From Dirt to Soil


'Soil' is a very precious material and therefore it is a rule at Green Art that we never call it 'Dirt'.

Just like pure water and clean air, soil is one of our most important resources. It is the stuff on which forests grow and fields yield the harvests that nourish us all. And yet, we are generally so unconscious about the irreplaceable value of soil.

Sadly, erosion of soils and destruction through urban development and infrastructure are tremendous threats to agriculture, forestry and nature in general in every country of the world. Furthermore, the soils that erode - with everything that is in them - pollute our freshwater bodies and oceans, threatening life even far from where the erosion took place. For all these reasons, taking care of our soils is a duty we owe ourselves and all future generations.


Soil is a very complex and variable matter. Soil scientists classify different kinds of soil and investigate their complicated physical, chemical and biological characteristics. For plants, soil provides water, nutrients, and a place for roots to anchor. The properties of a certain type of soil define, for example, how much water this soil can store, how much of the stored water can be used by plants, which nutrients will be available for plants, and in which amounts and at what times of the year.


In the following, we discuss three materials commonly applied on top of the soils in our gardens. We will describe what  compost, bark mulch and landscape fabric do for the gardener, and what they don't do (summary at end of page):

Compost - the matter of live!

One of the most important components of soil is organic matter. It can balance out a lot of weaknesses of certain types of soil, which is why compost is so precious for the gardener. Sandy soil,  dense clay or soils that are depleted and apparently life-less can all be dramatically improved with compost.


Composts tremendous effects: 

- compost improves aeration or the soil, allowing bad gases to get out and oxygen to get into the soil. This helps positive microorganisms and roots to live. This effect is especially helpful in clay, which has naturally a poor air flow. 

- compost improves water infiltration and retention. Compost  is spongy and holds water. Mixed into the soil, it keeps the soil surface porous so that water can easily penetrate the surface. Air pockets created by the compost can fill with water and retain the water quite well. This is important in dense clay, where there is little porous space. In fact, in clay soil compost improves water holding capacity and, at the some time, improves drainage of excess water. In sandy soil, where water drains out very quickly, the porous structure of compost particles helps to retain water like a sponge. 

- compost activates soil live by improving many physical properties of the soil, such as the above mentioned aeration and water retention, and by providing food for soil organisms (soil organisms need food to live, such as leaf litter!) Freshly applied compost can function as an inoculation of the soil with living micro-organisms, that are much needed for soil and plant health.  

- compost also provides plant nutrients to the soil, especially those very rich composts made from manure or fishery byproducts.


How to use compost: Leaf compost is the most common available compost and one of the best for general use. Compost should be worked into the top 4 to 8" layer of soil in generous amounts when a new planting bed is prepared. For established beds, mulching (= surface application) with compost or composted cow manure is efficient, and easiest to do in early spring or late fall, when fewer plants are in the way. Some gardeners use compost to fill the holes when digging up and transplanting perennials. 


Compost can help to suppress weeds, too. But it is not as efficient in this respect as bark mulch. Nevertheless, once a garden has been established for a few years and weeds are pretty much under control, then mulching should be done with compost instead of bark mulch, or with a mix of the two.

Bark mulch - less is more!

Bark mulch helps to protect the surface of the soil. It protects from the impact of rain, sun and wind, and thus helps to maintain a healthy soil structure up to the surface layers. Mulched soil (it can also be mulched with compost) has a much better water infiltration rate. Bark mulch is also very efficient in suppressing weeds, and in that respect better than compost.


But bark mulch can be a problem, too. Since it is very poor in nitrogen (C:N-ratio is very wide!), it can cause a nutrient imbalance in the soil. When microorganism decompose bark mulch, which they will do to nourish themselves, they will get plenty of carbon-based substances from the mulch, but not enough nitrogen. To make up for that they will take nitrogen from the soil, depriving plants of this important nutrient.


Bark mulch is big business in the landscaping industry. But excessive amounts of bark mulch are not what the customer would want, as can be observed very often on properties that have been under the pitiless rule of bark mulch-spreading landscape professionals: It is not uncommon to find bark mulch so thick that an average rainfall can not even reach the soil. It is easy to see that in such cases, oxygen does not sufficiently penetrate the soil, and of course plants suffer heavily from nitrogen deficiency.


Therefore, we recommend to use bark mulch sparingly. An application (1 - 2 inch thick) after a new bed has been established is very useful. After a few years, when the first flush of weeds has been suppressed and not so many seeds are left to germinate, it is better to mulch with compost. Mulch can only be part of the battle to suppress weeds - shading weeds with dense groundcovers can be very efficient, and some hand weeding will also be necessary.


Landscape fabric - mostly a waste!

A good soil is alive with organisms in a rich variety. Bacteria, algae and fungi, nematodes and earthworms, tiny arthropods like springtails,   and ants, insect larvae, microscopic mites, and the like all play an important role in the cycle of organic matter and nutrients in the soil.


These organisms are absolutely essential to a healthy soil. They decompose plant residues and transform them into nutrients available for plants. Soil organisms improve aeration and water infiltration, they bring nutrients up from deeper parts of the soil and other nutrients down into the sub-soil from the surface. They make nutrients available by transforming them into substances that plants can take up with their root system. Soil organisms also store large amounts of nutrients by digesting them and incorporating them into their own biomass - and as they die off and decompose these nutrients become available again to plants. 


What does this have to do with landscape fabric? When taking a look at a garden with landscape fabric (and perhaps a thick layer of bark mulch on top of it) - one will usually find unhealthy plants, often stunted in their growth, stressed and diseased. When removing the fabric, one will not find those precious worms that do such a fine job in mixing and enriching the soil. Neither are there any other larger soil organisms, because the fabric literally stops them in their tracks. Animals that need to come up for example during a heavy rain will drown under the fabric. Nothing can come up, nothing can get down. Just as the fabric stops plants from rooting into the ground, it also stops soil organisms from moving up or down - with other words, the fabric destroys their habitat.


Landscape fabric does work quite well to supress weeds, but it should only be used in limited areas and be removed after one or two seasons, so that soil life can recover. A fresh dose of compost would very much help at this time. If the fabric is left in place for many years, the lack of activity by soil organisms will degrade soil fertility, and plants will slowly become unhealthy and decline. The only places where landscape fabric makes sense permanently are under a walkway and along a foundation under a layer of gravel.



To obtain a healthy soil, and be successful in gardening, I recommend to:

- remove excessive amounts of bark mulch and make sure to remove old, unnecessary landscape fabric as well.

- use as much compost as possible to improve soil quality, and add to it regularly in order to maintain a healthy, rich soil.

- to suppress weeds, cover garden beds with bark mulch (or newspaper and bark mulch) after planting and hand-weed in the first seasons, so that weeds can not get established.

- use dense, vigorous groundcovers to suppress weeds where this is an option (not very efficient are myrtle (Vinca) and the very low and thin varieties of creeping junipers)

- avoid mounding up mulch against plants, especially avoid grasses, who are generally sensitive to mulch cover, and tree bark, which can rot under mulch cover.

- use landscape fabric sparingly, such as in problem locations, but not as a general bed cover under mulch. Remove the fabric after two or three seasons so that the soil can recover.




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