Botany · Ch 2 — Mineral Nutrition
Methods to Study the Mineral Requirements of Plants
Methods to Study the Mineral Requirements of Plants
Growing Plants Without Soil
To find out exactly which minerals a plant genuinely needs, scientists cannot simply analyse soil — soil is far too complex a mixture to isolate the effect of a single element. Instead, the standard approach is hydroponics: growing the plant with its roots suspended directly in a water-based nutrient solution, with no soil involved at all. The German botanist Julius von Sachs is credited with growing a plant to full maturity this way for the first time, in 1860, and the method has been refined steadily since.
How the Method Works
A seedling's roots are placed in a solution containing a defined mixture of mineral salts. Researchers then add, remove, substitute or vary the concentration of one element at a time and observe how the plant responds. If leaving out a particular element stops the plant growing or reproducing normally, that element is essential; the pattern of abnormal growth that appears is its deficiency symptom. Because the whole point of the technique is to isolate a single mineral variable, the water and salts used must be extremely pure — any stray contamination would introduce an uncontrolled element and confuse the result. The solution also has to be kept well aerated, since poor aeration by itself damages root growth regardless of mineral supply.
A Commercial Technique Too
Hydroponics is not confined to the laboratory — it is also used to grow crops commercially without soil, including tomato, seedless cucumber and lettuce.
Figure 12.1 shows a simple static hydroponic set-up: a seedling in a pot of nutrient solution, with a funnel for topping up water and nutrients, cotton packed around the stem at the pot mouth to hold it in place, and a tube feeding air into the solution to keep it aerated.
Figure 12.2 shows a larger, commercial-style flowing system: a tilted trough carrying a row of potted seedlings, with a pump continuously circulating nutrient solution from a reservoir up to the raised end of the trough. The solution flows back down over the roots by gravity and drains back into the reservoir, so the roots are constantly bathed in fresh, aerated solution.
What this figure shows. Labelled line drawing of a young leafy seedling growing hydroponically in a squat green pot filled with pale nutrient solution. The seedling's fine brown roots hang down into the solution. A funnel is inserted through the pot mouth at upper left with a leader line labelled 'Funnel for adding water and nutrients'; the stem is packed with cotton at the pot rim, labelled 'Cotton'; a bent orange tube enters the solution from the upper right, labelled 'Aerating tube'; a leader line from the liquid itself is labelled 'Nutrient solution'.
Figure 12.1: Diagram of a typical set-up for nutrient solution culture.
What this figure shows. Composite diagram of a commercial hydroponic set-up. The main drawing shows a long horizontal tube/trough tilted on a slight incline, holding a row of small potted seedlings along its top, with curved arrows on the tube showing the direction of solution flow; at the lower right the tube connects by tubing to a blue rectangular reservoir labelled 'Nutrient solution' next to a cylindrical 'Pump'. Above the trough, a circular inset (linked to one seedling in the row by a curved arrow) shows an enlarged view of a single potted seedling standing on a raised perforated platform above a shallow dish of nutrient solution, with its pale roots bathed in the liquid below.
Figure 12.2: Hydroponic plant production. Plants are grown in a tube or trough placed on a slight incline. A pump circulates a nutrient solution from a reservoir to the elevated end of the tube. The solution flows down the tube and returns to the reservoir due to gravity. Inset shows a plant whose roots are continuously bathed in aerated nutrient solution. The arrows indicates the direction of the flow..