A tomato plant shows yellowing in its older, lower leaves first, while young leaves stay green. Which deficiency best fits this pattern?
ANitrogen, which the plant can move from old leaves to young ones
BCalcium, which cannot be moved once deposited
CCarbon, because of low carbon dioxide
DOxygen, because the roots are flooded
Nitrogen is a mobile nutrient. When it is scarce, the plant breaks down molecules in older leaves and sends the nitrogen to growing tissue, so older leaves yellow first. Deficiencies of immobile nutrients such as calcium show up first in young tissue.
Question 2 True / False
Most of the dry mass of a large tree comes from minerals absorbed from the soil.
TTrue
FFalse
Answer: False
In the 1600s Jan Baptist van Helmont grew a willow in a weighed pot of soil for five years. The tree gained a great deal of mass while the soil lost only a tiny amount. We now know most of the dry mass comes from carbon dioxide fixed in photosynthesis.
Question 3 Short Answer
Why does nitrate fertilizer wash out of soil more easily than potassium?
Think about your answer, then reveal below.
Model answer: Clay and humus particles carry negative charges on their surfaces. Positively charged ions such as potassium (K+) cling to them, while negatively charged nitrate (NO3-) is repelled and stays dissolved in the soil water, so rain carries it away.
This is why nitrate runoff from farms can pollute streams. Roots obtain the bound cations by cation exchange: they release hydrogen ions, which take the place of the cations on soil particles and free them into solution.
Question 4 Multiple Choice
How do mycorrhizal fungi benefit their host plants?
AThey fix nitrogen gas inside root nodules
BTheir hyphae spread through the soil and supply minerals, especially phosphate, in exchange for sugars
CThey photosynthesize for the plant
DThey block water from entering the roots
Mycorrhizal hyphae are thin and extensive, so they reach far more soil than root hairs do. They pass phosphate and other minerals to the plant, and the plant pays with sugars. Nitrogen fixation in nodules is done by bacteria such as rhizobia, not by mycorrhizal fungi.