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Aeration and drainage

by editor

Water can be the provider of both life and death in the landscape

How we manage to maintain the right quantity of water in the soil and how we manage to remove the wrong quantity or excess water from the site is the difference between success and failure in our quest to achieve the designed intent.

When we get it right the first time then nobody notices. When it’s done wrong then everybody notices.

A large part of my job is problem solving, the most common problem I’m engaged to solve is water problems. Too much of it to be precise.

Once again, when we get the soil right for the specified plants then the landscape asset performs. Sure, aspect and climate play roles but generally speaking and for the purpose of this article, let’s get the soil right for the plant’s roots.

Putting chemistry aside, most general lines plants have similar physical needs from the soil: air, water and structure. The latter dictates how easily roots have access to air and water.

Roots are just like us. We both need oxygen to make new cells, repair old ones, to take up nutrients and to absorb water. Like all gases, oxygen moves through the atmosphere by means of diffusion. High concentrations of a gas move towards low concentrations. Well drained or porous conditions allows oxygen and other gasses to diffuse freely through the soil. As oxygen depletes from plant and soil organism consumption, more needs to move in to maintain a state of balance or equilibrium. Keep in mind that both air and water like to occupy and move through the same pore spaces within the soil. If there’s too much water in these spaces then we are waterlogged. Conversely, when there’s too much air in these spaces, we can have drought conditions. How we maintain or create this important balance of moisture and air requires us to factor or adjust our behaviours towards the soil.

The most common causes of poor aeration is from waterlogging and soil compaction. The former may see plant roots suffocate from experiencing periods of inundation from poorly drained conditions where the latter will inhibit or completely cease rootgrowth. Soil capping and layering are other factors that can suffocate the soil profile. This can be from something obvious like installing an impervious layer over the soil such as a concrete slab or raising soil levels beneath trees, down to the dressing of incompatible soil textures or spreading a finely graded mulch or compost layer over a silty or clay soil.

Changes take place within minutes of roots being cut off from precious oxygen. Uptake of important nutrients such as calcium, phosphorous and potassium almost ceases, and the water movement into roots is reduced to near zero. The roots develop toxins such as alcohol and ethylene, while vital hormone production is retarded.

The movement of oxygen through waterlogged soil is very slow indeed, around 10,000 times slower than atmospheric movement in fact. After a few hours or days, the effects of no oxygen from waterlogging becomes easy to see. Wilting in the middle of the day, dying roots, calcium deficiencies and yellowing of leaves (chlorosis) are a few of the many reactions.

After a short period, the effects of waterlogging to the soil soon occur. Anaerobic or anoxic soil conditions produce organic toxins such as ethylene, methane and organic acids. The build-up of rotten egg gas (hydrogen sulphide) will affect the plants and microorganisms within the soil profile. Some vital elements such as iron and manganese can dissolve from minerals to reach toxic levels.

Detecting poor aeration and waterlogging
Obviously pooling and puddling of water is one clue. A soddened soil sample taken more than two days after rain or irrigation will also indicate waterlogging. Often the problem can be deeper than the soil surface. In these cases, dig some holes and use your nose to detect poor aeration from saturated soil. A musty, dank smell with the unsavoury aroma of rotten eggs will quickly tell you that oxygen is needed.

Maintaining oxygen for trees within the built environment
Eighty-five per cent or more of a tree’s roots are in the upper 300mm of soil where oxygen levels are highest.

Based on the frequency that we see soil levels being increased beneath trees, it would seem evident that many people are not aware of how shallow tree root systems are and how important gaseous exchange is to these slow growing creatures. Applying layers of fill over an established mature root system greatly hinders the ability of oxygen to diffuse into the soil. As the roots use up the available oxygen and when it is not replenished, the roots gradually suffocate and die. As these die, they cease absorbing the water the tree needs, thus the tree gradually declines or dies from thirst.

This can occur slowly over years, quickly over a few months, or a tree could languish in a slow state of decline indefinitely. Trees take a long time to grow, so often they take a long time to die. While trees vary in their tolerance of altered soil levels, the amount considered safe to apply is less than 100mm annually. When you are presented with the request or need to increase soil levels beneath a tree, to pave over or within its root zone, always ensure that you’re maintaining the opportunity for gaseous exchange within the root system. Let’s cover root protection methods in further detail later in the year.

As I mentioned earlier, getting excesswater off site is just as important as keeping the right amounts on site. Effective drainageis one of the most important factors behind a healthy landscape asset. Consideration needs to be given towards where this water comes from, where it accumulates and how to either take advantage of it or effectively remove it.

Naturally, during rain events, all hard surfaces catch and concentrate large amounts of water. Effective stormwater design generally sees this water collected and transferred within a drainage network. Although often, the garden or lawn beside hard surfaces is expected to absorb or disperse concentrated amounts of surface water. How the garden copes with this burden often depends on how we’ve prepared the soil within the area for rain events. A well-structured soil will greatly improve infiltration rates, which in turn will serve to maintain a higher degree of porosity and air within the root zone

Although most of us know the process and installation of ag lines, often is the case that they aren’t installed or backfilled correctly to effectively intercept surface and sub surface water. Ag or French drains are often finished with a final layer of soil rather than gravel or drainage sand continuing all the way to the surface. Keep in mind that water can only enter a coarse layer of soil from a finer textured soil if it is given a ‘push’. It won’t naturally percolate into the coarse layer. The push is usually the weight of extra water arriving from above or beside the coarse layer. Only when there is a small ‘head’ of water pressure above the porous layer will water flow into it. We’ll cover this situation in detail when we discuss layered or inverted soil horizons later in the year. In the meantime, the best thing I suggest to you is to remove the layer of soil from above your ag lines and replace it with more aggregate or coarse mulch. Your surface water will then easily percolate into the drainage system rather than waiting to be pushed into the drainage network.

Another thing to consider when building impermeable layers such as paths or kerbs within the landscape is to give water somewhere to escape to. Installing drainpipes at the lowest points of paving or kerbing will enable water to move more freely downhill. A small amount of fore thought will greatly improve the finished product and assist you in the quest to achieve the design intent.

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