Green Garden Dreams

The Joy of San Diego Vegetable Gardening

Healthy Soil for Healthy Vegetables

Soil is a deep topic (no pun intended). Soil is very complex because it’s got a biological community, and has both a mineral and an organic component. But, it can be relatively easy to take care of, because a healthy balanced soil takes care of itself, and you just have to give the soil (and the plants) what they need: inputs of organic material, water and air, and understanding what the soil needs and the plants need.

Soil preparation and upkeep is essential for a good garden. By creating healthy soil, you create a place where the cycles of life take place: you help process organic materials into nutrients, you help the soil and create a root zone for organisms below ground and above ground, with plants that turn carbon dioxide into oxygen, create beauty and food for you and visitors and dwellers in the garden. From the butterflies, birds and insects above ground, to the earthworms and microbes under the soil, you are doing your small part to create a healthier, happier, more sustainable planet.

I’ve been growing plants, including vegetables (my favorite thing to grow) since I was about 10 years old. Over that time I’ve gotten a definite impression of what plants like, and how they survive. I’ve also done some reading about plants and botany that’s provided background theory to helps me understand what I see. I’ve also spent time in nature, such as in the desert, and been impressed by the strategies plants employ to keep alive, grow and reproduce, even under very challenging conditions. I’ve also grown very unusual plants, like the carnivorous Venus Flytrap, and various Sundews and Pitcher Plants, whose form and function are evolved in response to the nutrient-poor acidic soils in which they live. These strategies and adaptations point to their basic needs.

For example, for plants growing in very arid conditions of a desert, where drying winds, sandy soil, intense sun radiation, and high temperature all conspire to rob plants of moisture, one can see where a limiting factor such water determines the way in which plants and live (and animals too), both in relation to each other and to the environment. In such an environment, the conservation of water, and protection from predations are paramount. There is more than enough solar radiation. Therefore you see all kind of protective mechanisms to help the plant keep from drying out and being eaten: hairs, thorns, waxy coatings, small leaves (or no leaves), pungent chemicals, and so forth. You also see a large distance between plants, since resources of water and nutrients are limited.

For best vegetable success, you want as few stops to their growth as possible. And ideally, you want plants to reach their greatest genetic potential. You want as few *limiting factors* as possible.

I’ll go over several factors, including air, water and minerals, but start with one real-world experience ground us (no pun intended). It has to do with air in soils. My experience growing vegetables and herbs using hydroponics was very telling. Growing gardens in both a “fill-and-drain” hydroponic bed and an organic double-dug bed, and seeing how well they did, and reading that plants like lots of air in the root zone (there’s even a system of hydroponics called Aeroponics), helped make sense of what I was seeing. Because in both cases, plants grew exceptionally fast and large, when there was lots of loose material that allowed air into the soil. With hydroponics you have a very course “soilless” medium (in my latest hydro garden it was volcanic rock: white pumice) that allows lots of oxygen into the root zone, and a nutrient solution (water with dissolved minerals) that pulls air down into the medium as it drains. And with organic methods (“biointensive”) double digging, you create a very loose pile of soil, full of coarse organic material that also creates lots of spaces for air to penetrate into the soil.

Let’s look at the big picture. In nature, every basic need is also a potential limiting factor. These limiting factors determine the pattern of life in a particular environment.
For instance, in a jungle, the limiting factors for plants are space, niche competition, sunlight below the canopy, and the fact that most nutrients are held within the biomass, so the soil is very poor. There is an abundance of water, humidity and warmth.
In a desert the limiting factor is water and nutrients. There is an abundance of sunlight, air and space.
In a wet bog the limiting factor are nutrients (too much water and low PH limit them). There is an abundance of water and sunlight.

So getting back down to soil, plants need, and soil provides (and are potentially limits), via the root zone:
• Water
• Air
• Nutrients
• A support via a stratum or medium to anchor their roots
• A biotic community and Symbiosis with fungi (mycorrhiza) and bacteria (e.g. in Nitrogen-fixing nodules in legumes)
Soil also plays a role in:
• Defense against disease and predation.
• Proper temperature, thermal regulation, and protection from solar radiation (depending on the plant)

Soil does this via:

• Water
Flows through or is held, clinging to particles, or trapped in clay, or wicks up and through.

• Air
Space between soil particles
Water that carries dissolved Oxygen

• Nutrients & Support
Inert, Inorganic compounds – minerals
Un-dissolved minerals: sand, clay, rocks that provide support and transport
Dissolved minerals: salts and ions that provide nutrients (from both inorganic and organic sources)
Organic Materials and Compounds
Plant and animal material in various stages of decomposition
Miscellaneous enzymes and organic molecules provided by the biotic community and the plant roots themselves.

Organic materials provide:
Texture.
Nutrients: Microorganisms – Bacteria and fungi breakdown, provide elements in the form of ions. The cycle of life.
A whole community lives in soil: worms, bacteria (good, mostly), fungi, nematodes, sowbugs, etc. A teaspoon of productive soil generally contains between 100 million and 1 billion bacteria, representing thousands of species.
The ability to hold water.

Inorganic components provide:
Support.
Nutrients. Some minerals (rocks, originally) can form dissolved salts and ions that plants need.
Water retention. Ranges of particle size and their properties:

Clay and silt hold moisture and nutrients well but do not allow air into the root zone. Clay also holds the water so tightly (microscopic particles) that it is not available to plants. They have very poor drainage.

Sand holds less moisture and nutrients but provides good drainage. Same for rocks but even more so. Some plants need more drainage than others. For instance, cacti that evolved in habitats with sandy, rocky soils need soil that allows excellent drainage with very poor nutrient level. They grow very slowly. However they do need some organic material or nutrients. Epiphytes – plants that grown on trees for support, have roots that cling to a surface and get moisture and air directly from the air and dissolved nutrients from rain water that carries nutrients to them. At the other end of the spectrum, some plants that grow in swamps have ways of getting air in the root zone since their roots are underwater. Most plants, including vegetables, are somewhere between those two extremes, and grow best in a soil that provides good moisture retention but also provides good air and nutrient creation and transport. This generally means a soil rich in organic material, with enough inorganic material such as sand to provide better texture and drainage. A soil that was pure compost for example, would grow nice plants for a while, but would eventually become too dense as the material decomposed more, not breathing well, not allowing air and water to flow easily enough. This is why packaged potting soil always have some kind of mineral components such as sand or perlite in addition to the organic components such composted bark or peat moss. They tend to lack adequate sand though because it’s heavy to ship.

So a good soil has the proper balance of: texture, mineral and organic material (dead or dying plants and animals), a healthy biotic community, water and air, pH (roughly between 6.5 and 7.0 for most veggies). Soil also needs to be the proper temperature for whatever species you are sprouting or growing. For example, bean seeds need warm soil to germinate. Otherwise they will rot in the soil.
Good soil is sustainable, via added organic material over time, and a healthy biotic community, and rotation practices.

San Diego soils, having derived from ocean and river sediments, often contain a lot of clay and sand. There are often many rocks, usually the rounded ones that have been shaped by water erosion. The topsoil of the native chaparral habitat is generally thin. These factors mean you will want to add lots of organic material, such as compost to the soil, and of course remove the rocks. You will probably need to add more every year before planting, and sometimes as a supplement during the season. Worm castings are also a good organic source of nutrients, as are organic fertilizers.
Inorganic fertilizers do not help build the soil, being just mineral salts. They give the plants a quick boost, but then they become dependent on them. Over time they can make the soil more depleted.

So you can see that for many reasons, helping your garden by fostering a healthy balanced soil is good for the plants, the your own, health, and the earth. Enjoy the creative process and watch the miracle of life unfold in your own garden.

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