This section began innocently enough as a simple info blog about jujubes. But during the dormant winter periods with no live action to write about, I went back to my roots (ha!) to write more on soil, biochemistry, and soil microbiology in general. I found myself wanting to keep going, and this blog was becoming less and less jujube-specific.
Thus it made sense to restructure everything.
This blog is now The Biosphere Blog, where I will continue writing about these subjects very dear to me.
(And here is my passion project From Soil to Fruit, a combination of the two and very much a work in progress. This is where topics in this blog are arranged in a more structured book-chapter format, to be explored in far more detail.)
Did you know there is only one substance that absolutely must be available to all living things on planet Earth?
It isn’t oxygen — while that’s essential for animals and for non-photosynthetic processes within a plant, it’s a poisonous gas to many microbes.
It isn’t a particular vitamin, like, say vitamin C is for us — many organisms can make their own and many other important compounds.
The one essential thing required by all life as we know it, to the extent that NASA in its search for extraterrestrial life skips over planets without it, is…water.
But what is it about water that makes it so crucial for life?
(’pm’ stands for picometre, which is one-trillionth of a metre).
Most people would know that water is a molecule of one oxygen (O) atom and two hydrogen (H) atoms (hence the ‘H2O’ everyone is familiar with). But individual atoms, like O and H, have a neutral charge, and so to form reactions with each other to make molecules, they must first be in ionic form, with either a positive or negative charge overall. A positive charge results when there are more protons (+) than electrons (-), and a negative charge results when there are more electrons (-) than protons (+) in an ion.
A hydrogen ion has a charge of +1 (written as H+), which means it has one more proton than electron (in fact, a hydrogen ion is unique in that it has no electron). An oxygen ion has a charge of -2 (written as O2-), which means it has two less protons than electrons. Two hydrogen ions have a combined charge of +2, which is why a single oxygen ion attracts two hydrogen ions. Each of the oxygen ion’s ‘extra’ electrons bonds with one hydrogen ion, to make a neutrally-charged water molecule. We call the now molecular O and H here atoms again, and not ions, as once they are bonded they again have a neutral charge.
That explains the composition of a water molecule, but it doesn’t explain its structure. Why is the molecule bent, and not in a straight H-O-H arrangement?
While the oxygen atom in a water molecule has electrons bonded to hydrogen atoms, it also has two pairs of unbonded electrons opposite the two hydrogen atoms (to explain why this is so is quite involved and well beyond the scope of this post):
If it weren’t for the two electron pairs, water would be a linear molecule, with a hydrogen-oxygen-hydrogen bond arrangement of 180°. Instead, the two electron pairs exert a force that pushes each oxygen-hydrogen bond away to cause the roughly 104° bend between them.
The diagram above is a simple two-dimensional representation of what is really a tetrahedral shape — as shown in the second image on this page. There is a slightly negative charge (written as δ-) near the two electron pairs, from the electrons themselves, and a slightly positive charge (written as δ+) near the hydrogen atoms, due to their single proton each. This uneven distribution of electron density (more electrons in the electron pairs than in the hydrogen atoms) not only bends the molecule, but makes it polar. That is, it has a negative pole at the oxygen end and a positive pole at the hydrogen end.
The bend is a consequence of water’s polarity, but it’s both the bend and the polarity that makes water essential to life. Let’s investigate further.
Water’s polarity makes hydrogen bonding possible. This is where the slightly positive hydrogen ends of water molecules are attracted to the slightly negative oxygen ends of other water molecules. This attraction is strong, and brings the molecules closer together than otherwise without this bonding. Crucially, the bend makes this attraction even stronger, by enabling the oxygen and hydrogen atoms of different molecules to be in closer proximity than if they were in a linear arrangement:
Most molecules with a similar mass to water (for example atmospheric nitrogen, N2) are gases at room temperature. The individual molecules of nitrogen gas, for example, are at too high a level of excitation to come together and form a liquid. It takes a temperature of at least -196 °C to slow them down enough that they do form liquid nitrogen!
Water molecules are not only prevented from bouncing around in a gaseous state because of hydrogen bonding, but the presence of two hydrogen atoms for every one oxygen atom means each water molecule can bond with two others, making for a stronger liquid state than if a one to one ratio.
The importance of hydrogen bonding holding water molecules together in a strong liquid formation cannot be overstated: it means water evaporates at a higher temperature than otherwise, it means water has surface tension and can cling to surfaces and move due to capillary action, and it means water is slow to heat up. It ultimately means that water is a liquid over a much wider range of temperatures than would be possible in the absence of hydrogen bonding. This in turn means that life can exist across the temperature extremes that exist naturally throughout days, seasons and across the greater Earth.
And water is not just any liquid, but a liquid so special that it is often referred to as the universal solvent. This means that it can dissolve nearly anything because of its polar nature. The negative oxygen ends of water molecules don’t just attract positive hydrogen ends, they can attract (hold in the water, dissolve) many positive ions you’d recognise as plant nutrients, such as sodium (Na+), potassium (K+), calcium (Ca+) and magnesium (Mg2+). Similarly, the positive hydrogen ends enable many negative ionic nutrients such as phosphates (PO43-, HPO42- and H2PO4-), sulfate (SO42-), and bicarbonate (HCO3-) to dissolve in water.
In short, water’s structure is crucial to life because it enables water to flow (being a fluid) and carry (being a solvent) over a wide range of temperatures. It can flow on and into land, and collect into rivers, lakes and oceans where it becomes available to organisms. Water in an organism moves in and out of cells, delivering nutrients essential in, and removing wastes from, biochemical reactions.
No other substance has all the properties of water that life as we know it depends on, which is why water has no substitute. Some other compound may be polar for example, but be a gas or a solid at the temperatures water is liquid at — no good if it solidifies overnight when temperatures drop, and has evaporated by mid-afternoon!
Let’s compare ethanol with water to make this point. Ethanol is polar, freezes at -114 °C, and boils (evaporates) at 78 °C. Plants wouldn’t need to enter dormancy during freezing winter temperatures if they used ethanol!
But this is where water’s other properties become so important. Ethanol’s lower boiling point of 78 °C would make life harder in warm climates and especially so in hot summers. Water, being harder to heat up and turn to gas, is an excellent coolant and makes life possible in such areas.
Hydrogen bonding in ethanol is also much weaker than in water, because of its size, its linear structure, and having only one hydrogen available to bond with any other oxygen.
This weaker hydrogen bonding significantly affects the dissolution of nutrients and lowers surface tension. Plants especially would struggle in these conditions. And while water pre-existed life on this planet, ethanol, as far as I can tell (and I’m happy to be corrected, and to update this post accordingly), is not made naturally via non-biochemical means. In other words, water jump-started life (notwithstanding the water paradox!), but life jump-started ethanol (via fermentation).
(Theorising alternatives to water for life, whether here or on other planets, is a fascinating topic that has preoccupied scientists and science-fiction writers for quite a while. Please look up ‘hypothetical types of biochemistry’ elsewhere should you wish to explore this further.)
Water leaves soil in three ways: evaporation from a surface; drainage; and movement through a plant. Evaporation is due to heat, drainage is due to gravity, and movement through a plant begins with root uptake and ends with evaporation through the stems, leaves and flowers (a process called transpiration).
Evaporation can be controlled to some extent by using mulches to cool the soil and reduce exposure to the heating influence of the sun. Mulches from shredded plant materials such as leaves and grass are preferred over materials such as plastic sheeting, as these allow infiltration of water into the soil as well as breaking down to fertilise the soil and build structure. Do make sure the mulch doesn’t form an impenetrable mat that forces water to pool on the surface and wash away.
Water that drains out of a soil is removed via gravity. There is little control over this except to allow a soil rich in organic matter and structure to develop, which will naturally hold more water than a poorly-structured infertile one.
The water left after drainage is what resists that pull of gravity, and such ‘just-drained’ soils are said to be at field capacity. As this water resists the pull of gravity, roots must ‘pull’ a little harder again to overcome this resistance and extract this water.
Water in the largest pores is the easiest to remove, then that in subsequently smaller and smaller pores. As smaller and smaller pores are accessed, the amount of water becomes less and less, to a point that it cannot meet a plant’s needs. A plant begins to take on a wilted appearance that progresses during the heat of the day, but recovers during the cooler night. Eventually there is a point at which a plant cannot extract more water — this could be because the soil is too dry, or the water that is present is too tighly held to extract, or roots cannot reach further sources of water quickly enough.
When a soil is so devoid of water, or is held so tightly in the smallest pores (as small as 0.0002 mm across) that a plant cannot overcome the force needed to access it, the soil is at permanent wilting point. If water isn’t returned to the soil, a plant will shed leaves to reduce transpiration, and die if this condition persists.
The water held in soils between permanent wilting point and field capacity is called the available water. Soils with good structure will have more available water than those with poor structure.
Available water is measured as a concentration rather than a volume, often expressed as mm of water held per cm depth of soil. This concept helps to understand the interaction between water, soils and plants.
For example, sand has a lower ability to retain water than a well-structured loamy soil, and would have a lower concentration of available water because of this. However, if that sand is deep and consistently exposed to water, such as in sandy hills by a beach, infiltrating water will aggregate at those lower depths and the whole profile in general will be consistently at or near field capacity.
That field capacity will still be lower than that of a typical well-structured loam, but if the plants growing on those sandy hills have deep roots that reach deep down into the profile, there is actually more water available to them than if that well-structured loam is no more than a shallow layer over solid rock.
A plant needs to exert a pressure of around 10 kPa to pull, or suck the first water from a soil at field capacity. Higher and higher suctions are required to remove more and more water, as this water becomes more tightly bound within smaller and smaller pores. A soil at permanent wilting point requires a suction of at least 1,500 kPa to remove that water.
Sandier soils are able to release more water at lower suctions than heavier soils, as water does not cling to their larger particles as tightly as in clays. This means that while heavier soils hold larger volumes of water comparatively, more of it is locked away in the capillary pores to be of any use to plants.
Understanding the implications of soil type and depth on water availability will help with planting decisions. Plants in shallow sandy soil will need small, frequent waterings to reach the happy medium between fast drainage and subsoil pooling of water. It would probably be best to not plant in such areas in the first place, especially if the plants aren’t drought-tolerant. Plants in a deep sandy soil will benefit from long waterings to encourage deep rooting to where the water will pool.
Conversely, plants in well structured soils of high field capacity and which drain moderately can handle less frequent waterings, but of higher volumes. This too will encourage deep rooting. Plants in shallow soils that don’t drain quickly are best watered more frequently but with smaller amounts, to avoid waterlogging and deprivation of air to the roots.
In this ‘Water’ section so far, we have covered how water enters and moves through soil, and its availability or otherwise to plants. We’ve also seen how a soil’s texture and structure features in all three of these. It follows that allowing your soil to develop a sound, undisturbed structure rich in organic matter can only improve water infiltration, retention and availability to plants with time.
Are you familiar with capillary tubes, the very thin rods with very narrow bores? When placed into water, the water is pulled into and up the tube, against gravity. This is capillary action. And the narrower the bore, the higher the column of water will rise.
The smallest pores and channels in soil also act like capillaries. You can see this in action when water drips onto dry soil and spreads sideways across the soil. Similarly, should you place a small piece of soil on a drop of water, you’ll see how easily that piece soaks up the water.
An amount of soil that has been saturated with water and allowed to drain will still hold some water. (As does a sponge.) Just as water clings to the insides of narrow capillary tubes, or to the sponge particles that surround the pores, so too does water cling to soil particles surrounding the narrowest (smallest) pores in the soil. These pores are in fact often called the capillary pores because of this. The water that did drain was the water that was not held as tightly by the larger, non-capillary, pores.
Because of this attraction between water and soil particles, water can act as a glue and is one reason wet soils can be harder to work than their dry counterparts. Clayey soils especially become very sticky when wet.
When a wetted soil has been given time to drain and no more water leaves it — this could be several days after rain or irrigation — it is at field capacity. This is the amount of water the soil could theoretically retain in its capillary pores indefinitely, assuming no losses through transpiration from plants or evaporation from the surface. A soil at field capacity is moist, crumbles easily and is easiest to work in this state or a little drier. Water in a soil at field capacity fills all pores of up to 0.03 mm diameter, and clings as a thin film to soil particles surrounding the larger pores, something like this:
Field capacity will vary depending on soil texture and structure. With regards to texture, heavily sandy soils, with their large minerals and pores cannot retain as much water as the loams and clays with their smaller particles and pores. But it’s structure that is more important than texture — a sandy loam without organic matter will have a lower field capacity than one with organic matter. Likewise, a well-structured clay will have a higher field capacity than a poorly-structured clay.
Water on contact with a soil’s surface may do one of three things. Depending on the geography and soil, water may continue downhill, or pool on the surface, or soak in. Water that soaks into a soil is said to infiltrate it, and it follows that it is this water that will ever reach plant roots.
But infiltration in itself isn’t enough — water also needs to enter at an infiltration rate sufficient enough to wet enough soil at enough depth that plant roots can access it. Just think of a very light rain spotting a dirt surface. You only need to gently scratch that surface to see how dry the soil is underneath. It may as well not have rained at all.
Infiltration and infiltration rate will vary depending on the amount of water applied to a soil as well as the type and structure of that soil. A gentle, lengthy rain on dry, well-structured soil will readily enter the surface cracks and channels between peds, soaking into the pores until they fill. If that rain continues long enough, the water will continue to percolate deeper and deeper down the profile, filling even more pores as it goes. Eventually it will reach a permanent zone of saturation known as the water table, from which bores, wells and springwater are sourced.
Rain falling on heavily compacted, poorly-structured soil may not soak in much at all, but rather pool on surfaces, leading to runoffs and soil erosion. But this will be the fate of any water regardless of the soil, if the intensity of its fall is greater than the ability of the soil to soak it up.
Very sandy soils, with their large minerals and pores, as a rule will enable more water to infiltrate and at a higher infiltration rate than that same water into high clay soils with their smaller particles and pores. Conversely, the deep cracks that form in cracking clay soils when dry can enable both a high infiltration and infiltration rate of water deep into the soil profile before the clay minerals swell and seal the cracks.
Infiltration rate is measured as mm (height) of water soaking in per hour, and can range from zero in water-repelling sands to several hundreds of mm in very coarse sands. Infiltration rate into soil decreases the wetter the soil becomes, but over time reaches a steady rate known as the saturated hydraulic conductivity of the soil. This is the rate at which water enters a soil’s surface and percolates through the topsoil to drain below. Any compacted sublayers or underlying bedrock will reduce this rate. Eventually water already in the soil will no longer be able to drain away and the water table will rise. In extreme cases too much infiltration will lead to waterlogging and eventual root death if that water cannot drain away quickly enough.
Even very gentle rain falling on wet soil will pool or lead to runoff if the steady infiltration rate is very low, at less than 5 mm an hour. Another thing to be aware of if using an irrigation system, is that if the system delivers at a rate higher than the infiltration rate, water will pool and be wasted. And water that doesn’t infiltrate soils in low-rainfall areas is just too valuable to allow wastage through surface pooling and runoff!
It’s best to avoid traffic on any soil on which water has pooled for days after rain stops, so as to minimise compaction and disruption of the soil’s structure.
Increasing infiltration rates can be an easy solution for excessive pooling, and with quick results. Coring or spiking the soil to create instant large holes and channels may be enough. Some instances may require digging to break up compaction, whether of the soil itself, or a sublayer below, or of any thick compacted mats of dead and dried organic matter on its surface. Digging into the soil may reveal the compacted sublayers or other obstructions such as shallow bedrock that were the problem all along, which can then be remedied or avoided by relocating plants.
Next week we’ll examine how water, once it has infiltrated soil, then moves through that soil.
As with soil, there’s more to water when it comes to plants (and soil) than is evident at first glance. Elsewhere in this blog I’ve mentioned water secondarily and where relevant, but now it’s time to give the subject of water the attention it deserves, with its very own section! Some information mentioned elsewhere may be repeated in this section, but will be rephrased from the point of view of water and its interactions with soils and plants, rather than the other way around.
Plants, like all life, need water. And for the vast majority of plants that water is obtained via their roots. Water thus needs to be where the roots are, or roots need to be where the water is. And again for the vast majority of plants those roots tend to be in soil. Thus for water to be of use to most plants it needs to be in the soil where roots can access it.
All this is pretty obvious of course.
But how that water comes to be in the soil in the first place, and what happens after that, can be somewhat involved. And it’s that discussion that will underpin the topics in this new ‘Water’ section.
Water invariably enters soil via the surface. This entry could be from rain, surface runoff, overflowing creeks and rivers, us standing over with a watering can or hose, or an irrigation system. Even if we were to water with bore water we still must pump it up from underground first, just to reapply it to the surface. As growers of trees, we then expect that water to make its way through the soil to where the roots are.
Simple? Not really! For instance, how do we actually know that enough water has entered the soil? Too little or too much can have consequences. And has the water that entered then moved deeply enough for roots to access? Or has it moved too quickly down, draining out of reach before roots could uptake it? Or has it not moved much at all, and is collecting in the upper surface away from those roots?
And don’t forget that water is a solvent! As it travels through a soil profile it ‘collects’ various salts and other substances in that soil. If bore water, it would already have a good number of such compounds already dissolved in it, which would then enter the soil along with the water. Some, like sodium chloride (table salt) are harmful to plants, and some, like nitrates, are needed by plants. Some may deposit out into the upper soil, and some may travel deep down. This may be a good thing or a bad thing, depending on the compounds, their concentrations in the water, the soil they travel through, and how the water moves through that soil in the first place.
There are so many variables associated with water. These variables, along with the variables of soil composition and structure through which water travels, can make for a complex subject.
But to summarise very simply: how water enters soils, moves through soils, is retained by soils, drains from soils, as well as what it carries through soils, all ultimately determines how much water ever becomes available to plants. We will cover all of these, starting in the next post with the first variable, how water enters a soil, or its infiltration.