Phosphorus is:
1 found free in the atmosphere
2 returned to the soil by rhizobium
3 added to the soil through weathering
4 usually very abundant in an ecosystem

added to the soil through weathering  

Explanation:

1) found free in the atmosphere : in it is found in the form of phosphates etc.

2 returned to the soil by rhizobium : Rhizobium is a bacteria which is involved in nitrogen fixation.

3 added to the soil through weathering  : It is added to soil again by weathering and is absorbed by plants in the form of phosphates.

4 usually very abundant in an ecosystem: it is found in earth nearly 0.1% by weight.

b. examine and describe cells