Green Manures -- Growing Fertility

Legume green manures fix atmospheric nitrogen through bacteria living in nodules on their roots, converting it into a form plants can use. Both legume and non-legume green manures also build organic matter and soil structure simply through the physical action of their root systems growing, breaking up compaction and leaving channels behind as roots decay. Once a green manure has done its job, it's cut and either dug into the bed or left on the surface to break down, returning the fertility it built straight back into the soil it grew in.

• Cowpea and related fast tropical legumes, already established as reliable choices in the Tropical guide, work equally well here given how similarly warm this zone stays for much of the year.
• Sunn hemp, a genuinely fast, warm-season nitrogen fixer, establishes quickly and reliably nodulates in warm soil, making it a strong choice specifically for speed.

Because this zone's growing year is long and only mildly interrupted by a real winter, there's far more flexibility about when to fit a green manure into a rotation than a Mediterranean or boreal climate's tightly bounded windows allow (see Chapter 17). Even with that flexibility, it's still worth timing a sowing to coincide with a wetter stretch of the year rather than a dry one, so the crop gets properly established before it needs much supplemental watering, a scheduling decision that draws directly on the moisture-tracking skill covered in Chapter 11.

Cutting a green manure and leaving it where it fell, rather than digging it in or carting it to a heap, creates an instant mulch layer over the bed. That mulch does double duty: it breaks down to release the fertility the crop built, and in the meantime it helps retain soil moisture the same way the mulch caps described in Chapter 11 protect a compost heap, a genuine two-for-one benefit in a climate where bridging dry spells is the ongoing task.