http://soilquality.org.au/factsheets/water-availability

There's a chart here you'll find very, very interesting. Although "wet," the soil may not have all that much water available to plants. That tends to happen more with denser soils.

I have a silt loam, near (but slightly above) the maximum point of available water. Wilting tends not to occur until the soil feels pretty dry even deep down. In extremely light or extremely heavy soils, the difference between wet and dry is much more trim, and extremely wet still doesn't mean that there's a vast amount (or even enough) water available unless it's showing across a good percentage of the root.

Other considerations can block water absorption, including excess hydrogen in the soil, but we know from your soil test that this isn't an issue for you. Wicking of water onto particles that contact roots is slower in sandy soils (fewer, larger particles) than in heavier (many more contacting and smaller particles) soils.

Accuracy of soil moisture probes is always in doubt, with the dual prong ones tending to be more accurate (but also very sensitive to very low or very high levels of free ions in the soil solution). Plus the probe can easily detect water that's present in the soil, but not available to the plants--although that's going to be much more of an issue in clay soils.

Yes, interesting. Things are always more complicated that first thought. One thing about that study, though, is that they sample moisture all the way down to 100 cm - over a yard deep! We know my soil is sandy/loam with a good EC at least down to 4" so I wouldn't think there is an issue with much of that moisture being unavailable. In their chart, more than 50% of the water is unavailable, but I have to think that is partly due to the depth of the sample beyond many plants' root system. The report does make it clear that soil doesn't have to be completely dry in order for the plant to be wilting.

The other thing is that some of my lawn is stressed to begin with because I was watering only 1/2" all season, and in addition some areas weren't getting the best coverage from the sprinklers. So it is possible next year that the stressed areas won't be that way when getting the full 1" all year.

I also assumed the probe might not be that accurate, but relatively speaking, the drier looking grass registered drier readings than the healthy grass areas, which had good moisture. Repeatability also seemed to be good, so I don't think I'm getting a lot of spurious readings.

>>In their chart, more than 50% of the water is unavailable, but I have to think that is partly due to the depth of the sample beyond many plants' root system.

That's specifically the water-binding capacity of the soil particles. Sandy soils don't bind water very tightly because they're large. There's plenty of room for water to hang out in the middle between them, essentially bound only by surface tension to the water that's closer to the particle.

However, because of the large open spaces, not much water actually gets bound. The tension binding is too weak, and most of the water simply runs through, leaving air in the interstitial spaces.

Very fine clays have tiny spaces and bind enormous amounts of water...but it's bound so tightly the root can't exert enough osmotic pressure to break the electrostatic binding to the soil particles.

All things considered, most average roots can exert about an atmosphere of pressure to pull water away from the soil (or, they can pull as hard per square inch of area as a thirty foot column of water weighs per square inch of area, which is pretty hard).

Clays bind water at greater than 1 atmosphere of pressure pretty easily. Sand, not so much.

The storage limit is there to indicate what the soil can actually hold in its structure. Amounts of water above the upper storage limit simply drain away (eventually, anyway). Since soil doesn't hold vast amounts of water, the 100 cm basis is used to get good measurements and minimize the measurement inaccuracies.

One meter seems about right for that, as well as being a cross section of soil that's relatively easy to handle.

You also have to take into account whether the volume of root exposed to adequate water is enough to supply water to the entire plant. If the top three inches are too dry but the fourth inch is OK, the grass may or may not be fine. In very hot weather, I'd count on wilting--there isn't enough root system in the water to supply the whole plant. In fall or spring, wilting is less likely since water demands are so much lower.

That stress, with adequate water at the low range, does send the signal that more roots are necessary. Or, why we always recommend deep and infrequent watering--the plant "knows" the lower roots have water, it's just not enough.

For that reason, plants that may form six inch balls of root in the garden will fill an entire enormous pot on the patio. Garden water levels are usually adequate, patio water levels, not so much--and I always water until the pot pees, both to flush exuded waste products and to encourage the roots to go to the bottom.

So yes, water stress will reduce as your roots deepen--and organic matter certainly helps! I wouldn't say the process is all that fast in either case, but year on year you should notice your watering dates spreading further and further apart.

That all makes sense. The one point I was making is that in their study, rooting depth was a variable that caused some of the water to be unavailable, as mentioned in their Background section. For instance, if we're talking KBG in sandy soil, much of the water will be unavailable simply because it soaked into the 100 cm region beyond the root's reach. However, as long as that water remains in the 100 cm cross section, they count it as being unavailable. My point was that if a sandy soil had 50% of the water available and 50% unavailable as stated in their study across 100 cm (or 39ish inches), then the water available only in the top 4 or 6 inches would surely be higher than 50% since what is going to soak down deeper has already done so.

That probably didn't come across clearly... Practically speaking, all I'm suggesting is that in the top few inches were interested in, I should expect the majority of the water to be available to the grass, barring anything unusual like clay.

Could be, that read is really ambiguous on the sheet. I'm not sure if they count water that ran through as completely lost, or just not bound to the soil profile.

This evening, when I have time, I'll see if I can find any corroborating data. Right now, I'm puffing soap in the microwave to make laundry detergent...

I got sidetracked. Puffing 2 pounds of soap takes a while.

And no, seriously, that's literal. Put soap in the microwave and it puffs up like a marshmallow, plus drives off remaining water. When it cools, it's a dry, very friable solid that shatters easily. Since I was lazy and only cut the stuff instead of shredding it when it came out of the mold, it refused to shred and wouldn't chop in the food processor. Lesson learned.

On the up side, there's now 2 pounds of really dry, powdered soap in a Tupperware container atop my bookshelf. It's ready to be turned into 5.5 pounds of hyper-concentrated, very pleasantly scented powder laundry detergent when I have the time to mix everything. Probably tomorrow, unless I just make the next 2 pounds of soap for the mix instead...

I did locate two references, both of which explain things differently than the original link:

http://www.extension.org/pages/33617/soils-and-water-availability#.VdK9MPNVhBc

http://www.noble.org/ag/soils/soilwaterrelationships/

Or, pretty much ignoring run-through, it's only the water bound into the soil profile they worry about.

However, with a nice silt loam, 2" per foot of soil means that our Kentucky bluegrass, which can only access 6" or so of the profile, has only 0.5" of water available to it. Fortunately, water does wick up from below, and high levels of organic matter are also going to influence the situation. With my 14% OM, far and away beyond the nominal 1-5%, I can bind far more water than one might expect from just the soil structure.

I think I've posted the (amusing) flooding photo where the flood water entered my property on the north side and...disappeared. With my open soil and very high water capacity, it was soaking up inches of rainfall.