Most people understand that plants, being the wondrous organisms that they are, make sucrose from water and carbon dioxide through photosynthesis and that oxygen is released as a bi-product.
Yet, for many functions at different stages in the life cycle of a plant, other nutrients are needed to make organic compounds of proteins, lipids, enzymes and anynumber of other chemicals involved in the life processes of the plant. Glucose created by photosynthesis is the building block upon which the creation of other compounds is based by splitting the molecule, recombining ions, adding other ions onto the resulting compounds and so on in the metabolism processes of the plant. At different stages of its life cycle such as fruit production, flower production and rapid growth (such as directly after germination), plants need more of some nutrients to produce those products specifically involved in this process. So, how do they absorb these nutrients?
It is all due to soil moisture and the soil itself. Plants can only absorb nutrients which are dissolved and they take them in by a special diffusion processes called osmosis, whereby water and dissolved nutrients pass through a membrane, which is selective rather than allowing anything in until equilibrium is reached either side as in straight forward diffusion.
What is more, because plants may need more of one nutrient than another due to different stages of the life cycle, the membrane alters to select for different ions which it allows in greater quantities during certain processes and not in others. This is fascinating and creates a whole area of study for botanists.
Plant nutrients are classified into macro and micro nutrients. The distinction is not because some are more vital than others but because the plants needs more of the macro nutrients than the micro nutrients.
Macro nutrients include Nitrogen, Phossphorous and Potassium. Plants obtain nitrogen from the air and organic compounds. Nitrogen fixing bacteria in the soil ‘fix’ nitrogen from the air. The nitrogen is incorporated into ammonia and this is also suplied by the breakdown of organic material. Another set of bacteria – the deintrifying ones- break down the ammonia into soluble compounds (nitrates) which the plant absorbs and so obtains the nitrogen it needs for green growth.
Phosphorous is in the soil solution and results from the breakdown of organic matter and weatherin gof the parent rock material as well as added manures and compounds. Potassium is obtained from ash produced by buring and is freely available in soil water.
Micro nutrients include boron,barium, calcium, sodium, chlorine, iron and zinc and these too are released into soil solution by weathering of mineral rocks or from decay.
Nutrients are taken into plants with water in solution and first enter through the root hairs. By osmosis they then travel in the water, across the cortex until they reach a special ring of material called the Caspian strip. Water can go both ways until this point but tends to go across the cortex because as each cell loses water, a defecit is created in that cell and water is pulled from the adjacent cell. When it crosses the Caspian strip, the water and dissolved nutrients cannot go back and instead travels across tissue until it reaches the xylem vessels.
Now it travels up the xylem vessel , which is part of the vascular tissue of the plant. It travels up because, while root pressure pushes water and dissolved nutrients upwards, at the same time,water is being lost by transpiration at the top of the plant via stomata on the leaves. This creates a defecit and water is pulled up. This is leaf suction. The dissolved nutrients are therefore pulled up in the water stream and, when a tissue needs them, they diffuse into the cells of that tissue. Different cells in the plant are selective for different molecules according to their selective membrane so some ions of sodium perhaps are absorbed by one tissue but not by another. This means, each tissue gets what it needs (for example, stamens need boron to create pollen but a stem would not so cells in the stamens select for boron as a nutrient but those in the stem do not).
Nutrition in plants is usually from natural sources but we can obviously supplement the nutrients by testing the soil and seeing if there is a lack, checking the plants to see if they show signs of nutrient deficiency and giving them fertilisers as a base or top dressing.
Nutrients are absorbed by plants in complex ways and plants are amazing how they use limited resources efficiently and get everything they need. Amazing!
