How plants take up water

What plants need

Plants need water. We all know that. But why do they need water? These are the reasons:

  1. They need water in order to stand up. Some will eventually make woody tissue to help this process, but basically plants are full of pressurised water which makes them turgid. The leaves offer themselves to the sun….their stomata (pores) open….and moisture evaporates. Water is drawn upward from the roots and through the stems to replace this lost water. This process is called “evapotranspiration”. The more sun, the greater the pressure to take up water. This process takes energy from the plant, and obviously requires a healthy root system and the presence of AVAILABLE water in the root zone (I’ll explain the “availability” shortly). If it’s not there, the plant will wilt. In cases of root disease and diseases like Fusarium, you will see whole crops crash down.
  2. They need water to carry nutrients into themselves which are dissolved in the soil water. They can’t munch on dry fertiliser. No water…..or I should say, “no passage of water into the plant”….and no nutrient uptake. If the plant can’t take up water, it will become starved of nutrients. It’s not so uncommon to see high nutrient soils and pale, nutrient-starved crops because of an inability of the plant to take up water.
  3. Finally, plants need water to photosynthesize. To summarise a fairly complex process, photosynthesis is the synthesis of sugar (energy) from light, carbon dioxide and water, with oxygen as a by-product. Take away any of those factors, and the plant won’t grow. It has no energy.

What else do plants need? They need oxygen, and they need it in the root zone. Like all aerobic organisms (including us), they need to respire as part of the process of utilising the sugars they created in photosynthesis, and this requires oxygen. No oxygen, and no respiration. No respiration, and no functionality. The roots can’t grow….and can’t take up water….and can’t supply the plant with the nutrients and water that it needs. This is why we talk about a plant needing DRAINAGE. The problem in a waterlogged situation is not too much water……it’s too little oxygen!

Water in the soil

Soil is made up of soil particles in crumb-form (peds), and pore spaces around the soil crumbs. In a well-structured soil, these crumbs are nice and stable….but in a poorly structured soil, the crumbs are unstable which often limits pore-space. The pore-spaces are necessary for holding water, and for the free gaseous exchange of oxygen and carbon dioxide between the plant roots and the soil surface (respiration process).

There are three types of soil water (ie. water in the soil).

Gravitational water: This is the water which is susceptible to the forces of gravity. It exists after significant rainfall, and after substantial irrigation. This is the water which fills all the pore-space, and leaves no room for oxygen and gaseous exchange. In “light” soils, this tends to drain away quickly. In heavy soils, this can take time.

Capillary water: This is the water which is held with the force of SURFACE TENSION by the soil particles, and is resistent to the forces of gravity. This is the water which is present after the gravitational water has drained away, leaving spaces free for gaseous exchange. When the soil is holding it’s MAXIMUM capillary water (after the gravitational water has drained), this is called FIELD CAPACITY. At this point, the plant is able to take up water easily, and has the oxygen that it needs in the root zone.

Hygroscopic water: This is the water which is held so tightly (by surface tension) to the soil particles that the plant roots can’t take it up. It’s there…….but it’s unavailable. At this stage there’s generally sufficient oxygen, but there just isn’t enough available water. The plant wilts, and will eventually die if it doesn’t get water. When the plant wilts and is unable to recover, this is called the PERMANENT WILTING POINT.

Now…….a lot happens between field capacity and permanent wilting point. This is the key point:

The closer to the soil particle the water is held, the tighter it’s held. And the further from the particle, the looser it’s held. It takes little energy for the plant roots to take up the water that’s far from the particle and is present at the field capacity point. By contrast, as the water is used up (or evaporates), it takes more and more energy for the plant to take up water.

I often use the analogy of drinking through a straw. A short straw, ie. when a cup is 15cm away from you, is easy to use. A one-metre long straw takes a lot of energy to suck up a drink. A twenty-metre straw is impossible to use. It works much the same with plants. The more the soil dries out, the more energy the plant needs to output in order to get a decent drink.

Salinity and Sodicity (too much sodium)

These are two often-related areas……but not necessarily. Salt is NOT neccesarily sodium chloride. That’s table-salt! Salts can be Calcium Carbonate, or Magnesium Sulfate, or umpteen other combinations as well as sodium chloride. High levels of SODIUM in soil (either naturally there, or from high-sodium and/or high SAR waters) will make a soil SODIC. And sodic soils have a horrible dispersive, hard-setting and easily-compacted structure.

Well-structured soils have plenty of exchangeable Calcium. Poorly structured, dispersive, “sodic” soils contain high levels of exchangeable Sodium where there should be Calcium. Calcium holds soil particles together, ensuring stability, root penetration, water infiltration and aeration. Sodium ions have only half the charge as Calcium ions, so therefore holds particles weakly….which means the bonds fall apart, leading to compaction, poor aeration, poor infiltration and root penetration. High levels of soil Magnesium have a similar effect on structure. The solution is: replacement of Calcium ions (commonly through gypsum), drainage (ripping) and leaching of Sodium ions. Additions and maintenance of soil Organic Carbon will also assist in rebuilding healthy soil structure. High levels of soil magnesium, often indicated through a low calcium to magnesium ratio, tend to have a similarly detrimental effect on soil structure.

So this is what happens when you water your soil with high-sodium waters. Treatment with BioFarm’s magnetic water conditioners (water magnets) reduces enormously the build-up of sodium in the soil, and will generally assist in leaching built-up soil sodium thus allowing you to keep using the treated water without detrimental results.

What happens when a soil becomes sodic (too much sodium)

Salinity

Now, SALINITY is a different matter. Saline waters DO often contain high levels of sodium, therefore all the above may well still apply. Your water test will show this. Salinity will show up in your water test as a high EC (Electrical Conductivity). Read the article How plants take up water, and note the point made about how it takes energy for the plant to take up water from the soil. When there is salt in the water (or in the soil-water, because of salty water and/or salty soils), it is extremely hard for a plant to take a drink. The plant uses it’s energy from evapotranspiration to draw water from the soil. But when there is SALT present, it becomes harder and harder. Water moves from the soil into plant tissues by OSMOSIS (definition: “Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration”). Where there is salt in the soil and soil-water, the salt in the soil/water draws the water back to itself. Therefore the saltier the soil and soil-water, the less water is taken up by the plant. The plant is stunted. If it’s very salty, it may eventually die of thirst. The other important issue is this: When you apply saline water to the soil, much of this water evaporates. The water evaporates….the salt does not! Generally the more you use saline water, the high the concentration of salts you will create in the soil. So whilst a moderate salinity water may not show up immediate problems, it may well become a problem in the months and years ahead. BioFarm magnetic conditioners (water magnets) are used to treat all but the worst of saline waters (we will always tell you if we can’t help you!), and have consistently shown positive results both in plant growth and in soil test results.

The same osmosis issue applies when saline waters are applied to plant foliage, especially on the more sensitive crops. The water evaporates, leaving minute crystals of salt on the leaves. These salts will then draw moisture out of the leaves into themselves, resulting in dead patches and spots on leaves…..or even dead leaves or dead plants. Again, our magnetic water conditioners (water magnets) will prevent this happening in all but the worst of waters.

Salt-tolerant versus salt-sensitive!