A field holds 400 units of phosphate that the plants can take up…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
A field holds units of phosphate that the plants can take up. Each year the crop takes up units, and the crop is carried away. Only units return from roots and stubble. The rock gives units of new phosphate by weathering, and runoff washes away unit.
- What is the net change of the soil phosphate in a year?
- If nothing changes, in how many years is the pool used up? Give the answer to one decimal.
- How many units of fertiliser each year would balance the field?
Need a reminder? The key ideaShow
Carbon has a gaseous cycle, with its reservoir in the atmosphere and the ocean. Phosphorus has a sedimentary cycle, with its reservoir in rock, and no respiratory release into the air. For a pool in balance, .
The worked solution
Have you tried it?
Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Given. Pool . Uptake . Return . Weathering . Runoff (units a year).
Plan. Net change inputs outputs. Then divide.
Step 1. (a) Inputs . Outputs . Net change units a year.
Step 2. (b) years.
Step 3. (c) To balance the field, add units a year.
Answer: units a year; years; units a year.
Check. There is no gas phase for phosphorus, and its return is small, so a harvested field loses phosphate.
Watch out. Fertiliser that runs off can cause eutrophication in the water that receives it.
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