Cheap fertilizer injector mod

Jay Part Shade (Zone 10B, S21, Los Angeles)June 11, 2014

Hey guys, here's a little mod I put together so I could fertigate my ever growing collection of container plants. Wrestling with the watering can and teaspoons was just getting too time consuming. And the other cheap fertigation solutions out there (Ez-Flo, Hozon) have marginal reviews and seem difficult to dial in correctly, if they work at all. The Dosatron/Dosmatic route is the best, but at over $250 each, they're too expensive for the three areas around my house that need watering.

I'm using the Gilmour 362 sprayer ($18 Amazon) and a Melnor wand ($8 home depot). There's a number of Gilmour sprayers, but the 362 is the highest rated and built like a little brass tank. The ratio goes down to a single teaspoon per gallon, which is great for Dyna-Gro -- fill up a quarter to half the container with fertilizer and the rest with water and you're good to go. At the lowest setting, you'll get 100 gallons before needing to refill.

Also, it's easy to double check that the Gilmour is actually proportioning the fertilizer correctly. Set the dial to 10 and fill up a gallon jug. 10 tbs (5 oz) should disappear from the tank if your water pressure is a standard 50-60 psi.

The problem with the Gilmour is its nozzles -- either a flat 10' wide fan or a beam of water that'll strip paint. That's where the Melnor wand comes in. Just about any wand will work, though you'll need a hose thread adapter to connect the wand to the tube. I'm using 1/2" tubing from Home Depot along with metal screw clamps to tie the whole thing together. I'm sure one could rig up a nozzle directly to the end of the sprayer without the hose. If Gilmour strapped a watering wand on the end of this sprayer, it'd be the best hose fertigator around.

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I actually have one of those sitting around. Great idea!

    Bookmark   June 11, 2014 at 10:02PM
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Thanks for this!

I have recently been looking for a reasonable outdoor hose based fertigation solution myself. Have about 60 containers on a roof that range from ~3 gallon to whiskey barrel size, and using a watering can for all of it is very time consuming and at the edge of practical.

I did try a few cheap "hose end sprayers" and they were all garbage - leaky, crappy nozzles, impossible to adjust accurately, etc.

It looks like this requires 2 hands to operate - do you find it awkward in practice? I will try to frankenstein one up this weekend and hopefully my plants will appreciate it as much as I do.


    Bookmark   June 12, 2014 at 11:59AM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

Turns out attaching the water wand directly to the end of the sprayer was easier than I thought. Simply cut the clear hose, leaving 6" on the sprayer. The metal wand unscrews and fits over the plastic hose nice and snug. And it saves having to buy the brass pieces to attach the hose to the watering wand.

    Bookmark   June 12, 2014 at 11:59AM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

Hey Daniel,

Glad to help! Yeah, the plastic deals just aren't worth it. And your Brooklyn roof garden sounds awesome, post a pic if you can :)

The second version is one-handed. The two-handed version was a little awkward, mostly because I need the second hand to move the hose around.

The second version is much better (and cheaper). If your hose is strong enough (the better ones are rated to constant 100+ psi) you can just leave the sprayer attached to the end and the hose always on. Just make sure the bottle always stays upright, otherwise it can leak through the back flow valve.

    Bookmark   June 12, 2014 at 12:08PM
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rebuilder(7a-7b Snellville, GA)

Thank you so much! I have been mixing my fertilizer in a 40 gallon garbage can and using a 12 volt pump and a garden hose. I have a lot of containers so I have to fill it almost twice(65 gallons or so). I am tired of that laborious setup. My last fertilizer garden hose sprayer was a complete disaster(Pennington Smart Feed). It literally blew up in my hand plus it would go thru the whole cartridge of fertilizer discs in about 3 or 4 minutes(the binder they used didn't bind very well)
Thanks again!

    Bookmark   June 13, 2014 at 5:16PM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

Hey Tony, happy to help, I just ordered my second one :)

Update on maintenance: you may run into a clog if you're using the tsp setting and don't clean the sprayer. Cleaning is really easy -- simply fill the tank with water and spray away.

That said, to convert the sprayer from tbsp to tsp, you insert a plug into the tube inside that has a tiny pinhole to let the fluid through. I left my sprayer out all week with the plug in and sitting in the sun. The hole started to crust over with fertilizer salts or tap water calcification. Dyna-Gro is really pure, but one could run into an issue where it gets clogged frequently. Mine started to run slowly on the tsp setting. I took out the plug, rinsed it off and blew into it to make sure it was clear.

If you want to use the tbsp setting, adding 5oz out of 15oz in the sprayer will give you full strength Dyna-Gro. 2.5oz is half strength, etc. The tank needs to be refilled more often in the tbsp setting, but there's less chance of a stoppage.

    Bookmark   June 13, 2014 at 6:09PM
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I tried today and almost got it, but failed, may have gotten wrong tubing and clamp. The tubing I found is 1/2" inside diameter, 5/8" outside. The clamp I got is way too big, I guess a 1/4" instead of 1/2" clamp is probably what I need (?). The tubing won't fit inside the sleeve on the sprayer without slitting the end of it, and then stuff leaks all over (admittedly, I need a clamp that works for the end that goes over the gilmore nozzle as well, the one I got is way too big).


Do you have more specifics on the sizes of tubing (inside/outside diameters) and clamps you used? I may just take the whole contraption to the hardware store and ask them for suggestions. I don't know much about this stuff.



    Bookmark   June 18, 2014 at 5:55PM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

Hey Daniel, sorry to hear it didn't work. It looks like you got tubing one size too big. I'm using the 1/2 OD and 3/8 ID tubing. And the clamps I have are 1/4" x 5/8" (that's what it says on the receipt. They're the smallest ones available and fit the tubing perfectly).

And are you using the Melnor wand? When you have the right tubing, it should fit snug. I've also used the little rubber grommet that's left on the wand and slid it onto the tubing before putting on the wand. It makes the fit a little tighter.

Also, because of the back flow preventer built into the handle, there will always be a little water that dribbles out when you turn off the hose. But once everything fits together, it shouldn't have any leaks.

Let me know if you have any other issues :)

    Bookmark   June 18, 2014 at 6:16PM
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Jay, thanks for the help and super quick response! The correct parts made all the difference. However, I changed it up a little.

I am using the melnor wand, looks like it has the same handle as yours with rubber grommet etc. I found that with clear vinyl tubing, it was a bit too flexible. I had two problems - first water was leaking out from the metal sleeve that the tubing is inserted into. I fixed this by shoving the grommet in between tube and want sleeve, though it didn't really fit I made it ;) However, no matter how tight I made the clamp, water came out from the tube where it goes over the gilmore nozzle. I also found the vinyl tubing too flexible, and that I still had to use two hands, one to make sure the wand didn't fall off.

I ended up using polyethylene tubing, same size - its much more rigid and somewhat opaque in comparison to vinyl. My first try like this held up the wand great by itself, but water still came out from where the tube goes over the nozzle. I solved this by wrapping 2 layers of vinyl electrical tape over the nozzle before clamping the tubing to it. Now, it is truly a 1 hand device and does not leak (time will tell...). With the polyethylene tubing I also did not use the grommet, wasn't necessary and didn't fit.

The other thing I did was test various settings, which were not as expected. On TBSP setting (default) I filled up the bottle with 15 ounces of water, set the dial to "2", and sprayed out a measured gallon, expecting to see an ounce gone from the bottle, but much less had been used. Setting 4 seemed to get me closest to 1 ounce of liquid from the bottle for 1 gallon of output, and setting it to 1 appears to use pretty much no liquid from the bottle. Pretty sure I'm not clogged. My hose pressure is pretty good too, though being on the roof it may not be as good as I think. Have never measured it.

Since I'm not even confident that the scale is necessarily linear, I'll stick with the fact that a setting of 4 uses 1 ounce, or 2 TBSP per gallon, and mix accordingly and use setting 4.

Anyway, for anyone else trying to use one of these, do measure the output.

    Bookmark   June 19, 2014 at 1:30PM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

Awesome, glad you got it to work! I now have two setups, one for the front yard, one for the back. I've changed out my hoses to much higher strength air hoses that won't burst so I can just leave the water valve open all the time and spray whenever I want.

Yes, the hose PSI does determine the mixing ratio. To get the standard ratio, PSI needs to be between 50 and 60, which is standard residential. Maybe it's higher because you're in a building that needs to service multiple residences? I'm not sure if higher PSI means lower mixing ratios or the other way around, but Amazon sells brass pressure regulators that reduce the PSI to 60.

    Bookmark   June 19, 2014 at 2:39PM
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I presume the gilmore mixer works off a standard venturi injector principle. If it does, then that means higher source pressure would increase the amount being injected. Also keep in mind that any restrictions beyond the injector will decrease the amount of injection. A long hose, a small hose, a garden sprayer, etc will create some amoumt of back pressure and reduce the effectiveness of the venturi injector. It's still a great solution, just need to be aware that the mixing ratio won't be exactly as expected on the dial.

    Bookmark   June 19, 2014 at 5:30PM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

Hey Charina, thanks for the info. I believe it is venturi. It definitely doesn't mix water in like a Ez-Flo. Hozon and their ilk have the injector at the base of the hose, not at the end, and that's where I think people have run into problems -- length of hose, etc. It really screws with the ratios.

But I think hose-end mixers like the Gilmour are more accurate because there's a lot less back pressure. At least, I tested mine with 50ft of 3/8" hose and the whole contraption on the end and it worked perfectly. Honestly, I wasn't expecting it to but it stayed accurate. The hole at the end of the Gilmour is so small, I don't think any sprayer attachment could cause back pressure (when I had the wand set up on a hose like the first picture, I did get back pressure if I didn't press the handle -- water would shoot out of the top of the gilmour).

Like Daniel mentioned, it's easy to test. I simply put the dial on "10" and fill up a gallon water jug. 5 oz (or 10 tbsp) should disappear from the bottle. If not, adjust your ratios accordingly.

    Bookmark   June 19, 2014 at 6:56PM
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hairmetal4ever(Z7 MD)

May I ask a REALLY STUPID question?

Does this property mix and distribute something like FoliagePro?

The only hose-end fertilizer sprayers I've ever used where one that Miracle Gro used to sell ages ago - and when you first set it up, it comes out about 10X stronger than towards the time the instructions said it was time to refill.

Does this setup give you even concentration from start to finish?

    Bookmark   June 20, 2014 at 2:23PM
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If it is a venturi injector, as it appears to be, then yes, it is a constant rate of injection whether newly filled, or almost empty. Changing your supply pressure, or any changes in backpressure beyond the gilmour will alter your mixing ratio, but otherwise time won't change the ratio much - they are pretty steady (until you run out and begin pulling air rather than mix).

    Bookmark   June 20, 2014 at 3:12PM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

Hey Hair Metal, yup, I use it with Foliage Pro. And, as far as I've tested, yes, it's even from start to finish. With Foliage Pro, put the Gilmour on the teaspoon setting and then fill up the bottle with 1/4 or 1/2 (depending on the strength you want) and the rest with water. Or, if you want full strength, just fill the entire thing up with fertilizer and you're good to go.

    Bookmark   June 20, 2014 at 3:23PM
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hairmetal4ever(Z7 MD)


1-if your water pressure is higher or lower than 50-60 psi, does the concentration change?

2-if you put the Gilmour sprayer directly on the end of the hose and leave it "pressured" until you pull the trigger on your sprayer to water, does it stay put or will it leak water into the reservoir and dilute your solution?

    Bookmark   June 20, 2014 at 4:23PM
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I can answer 1) from playing with other non-gilmour venturi devices.

The rate of injection by a venturi is impacted primarily by the pressure differential between the source side of the restrictive orifice in the injector, and the output side. A higher pressure on the input side generally will cause a higher injection rate. I say "generally", because it is dependent on the pressure differential on the two sides of the orifice. Backpressure from a sprayer, or a bypass (not applicable to Gilmour) on the downstream side can affect the differential, despite what the source pressure may be.

Lower pressure from the source (or an inadequate flow that doesn't allow pressure to build) will reduce the injection rate (lower concentration of end product). A higher pressure will increase the injection rate.

    Bookmark   June 20, 2014 at 4:49PM
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Jay Part Shade (Zone 10B, S21, Los Angeles)

I leave my hose on all the time and there's no drips. It's when you turn the hose off that the water leaks out the backflow preventer in the handle.

    Bookmark   June 20, 2014 at 9:41PM
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I made the below post on the Fruits forum in relation to acidification of water for blueberries. I thought I would repeat it here, as it could just as easily be used as a less expensive mode of fertilizer injection, or pH + fert mix injection. The injector is attached to my hose bib by a 5 ft hose, and a 1 gallon milk jug acts as my reservoir of dilute acid (for now, a more permanent and secure 5 gal bucket is in the works). Then, and 50 ft hose and a standard shower wand is attached as my distribution device. Just be aware, that changing the settings on a sprayer, or adding a second hose will add backpressure, and change the mix. It still works, but will need adjustment to maintain the same ratio.


I spent more than my budgeted amount on plants this year, so the addition of a fancy injector for acidification of water for lowering the pH of my irrigation water as out of the question. With almost 30 blueberry bushes, and living in a very dry climate (both low humidity, and very little rain), watering via buckets or watering cans, each treated to adjust pH was also not a viable option.

My solution is a venturi device to pull acidified water into a stream of water and then distributed via a hose. Here is the second attempt to build a satisfactorily working injector.

The first attempt was to use a ½ inch injector bought off ebay. Unfortunately, the smallest injector I could find on ebay required a flow rate that could not be delivered through a hose and hose end sprayer. Already on order was the Mazzei 283 in the picture above. Although still ½ inch threads, the injector is much smaller, and hence, designed to operate at much lower flows. I had to cut and mend the first attempted setup in order to fit the Mazzei, as the Mazzei was about an inch shorter. Hence the coupling next to the ball valve.

This setup worked, but the ball valve left lots to be desired in the way of adjustment of the pressure differential on the two sides of the injector device. The injectors function is regulated by the amount of pressure differential between the input and the output, and the resulting flow rate. By opening the bypass ball valve, and letting pressure increase on the downstream side of the injector, less injected material is pulled into the stream at the venturi. Conversely, the injection rate can be increased by closing the valve, and increasing the pressure differential.

I’m using 98% sulfuric acid, significantly diluted into a jug. This mixture, of which I’ve tried several concentration rates, ranged from 12 ml to 21 ml per gallon of water. This diluted mixture is then piped to the injector, and when a portion of this is mixed with the irrigation water, the overall resulting pH coming out the hose end is adjusted.

As I said, the ability to precisely adjust the pH of the resulting water was lacking. And, if I wanted to use regular water, or less acidified water for fruit trees, or other uses, I would either have to pull off the hose and rearrange things, or open the ball valve all the way, and then readjust the pH through titration, and attempt to get the ball valve in just the right position.

To address the adjustability, and add an additional bypass to simplify changeover to non-adjusted water, I built the following injector setup.

Here is an exploded view of most of the parts, prior to assembly.

I ended up adding a 200 mesh filter, as the secondary irrigation water here can be very dirty, and the orifice in the injector is small enough that it could get clogged.

The use of a gate valve provides sufficient control, even if it still is not as precise as I would like it to be. I suppose one could use a needle valve for fine adjustments, but incorporating one wouldn't be as simple. And the additional ball valve allows me to simply open it to not inject acid, or close it to use the injection capabilities. The pH in my potting mix is a little high right now, so I’m adjusting the water coming out the end of my hose sprayer to a pH of 4.5 for the time being. After a few waterings, I’ll crack the gate valve open a little bit more (or I could add less sulfuric acid to the dilute I’m injecting), and put the pH closer to 5 or 5.5.

I haven’t carefully tracked the costs of this setup, and I’ve spent extra in the various tinkering and rebuilds, but I think it is reasonable to say that one can be built for less than $50, presuming you already have tools and pvc cement. $60 if not. The injector is the most expensive part, about $30. A pvc gate valve is spendy, at about $7, which I found online, although some people say they can be found at their big box hardware stores. I suppose one could use a metal one, since it doesn’t come in direct contact with the acid, but I didn’t feel like dealing with any corrosion that could be possible. The rest can be obtained at any hardware store with a decent sprinkler component selection.

    Bookmark   June 27, 2014 at 6:11PM
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rebuilder(7a-7b Snellville, GA)

Thanks again Jay and Charina (for design of future injector). I finally got around to hooking mine up and after trying various wand setups I finally just added a section of open pipe with a bend to slow down the water stream. Works great except for small containers. It is so much easier than mixing three separate barrels(my first post(65 gallons) was just for the back yard). Also using pumps for the pressure was a pain to setup and maintain.
You've helped make my gardening chore so much easier!

My next project is to build charina's injector with a large stationary tank.

Have a great growing season!

    Bookmark   July 28, 2014 at 7:02AM
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Nice modification! One of these days I'm probably going to figure out how to rig a custom fert injector to my rain barrels and lines for drip watering of my container garden while I'm on vacation. For now your mod would be better than me power washing my veggies with the Gilmour directly, which is what I do today along with adding compost, vermicastings, and CRF to my containers and fluffing the perlite and peat moss.

    Bookmark   August 4, 2014 at 3:42PM
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