# Soil Moisture And Saturation



## go_hercules (Jul 29, 2018)

I understand and practice the "water deeply and infrequently" approach. However, I find that my tall fescue does best with about 2 inches of water per week in the summer. This is done at 1 inch twice per week. However, what I find by probing the soil is that it is almost saturated down to over 2 feet. If I cut back the frequency the grass suffers. So how do I ever dry that soil below the root zone? I think it is so saturated below the root zone that the newly applied water doesn't drain very well. What I do notice is some brown spotting on the leaves of the grass which I think is a sign of overwatering. I am a little confused on what to do next. It seems that ideally the soil would be moist to just a little below root level (maybe 6-8 inches in my case) and pretty dry below that. How do I get there ???? Thanks.


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## g-man (Jun 15, 2017)

When you say you cut back and it suffers, what do you mean? Can you define suffer?

California has a lot of different climates. In what part of california are you?


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## go_hercules (Jul 29, 2018)

Hi, I am in Ventura County about an hour north of Los Angeles. When I cut back the amount of water the grass wilts by the time I get to the second of two weekly waterings. The grass blades begin curling, the lawn takes on a grayish shade, and footprints are definitely left. I have a tubular soil probe that I use and I can see roots down to at least 6-8 inches. I would think that with those roots I should be able to get by on two waterings per week, but I can only if I put down quite a bit of water.


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## Delmarva Keith (May 12, 2018)

You may need to switch to 3x (or even more) per week, but much less total water. Typical root zone is like a horizontal sponge. It takes water until saturated and then water will slowly flow through it and run off of it. What may be happening is when you give it the full inch, a lot of water flows through it to the subsoil. Between the twice weekly watering, subsoil water can move back up into the root zone. When you give it less than an inch each time, there's not enough subsoil water stored to get enough hydration to the root zone between the twice weekly waterings. This may be why you need to apply so much water.

There are calculations and charts you can use to figure out the root zone soil water capacity and then your local extension service likely can give you real time daily ET. What comes out via daily ET must be replaced via irrigation within the time limits of not depleting the root zone beyond the wilting point of the plant.

As an example, on a typical full sun Summer day, ET where I am will run about 0.2" per day. I obtained soil numbers for my soil from my local university extension. Root zone soil water capacity for my soil is about 0.32 per inch and I use a root zone depth of 8" for total soil water capacity of 2.56". Permanent wilting point (where the partial pressure of the water in the soil is so low the plant can't physically take up water and it unrecoverably wilts) is 0.23 per inch or 1.84". So, looking at this so far, if my root zone starts at field capacity of 2.56" and I let it deplete to 1.84" at 0.2" of ET per day, I will reach permanent wilting point in 3.6 days and the plants will wilt and some types of plants would actually die (in reality, as the grass has more and more trouble obtaining water from the soil, ET will reduce and it won't wilt so fast or die, but let's not overcomplicate things for now). So in other words, no matter how much water I might apply twice per week, my grass might indeed still wilt between waterings.

So I don't want it to reach permanent wilting point but an amount of depletion that keeps it looking acceptable without being overly wet or dry. My compromise has been to water 3x per week. This allows for a 0.4" depletion between waterings with 0.6" depletion once per week. Not technically ideal, but it works due to the effects of reduced ET when the grass gets on the dry side and from a little water coming up from subsoil stores. Total weekly water applied in full sun areas matches average weekly ET of close to 1.4" per week. If it rains a lot (more than half inch) I back off the water accordingly.

You can do similar calculations for your soil and your ET. Typical allowable depletion would be around 50%. In other words, water often enough so the root zone doesn't get below half the amount between the soil water capacity and the permanent wilting point. Total weekly water should match ET and just divide total weekly water needed by number of waterings per week for how much to apply at each irrigation event. Given what you've observed on your ground, twice a week very likely isn't enough.


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## go_hercules (Jul 29, 2018)

Thank you very much for that. When I first started studying up on ET, field capacity, etc. I would read a lot of random articles that said that lawns needed about an inch of water per week. I wondered where that number came from and dismissed it as oversimplified. Well, after studying it more, and even measuring the ET in my own yard with an evaporation pan, I think about an inch is actually pretty accurate (water in my pan actually evaporated at about 0.1 per day, or only about 3/4 inch per week). That's why I was so confused that I needed so much. I think you are right that I may need three waterings instead of two. Do you think if I actually shoot for an inch total over three waterings that the subsoil will eventually dry out? I was thinking the subsoil water would just continue downward by gravity, but you are right that it appears to wick back up to the root zone.


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## Delmarva Keith (May 12, 2018)

go_hercules said:


> Thank you very much for that. When I first started studying up on ET, field capacity, etc. I would read a lot of random articles that said that lawns needed about an inch of water per week. I wondered where that number came from and dismissed it as oversimplified. Well, after studying it more, and even measuring the ET in my own yard with an evaporation pan, I think about an inch is actually pretty accurate (water in my pan actually evaporated at about 0.1 per day, or only about 3/4 inch per week). That's why I was so confused that I needed so much. I think you are right that I may need three waterings instead of two. Do you think if I actually shoot for an inch total over three waterings that the subsoil will eventually dry out? I was thinking the subsoil water would just continue downward by gravity, but you are right that it appears to wick back up to the root zone.


ET includes evaporation but more than evaporation. Simplified, they placed cool season grass cut to a standardized height in a cylinder of soil and weighed it. They then placed the cylinder back in the ground and let it do it's thing. They removed the cylinder from the ground and weighed it to determine amount of water used. They did this over and over a lot and derived an equation which includes sun, wind, temp, plant use, etc., etc. The equation pretty accurately predicts total water use (plant use and evaporation) based on all the other factors.

Here is an average ET map. http://missionrcd.org/wp-content/uploads/2014/04/CIMIS-Reference-Evapotranspiration-Zones.pdf

You can also likely get real time daily ET from your local university extension. Also note that reported ET is a reference which for grass is ETo. A crop coefficient is then applied to relate what you're growing to the reference. For tall fescue, it's going to be pretty close to 1 so you can ignore that for now or discuss with the university extension office.

I'm not familiar with your climate and Ventura County is a big place with several different climates so I can't zero in on your average monthy ET. Where I am in the Summer we can average 1.4" per week but it's generally not scortching hot or dry in terms of low humidity here.

If I were in your shoes, I would either go through the calculations as in my example or as a shortcut, figure average ET for the month based on the chart or a phone call to the university extension office, divide by 4.3 (there are 4.3 weeks in a month) and then divide that by three waterings per week and see how it does. Depending on ET and soil composition, it might need more than 3x a week.

Based on what you've said, yes, less total water more frequently will give you less waste into the subsoil and less potential for runoff. Deeply and infrequently is the mantra but for some soils and climates, it can be too infrequent and therefore too deep. If you figure a weekly amount based on weekly ET and figure frequency based on root zone depletion, adjust that as needed based on actual observation of the plants, as far as I know that's the most efficient use of water to keep the plants healthy and hydrated.

For the subsoil water, like I said, the root zone is like a sponge. Just like a sponge, the root zone will absorb water until it's saturated, and then the excess will flow through it due to gravity. That's basically the definition of field capacity - how much water can you add before it starts to "overflow" downward due to gravity. Whether the subsoil dries or not doesn't matter (and it won't completely dry due to capillary action and basic entropy - more wet will always eventually flow to more dry), as long as your root zone is adequately hydrated.


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## go_hercules (Jul 29, 2018)

I went to your link and found that I was in zone 3. That says 5.89 inches per month in July, or 1.47 inches per week. I had read you should replace about 75% of ET for cool season grasses. So my case would be 1.47 X .75 or about 1.1 inches. Anyway, anything between an inch and and an inch and a half seems to be my target. NOT the two inches I'm using. I think I will try cutting back to maybe a half inch or so twice a week. I will try to be patient if it wilts a little to get it through. Depending on how that goes, then maybe I will go to 3/8 inch three times a week. I have to say that in the past I watered three times a week before getting so scientific and the lawn looked better! But after all that reading I was convinced that I should cut the frequency. Now when all is said and done, I will probably end up where you are (and I was) back at three times per week. It's still fun figuring things out. Thanks.


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## Delmarva Keith (May 12, 2018)

go_hercules said:


> I went to your link and found that I was in zone 3. That says 5.89 inches per month in July, or 1.47 inches per week. I had read you should replace about 75% of ET for cool season grasses. So my case would be 1.47 X .75 or about 1.1 inches. Anyway, anything between an inch and and an inch and a half seems to be my target. NOT the two inches I'm using. I think I will try cutting back to maybe a half inch or so twice a week. I will try to be patient if it wilts a little to get it through. Depending on how that goes, then maybe I will go to 3/8 inch three times a week. I have to say that in the past I watered three times a week before getting so scientific and the lawn looked better! But after all that reading I was convinced that I should cut the frequency. Now when all is said and done, I will probably end up where you are (and I was) back at three times per week. It's still fun figuring things out. Thanks.


That's funny. I did basically the same thing when I first moved here from farther North. Everybody who should know better was watering at least three times a week, some every day and some more than once a day. But I knew better - lots of water twice a week. When the first drought really took hold I needed to "reevaluate" how smart I thought I was. :nod: After doing the calculations and discovering I could water whatever amount twice a week and still have turf in distress, my thinking changed quite a bit.

My local university extension is phenomenal (U. Delaware). They even have an online irrigation management tool that tracks hyperlocal ET, rainfall and lets you input irrigation amounts and then produces raw data as well as a graph over time depicting soil water content versus field capacity, allowable depletion and permanent wilting point.

If you basically cut the water in half twice a week, keep a very close eye on root zone soil water content with a soil moisture meter or even just the screwdriver test. There's a fine line between barely enough and dormant, the crash happens very quickly, and it can take a long time and a lot of water to catch back up if you let the root zone get too dry.


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## go_hercules (Jul 29, 2018)

I actually did buy a cheap moisture meter that seems to do a pretty good job. I "calibrated" it by using a tubular probe that I made to pull up a soil sample and comparing it to what the meter said. So I have a good idea what the meter is telling me. On another note, I live in a community with a homeowner's association that waters all the front lawns. For two weeks about a month ago I left cups in the front yard to see what watering schedule they are using since the grass looks very nice and healthy. Believe it or not, they water six out of seven days a week about 1/2 inch per day. That's three inches per week. Anything you read would tell you the grass should have shallow roots, etc. Well I have dug it up in areas and the roots are at least 8 inches or better. Go figure .....


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## Delmarva Keith (May 12, 2018)

Ha! That's funny. Water must be cheap there.

Roots continue to grow where soil water is adequate. I'm sure you know the old wive's tail about roots "finding" water is a myth. If the soil is hydrated, they continue to grow into it until the top growth can no longer support additional growth. If it's not hydrated they stop. If the topgrowth could support it, grass roots would just continue to grow deeper and deeper as long as soil water content stimulated that growth. Corn is a good example of a grass plant with very deep roots - corn roots go deep, several feet. The topgrowth allows the plant to feed such a deep root system.

Frequent shallow watering generally results in a shallow layer of hydrated soil and thus shallow roots. Three inches a week would result in deeper roots.

I limit water to 3x per week because I don't want to waste water (even though I have a dedicated irrigation well, I just don't like squandering water) plus the fungus pressure is pretty extreme here. Watering at 3" per week, 6x per week here would almost certainly result in a fungal slime instead of grass, interspersed with incredibly healthy sedges growing like a lawn. :lol:


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## g-man (Jun 15, 2017)

This is a great thread. I second what Keith is saying. One thing to add, if you notice wilting, trigger the irrigation regardless of your schedule.

Also, i have seen studies of root depth (in ideal soil conditions) of kbg going 30in. I think tttf was around 4ft. If there is water and nutrients, the roots will keep growing.


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## go_hercules (Jul 29, 2018)

One more thing that confuses me is that when we talk about the "root zone" it's as if the grass ONLY absorbs moisture through the tips of the deepest roots. But doesn't the grass have deep AND shallow roots? If so, like I'm guessing, then letting the top few inches of soil dry out between waterings would starve the upper shallow roots. Assuming the roots are as deep as you would like with no reason to coax them into growing deeper, then why not water just enough to always keep the soil JUST DOWN to the root zone wet? That might mean a medium amount pretty regularly, maybe almost daily. In other words, if the roots are already deep enough, then why not try to keep the soil at field capacity constantly - not dry and not wet but just right. So instead of soaking and drying, you would just maintain the ideal condition all the time. And like I mentioned, this is not like the scenario of watering a tiny bit every day just down an inch or so where the roots never get deeper. Seems like once the roots are full grown, then just keep the soil perpetually just right. Is this possible or am I overlooking something? Like maybe the time it takes to go from watering down to field capacity or something? Must be a reason not to shoot for this or I would have heard of it !!!!


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## g-man (Jun 15, 2017)

Some reasons to avoid daily watering.

1) avoid fungus. Daily will get the leaf blades wet every day. Heat + water promotes fungus. So avoiding watering everyday helps that.

2) deep roots would also be in the colder layers of the soil, which help in the summer heat. During the summer, roots die with the heat. Having deep roots,

3) I think too wet roots also grow fungus, but don't quote me on that.

4) save $$. I would like the roots as deep as my soil will let them, so they could use all the available water (think rain). If i just chose 6in as my target, then I would not take advantage of the deeper moisture that is there.


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## go_hercules (Jul 29, 2018)

Good points. I had not considered some of this since I live in a very dry zone where rain is rare. But I do get mushrooms if things stay too wet. Thanks for the reply. I will re-read all these posts and let it sink in.


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## Delmarva Keith (May 12, 2018)

In addition to g-man's thoughts, the root zone, when talking grass anyway, is the zone from the surface to the depth of the deepest effective roots. You can take soil profiles to get a good idea of how deep this is or make an educated guess based on grass type. The grass will use the water in this entire zone. Like we said before, roots exist where the soil is hydrated. So if the root zone is kept reasonably hydrated, roots will exist throughout.

Root growth can be encouraged but there's a practical limit based on a number of factors that we can talk about at length another time.

In terms of irrigation strategy, like g-man says, just watering all the time is generally bad. fungus, lack of oxygen, encouraging shallow rooted weeds to flourish, on and on. The ideal irrigation strategy would be the most infrequent possible, i.e., irrigate to full field capacity only when the root zone soil water content falls to maximum allowable depletion. With typical grasses, soils and climates, this is usually two or three times a week in the Summer. In very sandy soils with shallow rooted grass types (like a putting green), it may be more than once a day.

If fungus is not an issue and the turf is thick enough to crowd weeds, watering to full field capacity and then daily watering to replace ET should work just fine. Lots of golf courses water every day (but they also have a full time staff and a fungicide budget to deal with the consequences of that - and they often still get fungus).


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## go_hercules (Jul 29, 2018)

What would happen to a golf course that was watered daily if it had to go a few days without? Would it die off like a piece of unlaid sod?


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