Plants flourish when their roots thrive. Conventional wisdom is that roots thrive in soil that provides nutrients and structural support and allows water and air to circulate freely. Soil must also sustain the beneficial bacteria and fungi that help roots take up nutrients. The roots of most plants function best in soil ranging from pH 6 to pH 7.5.
All true, but gardening in the Flagstaff area often requires adding an element to this sage advice. Creating an open patch of ideal soil that is exposed to the elements is rarely the best approach in our climate. The soil will dry out and the plants will be battered by wind and sun. Consider how you can hardscape to hold your soil in place, keep it moist, and protect your plants from the wind and overnight frosts.
Soils in the Flagstaff area tend to be shallow, heavy in clay, and alkaline. Shallow, clayey soils prevent the development of strong, deep roots. Clay-heavy soils also prevent the free circulation of water, oxygen, and carbon. Alkaline soils chemically inhibit the uptake of nutrients especially iron, phosphorus, and manganese.
- Determine the pH and texture of your soil. Inexpensive devices for measuring pH are available in garden supply stores. A simple test for evaluating texture is to moisten your soil and squeeze some of it in your hand. If it falls apart easily, it is loose and will probably not retain water well. If it sticks together tightly, it is probably heavy in clay and will not allow adequate circulation of water and air. If it holds together yet appears porous, the texture is probably fine but the soil may still lack nutrients. Nutrient levels in local soils are typically quite low but if you’re unsure, consider having your soil professionally tested.
- Amending problem soils with organic compost is the single most effective way to adjust pH, improve texture, add nutrients, and increase water-holding capacity. Compost is commercially available but read the label to be sure it is made from natural, uncontaminated ingredients. Ideally, begin making your own compost in a rodent-proof bin and use it when establishing new plantings or as a top dressing and mulch. Adding acidic amendments such as sulphur or coffee grounds can benefit alkaline soils in our area, but proceed with caution and test occasionally if you decide to do this, to avoid creating excessively acidic conditions.
Native plants have adapted to our poor local soils in several ways, such as exuding acids from their roots to extract phosphorus and iron in spite of alkaline conditions. Yet in most cases, even native plants benefit when soils are improved.
- Carbon from atmospheric carbon dioxide is an essential component in starches, cellulose, and the carbohydrates that store energy in plants.
- Hydrogen from water is used in photosynthesis, respiration, and creating sugars.
- Oxygen from the atmosphere or from water is necessary for plant cells to breathe.
- Nitrogen—a component of all amino acids and proteins—is necessary for growth.
- Phosphorus is used in photosynthesis, root development, plant growth, flowering, and setting seed.
- Potassium reduces water loss from leaves and helps form healthy fruit.
- Calcium is needed for photosynthesis and structural development, regulates transport of other nutrients, and activates enzymes.
- Sulfur is used in the formation of chloroplasts.
- Magnesium forms the central atom of chlorophyll.
- Silicon strenthens cell walls and increases plant mass and root density.
- The micronutrients iron, manganese, and copper are necessary for photosynthesis, while other micronutrients perform more obscure but important roles in plant metabolism.