Figure 4 The bottom third of the diagram is shaded green to represent the land, and the area above that is shaded blue to represent the sky. At the far left, in the blue area, it says 'N subscript 2'. An arrow points vertically down into the green area, and then turns and points to the right where there is a plant, with its roots in the green area. Beneath the plant it says 'N subscript 2 fixing in roots (N H subscript 3 )'.
An arrow points to the right to the words 'amino acids'.
From here another arrow points to the right where it says 'nitrates nitrites'.
From here another arrow points to the right to 'denitrifying bacteria', and from there an arrow points up into the blue layer, where it says 'N subscript 2'.
From 'nitrates nitrites' an arrow points to the left and then up to the surface of the green layer, where there is some grass growing. An arrow points up from the grass to the mouth of a cow. The cow is defecating. An arrow from the faeces down to the words 'amino acids', previously described.
On the top of the green layer is the word 'fertiliser'. An arrow points from the fertiliser down to the words 'nitrates nitrites'.
At the top of the blue layer is a cloud. Rain is falling from the cloud, and there are two lightning bolts below the cloud. An arrow labelled 'N O subscript 3 superscript minus (aq)' points down to the words 'nitrates nitrites' in the green layer.