"One inch of water"

[IDemo]

This is often given as a unit of measure for how much water is required by a given plant. But it's a subjective term, not like "acre-foot". It apparently means the equivalent volume of water applied to an area if a container has one inch of water after a rainfall or sprinkler irrigation. But how is that in any way more useful than simply stating the volume directly in gallons per tomato plant?

[jeff47]

1. Because they don't know how much dirt is around that plant

What you're looking for is a particular moisture level in the soil. That's going to take a different amount of water for a plant in a small container versus one in the ground.

Say you're in southern California, so the ground surrounding the plant is bone-dry. A significant portion of the water will be taken up by the dirt away from the plant.

Same plant in the same area, but in a container instead of the ground. The only water loss will be due to evaporation, since the potting soil isn't in contact with the rest of the ground.

Same plant in the ground in the Southeastern US. The clay soil soaked by thunderstorms will not absorb much water.

[IDemo]

2. So it's still a subjective measure

What it comes down to is the variables of local climate, soil conditions and container versus earth planting. Wouldn't it be more helpful to simply advise the optimal soil moisture for a given plant? Say, "keep uniformly damp without allowing soil to dry out, but not drenched". Most people (well me, anyway) are going to have a much better idea of how much water is needed to meet this description than by having to guess what is meant by 'inch of water' or by having to calculate that volume.

[jeff47]

4. Your measure is far more subjective

"keep uniformly damp without allowing soil to dry out, but not drenched"

That's way more subjective. And wrong - you'd want the tomato plant to be a little drier than you describe.

Most people (well me, anyway) are going to have a much better idea of how much water is needed to meet this description than by having to guess what is meant by 'inch of water' or by having to calculate that volume.

It's really not that hard to calculate.

If watering by hose, see how long it takes to fill a small container 1" deep. Multiply by the size of the area you're watering vs. the size of the container.

If watering by sprinkler, put the same container in the area covered by the sprinkler. Water for a few minutes. Measure how deep the water is. From that you can calculate how long it takes to put out 1" of water.

[IDemo]

6. Still a nonsensical measure

As I mentioned in the OP, "acre-foot" is an exact amount of water. "Inch of water" relies on the less than scientific methodology of capturing an amount of water in a container, estimating its area and multiplying that by an area assumed to be within the plants root zone. No one who has a clue is using sprinklers on tomato plants, why employ them to guess (and that's all it is) how much to drip or soak your plants with? "One gallon per square yard" is far simpler to understand.

And yes, I know you don't keep tomatoes soaked, or that gallon per sq. yard is the recommended amount. these are examples of the type of phrasing I could relate to.

[jeff47]

7. Do you have a water meter on your hose?

How, exactly, do you measure acre-feet or even gallons that your hose puts out?

The actual volume your hose puts out will depend on the water pressure at your house, which is not uniform throughout the country. Heck, it isn't uniform throughout a single neighborhood. You could even get different flow rates from different faucets on your house - back when we built houses, we'd regularly put a faucet or two in front of the pressure reducer.

As a result, any measure in volume either requires spending a lot of money for a water meter, or it requires measurements as I described.

Since nobody's going to buy a water meter, they're going to use those measurement methods. Labeling it "an inch of water" takes one step out of the calculations - you don't have to figure out how many gallons it takes to cover the relevant area in 1" of water.

[IDemo]

8. OK, trial and error by soaker hose it is, then

thanks for your input.

[jeff47]

9. Just put the hose in a container. That will let you know its output (nt)

[JayhawkSD]

3. It has nothing to do with "how much dirt is around the plant."

Put a can, or any container with an open top and straight sides, on the ground near the plant and run your sprinkler for ten minutes. Then measure the water in the can. If it is half an inch deep the you need to run the sprinklers for twenty minutes to deliver an inch of water. That remains true no matter what kind of soil, what the temterature is, or the time of day.

"A significant portion of the water will be taken up by the dirt away from the plant."

This is a nonsensical statement. The dirt will take up all of the water, and the plant will then take up the water from the dirt. That is the way horticulture works. Other than bromeliads, which don't have roots, plants take water only through their roots, and they take it from the surrounding soil. They will send roots outward and downward to find moisture in soil that is distand from them.

Regardless of the area, type of soil, temperature and humidity, soil moisture is lost only by being taken up by plants and by drainage. Essentially no soil moisture is lost to evaporation; only a tiny portion of moisture from the top 1/16th inch or so. Tight clay soils can remain moist below the surface for a couple of weeks after a significant rain because they do not drain. Very sandy soil will dry quickly because the water drains quickly.

IDemo: yes, that form of watering instruction is far more useful.

[jeff47]

5. Not what I'm saying

Most people (well me, anyway) are going to have a much better idea of how much water is needed to meet this description than by having to guess what is meant by 'inch of water' or by having to calculate that volume.

If the ground is very dry, not all of the water will remain near the roots, and thus not available to the plant. Which is why I used drought-ridden Southern California as that example.

If you're not in such severe conditions, the water will remain near the plant.

Essentially no soil moisture is lost to evaporation

In the ground. In container gardening, evaporation can be a significant loss of water, especially in a dry climate.

[JayhawkSD]

10. Your thinking is really muddy

(pun not intended)

"If you're not in such severe conditions, the water will remain near the plant."

Presumably because the soil is more moist? In which case, why are you applying water? And your thinking is based on the assumption that moisture travels laterally through soil a lot more freely than it actually does. The lateral travel of moisture through any kind of soil is very limited. Water travels through soil only due to gravity, and that is downward. The "wicking" effect is very limited and moves it very short distances; like small fractions of an inch.

Besides, the plant will send its roots in pursuit of water. Why else do you think your stupid pine tree has roots tearing up your sewers? So even if the water did not "remain near the plant" (which it actually will), the plant will send roots out to get it.

"In container gardening, evaporation can be a significant loss of water"

Not really. Clay pots will allow the water to leak out, so they should be provided with a waterproof lining, but otherwise once the surface of the soil has dried out no more evaporation can occur because the water cannot move upwards to do any evaporation. Gravity rules.

[jeff47]

11. No, because the soil is really dry.

Presumably because the soil is more moist? In which case, why are you applying water?

Because in the drought scenario, the soil is absolutely dry. Some of the water will be wicked away by the utterly dry soil. We're talking about cracked desert soil, not anything you'd expect to grow anything, including cactus.

So in more normal scenario you still need to water the plant. But the surrounding dirt still has a little moisture, and so it doesn't wick away as much water.

Think utterly dry sponge vs. very slightly damp sponge.

Water travels through soil only due to gravity, and that is downward

Nope. Capillary action still works. If you water only the center of a houseplant, you'll find the edges of soil are damp after a little bit.

Besides, the plant will send its roots in pursuit of water. Why else do you think your stupid pine tree has roots tearing up your sewers? So even if the water did not "remain near the plant" (which it actually will), the plant will send roots out to get it.

This isn't a scenario where the water completely disappears. It's a scenario where you need to add additional water to overcome what is lost by water being drawn away by the "utterly dry sponge" soil. You just need to add a little more to get the same concentration of water at the plant.

Not really. Clay pots will allow the water to leak out, so they should be provided with a waterproof lining, but otherwise once the surface of the soil has dried out no more evaporation can occur because the water cannot move upwards to do any evaporation. Gravity rules.

First, "water cannot move upwards" is false. Capillary action still works. That's why you don't end up with super-wet soil at the bottom of the plant, and utterly dry soil starting half-way up.

Second, pots have bottoms. Even if you were correct about gravity being the only thing that has an effect, water can still leave the container.

[JayhawkSD]

12. Practical application

I worked as a landscaper and landscape supervisor in Tucson, AZ for twelve years, installing several dozen miles of drip irrigation supplying several thousand plants. I also spent two years at the University of Arizona Agricultural Extension in Marana studying irrigation and horticulture.

A typical dripper applies water onto a single plant ar a rate of .5 to 2 gallons per hour. After the irrigation cycle is complete you have a patch of moist soil about 10" in diameter and about 18" deep for the small dripper, larger for the higher rate drippers. The boundary between moist soil and dry soil is quite sharp and well defined, and beyond that boundary the soil is bone dry. I doubt you will find a climate more arid and hot than the Southern Arizona desert, so I'm not prepared to accept your theory that water moves away from the plant because the climate is arid and hot and the surrounding soil is dry. That is simply not consistent either with my education, nor with my twelve years of experience.

One of the problems we encountered was layers of compacted sandstone called "caliche," which is impervious to water. When we hit such a layer we had to either punch through it for drainage or lay a drainage line to drain the plant hole, otherwise the plant would die because water would stay in the hole and drown the plant. To test the drainage we would fill the plant hole with water and leave it overnight, and when we found it still full of water the next day the soil surrounding the hole would still be bone dry. The water would not have penetrated the surrounding soil more than an inch or so, even after standing in the hole for eight to ten hours.