How much water am I putting down?

Using drip tape or line…after 1 hour, the surface is moist only about 3” on either side of the tape, so maybe a 6” strip of moist soil. How much is it spreading below the surface? Since the ground was slightly moist it’s hard to tell
by digging down (I’m not comfortable letting it get THAY dry before watering for onions.)

It depends on how well draining the soil is. With loose / sandy soil there is minimal spread; with dense / clay soil it spreads out wide, but slowly.

I had a bed that was filled with mostly bagged raised bed soil and it was almost TOO well draining. What I ended up doing was just watering every 6 hours (even at night) for 3-5 minutes.

Its a myth that you need to let soil dry out between watering. Plants can easily grow with their roots completely submerged in water (hydroponics), the key thing they need is oxygen. So as long as you don’t saturate your media (which is very hard to do with well draining soil) you can keep it wet at all times.

One thing to note is that this could be detrimental to growing root crops like beets because most of your root development will happen close to the surface. But I didn’t test this out too much.

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It’s loam but trending towards sandy loam.

Still, it’s really a question of whether I’m watering long enough and if I have the correct spacing of my lines.

Example: my onions are planted 7" apart in a “block” of four rows, each 7" apart.

I ran two lengths of 1/4" soaker drip line, 6" emitter spacing, between the first and second row, and one between the third and fourth row, so each run is 14” apart, and 13’ long (length of the bed).

The “wetted” area is only about 5-6" wide at the surface after 75 minutes of watering per my “test” area of bare soil. My concern is the water went straight down, well below the shallow root zone of onions, but did not spread laterally enough to cover the whole rooted area.

Yet, I can’t see how much lateral spread happens under the surface, either.

The “how long” question is tough, too. My 6” soaker dripline is rated for 0.8 GPH, but multiple tests I have done at multiple locations in the system shows it putting out about 0.5 GPH.

So just for kicks, let’s assume the actual lateral spread is 10” including under-surface spread. That means I’m watering an area 24” wide. The bed is 13’ long, so this is 26 square feet.

In that 26 square feet, I have 52 emitters. So using my observed 0.5GPH, that’s 52 * 0.5=26 gallons of water in an hour. I ran it for 75 minutes (1.25 hrs) so that’s 32.5 gallons.
One inch of water over one acre (43,560 square feet) is 27,154 gallons.
0.0006*27,154=16.29 gallons equals the equivalent of 1” of water

So if this is correct, I put down the equivalent of about 2” of rain. It could be a lot more or less than that depending how much it spreads. Although I probably only have to run it about half as long as I did!

You’re overthinking this.

Let the soil dry out decently and then run the system for 10 minutes, let it absorb for an hour and then dig down to see how far the spread is. If its not enough run it for another 10-20 minutes, wait an hour then check again in another spot.

If it looks like spread is really poor you will need to run at a much higher frequency for shorter duration. Let the soil dry out and try watering for 5 minutes every 4 hours for a day, and see how that works. If this fails then you will just need to reduce spacing between lines.

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Also forget the whole “inches of water” thing, there are too many factors that can influence water need: planting density, plant canopy size, temperature, humidity, soil water retention, etc.

Trust me, I’ve already been down this road and wasted many hours in Excel.


Maybe I’ll just run a line to an area that’s currently barren of plants, so I don’t risk letting something dry out too much while I experiment.

Of course, in my climate, we could get a rain and I have to start over again.

It’s almost unfortunate (in this sense) that I’m not somewhere like California where it never rains in the summer.

You could, but the whole point is to test it in the “working environment”, so to speak. It will be like comparing apples to oranges. The water distribution will very likely be different in the planted area because of the plant roots.

I’m not talking about making it bone dry, I’m talking about making the top 2-3 inches just dry enough to see the water distribution.

Don’t forget that how dry a soil is also impacts capillary action - ever try to water a potted plant that you forgot to water for way too long? The water just shoots right out of the bottom with minimal absorption. You need to water it a bit and then come back later to water again to get it to saturate.

Oh and if you ever need to decrease your 0.8 GPH drip line to below what its rated you can just add a valve on the 1/2" main line (which you should already have for each zone) and close it until you get the decrease you want.

I run mine closer to 0.1 GPH (about 2-3 drips every 1 second) by closing the valve almost completely. This impacts pressure in the line and is probably not suitable for drip line runs longer than 10-15’. Works really well for me though.

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A related question:

For large plants like tomatoes, might be better to put the drip tubing in “rings” around each plant rather than a line?

Fruit trees as well? (Although fruit trees rarely need irrigation my climate)

Yeah I’ve used rings before. Downside is they are less versatile than lines if you decide to change planting locations - I mostly use rings for potted plants now. You don’t want to be rebuilding your watering system every year.

If you do decide to use drip line in rings, instead of making a closed loop with a barbed T piece I use a barbed L piece and then a goof plug at the end. Then you just zip-tie the loop closed. This way you can easily add or remove the ring as needed just by cutting the ziptie.

I use drip irrigation to do the bulk of the work and water everything else by hand. There’s a point of diminishing returns if you try to water EVERYTHING off drip irrigation.

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I agree. I have a couple small plants that are easier to hand water.

Could I use a soil moisture meter to figure this out?

Yeah that could work if you already have one, but its just as easy to tell how damp soil is just by pushing it back with your hand a bit.

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