Purpose of the soil
Why do plants need soil? Of course, not all plants need soil, epiphytes such as orchids don’t, but 99% of terrestrial plants do.
The soil provides the plant with a number of things such as anchorage, a “reservoir” to store and release water, a surface area to provide points of attachment for nutrients, a large surface area, a medium to facilitate oxygen exchange as well as a home for microbes which are necessary to break up waste products and convert nutrients to usable forms for the plant to use (I’m sure I may have left a few out).
You are well aware of what you see above ground, but in the soil of a normal pot you will find literally hundreds of meters (in some cases) of the root system, billions of microbes, a “soil solution” which is the water contained within the pot filled with dissolved nutrients - the blood of the soil for want of a better description, there are square meters of soil (if you spread out the individual particles thinly over a surface). This is a dynamic system but relies heavily on water and moisture content in order to drive chemical reactions. Without water, it's dry and seemingly devoid of life.
Organic and Inorganic components
Natural soils (generally speaking) contain two main things, an inorganic component and an organic one.
The organic component consists of dead and decaying plant and animal matter – imagine a forest floor covered with leaf litter, through the process of decay, fungi, and microbes break down these larger components into ever smaller and smaller particles until they finally decay into the simplest molecules that the environment and soil chemistry will allow. These are then able to be absorbed into the plant through osmosis, which if you remember from school biology is the process of moving something from a high concentration to one of a low concentration across a (semi-permeable) membrane (in our case, the root membrane) and into the plant. Various organic components decay over differing time periods according to their initial size and composition and what can feed off of them. This is why organic fertilizers take longer to be released into the soil and affect plant growth, they first have to be decomposed into molecules (by bacteria and fungi) and in the right chemical form first, in order to be absorbed by the plant.
This process has to take place in a water medium, hence the terminology “soil solution” imagine a chemical soup, with all the bits and pieces suspended in this medium, sometimes interacting with other bits depending on their chemical make-up and then finally being drawn into the root system.
The inorganic component is simply that, any form of sand, silt, clay, stones, or pebbles that are not organic by nature and have little impact on the soil chemistry as they are fairly inert. Of course, there are exceptions such as limestone for example, which dissolves in water affecting the soil chemistry. While these may be chemically inert themselves they do provide points of attachment for nutrients/ elements in the soil which bind or bond to them depending on the structure of the actual component. I won’t go into specifics here as this is a whole topic in itself. What you need to take away from this is, they are in general, chemically inert, they are heavier (denser) than organics and they have structure and don’t compact like organics. Imagine a bag of pebbles, magnify it a number of times and you will see the individual stones have spaces between them, they are not solid blocks, in soils, these are called “pore spaces” and are filled by the soil solution and small pockets of air. The larger the grains the larger the pore spaces and the more water and oxygen the soil can hold, this is influenced by the actual material as well as the physical shape of these particles (think round pebbles versus crushed gravel and how the two interact with each other).
This takes us on to...
The nature of roots
If you were to upend your favorite pot plant you would see a network of roots extending through the soil, in a branched fashion and getting progressively smaller and smaller until they reached a microscopic level. These microscopic “root hairs” are actually responsible for the uptake of water and nutrients. Due to this extremely small size, they cover an enormous surface area within the soil, far more than you can see with the naked eye. I smile a lot when I see people simply pull a plant from the ground with a few thicker visible roots attached and then try and replant it, without realizing that they have actually left about 98% of the root system behind (transplanting is another discussion). This very large surface area that the roots have assisted with the anchorage of the plants, in other words, their ability to stay upright in the soil without being blown flat.
Oxygen in the soil – a slight detour
This is an extremely important factor, and largely responsible for the death of millions of houseplants (and outdoor ones) every year. In fact, the bulk of the plant's oxygen requirements is taken up through its roots and not through its leaves. As with humans, plants require oxygen for respiration and metabolic processes, this is not to be confused with photosynthesis which is the conversion of CO2 into water vapor and oxygen – photosynthesis takes place in the leaves, so oxygen levels are generally high there, the difficulty is the translocation of oxygen to the roots – this is why aeration of the oil is so important.
The way the system is supposed to work is that water contains some dissolved oxygen it, when you water your plant this moves into the soil (and soil solution), but at the same time the process of physically watering also actually draws oxygen into the soil, as the water drains down through the soil by gravity it draws fresh air in behind it from the surface, like a suction, (imagine pouring water on a sponge till it's saturated and gravity starts to drain the water sucking in the air behind it), this air and oxygen fills these small pore spaces and is available to the root hairs to absorb.
If the container drainage is blocked, or the soil is waterlogged this process cannot occur, the water in the soil starts to stagnate and the root hairs start to die off as they don’t have enough oxygen, this is physical damage and they are unable to absorb water or oxygen and the plant starts to wilt. To complicate matters, once the root membrane is damaged, osmosis can no longer occur and in fact, the opposite happens, water and minerals from the plants start to leak back into the soil solution causing first the yellowing of the basal leaves – and then gets progressively worse if the situation continues.
Does this sound familiar? This is what happens with overwatering (or lack of drainage).
Even if you are able to drain the water the root hairs don’t simply bounce back overnight, they have been physically damaged and new ones need to regrow and the plant re-balance itself, which means the plant will die back until an equilibrium point is reached between what water the damaged roots can supply, versus what the demands of the plant are – this is referred to as the root-shoot ratio. If the damage is too severe (i.e. the shoot ratio is far greater than the root ratio) the entire plant suffers and dies.
Soil pH
Soil pH follows directly on the issue of over-watering, as it can result in similar symptoms, but is caused by different factors. The pH is the measure of acidity or alkalinity of a substance, this includes the soil, which if you want to imagine with all the roots and microbes and fungi is a “living organism”. Like our bodies, this organism has a preferred pH range in which it will thrive and function, organic components, and chemical reactions alike. Small changes in pH can be absorbed by the soil solution and also the soil medium (known as buffering), however, if the change is too extreme, buffering cannot absorb these changes and the system is damaged. Organic components like roots, microbes, and fungi are killed. This can occur by spilling acidic or alkaline substances onto the soil, or adding too much chemical fertilizer – resulting in “fertilizer burn”. Again like over-watering the symptoms are similar as the root system is physically damaged, but the symptoms appear much faster; symptoms of over-watering may take a week or two to become fully apparent, fertilizer burn will show within a few hours, and in a summer garden within 20 or 30 minutes. The plant will wilt as it can no longer absorb water through damaged roots and eventually die of drought.
Native soils of the UAE
Speaking very generally there are two main native soil types that affect gardeners in the UAE, these are coastal calcium-based soils and red dune sand.
Red dune sand is affectionately referred to as “sweet soil” although there is nothing really sweet about it, it simply has less naturally occurring salts than the coastal soils, and is more conducive to plant growth and landscaping; the coastal soils by comparison are the remnants of the old seashore and cretaceous period calcium deposits, so are high in a variety of salts. High concentrations of salts are bad for plants as they prevent the osmotic system from working, preventing the uptake of nutrients and water for the plant. Therefore, close to the coast, the only native plants that thrive are halophytic in nature, which enables them to metabolize salt. Many years ago they figured out that by importing red dune sand from inland areas, they could cover or replace the coastal soils and this would allow a broader range of plant materials to be grown.
While red dune sand has less salt it also comes with its own set of problems. As you will see when you pick up a handful of this sand, it is extremely fine and the particles are naturally rounded. So, when this is wet it is extremely dense and has little pore spaces between it to trap oxygen and quickly becomes waterlogged, it’s also chemically inert and doesn’t provide much in the way of attachment points for chemical elements, so these can quickly be leached from the soil by overwatering.
What is potting soil?
Potting soil is a generic term that is used to describe a blend of organics and inorganics that have been formulated to provide the best possible soil environment for a broad range of plants. Obviously, they are not all the same, different brands, blends, formulations, etc. like washing powders. Some are “chemically charged”, this is to say they have chemical fertilizers added to them to improve growth, while others do not. There is a range of blends for different types of plants, for example, indoor plants and succulents. The objective of the potting soil is to mimic the natural soil environment of the plants, as many indoor plants originate in tropical areas, the standard medium would be compost-rich and have a slightly acidic pH (due to the organic component); potting soils normally have a pH between 5.5 and 7.5.
So building your own potting soil is much like baking a cake, there are general components, combinations, and ratios that work well (for different plants) and different methods in order to achieve satisfactory results and a host of recipes to choose from.
Let’s take an example, say someone gifts you some kind of succulent plant and you would like to re-pot it and perhaps upgrade the soil medium. The first step is to research and identify the plant, where it comes from, what are its native climatic conditions, and what type of soil is it at home in. This will provide you with the care/ maintenance information you require as well as information about the light and exposure of the plant as well as what sort of soil medium you need to prepare.
Weight and volume
The weight and volume of the soil mixture are particularly important with outdoor plants that stand in the wind. If the medium is too “fluffy and light” the plant stands a very good chance of being moved around and dislodged in high wind – resulting in damage to the plant. Additionally, the plant can also act as a sail in high wind and cause the container to blow over if the container is not heavy enough – a very common problem with tall plants and small containers.
You also don’t want to add too much or excessive weight so that the container can’t be moved around. To take this to perhaps the extreme where planters are constructed on balconies or rooftops you have to consider static weight loading, where you don’t exceed the design weight loading point of the actual roof structure – another topic together.
Building your own soil
You are obviously not going to find the plant's native soil in a bag at the shop, so you have to come up with a close approximation of the soil.
Start with the inorganic component, sand, in “sophisticated” gardening countries there is a wide range of even base soil types to select from, in the UAE, not so, where you normally use the course, washed river sand as the basis for potting soil, we have red dune sand, but we also have “silica sand”. Silica sand is used to manufacture glass but is also used in children’s play areas, its white, and the grains are a lot larger than dune sand; it’s not easy to find, but also not impossible to find; but costs a lot more than a bag of dune sand. You won’t find much washed river sand in the region as we don’t have any perennial rivers and it is also illegal to harvest sand, stones, or rocks from the local environment or beaches.
Once you have a base to work from you then add the organic component, and in this case compost from any suitable source; this must not be raw compost and needs to be fairly well-decomposed and weed-free. If you feel it’s too coarse then it can be sieved, but I would suggest that you retain some larger pieces else it may compact too quickly. The key to remember is that most compost is going to decompose and compact over time resulting in a loss of volume of the soil (very noticeable with container plants), this then has to be “topped up” from time to time, or the plant re-potted.
Potting soils are constructed proportionately or in ratios, these are easy to scale and use; for example, 1-part sand, and 2-part compost – the same holds true whether you are mixing for a small container or a wheelbarrow full.
Let’s assume you have assembled all your components for the mixture, you measure them out into a bucket or bowl or even on the floor. You now have to mix them; this is key as if the mixture is not thoroughly mixed you will end up with areas that are not consistent.
Outdoor plants containers – general shrubs: 3-parts coco peat, 2-parts red sand, 1-part compost
Acid-loving outdoor plants: 3-parts coco peat, 2-parts red sand, 2-parts peat
Outdoor vegetable containers: 4-parts potting soil, 1-part compost, 1-part red sand
Outdoor succulent containers: 4-parts red dune sand (even better if you can use silica sand), 1-part compost
Indoor containers – general houseplants: 3-parts potting soil, 1-part perlite
Remember in the above examples – the potting soil already contains compost, so in some instances, we are just adding a little more to the mixture.
Water retention agents
And lastly, one final word on water retention agents. There are a number of water retention-type products on the market. These absorb and then store water in various ways depending on the type of product and release the moisture back into the soil as the medium dries. There are natural products such as clay/ bentonite that perform this action, as well as synthetic products like polymers (similar to what is found in nappies). There are advantages as well as disadvantages to both of these types of products, but the key here is if you do use them, do not use too much in your potting media, as they can retain too much water and turn your soil into mush.
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