Ever wonder what it takes to grow amazing plants: could it be luck, skills learned from your grandpa, a little bit of witchiness in the family tree? No matter what other factors you have (or not!) on your side, one thing that everyone can do to help their plants flourish is make great soil. That’s right: MAKE great soil.
Now dirt in all of of its mud-pie glory glory has a lot going on. In one cup of healthy soil you will find up to 1 billion bacteria, 20 million protozoa, 100,000 fungi, 100,000 nematodes and 50,000 other arthropods plus a healthy collection of earth worms. These tiny wonders are the collective super heros of the garden. Some of the amazing functions supported by a healthy soil eco-system include diverse mechanisms to ward of pathogens, disease and pollutants, varied strategies to ensure nutrients are released and available to plant roots, and also systems for holding onto water and facilitating aeration. Without all of this subterranean action plants in nature would not survive; but what about in cultivation? What can we as home gardeners do to improve soil?
Gardens big and small generally differ from their natural counterparts in that they are slices of the bigger picture. Since the 1950’s the focus of agriculture and horticulture has been on reducing the diversity found in gardens and especially soil, through the use of chemical approaches to management. Most potting soils have become completely neutral with the explicit goal of having no living components at all, and garden soils have suffered from years of pesticide and herbicide pressures as well as from urban development (soil compaction is a big issue). So the slice has gotten thinner. Now its time to reclaim living soil - there is so much evidence that a healthy, living soil allows plants to thrive with less interventions by humans... you just have to trust the dirt.
Plants require water, nutrients and proper soil structure (plus correct exposure and temperature) to perform at their best, and it is the synergy between these elements that counts. No one component on its own is enough to keep gardens happy. Healthy soil improves the ability of plants to find and make use of the necessary water and nutrients, and optimal soil literally anchors plants’ roots. So, let’s break soil know-how down into the following pillars, and take a deeper look at each:
Adequate water is, of course, essential to plants. Healthy soil has three components that relate to H2O: the ability to wick water up to roots; the ability to hold moisture, keeping roots hydrated; and the contrasting capacity to breathe (drain) so that roots do not rot. These last two qualities are very important and involve balancing opposing properties. Oxygen is essential for many root and soil activities, including strong root development. Well functioning roots (working in conjunction with soil microbes) absorb nutrients and water for plant growth. While moisture is necessary, soggy, poorly drained soil limits aeration and pushes oxygen out of the soil. So balance is achieved by having a soil with good water holding (slow release) properties but also lots of grit and other elements that create air spaces. Loamy, humus rich soils manage water very well.
It is the sun that *feeds plants while soil minerals supply critical catalysts for photosynthesis and other key plant functions. Major nutrients (nitrogen, phosphorus and potassium, aka the N-P-K displayed on fertilizer packs) along with a wide spectrum of trace minerals (micro nutrients) must be available to plants if they are to flourish. Healthy soil has nutrients built into the mix - but simply being present does not make minerals accessible; releasing nutrients from organic matter is one of the key functions of soil micro-organisms and the end goal is for garden soil to contain 50 - 80% humus (stabilized organic material that slowly but steadily releases nutrients while keeping minerals from leaching away). Living soil has nutrients and beneficial micro-organisms built right in. By contrast, many potting soils and conventional garden soils lack a full spectrum of nutrients or the nutrient / beneficial micro-organism combination that support strong plant growth.
*It is somewhat of a misnomer to call fertilizer “plant food”... the real food is the sun
Garden soil comes in many forms ranging from sandy to clay. Soils from each end of this spectrum have advantages, but the widest range of plants thrive in a balanced soil that possesses both the drainage qualities of sandy soils and the higher nutrient content of clay soils. Broadly speaking the goal of soil building is to create nutritious soil with good water holding capacity and high aeration. Wonderful soil should literally “feel” good to the touch, be slightly light and fluffy, not tend to be soggy nor should it dry out and crack.
Tip: Potting mixes, which are nearly always peat based, are high on aeration but low on nutrients (often nil on nutrients) and low for water holding capacity. Ever had a houseplant dry out, then find that water just runs through the soil no matter how much you water? This can be permanently corrected by blending your own mix and adding 1/3 compost and 1/3 coconut husk fibres. A short term solution is to let your dried-out pot sit in a basin of water and soak from the bottom up; this will slowly rehydrate the peat.
Soil structure is the variable that changes most over time, for both garden and potting situations. In the garden, tilling and compaction from foot traffic negatively impact structure. Strategies to retain structure include creating garden beds that are accessible without walking on or through them, and replacing tilling with top dressing with compost and mulches on an annual basis. Potting mixes, frequently peat based, break down with in one growing season and loose their fluffiness. As a result it is recommended to replace soil in seasonal pots annually, and top or side dress longer term potted plants with compost at least once per year.
The Nuts and Bolts
Soil is the engine that keeps plants in top form, and it is variables relating to moisture, nutrients and physical structure that differentiate healthy, living soil from the alternatives. With this knowledge in hand, it is time to talk about actually making soil. The process is going to be different for potting mixes versus garden soils (potting mixes need to be lighter than garden soils):
Container mixes are easy to make! The basic ingredients for container soils are something bulky, that provides the structure (often peat, but there are alternatives), compost (provides nutrients, micro-organisms, water holding capacity, and improved structure) and some type of grit (improves aeration).
The ratio of these ingredients can be played with depending on what is going to grow in the soil. For example, many herbs like a very well drained soil and require less nutrients compared to a tropical fruiting plant (such as bananas) that thrive on moisture and higher nutrients. To start, blend 1/3 basic organic peat potting mix with 1/3 quality compost and 1/3 gritty sand. This will yield a very well drained living soil that is ideal for herbs and succulent houseplants. Homemade compost is fine, but a commercial compost will be more consistent and free of pests or weed seeds. Coconut husk fibres can be substituted for the peat, but the mix will be heavier and hold about twice as much water (great for tropicals... less so for succulents and herbs). If you want to reduce your use of peat moss, doing half peat / half coconut fibre for the bulky 1/3 will yield a balanced result.
Some plants use up nutrients faster (called heavy feeders); for these varieties special types of compost such as worm castings can be added for excellent results. Speaking of worm castings, these are the ultimate form of compost, with an incredible array of beneficial micro-organisms, enzymes and major nutrients in a quickly available form. Generally worm castings are added at about 10% of the total mix, and are especially beneficial for seed starting mixes and for those container plants that are heavy feeders.
One of the best things about creating living potting soil with compost is that you can never really add too much. In fact, some types of plants such as potted citrus and bananas love straight compost soils. Just keep in mind that for most plants there is no direct benefit to growing in straight compost compared to 30 - 50% compost, so it is usually better to blend the compost down and make this resource go farther.
The more one experiments with growing different plants, and gets comfortable mixing home made soil recipes, the more fun you can have with adding micronutrients and other ingredients that add finesse to your growing. For example, vegetables benefit significantly from trace mineral amendments such as glacial rock dust. Other varieties respond to tweaking of soil pH; for example the exotic miracle fruit berry (the fruits tinker with the taste buds to make everything bitter or sour taste deliciously sweet). Miracle fruits like an acid soil, so adding pine needles or coffee grounds is a good idea. There are many other examples of situations where plants perform better with some special attention to the soil... and with a little practice creating your own mixes with simple organic ingredients becomes second nature.
Attending to garden soil is even easier. The basic rule of thumb is add lots of organic matter. This can be in the form of fully cured compost (home made or commercial), or readily available organic resources such as mulched grass clippings or mulched leaf litter. Whether you are starting with clay or sandy soil, the addition of ample composts will dramatically improve the situation. Organic material can be top dressed any time through the season, but the easiest time is in fall then the garden is less busy. The ideal method for improving garden soil is to simply add compost liberally on top of the existing beds; tilling negatively impacts structure and also opens things up for weeds. That said many gardeners find it hard to resist turning compost or other amendments into the top eight inches or so of the existing soil - the result feels immediate and tangible.
What if you do not have existing beds? In this case, soil can be made 100% from scratch using a combination of greens and browns (wet and dry organic material). The terms “lasagna gardening” was coined by garden writer Patricia Lanza to encapsulate this technique: layers of greens and browns are built up from the sod layer, and allowed to cure over winter on top of newspaper or cardboard. Lasagna gardening effectively yields amazing living soil free from weeds and without tilling or digging up the sod layer. Very cool! The catch is some time is required to allow the materials to fully compost. If planning a lasagna style soil project, it is best to allow a full summer season followed by one winter before planting.
Just as with potting mixes, a variety of specialized amendments can be added to garden soils. It is particularly useful to add trace minerals after a few years of growing in the same space, and also to follow through on preferences for pH when indicated (for example blueberries simply will not grow unless the soil is very acidic).
Keep Your Soil Alive
After putting in the effort to make amazing soil, it is important to keep it healthy! Choosing certified organic seeds, plants grown without harmful chemicals, avoiding non-organic fertilizers and committing to a pesticide-free approach to your green spaces are all easy choices that will promote soil vitality. Your thriving plants will inspire others to think about all of the amazing things that happen when you start with great dirt.
© 1996 - 2015 Dave Hanson, Sage Garden