#54 - Nov 2021 - Garden Stuff - © - Sandy Lang - slang@xtra.co.nz
HYDRAULIC REDISTRIBUTION November/December: Late spring/early summer. Still time to plant seedlings. Stick with the well-tried classics - but try something new too.
Tree roots: We think of roots as sucking up water from the soil but that’s only half the story. Roots also exude water into the soil. The physics is reversable…
Water potential: (Gk. symbol Ψ, pronounced ‘psi’) is a measure of water availability. The units of Ψ are kPa (kilo Pascals). By definition, a glass of water (or puddle, or lake, or river) has a Ψ value of zero (0 kPa) but water is usually less available than this in the soil (drier), less still in plants (even drier) and much less available in the air (very dry). Hence, in all these places the value of Ψ is negative.
Ψ values: •Soil sits between 0 kPa (just after heavy rain) and -100 kPa (drought). •Air is much drier. At 80% relative humidity (RH) the air Ψ is at -30,000 kPa. At 50% RH air Ψ plummets to -90,000 kPa. •Plant water potential sits somewhere between the soil and air extremes, maybe at about -1,000 kPa.
Flow: If a plant has its toes in the soil (say, -50 kPa) and its leaves in the air (say, -30,000 kPa) then water runs through it ‘downhill’ from -50 kPa to -30,000 kPa. That’s from a low Ψ to a very low Ψ. Wicks: Think of a plant as a wick, one end in the damp soil, the other in the dry air. Water is sucked into the roots from the soil, rises to the leaves, then evaporates to the air. The bigger the Ψ difference between soil and air, the faster it goes. But plants are ‘super-wicks’. Water travels very much faster through a plant - at about 5 metres per hour (tiny pipes) - than along a wick.
# A simple wick experiment. You might like to roll up a newspaper tightly (a couple of runbber bands will keep it in place) and 'plant' it in a pot of soil - half belowground, half in the air. Then water the soil. After a few days, you will see it becomes discoloured as moisture tracks up the newspaper (capillarity) and evaporates to the air, leaving behind the solids dissolved in the soil solution - the soil mineral nutrients (on which the plant feeds) and tannins (the brown discolouration derived from rotting plant material - Google humus wiki).
Water redistribution: What if a tree has some roots in wetter soil and others in drier soil? •The roots suck up water from the wetter soil, shift it several metres, then exude it into the drier soil. So, tree roots even-up, uneven soil moistures. •Trees also drag water up from deep below and exude it into drier soil above, where tree seedlings and small plants live. •Trees also help dry out over-wet soil (near a creek, after flooding) and dump the surplus water in over-dry soil (where it’s needed). Pretty smart… Google Hydraulic redistribution wiki___________________________________
HYDRAULIC REDISTRIBUTION November/December: Late spring/early summer. Still time to plant seedlings. Stick with the well-tried classics - but try something new too.
Tree roots: We think of roots as sucking up water from the soil but that’s only half the story. Roots also exude water into the soil. The physics is reversable…
Water potential: (Gk. symbol Ψ, pronounced ‘psi’) is a measure of water availability. The units of Ψ are kPa (kilo Pascals). By definition, a glass of water (or puddle, or lake, or river) has a Ψ value of zero (0 kPa) but water is usually less available than this in the soil (drier), less still in plants (even drier) and much less available in the air (very dry). Hence, in all these places the value of Ψ is negative.
Ψ values: •Soil sits between 0 kPa (just after heavy rain) and -100 kPa (drought). •Air is much drier. At 80% relative humidity (RH) the air Ψ is at -30,000 kPa. At 50% RH air Ψ plummets to -90,000 kPa. •Plant water potential sits somewhere between the soil and air extremes, maybe at about -1,000 kPa.
Flow: If a plant has its toes in the soil (say, -50 kPa) and its leaves in the air (say, -30,000 kPa) then water runs through it ‘downhill’ from -50 kPa to -30,000 kPa. That’s from a low Ψ to a very low Ψ. Wicks: Think of a plant as a wick, one end in the damp soil, the other in the dry air. Water is sucked into the roots from the soil, rises to the leaves, then evaporates to the air. The bigger the Ψ difference between soil and air, the faster it goes. But plants are ‘super-wicks’. Water travels very much faster through a plant - at about 5 metres per hour (tiny pipes) - than along a wick.
# A simple wick experiment. You might like to roll up a newspaper tightly (a couple of runbber bands will keep it in place) and 'plant' it in a pot of soil - half belowground, half in the air. Then water the soil. After a few days, you will see it becomes discoloured as moisture tracks up the newspaper (capillarity) and evaporates to the air, leaving behind the solids dissolved in the soil solution - the soil mineral nutrients (on which the plant feeds) and tannins (the brown discolouration derived from rotting plant material - Google humus wiki).
Water redistribution: What if a tree has some roots in wetter soil and others in drier soil? •The roots suck up water from the wetter soil, shift it several metres, then exude it into the drier soil. So, tree roots even-up, uneven soil moistures. •Trees also drag water up from deep below and exude it into drier soil above, where tree seedlings and small plants live. •Trees also help dry out over-wet soil (near a creek, after flooding) and dump the surplus water in over-dry soil (where it’s needed). Pretty smart… Google Hydraulic redistribution wiki___________________________________