Soil

Greenery and technology

Trees in the city are often in and near pavements. Their access to water, food and oxygen can be limited. Supporting trees to thrive while protecting the pavement and road structure requires advanced solutions. Tree roots need space and a good growing medium to provide water and nutrients.

The right choice of tree species and varieties, and provision of a suitable tree pit reduce the potential for damage to pavement and road structures caused by root pressure.

Tree pits

A tree pit is a type of underground structure. Such a structure offers the tree roots a relatively large amount of space in the pavement and protects against the pressure and compaction of the soil by traffic. There is no damage to paving by root pressure. At the base there may be a water buffer that helps the trees to bridge longer dry periods.

A well designed tree pit may not require a major change to the network of pipes and cables in the street. These often can be incorporated within the structure. Advances in the materials used support recycling if changes are made after many years.

Solutions to improve growing sites depend, among other things, on the desired age of the tree, the load-bearing capacity, and specific properties of the existing pavement.

Preservation of growing sites

Trees provide numerous ecosystem services and therefore represent a significant value. With good planning, design, and construction, a tree can perform well in its location to maintain its predicted lifespan and functions. Information about guidelines can be found in the Tree Effect Analysis of the Tree Foundation and at CROW.

Growing site improvements

Trees in their natural habitat grow with a surface layer of leaf litter. This layer protects the roots, provides soil life, and retains moisture. The soil life present ensures that the nutrients from the dead plant material return to the ecosystem.

In the city, trees are often surrounded by paved surfaces and leaves are removed, preventing formation of the leaf litter layer and disrupting the natural supply of water and nutrients. In non-paved areas, a mulch layer of decomposed plant material can be applied around the tree. This also reduces evaporation and soil compaction and is highly recommended for  green areas and parks.

In addition to organic matter, soil life, minerals, oxygen and a balanced water balance are needed in a good growing site

Soil life

In any living public space, the soil life, together with the plant roots, support the soil ecosystem. Soil life includes bacteria, soil fungi, insects, algae, and earthworms. All these organisms together contribute to a good soil structure, water retention, aeration (oxygen content), breakdown of dead material and provision of nutrients.

Soil life supports healthy root growth of plants and protects against damage from rainwater and drought. The soil can be improved by providing the right mycorrhizae and soil bacteria at the time of planting. This minimises the loss of trees after planting.

Research shows that nature restoration can be promoted with soil amelioration. From healthy nature reserves, a small layer of the soil may be transferred to the new site. This supports healthy tree growth through provision of soil fauna and flora.

Worms

Worms do incredible things for soil. By digging, ‘ploughing’ and decomposition, they contribute to fertile soil. There are different types of earthworms that live at different depths the soil. The so-called commuter worm makes vertical passages in the ground up to three meters deep. This allows twice as much water to infiltrate into the soil. No other soil life can achieve this. Artificial processing does not have the same result either.

Natural fertilisers and soil conditioners

General advice for soil improvement is: start from the soil that is available and use compost from the local environment. This can be your own compost or leaf compost from other locations. Consider enrichment with other soil improvers, gardener’s bokashi or natural fertilisers.

Soil biodiversity in cities is often low due to many factors:

1. Application of artificial fertilisers
2. Destruction of soil structure (tillage)
3. Low organic matter and humic acids

Artificial fertilisers can make the use of chemical pesticides necessary by disturbing natural soil life.

There are alternatives to fertiliser. Fast-growing ground cover plants can be used as green manure. Legumes such as clover, lupin and legumes are able to fix nitrogen and enrich the soil for other plants.

Alternatives to chemical fertilisers are natural organic fertilisers. There are many kinds of ready-to-use natural soil improvers made from plant residues, that respect and stimulate soil life. In combination with earthworms, these can provide sufficient nutrition and a good soil structure.

Less application of artificial fertilisers increases the soil’s capacity to store CO2. In addition, this leads to reduced CO2 emissions in the production of fertiliser.

Prevent soil erosion

Planting prevents erosion of a bare soil. Ground covers and the right perennials are low-maintenance and keep out unwanted weeds.

By covering a soil with plants, it dries out much less quickly and soil erosion is prevented. This is even more important on slopes.

Dikes and embankments

Monotonous shallow-rooted grass is often used on dikes. However, flood protection and biodiversity can go hand in hand by making the top layer of the dike from lighter soil. Sandy slopes are suitable for deeper-rooted flowery mixtures. These dry faster and are therefore warmer and attractive for insects. This is in contrast to heavy clay, which cracks easily in drought and recovers slowly.

Soil remediation with (temporary) nature

Plants can be used in combination with suitable soil life for soil remediation. This process is called bioremediation or phytoremediation. If the top layer of the soil is contaminated, plants can work with bacteria and fungi to break down or absorb the contamination from the soil. For example, next to driveways (oil spills) or on old industrial sites (brownfields). The soil does not have to be excavated or transported. In addition, the soil life is not disturbed.

Various plant species with high root activity are used for soil remediation, each with a specific remediation property. The breakdown of organic compounds (PAH), such as (diesel) oil, is possible with, for example, reeds, poplars or willow.

The degradation products that are released during this process are harmless.

Contamination by heavy metals (copper, lead, zinc, cadmium, chromium and nickel) can be absorbed by, for example, the brassica family (rapeseed, Indian mustard) and alfalfa.

Some grasses can be used for both contaminants. The plants are eventually uprooted and converted into biogas and ash (via biogasification). Plants that are used for soil purification are obviously not suitable for consumption.