Desert Soils

The playa at low tide near Punta Estrella, south of San Felilpe.

Soils play a major role in a desert ecosystem. A soil is the result of weathering of geologic material. Once exposed, the forces of nature, break this material into ever smaller pieces. Soil is not just the accumulation of this degraded material however. Over long periods of time, through the interplay of organic and inorganic and forces, soils are formed. Soils that look different and have different properties from their geologic parent material are said to be mature soils. A good example of this is the rich farmland of the American Midwest. Young soils on the other hand have characteristics similar to their parent material.

The desert has examples of both mature and immature soils. For example, the sand in a shifting dune is so young it is not considered soil because it is not sufficiently different from it’s parent material. Typical desert soils are called aridisols. Aridisols are formed under the influence of strong winds, scattered but torrential rains, and high temperatures. They have a surface layer low in organic matter, as well as other layers, or horizons, including clays, salts, and minerals. The materials in these layers often are cemented together forming water-impervious hardpans.

It is important to realize that soils are not dead, sterile substances. Algae, lichens, and fungi all live within the soil and become a part of it. Two important examples of this fact are mycorrhiza and crytogramic soil. Mycorrhiza is a fungi that grows in the same location as, and closely resembles, plant roots. This fungi forms a symbiotic relationship with higher plants, helping them take up water and phosphorus. Crypyogramic soil can almost be described as living soil. This combination of soil, algae, and lichens produces a gas, forming a crusty, tile-like surface. Full of tiny pockets, this is a storehouse for organic material. Its harder outside crust makes it less susceptible to wind and water erosion.

Another important aspect of desert soil development is dentrification which is the process of compaction of soil by natural elements. Light rainfall and baking sunlight cause the ground to compact. Over time, a fairly hard-rigid surface is established. Ironically it is more important to have compacted soils in the desert than loose soil. The reason is wind and water erosion. The nutrients trapped in cryprogramic and dentrified soil can easily be lost when churned into powder by grazing animals and motorized vehicles. Because much of the San Felipe region was under seawater for millions of years, local soils are more alkaline than other soils, low in lime, organic matter, nitrates and phosphorus while high in potassium, zinc, iron, manganese and copper.

Gardening With Desert Soils

If you were to examine your desert garden soil, you would find it composed of tiny particles of dirt, sand, gravel, rock and some residual percentage of salt. The fact is, former sea floor is composed of approximately 80% silica, 8 % alumina, 4 % sodium, potassium, calcium and magnesium oxides, 3% iron oxide, 1 % titanium oxide and about 1 % of all other oxides and a varying amount of residual salt.

Whereas it contains sufficient percentages of soluble sodium, potassium, calcium and magnesium oxides to meet the needs of natural vegetative cover, the normal decay of plant residues-and the resultant production of carbonic, nitric and sulfuric acids-is restricted by the presence of secondary clay minerals that inhibit the development of active acidity.

As regards gardening with desert soils, the initial question is whether your soil is alkaline or acidic. Acid Intensity determines which plants can be grown in the soil as it is. As you drive back and forth across the desert, you will notice different plants growing in different areas. Because every plant has its own nutritional requirements, the appearance of different plants tends to identify the acid intensity of a given soil.

Acid intensity is a product of the solubility of acid-containing minerals in soil and that acid’s degree of ionization or what is otherwise known as the HYDROGEN-ION CONCENTRATION of a soil solution. Hydrogen-ion concentrations are usually ex-pressed as pH values. A standard pH chart would show a series of numbers ranging from 1 to 14 with 1 being highly acid, 14 being highly alkaline and 7 being neutral.

Because pH numbers are a logarithmic representation, 5 is ten times more acid than 6 and 10 is ten times more alkaline than 9. If you were to examine plants you found growing in the desert, you would learn the dominant plant species-comprising nearly 90 % of the ground cover surrounding San Felipe-are bursage and creosote bush which tend to indicate a relatively low presence of alkalinity. Other plants common to the same region are brittlebush, cardón, cholla, desert mallow, indigo bush, ironwood, mesquite, ocotillo, palo verde, pitaya agria, smoke tree, elephant tree, and yucca. It is due to these plants-which grow only in acidic rather than alkaline soils-that we know the desert surrounding San Felipe will support or can be made to support a variety of garden plants.

To prepare your flower or vegetable garden bed for planting, take a soup can size soil sample to an American county agricultural agent for analysis where, for a small fee, he will be able to tell you precisely how alkaline or acidic your soil is, how to correct it and what soil builders and fertilizers you should use. Should you choose not to spend money on soil samples, we recommend the addition of 1 lb of agricultural sulfur and 2 bales of peat moss per one hundred square feet of garden bed turned to a depth of 2 feet. All plants require nitrogen for flower and fruit development, Phosphoric Acid for stem, branch and leaf growth, and Potassium for root and stem development. You will find those nutrients in mixtures sold either a Commercial or Organic fertilizers.

If you examine a commercial fertilizer label, you will see it lists a mixture of these three all-important chemicals by percentage numbers such as 6-10-4 which indicates 6 lbs of nitrogen per 100 lbs of fertilizer mix-usually derived from ammonium Nitrate, 10 lbs of phosphoric acid per 100 lbs of mix-usually derived from chemically formulated from Treble-superphosphate, and 4 lbs of potash per 100 lbs of mix-derived from Sulfate of Potash. In addition, most chemical fertilizers contain inorganic filler material to add weight to the bag.

Organic fertilizers include manure, ground cottonseed meal, ground rock phosphate and Muriate of Potash. Manure, whether cow, steer, horse, chicken, turkey or human, is an organic fertilizer containing 2% nitrogen. Two popular fertilizers-Mil Organite, originated years ago in Chicago, and Soil Organite, originated by California’s Orchard Supply Company, are mixture of dried human fertilizer, also known as sewage sludge, and commercially mined Urea to create a 6% nitrogen fertilizer that is perfect for new lawns and other seedlings.

Because organic fertilizers release their chemicals slower than chemical fertilizers, you usually do not need more nitrogen than they contain although commercial sales people have long ago convinced the American public they need a 10% nitrogen mix for tomatoes, 6% for strawberries and other amounts for other plants.

Higher percentages are required by farmers striving for maximum crop production in the shortest possible time. The problem with chemical fertilizers is a) they remain in the soil for relatively short periods of time whereas the slower releasing organic fertilizers remain in the soil considerably longer. b) They usually contain undesirable chemicals that can compound a problem created by desert soils. That is, chemical fertilizers contain chemically formulated salts such as those found in Super phosphates and Sulfate of Potash.

If you are a serious gardener, who wants the best vegetables or the prettiest flowers your garden can produce, use organic fertilizers at a rate of 6 lbs of cottonseed meal, 10 lbs of ground rock phosphate and 2 lbs of Muriate of Potash per 100 sq ft of garden surface applied in late fall. (Cottonseed meal is recommended because it has none of the seeds found in animal fertilizers.) Never add nitrogen after fruit has set. If your soil is prepared and fertilized properly, you should never need to add fertilizer but once per growing season. Also, because you are in the desert where both sun and wind exact a toll, use a ground cover to retain as much soil moisture as possible.

If your garden is plagued by coyotes and rabbits, fence it with a hardwarecloth buried 1 ft underground and purchase and distribute mothballs around the base of each plant. Regarding insects, surround your garden with a planting of marigolds, a plant most bugs don’t like.

Finally, for the serious gardener, purchase a soil fumigant and a large enough sheet of black plastic to hold that fumigant in the soil. Then, before you add your soil builders and fertilizers, fumigate the soil including a three-foot border around your planting bed, as described on the package label. When fumigation is completed, remove the plastic sheets, prepare the two feet deep garden bed by adding your soil builders and fertilizers, run your furrows, plant your seeds or seedlings and stand back and watch your plants grow to their hearts’ content.