The first and fundamental principle of implementing the Watershed Approach to landscaping requires that we grow a Living Soil Sponge in our landscapes. Building a Soil Sponge is critical to improving the water holding capacity of the soil, but it also is critical for increasing the porosity and permeability of the soil. Think about the sponge sitting on your kitchen counter. When it is dry and you add water, the sponge soaks it up quickly. When you squeeze the water out of the sponge, it is still damp. And, when you oversaturate the sponge, it begins to shed the excess water. If all of our soils could exhibit these three spongey qualities, our landscapes would look and perform completely differently from the way they do now. Building a Soil Sponge requires that we begin to think of soil in a different way; we shift our viewpoint from a focus on soil type (sand, silt, clay) to a focus on soil structure.
When we talk about soil texture, we are essentially asking the question: Do you have a “brick” in your landscape or a “sponge”? And while soil particle type (sand, silt, clay) is immutable, soil structure can be created or destroyed by human intervention. So, this shift to emphasizing soil structure allows us to develop and implement strategies for intervening in a brick-like landscape and turning it into a more spongey soil, regardless of soil type.
Protect Your O.W.L.
One way to further the dialogue about Living Soil is to refer to it as OWL (Oxygen + Water + Life). OWL is a convenient shorthand for facilitating a discussion about growing healthy, living soil and managing soil that already is acting like a sponge. OWL allows us to bring biology into the soil science discussion that traditionally has been dominated by physics and chemistry.
A teaspoon of good garden soil contains billions of invisible bacteria, several yards of equally invisible fungal hyphae, several thousand protozoa, a few dozen beneficial nematodes and other occasional charismatic micro and macrofauna (think worms, pill bugs, etc.). There are more individuals residing in a teaspoon of soil than there are humans occupying the space on top of the earth. Microbes bind soil together and, when OWL is balanced, these billions of microbes, working in concert with the roots of plants, are the change agents that transform brick-like dirt into a healthy, living soil sponge.
Oxygenis needed by healthy plant roots and soil organisms. Healthy soil has lots of tiny, little pockets of air (hence the “sponge-like” appearance). When soils are eroded, graded, or disturbed, their structure becomes compacted. Compaction is caused when the tiny air and water bubbles are squeezed out of the soil and the microbes are killed or demobilized. Microbes can be killed by fertilizer and pesticide use or even heavy traffic (foot or vehicular). We’ll talk about this more later.
Water is needed by both plants and microbes. But too much water in the soil will displace the oxygen, saturating the soil and creating an anaerobic condition. Pathogenic microbes prefer anaerobic soil, and if this condition persists, diseases may develop, thus endangering the health of the landscape.
Water is constantly moving through the soil. Any water in the soil needs to be replenished as the plants and microbes use it, as it evaporates from the soil surface, and as gravity and capillary action pull it down past the root zone or away from a planted area. Irrigation is often defined as managing the balance of Oxygen and Water in the soil for the benefit of the plants. So, from a human intervention perspective, proper irrigation management is critical to maintaining healthy OWL.
Life in the soil includes all of the bacteria, protozoa, nematodes, and fungi, the food they eat, the excretions they make, and the root systems they sustain. Adding good quality organic matter (in the form of compost) into the soil is the fastest method of incorporating living microbes and getting the biological community activity kickstarted.
Plants Are Microbe Farmers
For more than 400 million years, plants have been playing the powerful role of microbe farmers. In order to obtain the services they need from the microbial community, plants attract microbes to their roots by feeding them complex strings of carbon. Plants take the atmospheric carbon they have obtained through photosynthesis, and convert it into sugars, carbohydrates and fats; the plants are able to exude these carbon packet goodies into the soil to attract and feed the microbial community. Plants can be very specific in the compounds they produce for the community, and they know what they need in order to grow strong and stay healthy; the compounds are called exudates (because the plant exudes them.) The plants can target members of the microbial community in order to get what they need at the exact time when they need it.
These billions of microbes concentrating around the root zone of the plants all need water and oxygen, so there is probably a lot of pushing and shoving as they find their little nook close enough to the plant roots in order to receive the exudates. All that activity starts to create the microscopic air pockets in the soil. Bacteria and fungi hold the soil together with microscopic superglues and binders created from incredibly complex strands of carbon and other elements. The microbes consume organic matter from the plants and are then consumed themselves by larger creatures (worms, ants, slugs, centipedes, insect larvae, etc.) In turn, these creatures are consumed by creatures further up the food chain. Carbon and other nutrients are cycled through these many life forms, creating healthy, living, well-structured soil, no matter what the soil type.
Converting From Brick to Sponge: Eliminate Compaction by Loosening Soil
So how do we grow a healthy Soil Sponge? First, we try to avoid excessive disturbance of our soil. But, if it happens, we make sure to add Oxygen, Water and Life in the form of really good compost or worm castings as soon as possible to get the soil flora and fauna working again. Good organisms do the heavy lifting to turn a dirt brick into a great living soil Sponge.
Compaction occurs when the structure of the soil has been destroyed, and the air pockets are compressed. Compaction is the worst enemy of a landscape. With a compacted site, your plant material will suffer, irrigation will run off, rainwater will not infiltrate. Plants that are typically considered “weeds” will be attracted to compacted soil conditions; many of these pioneer species of grasses and forbes have strong tap roots designed specifically to survive in low nutrient-cycling conditions (i.e. not very diverse or extensive microbial community) and to break up compacted soil to build hospitable conditions for roots, microbes, and microfauna. You need to eliminate compaction stat!
Water the site first. None of these de-compaction techniques will be effective if you are working with bone dry soil. If you can press a pitchfork into the soil, then that is all you need to do to create air holes.
If the soil is heavy clay or very compacted, then augering or tilling may be necessary. Tilling is probably the worst thing you can do to your soil structure, as it destroys virtually all of the connections made by the microbial community. But, if you are working with a very compacted site, or a site that has been sitting under turfgrass for a decade or more, then tilling may be your only option for opening up the compacted area and getting oxygen into the soil. Augering is a more targeted approach; you may find it useful if you need to de-compact around trees or other previously planted areas, but you don’t want to disturb the existing plants. Immediately after augering or tilling heavily compacted areas, spread out or fill the holes with good compost or earthworm castings. Then water the whole thing thoroughly to get the biology processes revved up. Remember that augering and tilling damage any web already existing in the soil, so they should be employed only when absolutely necessary. If you have a lawn or unified groundcover area, aerating twice a year will help eliminate compaction.
After de-compacting your landscape, two essential practices for maintaining soil oxygen are: 1) Feed the soil good organic matter; 2) Manage water so things don’t get too saturated or too dry.
Yes, You Have to Feed Your Soil
Organic matter improves the water holding capacity of soil. You can get good organic matter from a wide variety of sources, including compost and living mulch. Once you get things started, plants manufacture their own organic matter to continue building the soil. Leaf litter and grass clippings should be allowed to remain on the soil surface, under the plants from which they fall, instead of being removed from the landscape during maintenance. When plants drop leaves, fruit, flowers, or bark, they are bringing food to the microbes they are farming. We need to respect the work the plants do and remember that nature never lets anything go to waste.
Mulch, compost and compost tea can be applied to the surface of the soil and used as amendments during planting and soil preparation. The most important thing to know about compost is that it should never smell bad (like feces, sulfur, or vomit), and that you should not be able to identify the components of the compost – it should look like “70% chocolate”-colored soil, so yummy you’d like to eat it. Worm castings are a good substitute for compost, especially if you are working in small quantities. The most important thing to know about mulch is that it should be very well chopped up and provide a mixture of green and brown plant matter; the best mulch for an establishing garden (up to 3 years old) is chipped to an average size of 2” or less (less is more in this case). We’ll talk more about mulch and compost in future posts.
Fertilizers Make Plant Farmers Lazy
Ornamental plants, particularly place-adapted native plants, do not need to be fed with fertilizers (even organic ones) if you maintain OWL. Nutrients in fertilizers easily are taken up by the plant and used to build their bodies and support reproduction. The challenge is that once the plant becomes accustomed to accessing fertilizer-induced nutrient, it no longer has an incentive to continue farming microbes who manufacture nutrients. When microbes are no longer being farmed, the soil structure begins to decline. The plants may look great and perform great (bear flowers and fruit) above ground, but their long term resilience is compromised by the diminishment of the soil microbial community that supports their roots. So, skip the fertilizers and concentrate on getting OWL going; you want your plants to be strong, resilient farmers, not weak, lazy, couch potatoes.
Rainwater and Soil Moisture Management Are Necessary For Healthy Soil
Rainwater should be directed into landscapes. There are no chloramines or other microbe-killing additives in rainwater; and it is usually slightly acidic, making it the perfect chemistry for both plants and microbes.
Watering wisely requires a careful management of Plant Available Water (PAW). This is the water available to plants at any snapshot in time; it is dependent upon the Available Water Holding Capacity of the Soil Type, and the Root Depth and Evapotranspiration Rate of the plants. PAW is a fundamental irrigation management concept, as it is the PAW we manage in irrigation controller scheduling. We also focus on PAW when we are explaining how plants provided with rainwater can rebalance the “small” water cycle and produce reliable rainfall. (Now you’re getting a preview of where these posts are going over the next few months.)
Studies have shown that PAW approaches 30% as soil organic matter approaches 5%. As soil organic carbon increases by a factor of four, the PAW increases by about 2.5 times (Hudson, B.D. 1994, Soil organic matter and available water capacity, Journal of Soil and Water Conservation, 49, 189-194). In simpler terms, every 1% increase in soil organic matter helps the soil hold on to about 20,000 gallons more water per acre.
Organic matter added to soil boosts the microbial community, improves the structure of the soil (making it spongier) and makes it easier for roots to extend the root zone. The result is that the soil is able to hold more PAW. This has huge implications for landscapes that are responding to climate change, particularly in areas where seasonal drought is a factor.
For many soil scientists, engineers and landscape professionals this shift to managing soil structure with OWL is not intuitive. Take irrigation management as an example; a “smart” irrigation controller is ready to receive information about soil type and calculate the Total Irrigation Run Time accordingly, but have you ever seen “spongey” or “compacted” as data queries? Yet, it is the sponginess or the amount of compaction of the landscape that really determines both the PAW and the infiltration rate of the water applied to a landscape.
Stormwater engineers design retention systems based on soil type charts; this leads to over-excavation of sites (oh, the soil disturbance!), design specifications calling for replacement of native soils with “sandy structured man-made soils,” and the prohibition of capturing rainwater in soil on sites with clayey situations or “slow infiltration.” Yet, extremely shallow depressions heavily planted in native healthy Living Soil Sponges could infiltrate huge quantities of rainwater into the plant root zone, even in the heaviest of clay soils.
The impetus falls on landscape professionals to educate our professional colleagues and our clients about the importance of healthy Living soil. Let’s start by spreading our knowledge of OWL, and bringing the Watershed approach into all of our future landscape projects.
