Theory 081/Food & Agriculture /Crop physiology
Yield Is a Root-to-Demand Ratio
Irrigation is budgeted by litres. The plant experiences a ratio.
Crop yield is set by the ratio of rooting depth to evaporative demand rather than by water supply alone, so a shallow-rooted crop in a cool climate can out-yield a deep-rooted one in a hot one at equal irrigation.
01The evidence
Yield responses to irrigation vary widely between environments at equal water application, and the variation tracks evaporative demand.
02The tension
Water budgets use applied volume, which predicts yield from supply and fails when demand differs between sites.
03The mechanism
A plant's water status depends on how much soil volume it can explore relative to how much the atmosphere demands. Depth sets supply; demand sets loss; the ratio sets stress.
That makes the ratio the yield determinant, and predicts that deep roots should pay off most where demand is high, and not at all where it is low.
04What this is not
Volume models predict from supply. This predicts from the ratio, which explains site differences at equal irrigation.
05Predictions
- P1Yield should correlate with the depth-to-demand ratio.
- P2Deep roots should pay off only in high demand.
- P3Shallow roots in cool climates should match deep roots in hot ones.
06Refutation
Yield tracking applied water at controlled demand would restore the volume model.
07What it changes
It would change irrigation and breeding targets by environment.