Plant philosophy
Water roots vs. soil roots
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Water roots vs. soil roots

Is there a difference?

Water and soil roots

Let’s talk roots! Why are all water-propped plants not able to survive their first weeks in the soil? I will try to answer this and other questions about the root systems of houseplants.

The first thing that we have to acknowledge is that both water and soil roots are a consequence of adaptation of the plant to their environment. Not all species all flexible, and not all can produce different kinds of roots in different kinds of mediums. In this article I want to focus on the two most popular ways of growing house plants. They also happen to rely on different root systems.

We can see the differences between these roots with the naked eye. Water roots are white, have more hair, and are more fragile than the soil roots. But this is where our superficial observation ends. Let’s focus on the functional differences of those roots and their consequences.

Do plants breathe?

the oxygen demand

It’s common knowledge that plants take in carbon dioxide and release oxygen into the atmosphere during the process of photosynthesis. It may be a surprise that plants also need oxygen to survive.

How does it work? Plants take in carbon dioxide from the air and combine it with water, which is absorbed through their roots. They use energy from sunlight to turn these two ingredients into carbohydrates (sugars) and oxygen (a by-product). They release extra oxygen into the air. This is photosynthesis.

But can plants live without oxygen? No. Every cell of every living organism (except maybe anaerobic bacteria) need oxygen to live. Plant cells are respiring constantly and for that, they need oxygen. The oxygen content has a visible effect on the development of the plant. It applies both to foliage growth as well as the development of the roots themselves.

Now you might wonder if maybe plants can live on the oxygen that they produce themselves during photosynthesis. This is the case only in the times where they are photosynthesising faster than they are respiring.

As it is crucial that there is enough oxygen for the respiration, let’s consider how does it get into the cells.

Functional differences between roots

About the oxygen uptake

Water, typically contains up to around 12 milligram per liter. The air contains 21% oxygen. The oxygen uptake occurs via the roots, so we have to be sure that enough of it gets there. Now the way in which roots can absorb oxygen may differ. That is how we arrive at the morphological differences of the water and soil roots as they get the oxygen in different ways. In both cases, a good oxygen level is crucial for the health of the roots and resistance from pathogen fungi.

In case of a shortage of oxygen in the root environment fewer energy-rich chemical bonds will be formed. This leads to all kinds of negative consequences for the plant. Firstly, root growth diminishes strongly or even comes to a full stop. Secondly, the uptake of nutrients is compromised, which may lead to deficiency symptoms.

In practice, the worst outcome in this case is that higher nitrite values occur in the root environment. This is because in case of a lack of oxygen (O), nitrate (NO3) is converted to nitrite (NO2), which is toxic in high concentration. That applies to both plants and animals (including humans ;)). 

Soil Roots

characteristic and function

Let’s explore how the oxygen can get into the cells when we grow our plants in the soil mix. As to the anatomical structure, the soil roots are thicker, bacause they have to overcome greater resistance and reach further to access enough nutrients and water. The oxygen has to be present in between the soil mix ingredients as the plant absorbs it from the air, not from the water. That is why we opt for “airy” mixes.

Access to oxygen is important for root development and metabolic functions, therefore for the health of the plant as a whole. In the soil or substrate, the proportions of water and air are largely determining the oxygen quantity. Unfortunately, before we see on the upper part of the plant, that the plant has been overwatered (which usually means that the roots has no access to the oxygen from the air), the root system is already destroyed.

We can do a couple of things to prevent this to happen and promote the root development. Firstly of all – the manual method – a chop stick ;). Loosen the soil by gently poking the soil with a stick. Carefully, don’t disturb the roots. This will allow the air to penetrate inside the pot. Secondly – a golden rule of every indoor-jungle keeper. Secondly, it is better to water less than more – a golden rule of every indoor-jungle keeper. And finally, you can use beneficial fungi and aerobic bacteria, that work in the soil, to generate an optimal environment for the root growth (I currently use Piranha and Voodoo Juice from Advanced Nutrients)

Water Roots

characteristic and function

Water roots require less energy to develop. Due to easier access to water (duh) and nutrients, they are thinner and relatively fragile.

As we normally don’t aerate the water residue in the hydroponic pots, organic pollution or biofilm emerging over time will reduce the oxygen content. When the water is stagnant, different types of bacteria and fungi will consume the oxygen. Over time the oxygen level will drop. In that case, roots will not have the opportunity to optimally take up oxygen or the available nutrients. In the worst case scenario the oxygen level can diminish to critical values (<4 mg/l) within an hour. For that reason, regular cleaning and changing the solution is an important measure to safeguard against oxygen shortage.

In addition, if we keep our plants in hydro on a heating mat, this process can speed up. High mat temperature and a too wet mat are ideal growth conditions for bacteria and fungi (Pythium in particular), making them reproduce easily. In addition, the higher the temperature, the worse the ability of oxygen to dissolve in water.

Why do my cuttings die when transferred to soil from water?

This brings us back to the question of the water-rooted cutting survival after potting in into soil mix. Another difference between water and soil roots is in their energy requirement. Water roots require less energy and time to grow compared to soil roots. So indeed it is easier to root in water and it is a common practice. We root the plants in the water environment and transfer them into soil after.

A water root extracts oxygen that naturally dissolves in water through a high surface porosity (more gas can go in and out). Soil roots are not able to do that. That explains why they tend to suffocate (and then rot) when they are waterlogged.

So why do we root the plants in water? To strengthen them enough to enable them to form the soil roots after the transfer.

Aroid roots

Dimorphism

As to Aroid vines, the subject is a bit more complicated. In nature, the vines begin life growing over the forest floor. Once a vine locates a suitable tree, it grows upward and into the canopy producing both anchor and feeder roots . This change in habitat exerts stress on the aerial part of the plant. Both types of roots maintain contact with the tree. These changes improve the roots light and water uptake in canopy conditions. The principal modification in the aerial roots is a loss of exterior tissue (epidermis, exodermis and outer cortex). It is replaced by a layer of lignified cork.

These green aerial roots have an increased water retention capacity and at least a weak ability to photosynthesise. This actually makes Aroids easier to transfer from one medium to another and to grow them in hydro. The area of the root in contact with the host retains both exodermis and lignified cork, as well as epidermal hairs. It is specialised for water absorption. Meanwhile, the outer surface is more specialised for water retention, which creates a functional dimorphism.

Nonetheless, the transition from soil to canopy in natural conditions imposes abiotic changes and potentially stressful (e.g., drought, photo-damage) situations. Same, when we transfer a cutting from water to soil or from water to nutritional solution, it will cause stress to the plant. I think that we should carry this out in stages. This means for example using a slightly weaker dose of fertiliser at the beginning and changing the hydro solution more often.

What do you think? Tell me in the comments and on instagram! @leafy_jungle

Sources:

  1. U. Videmsek, B. Turk, D. Vodnik. Aerenchyma – formation and function. Acta agriculturae Slovenica, 87 – 2, September 2006.
  2. What is the importance of the correct oxygen content in the roots of a plant? from Royal Birkman
  3. A.L. Filartiga, R.C. Vieira, A. Montovan. Size-correlated morpho-physiology of the aroid vine Rhodospatha oblongata along a vertical gradient in a Brazilian rain forest. Plant biology, April 2013.

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