Interaction of Soil, Nutrient, Plant and Water


Written by Sharing Sustainable Solutions

Plant Nutrient Uptake

Plants gather their nutrition from water-born elements dissolved by and suspended in the water surrounding their root systems. (They use atmospheric carbon and light through their leaves, too, but that’s beyond the scope of our interest here.) They also need air around their roots.

The smallest tips of the roots grow “hairs” who’s cell walls are semi-permeable; that is, water and very small nutrient molecules can pass through, driven by two forces: Osmotic Pressure and Water Potential.

Osmotic Pressure is nature’s desire for a balance of dissolved elements in a solution. If there is more Nitrogen, Potassium or Phosphorous in the water outside a plant root’s cell wall, for example, than inside, osmotic pressure will drive these elements through the cell wall and into the more dilute solution inside the cells.

Conversely, too high a concentration of nutrients outside the cell walls can actually draw water out of the plant, since nature wants that balance of solvent and solute on each side of the cell membrane.

Water Potential is similar to osmotic pressure in that water tends to move to areas of lower concentrations from areas of higher concentrations. If you’ve ever left a wash rag in a wet sink you’ve seen it “soak up” the water until it was saturated. This is water potential moving the water from a high concentration (the wet sink) to a lower concentration (the dry rag). This happens because water molecules are both positively and negatively charged. The positive end of one water molecule (the Oxygen part ofH2O) is attracted to one of the two negative ends of another water molecule (the Hydrogens..the H2’s in H2O).

They form an unbroken chain of molecules from the driest part of the washrag straight to the standing pool in the bottom of the sink. And as the first molecule moves to dryer areas, it pulls the chain along step by step until a stasis is achieved and every part of the washrag is equally wet. If the air around the washrag is dry enough, the water will evaporate and move from the wet rag to the dryer air, again pulling the water molecule chain along with it until the sink is empty and the rag is totally dry.

This nutrient rich chain of water moves the same way into, through and out of plants. Plants have tiny openings in their leaves called STOMATA. Very simply put, a stoma opens and closes through water pressure in the leaf tissue. When the plant is full, the stomata close.When its thirsty they open. Water evaporates out of the stomata, lowering the water potential in the plant and literally pulling the unbroken chain of water from the soil, thru the semi-permiable cellwalls of the roots and up thru hollow cells called XYLOM TISSUE to the leaves. The dissolved nutrients in the water move with it into the plant.

SOIL CHARACTERISTICS. Soil is more than broken down minerals. A good soil is a balance of “separates” called sand, clay and silt. “Sand” particles run from 2 milimeters down to .02mm; “Silt” particles run from .02mm down to .002 mm; “Clay” particles run from .002 mm down to submicroscopic sizes. The proportions of these constituents is called the soil’s TEXTURE.

Sand particles are the comparatively large units, with a huge volume and small surface area. They help hold the smaller particles apart, making spaces for air and water to move in the soil. Too much sand, however,and the soil can’t hold water or nutrients well. The water runs by gravity off of the small surface area rather than “sticking” to it by atomic attraction.

Silt and clay are tremendously smaller, with a proportionately larger surface area. There are about 100 particles of sand in a gram, with a surface area of about 10 cubic centimeters. There are over 100 TRILLION particles of clay in the same gram with a surface area of 8 million cm2! This tremendous surface area is negatively charged and attracts water and nutrients, holding them in the soil. Too much silt and clay,however, will form a sticky glop with no spaces between particles for air to reach plant roots.

Originally posted @ Sharing Sustainable Solutions


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