plants take up water
the plant needs
Seems like a good place to start? It's
all about the plants afterall.
need water. We all know that. Why do
they need water? For the following reasons:
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
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.
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.
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!
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
are three types of soil water (ie. water
in the soil).
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.
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.
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.
lot happens between field capacity and
permanent wilting point. Try to understand
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.
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.
and Sodicity (too much soil sodium)
to read up on this important area, and
how it relates to soil water.