Seed vigour versus germination
A Socratic walk-through of seed vigour versus germination — reasoned out one step at a time, not lectured.
The question we started with
THE QUESTION #Why does a seed lot that sprouts almost perfectly on a laboratory bench give a thin, ragged stand in the field?
A seed lot arrives with a germination figure on the label: 96 percent. The test behind it is not a marketing claim — it is a legally standardised procedure, run on counted seeds in controlled warmth and moisture, and a second laboratory would return nearly the same number. Then the lot goes into the ground and two-thirds of it comes up, unevenly, over ten days instead of four.
That gap is not a testing failure in the ordinary sense. The laboratory measured something real, carefully, and got it right. So what did it measure, that a field does not care about?
Reasoning it through
REASONING #Start with what the standard germination test actually asks. It supplies ideal moisture, an ideal temperature, no crust, no pathogens, no drought, no compaction, and asks: can this embryo produce a normal seedling? It is a question about capability under the best conditions anyone can arrange. A field asks a different question: can this embryo produce a seedling here, through four centimetres of cooling, crusting soil, with fungi in it.
Notice the shape of what has happened. To make the test reproducible between laboratories, every variable that differs between fields has to be removed — and those variables are precisely the ones that decide field emergence. Reproducibility and field realism are pulling in opposite directions, and the standard test resolves the conflict entirely in favour of reproducibility. It has to; a number that changed with the tester would be useless in a contract.
Now ask what could vary between two lots that both score 96. Seeds deteriorate from the moment they mature — slowly if dry and cool, quickly if warm and damp. Lipids oxidise, membranes lose their integrity, and enzymes and DNA take injuries the seed can only mend once it takes up water. So a deteriorated seed is not dead; it is slow, spending longer repairing before it can grow.
And that is the whole story, because time in the soil is exposure. A seedling taking seven days to reach the surface instead of three has spent four extra days available to damping-off fungi, four extra days for the surface to crust over, four extra days for a cold snap. In the laboratory none of those clocks are running, so slow seed and fast seed both score as normal seedlings and the difference vanishes into a single count.
Push the reasoning to its natural prediction. If deterioration is gradual but the germination test is a pass-or-fail count, then the two must come apart: vigour should decline steadily for a long time while germination stays flat near the top, and only when the damage is severe enough to stop seedlings entirely does the germination figure fall — suddenly, and too late to be a warning. That is exactly what deterioration curves look like. The label reports a threshold that is crossed long after the property you care about began to slide.
So the useful tests are the ones that put a stress back. Accelerated ageing holds seeds hot and humid before germinating them, compressing years of storage into days; a cold test germinates them in unsterilised field soil; a conductivity test soaks them and measures the electrolytes leaking from damaged membranes, reading the mechanism directly. None replaces the germination figure; they restore what standardisation removed.
The analogy
THE ANALOGY #Think of a driving test conducted in an empty car park on a dry morning. It genuinely establishes that the candidate can steer, brake and park, and two examiners would agree on the result. It has told you almost nothing about the candidate in sleet on a motorway at night, because every source of difficulty was cleared away in order to make the assessment fair and repeatable. The pass rate is not a lie; it is an answer to a narrower question than the one the road will ask.
a driver improves with practice, so the car-park test is at worst a floor, whereas a seed only deteriorates — its best performance was in the past, and every stress test is measuring how far it has already fallen.
Clarifying the model
THE MODEL #The correction most worth making is to the phrase "the germination test was wrong". It was not. The failure lives in the inference from the number to a claim about the field — the same failure a psychometrician calls construct under-representation, which a sibling walk-through on test validity takes up in the setting of examinations. The fixed point of difference is instructive: in an exam, standardising conditions is largely a virtue, and irrelevant variance a defect to engineer out. In seed testing the removed conditions are the subject matter, and standardisation is a trade accepted because seed is sold on contract and the number must survive dispute. Same logical fault, opposite intent.
Second refinement: vigour is not one property. It names the reserve of performance a lot has under stress, and different tests catch different parts of it — ageing catches membrane damage, cold testing fungal susceptibility, conductivity leakage. Hence no single vigour number is universally accepted, and the trade labels germination instead. That is an honest limit of the concept, not a gap better instruments would close.
Third: seed is not the only cause of a ragged stand. Sowing depth, seedbed moisture, drill calibration and crusting can produce the same symptom with a perfectly vigorous lot. That gives the falsification test. Take two lots of matched germination percentage but different vigour scores, and sow them side by side into both a good seedbed and a stressed one — cold, deep, or crusted. They should emerge alike in the good bed and differ in the stressed one, the gap widening with stress. If they emerged identically under stress, or diverged just as much in the ideal seedbed, vigour would not be measuring a reserve against field conditions and this account would be wrong.
One relation is definitional rather than empirical, and worth stating: field emergence is bounded above by the proportion of capable seed, so germination percentage sets a ceiling. Everything below it is the field's doing, and vigour estimates how much of that gap belongs to the seed rather than the drill.
A picture of it
THE PICTURE #How to readTrace a single lot from the left. The first split is what the laboratory measures — 96 in a hundred capable of a normal seedling, and that is the only fact on the label. The lot is then divided and sown into two seedbeds, so the middle of the diagram is one lot in two worlds, not two lots. Follow each branch right: in the ideal bed nearly all the capable seed arrives, so the label predicts the stand well, while in the cold crusted bed the same capable seed splits three ways and a third never reaches the surface. The widths are a schematic accounting of the structure of the loss, not measurements from a trial.
What became clearer
WHAT CLEARED #Germination percentage answers whether a seed can grow, under conditions arranged so that nothing else interferes. Vigour is how much margin it has when things do interfere, and it declines steadily while the germination figure sits reassuringly flat — so the label goes on saying 96 through most of the deterioration it is supposed to warn about. The ragged stand is what happens when slow seed meets a soil whose hazards are all functions of time: the laboratory stopped every clock the field runs on.
Where to go next
ONWARD #- Why priming seed — letting it begin germination and then re-drying it — buys speed at the cost of storage life.
Key terms
TERMS #| Term | What it means |
|---|---|
| Standard germination test | a legally recognised procedure counting normal seedlings under optimal, controlled conditions. |
| Seed vigour | the capacity of a lot to establish rapidly and uniformly across the range of conditions it may meet in the field. |
| Accelerated ageing test | a vigour test exposing seed to heat and humidity to compress storage deterioration before germinating it. |
| Conductivity test | a vigour test measuring electrolytes leaking from imbibing seed, reading membrane damage directly. |
Every term the collection defines is gathered in the glossary.