Soil and Fertility Plan

The soil is the bank, and the rule is to spend its interest and never its principle. The fourth deliverable is a nutrient budget and a fertility plan: the arithmetic of what your crops take out and what your system puts back, and the plan for closing any gap without a bag of anything.

What you produce

A nutrient ledger, one row per field, and a fertility plan that answers the ledger. The ledger is a stock-and-flow statement. Fertility is the stock; harvest, leaching, and erosion are the outflows; legume fixation, manure, compost, and mulch are the inflows. The change in the stock is inflows minus outflows, and the design aims at a stock that does not fall.

The exact format

One row per field, one column each for nitrogen, phosphorus, and potassium:

| Field | Area (sq ft) | N taken out (lb) | N put back (lb) | P out | P back | K out | K back |

The numbers you need fall into two kinds and you must not mix them. The outflows are your own harvest, and the honest way to get them is to take your yield from Lesson 606 and multiply by a crop-removal figure you look up for that crop. The inflows are your own cover crops and animals, and those have published figures you can use directly. Nitrogen fixation is the first-class inflow: crimson clover at about 100 pounds of nitrogen per acre, hairy vetch at 80, field peas at 60. A laying flock of six hens returns about 9 pounds of nitrogen a year. A dairy goat, 50. You are not buying fertility. You are routing it.

Then the rotation, because the rotation is the fertility plan wearing a different shirt. The legume goes before the heavy feeder. The fava fixes nitrogen and the wheat that follows it spends it. The field that grew the grain goes to the legume next year.

The old arithmetic

Worked example at the 10,000 square foot scale

Our 10,000 square feet is 0.23 acre. The staple block is 8,000 square feet, 0.184 acre. Under-sow the whole staple block with crimson clover in the autumn and the fixation return is 0.184 × 100, about 18 pounds of nitrogen a year. A laying flock of six hens adds 9. The total return to the staple block is about 27 pounds of nitrogen a year, all of it grown on the site.

Now the ledger has to show the grain crop taking no more than that. If the rye from Lesson 605 takes out 25 pounds and the clover and the hens return 27, the field is holding and the plan passes. If the rye takes out 40, the ledger is short by 13, and the design must find 13 pounds of nitrogen before it is built. The fix is never a bag. It is more legume in the rotation, more hens on the field in their tractor, or a shallower cut to the grain yield. A shortfall you write down and fix is a design. A shortfall you do not notice is a field that fails in its fourth year, when the soil has quietly run out.

Phosphorus and potassium ride on the same ledger, but the deep-rooted plants change the terms. Comfrey and the trees lift minerals from below the reach of the crop roots and drop them on the surface in their leaves. The house research is careful about what this is: an elevator, not a mine. It moves minerals up; it does not make new ones. So the ledger must still balance on the minerals the harvest carries away.

The test

The budget passes when extraction does not exceed return, and when it does, the plan names the fix. A ledger that balances only because the inputs column is empty has failed, because empty means the number was never found, not that the gap was closed. The second test is the hard one: nothing is allowed into the fertility column that was bought off the farm.