Leonardo da Vinci famously mused in the fifteenth century that “we know more about the movement of celestial bodies than about the soil underfoot.” Just as mind-boggling, the U.S. Forest Service tells us that “Soil is a complex, living system that serves as the skin of the earth. In just one thimble full of soil, you can expect to find 100 million to one billion bacteria, several thousand protozoa and 10 to several hundred nematodes. Dirt is literally squirming with insects, nutrients and life.”

The soil food web is the biologically active part of the soil, which is vital for growing healthy plants. As gardeners, our hands are often in contact with the soil, but unless we have studied botany or horticulture, most of us know very little about the soil in our gardens, other than the fact that our soil anchors and supports our plants’ roots and nourishes the parts of the plant aboveground. It is critical that we gardeners care as much for our soil as we do for our plants.

‘Just-right’ soil

Goldilocks was on a quest to find the “just-right” porridge and a “just-right” bed. If we were on a quest to pinpoint “just-right” soil, these are the attributes we would be looking for.

Texture: Soil can be sandy, silty, loamy or clayey (yes, this is the adjective for clay soil). Soil texture refers to the proportion of sizes of the various particles that make up the soil. Sandy soil has the largest particles. The next largest are silt and the smallest particles are clay. Loam is the best textured soil and contains about 40 percent sand, 40 percent silt and 20 percent clay. Loamy soil is ideal for many reasons including being well aerated and easily penetrated by roots.

Structure: Soil structure refers to the arrangement or aggregation of mineral and organic soil particles and the spaces between them. A just-right soil structure allows the soil to “breathe” and enables both movement and retention of water, air and nutrients. It also provides space for root growth.

Water-holding capacity: Water is delivered to plants from their roots. Water keeps plants turgid and is indispensable to the processes of photosynthesis, transpiration and the transport of nutrients from the soil to the plant. For these reasons, soil needs to hold and release just the right amount of water. Do a drainage test by digging a 1-foot-by-1-foot hole and filling it with water. Let it drain, refill it and time how long it takes to drain. In well-drained soil, the water level will go down at a rate of around 1 inch an hour. Less than that and drainage is inadequate, which is the case in very clayey and compacted soils. Very sandy soils drain too fast, which can deprive plants of sufficient water for their needs. Both sandy and clay soils can be improved with the addition of compost.

Fertility: A fertile, nutrient-rich soil is necessary to supply plants with the macronutrients and micronutrients needed in the proper form for plants’ roots to absorb them. Organic material such as well-rotted plant and animal residues provide soil fertility. Good soils contain 3 to 6 percent organic matter. We can improve our soil’s fertility, texture, water-holding capacity and soil aeration when we amend our soil with organic amendments such as worm castings, manure and compost. These amendments provide a food source for microbes, which can help soil structure regenerate.

Soil pH: An optimal pH is vital to the release of nutrients in the soil to the plant, so a correct pH level makes a huge difference to the health of our plants. Most plants grow best in slightly acidic to neutral soil (a pH between 6.0 and 7.0). The “just-right” pH for roses is a soil that tests between 6.0 and 6.5. An alkaline pH (typical in San Diego) can be adjusted with the addition of sulfur. A too-low pH can be adjusted with the addition of lime.

Soil porosity: Maintaining good air-filled porosity is important, as plants’ roots and soil organisms need oxygen. In water-logged clay soils and compacted soils, porosity decreases, which can limit root growth. Especially under wet conditions, we should refrain from tilling the soil or stepping on the soil, as both actions can lead to compaction.

Soil management and improvement

We may not be able to make our garden soil ideal or “just-right,” but we can take certain actions that will help improve our soil.

Avoid compaction: The lack of oxygen in compacted soils greatly limits plant growth because the lack of oxygen prevents good root growth. Compaction also decreases the micro-organisms inhabiting the soil that change organic matter in the soil into nutrients that plants can use. Is there anything we can do to avoid compacting our soil? Absolutely! Try to avoid walking in garden beds and step as much as possible only on steppingstones. Postpone your plans to work in your garden when the soil is wet after a rain. Limit tilling and spading the soil, as this damages soil structure, especially when the soil is wet.

Amendments to improve soil: Soil amendments such as worm castings and compost improve soil texture, structure, water-holding capacity, fertility, pH and soil porosity. In addition, the nutrients contribute to the soil biota, which help the plant uptake nutrients from the soil. Instead of digging the amendments into the soil, layer the worm castings and compost on top of the soil. Earthworms and other soil organisms will move the organic material into the soil. Adding liquid amendments like liquid humic acid, liquid kelp or seaweed, alfalfa tea or liquid fish emulsion is also a good way to enrich the soil without digging, and these products stimulate the growth of micro-organisms in the soil. Our best option to improve texture, drainage and nutrients in a clay soil and a sandy soil is to add organic materials.

Some rosarians will add gypsum (calcium sulfate) to clay soil to improve structure, compaction and drainage. Jolene Adams, master rosarian and past American Rose Society president, provides evidence in her article, “Will Gypsum Improve Clay Soil” that gypsum can help only a “sodic soil” — a soil with high levels of sodium and low levels of calcium and magnesium. Since the majority of clay soils are not sodic, Adams advises not giving in to the marketing hype and to get a lab test before using gypsum.

Organic mulch: There is a lot to be said for all the benefits that a good organic mulch can provide to the soil. It helps soil retain moisture for plant use, reduces erosion, improves soil structure over time, improves biological activity in our soil, impedes weed growth and it gives the garden beds a finished and attractive appearance. Mulch needs to be applied annually. When applying mulch, do not cover the canes or trunk of your plants.

Kick the chemical fertilizer habit: Synthetic fertilizers are lab-made and faster acting so they boost and expedite plant growth and bloom rate. However, these fertilizers are high in salts and do nothing to improve our soil’s health, texture or long-term fertility. They can be detrimental to populations of beneficial micro-organisms, and too much can “burn” our plants.

Cut out unnecessary fertilizers, pesticides and fungicides: Proper fertilization enhances plant growth without polluting the environment. Too much unnecessary fertilizer, pesticide and fungicide can not only injure, burn and kill a plant, they can also mess up and poison our soil and the soil organisms. Additionally, when these products are overused, they can run off into the ocean, lakes and streams.

Soil test: How can you tell if you are using an unnecessary amount of fertilizer? In addition to the pH of your soil, a soil test will tell you if your soil is deficient in nitrogen, phosphorus or potassium or any of the micronutrients. For a healthy soil and healthy plants, add only the nutrients your soil needs. Epsom salt has been added to rose gardens for decades by conscientious rosarians in the belief that epsom salt prompts basal breaks. But research shows there is no value to this practice unless soil is deficient in magnesium.

What is the top secret to success for gardeners? Take care of the soil, and it will take care of the rest.

Perwich is a member of the San Diego Rose Society, a Consulting Rosarian and a Master Gardener with UC Cooperative Extension.