Answer
Fencing is measured in linear units and the fertilizers are measured in square units.
Work Step by Step
In a garden, the fencing is done to protect the garden from the outside harm. Fencing covers all the outside boundaries of the garden. The length of the same is measured in linear units i.e. in single units such as meter, kilometer etc.
Fertilizing is done inside the garden for the crops or something which is being grown in the garden. It is done according to the area of the garden. Garden being a two-dimensional thing, the fertilizers shall be measured in square units i.e. square centimeters, square meters etc.