, and of importance because of the high clay content in a lot of our soils. The difficulty of clay is due to its water retentive nature—it absorbs water slowly, and once saturated, stays wet too long for the roots of most plants. To compound that problem, clay soils tend to crack at the surface as they dry, creating fissures which expose tender roots to our bright sun and arid heat. Lastly, as it dries, clay pulls water OUT of the roots of plants to maintain its moisture level, so you’re left wondering when to water—even when the soil still feels wet this reverse osmosis effect results in wilted plants. You can combat these problems by following the soil amendment solutions above, and mulching to reduce surface evaporation.

3.) Nutrients. Luckily, our desert soils are actually rich in many nutrients, but they are lacking in some major ones, like phosphorus and nitrogen. Nitrogen moves through the soil fairly quickly, so it is effective if applied at the surface, enabling it to move down through the soil. Phosphorus, on the other hand, moves very little in the soil, so using phosphate products as a top dressing will actually induce chlorosis. It is quite effective to add a phosphorus source to the soil when planting. We recommend super phosphate as a planting amendment in our soil. And remember: our soils are notoriously alkaline, so a lot of the remedies you read about in national gardening magazines, like using wood ashes for potassium, will actually raise pH levels with their high alkalinity. (Use Greensand instead of wood ashes as a potassium source.)

Finally, we can’t talk about soil without mentioning the bugaboo of a lot of southwestern gardeners: caliche. Caliche is calcium carbonate, a compound that is common in our soils, but only becomes a problem when it is compacted to form a hardpan. This occurs naturally when a porous soil is above harder subsoil, so that water that drains easily through the porous soil is stopped and pools on the hard surface. The calcium carbonate then leaches out of the porous soil and crusts in the contact zone, making drainage an even bigger problem. In your garden, you can accidentally create a caliche problem by not allowing the soil to dry between waterings, or by watering too shallowly. Dealing with caliche isn’t easy. Digging it out can be a monumental task if there are no fissures in the surface. There are two good solutions for this problem, should you be so unfortunate as to encounter it:

1.) Do you need to plant right there? Our soil can be capricious enough that if you just move the hole three feet you’ll encounter a caliche-free zone, ready for no-pick-axe gardening. If you must plant in that spot—

2.) Consider a raised bed. There are many ways to make an attractive raised bed—rock, aged latillas, traditional lumber (avoid treated wood for vegetable gardens)—so that it looks like it belongs in your garden. Plus, you get the benefit of filling it with a high-quality amended soil.

Don’t feel that you need to make vast beds of enriched soil in your garden. For starters, there are many plants that thrive in less than ideal soil. Our High Country Gardens catalog has a broad array of plants that require a soil composition like the one outside your door. But because even plants native to this area have problems with caliche shelves and heavy clays, you’ll need to understand what you’re looking at when envisioning your garden.