I suspect your instructor would have you pair up the forest and fallow measurements for each quadrate, so you would have data matrix / table like

$$ \left[ \begin{array}{cc} AFo1 & AFa1 \\ AFo2 & AFa2 \\ \vdots & \vdots \\ DFo4 & DFa4 \end{array} \right] $$

Then you would perform a paired t-test with the first column as one sample and the second column as the other (so each row would be a pair).

The paired t test takes advantage of natural pairings of observations between samples. In this case the pairs would be one measurement in a forest quadrate and the same measurement in the corresponding fallow quadrate, for example, pH in AFo1 and pH in AFa1. For any type of measurement (pH, conductivity, etc.) this gives you a data matrix / table like

$$ \left[ \begin{array}{cc} AFo1 & AFa1 \\ AFo2 & AFa2 \\ \vdots & \vdots \\ DFo4 & DFa4 \end{array} \right] $$

Then you would perform a paired t-test with the first column as one sample and the second column as the other (so each row would be a pair).