Sunn Hemp vs. Hardpan: What 90 Days Did

The field that feeds itself

The oldest complete Chinese farming manual, the Fan Shengzhi Shu (氾胜之书), was written in the first century BC, and its whole argument is that fertility is something the field grows, not something it receives: till deeply once, treat the seed, and let the ground feed itself. By 544 AD the Qimin Yaoshu (齐民要術) had it in writing that the soil is renewed by what is returned to it — green manure, crop rotation, the residue left in place. Six hundred years before the European agricultural writers, the Chinese had the whole method.

This bed is that method, run on sandy ground. The spade test said it all: sandy topsoil, a hardpan layer underneath, water sitting where it should not sit. The usual fix comes in a bag from a truck. The old fix grows in place.

Sunn hemp (Crotalaria juncea)

A tropical legume that grows fast in summer heat, stands a meter and a half to two and a half tall, and fixes nitrogen as it goes. Its thick taproot and vigorous growth make it the closest thing to a free fertilizer plant for sandy, nutrient-poor ground. It is a tool, and like any tool, it is about timing.

What the 90 days did

Sown after the soil warmed — in central Florida, that means a May to June sowing — and cut sixty to ninety days later, before it flowered. The sequence, as it ran:

• Read the bed first. The spade test answered the three questions that matter: is the topsoil sandy, is there a hardpan layer, where does the water sit? The old manuals called this knowing the strength of the soil before asking anything of it.
• Sow into a real seedbed. Cover crops need contact, not depth — rake, broadcast, rake again, and roll lightly.
• Let it grow to its window, not to seed. The taproot went at the hardpan from above; the stems stood in the heat and shaded the sand.
• Cut at flowering, while the stems are green and succulent. Leave it longer and the stems harden and the plant runs to seed, and the fertility goes to waste.
• Leave the residue in place. As mulch it feeds the top; the roots rot below and open the structure. The Qimin Yaoshu says it plainly: what the field gives, the field gets back. Then the bed rested two to three weeks before planting.

That last part is the part most people skip. The green top is only half the harvest ' + E ' + the roots, rotting in place, are the other half, and they are the half that goes at the hardpan from below.

Biological Soil Building — PDF ($7) →

or read the cover crops guide · see the Water-Wise bundle

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🌿 Ninety days, no bag — the bed grew its own fix, as it has for two thousand years.