Soils in the Flagstaff area are generally shallow, nutrient-poor, and alkaline, with a pH of 7 or higher. This is mostly because both rock and plant material break down slowly in our dry climate. Bedrock is often very near the surface and there is little organic matter to hold moisture or to nourish plants.
As a result, the bedrock strongly influences the meager soil that does develop. Our two main rock types—basalt and limestone—break down into soil that is alkaline and often high in clay. Alkaline soil interferes with a plant’s ability to take up nutrients. Clay soil is practically impermeable and should be amended, because most plants require porous soil that allows water and oxygen to circulate between its particles, carbon dioxide to escape, and excess water to drain.
Cinder soils east of the city limits do allow rain to percolate into the ground and permit both water and carbon dioxide to circulate. Meadow soils are also different; they are relatively deep, fine textured, and rich in organic matter.
Soils on the windy (usually southwest) side of slopes tend to be shallower than on the sheltered side of slopes, where the wind slows and drops airborne particles. Soils on the sunny (south) side of slopes will be drier and have the lowest organic content than soils in protected areas.
Note: See Improve Your Soil under Advice for suggestions about improving the soil in your garden.
Volcanic vents spewed lava flows, cinder cones, and ash throughout the Flagstaff area fairly recently in geologic time. In our dry climate, these young volcanic rocks and their overlying vegetation have broken down slowly, resulting in shallow soils with only meager organic content. Yet volcanics are naturally fertile as weathering gradually releases their abundant mineral nutrients. Some volcanic soils are higher in sulfur, which makes them naturally more acidic. If in doubt, have your soil tested.
Basaltic soil tends to be heavy in clay. Cindery soil allows roots to extend deeply and holds water well, and volcanic ash has been described as a “time-release capsule, rich in nutrients.”
Kaibab Limestone is mostly calcium carbonate deposited in ancient, inland seas. Limestone soils tend to be salty, with a high pH that inhibits the ability of plants to take up nutrients.
Within the limestone parts of the Flagstaff area, some of the soil that appears sandy is actually up to 50% dolomite. Dolomite contains large amounts of magnesium, which is the central atom in chlorophyll. These patches of gritty dolomitic soil have better texture and nutrient availability than most limestone soils.