Mineral Nutrition
Essential elements
The exam tests criteria of essentiality, macro- and micronutrients, deficiency symptoms, hydroponics, nitrogen metabolism and symbiotic nitrogen fixation.
Core rules
- An element is essential if it is indispensable, has a specific function, and its deficiency cannot be corrected by another element.
- Seventeen elements are essential: C, H, O, N, P, K, Ca, Mg and S are macronutrients; Fe, Mn, Zn, Cu, B, Mo, Cl and Ni are micronutrients.
- Hydroponics grows plants in nutrient solution; it is used to identify deficiency symptoms.
- Mobile nutrients such as N, P, K and Mg show deficiency first in older leaves; immobile nutrients such as Ca, Fe and B show it in younger leaves.
- Nitrate is reduced to nitrite by nitrate reductase and then to ammonia; ammonia is assimilated into amino acids.
- Symbiotic fixation occurs in Rhizobium-legume nodules with leghaemoglobin; free-living fixers include Azotobacter and cyanobacteria.
Worked example
A plant shows chlorosis in older leaves first. Reasoning: chlorophyll loss suggests N or Mg deficiency; because both are mobile, the plant moves them from older to younger leaves. If chlorosis appears first in young leaves, suspect an immobile element such as Fe or Ca.
Traps
C, H and O are essential but are not soil minerals. Micronutrients are needed in small amounts but are still essential and can be toxic in excess. Mo is needed for nitrate reductase. Do not confuse chlorosis with necrosis.