Why a Seed's Soak Time Goes as the Square of Its Size

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No, and for a reason that is worth more than the answer: a saturated sponge is already a soak. The seed begins taking up water the moment it goes in, so a pre-soak buys you only the hours before planting, and the number of hours it buys is set almost entirely by how big the seed is.

Water uptake is a diffusion problem, which means it goes as the square of the seed. At a representative coat-limited diffusivity, a mountain mint seed is done in 1.9 minutes and a runner bean takes 12.7 hours. A twenty-fold step in size, a four hundred fold step in time. For anything sown into a wet plug the soak is therefore pointless at the small end and genuinely useful at the large end, and the dividing line is a seed diameter rather than a species.

And then the part that actually matters, because the failure mode runs the other way. One millimetre of standing water around a seed carries the same oxygen resistance as ten metres of still air. The risk is on the wet side.

Why the time is a square

A dry seed takes up water by diffusion through its coat and into the cotyledons, and for a sphere of radius a the time to reach ninety percent of equilibrium uptake is 0.183 times the radius squared, divided by an effective diffusivity. The 0.183 is exact for a sphere. The diffusivity is representative: a coat-limited value of one times ten to the minus ten square metres a second is the right order of magnitude for an intact seed, and it varies between species by more than tenfold, which is why the minutes below are soft and the ratios between them are not.

Put numbers in it and the spread is startling:

Mountain mint at a quarter millimetre radius, 1.9 minutes. Basil at three quarters of a millimetre, 17 minutes. Chives and flat parsley at a millimetre, 30 minutes each. A coriander schizocarp at 1.75 millimetres, 1.6 hours. A pea at three and a half, 6.2 hours. A runner bean at five, 12.7 hours.

So the overnight soak that gets applied to everything is between fifteen and three hundred times longer than a small seed needs, and about right for a bean. It is not that the advice is wrong. It is that it was written for the seeds where it matters and then repeated for the ones where it does not.

The sponge was the soak, so what does the plug change

A Patigrow 120-cell self-watering seed starter kit with a grow light publishes 120 cells across two separate 60-cell trays, a footprint of 16.6 by 11 inches at 7.28 high, 1.5 millimetre BPA-free PET for the dome and base tray, domes 4.07 inches tall with two adjustable vents each, capillary mats between tray and base, enlarged drainage holes in the cell trays, 56 LED beads per tray on an adjustable timer and plant stands to set the tray height.

Two of those features point in opposite directions and the record does not say how they balance. A capillary mat holds the base of the medium at effectively zero suction, which keeps the plug wet. Enlarged drainage holes let water leave, which keeps the plug drained. Both are sold as benefits and only one of them can be winning at any moment, and which one it is decides whether the seed in that cell is soaking or breathing. Full record in our review of the Patigrow kit.

The useful consequence is that the question changes shape. Once the plug is saturated the seed cannot soak any faster or any slower than the arithmetic above allows. What a pre-soak does is move those minutes or hours to a time when you are watching, which for a bean is half a day of progress you can see and for mint is no progress at all.

One kit, five seed packs, a forty nine fold spread

This is where the square law stops being an abstraction. A HOME GROWN 5 herb indoor garden starter kit publishes five named seed packs, five soil pellets, reusable pots with trays, wooden markers, a nutrition pack and a grow guide, with the seeds stated as USA grown, non-GMO heirloom Genovese basil, chives, cilantro, mountain mint and Italian giant parsley, in hermetically sealed envelopes rated to stay fresh for up to two years.

Those five names are five different radii. Mountain mint at about a quarter millimetre is the smallest, the coriander that cilantro is grown from at about 1.75 millimetres the largest. Seven times the size, so forty nine times the imbibition time: 1.9 minutes against 1.6 hours, inside one box with one instruction sheet.

The soil pellets are the other half of the same physics pointed at a different object. A compressed pellet has to be hydrated before it will accept a seed at all, and a pellet is both larger than any of these seeds and much more porous, so it wets in a minute or two rather than in hours. That is the one thing in the kit for which the instruction "soak first" is unambiguous, and it is the one thing in the kit that is not a seed. More in our review of the HOME GROWN 5 herb kit, whose own drawbacks are properly cautious: "Germination results vary by home conditions" and "Pot and pellet sizes are not listed".

The real answer, and it is four orders of magnitude

An imbibed seed is metabolically awake and it needs oxygen. Oxygen reaches it by diffusion, and the two media it might diffuse through are not remotely comparable.

Oxygen's diffusivity in air is about two times ten to the minus five square metres a second. In water it is about two times ten to the minus nine. A ratio of ten thousand.

The diffusive resistance of a layer is its thickness divided by its diffusivity, so a one millimetre film of water presents five hundred thousand seconds per metre, and the thickness of still air carrying the same resistance is ten metres.

That is the whole of it. A merely damp plug has continuous air channels running through it and a saturated plug has none, and the step between those two states is not a matter of degree, it is four orders of magnitude in the one supply the seed cannot do without. Which is why over-soaking kills, why a plug standing in water kills, and why the advice that sounds fussiest, bottom water and let it drain, is the advice doing the real work.

And it reframes a feature on another tray. A Burpee SuperSeed 36-cell reusable seed starting tray publishes food-grade silicone pop-out cells, a tray of 11.5 by 10 and three eighths by 2 and a quarter inches, moulded corner slots in every cell for drainage and soil aeration, deep channels in the watering tray to draw excess away from the cell bases, a raised perimeter edge, alphanumeric rows with a planting diagram, side handles and dishwasher safety.

Its own record uses exactly the right pair of words, drainage and aeration, and it is worth saying that the second one is not a bonus. The slots are an oxygen feature wearing a water name. Water leaving is the mechanism; air arriving is the point. Our review of the Burpee SuperSeed tray has the rest, and notes the real limit on a single tray: 36 cells is one sowing's worth.

Cell count is not the variable. Depth is.

Compare the two trays on geometry rather than on features and something useful falls out.

A Kokopro seed starter kit with grow lights and a heat mat publishes cells divided into equal 1.96 inch squares at 1.85 inches deep, a far-infrared heat mat holding 70 to 85 degrees Fahrenheit with double insulation and an IP67 rating, three grow lights on an intelligent controller with four timing settings, heightened humidity domes, and drainage holes sized to let water through while the bottom tray retains overflow.

That cell is 116 cubic centimetres with a top area of 24.8 square centimetres. The Patigrow's 120 cells in a 41 percent larger footprint work out at about 37 cubic centimetres each with a top area of 9.8.

A volume ratio of 3.11 and a drying time ratio of only 1.23, because the evaporating area shrinks along with the volume and the only term that survives the division is the depth. So the headline difference between a 36-cell kit and a 120-cell kit is not how fast the cells dry out. It is how long a seedling can stay in one before it is pot bound, which the Kokopro record identifies correctly: "a shallow cell forces an early transplant or a check in growth."

That listing also carries an ambiguity its own record flags and declines to settle, which is the right call: the title says 36 extra large cells and describes a three pack, and nothing says whether 36 is per tray or across all three. We are not resolving it either. See our review of the Kokopro kit.

What the heat mat is worth on this particular clock

Our page on whether heat mats make seeds germinate faster owns the temperature argument properly, including the part most guides miss: "in lettuce, spinach and several brassicas warmth past the high 70s does not merely slow germination, it suppresses it." It also makes the point that decides the whole question: "the correct temperature is a property of the seed and not of the kit." Both are right and neither is about water uptake, so here is the missing half.

Diffusivity rises with absolute temperature and falls with viscosity. Across the mat's own published band, 70 to 85 Fahrenheit, which is 21.1 to 29.4 Celsius, water's viscosity falls by a factor of 1.200 while absolute temperature rises by 1.028. So the diffusivity rises by 1.234 and imbibition time falls by 19 percent.

Nineteen percent, from the entire range of the mat. Against four hundred fold from the choice of seed. The mat is a strong lever on the enzymes of germination and a weak one on the water getting in, which is a clean way to keep the two clocks separate: the mat is for after the seed is wet, not for getting it wet. The lighting side of these same kits, which is a third clock again, is worked through in why a grow light's coverage area needs a height.

The instrument that reads the wrong half

A 3-in-1 houseplant health meter reading moisture, light and pH publishes a dual-sensor probe with a three-position toggle and no batteries, and its own drawbacks are unusually useful here: "Best results only in loose, aerated soil", "Dense or compacted mixes can skew results", and "No listed probe length or dimensions".

Read those three together and the meter is telling you something about itself. It measures water in a medium whose air is the question, and it works best precisely where the air problem is least. A reading of "moist" is consistent with a plug that is perfectly aerated and with one that is sealed, because the quantity that separates them is the air-filled fraction and no consumer probe reads it. More in our review of the houseplant health meter.

Which leaves the practical test as the one that needs no equipment at all. Lift the tray. A plug heavy enough to feel saturated and a plug that has drained differ by a few grams of water and four orders of magnitude in oxygen supply, and your hand can tell the difference even though the probe cannot.

The honest limit, and what it leaves alone

What is exact here is the structure. Imbibition goes as the square of the radius, so the twenty fold size step giving four hundred fold in time, and the seven fold step inside one kit giving forty nine fold, are exact whatever diffusivity you assume. The 0.183 coefficient is the exact sphere solution at ninety percent uptake. The ten thousand between oxygen in air and oxygen in water, and the ten metre equivalence that follows, are exact to the precision of two textbook diffusivities. The 3.11 volume ratio against the 1.23 drying ratio follows from published dimensions. The nineteen percent follows from the mat's own published band and water's viscosity table.

What is representative rather than published is the list that sets every absolute number. The coat-limited diffusivity of one times ten to the minus ten square metres a second is an order of magnitude, not a measurement, and it varies between species by more than tenfold, so every minute and every hour above should be read as the right size rather than the right value. The seven seed radii are typical for their species and the seeds themselves are not spheres, which the sphere solution handles only approximately. The Patigrow cell depth of an inch and a half is derived from a published kit height minus a published dome height rather than stated. And the one millimetre water film is an illustration: the real geometry inside a wet plug is a network, not a shell.

Three things this page deliberately does not compute. It does not give a soak time for any named species, because the diffusivity that would make that honest is not published for any of them and the spread between species is larger than the effect being recommended. It does not touch germination percentage, viability or dormancy, which are different gates entirely and which the heat mat page handles properly, including stratification and scarification. And it does not compute the time a saturated plug takes to suffocate a seed, which would need an oxygen consumption rate for a germinating seed, and that is the real limit on the central practical advice here: the arithmetic establishes that saturation is catastrophic for oxygen supply by four orders of magnitude, and it cannot tell you how many hours of it a particular seed survives.

Three numbers to carry. Soak time goes as the square of the seed, so mint is two minutes and a bean is half a day, and anything under about a millimetre of radius is not worth soaking at all. One millimetre of standing water is ten metres of air to a seed's oxygen, so the fatal error is wet rather than dry. And a heat mat's whole published range is worth nineteen percent on getting water in, which means it is on the tray for a different reason and should be judged on that one.