Testing fertiliser solutions

[bpolky]

I have a large collection, mostly local species. I have automated most things as I travel regularly. I have watering on timers, and use a venturi pick up system to provide low concentration fertilizing in every watering. I create my own fertilizer mix using worm soup from my own worm bins, inorganic fertlizer, egg shells for added calcium and Magnesium sulphate. (This part of the world has high Ca/Mg ultramafic soils.) The problem with this approach is that I do not know exactly what fertiliser concentration is reaching the plants. I manage the pH at around 6-6.5 when I can (local water supply varies wildly). Is there an easy way to measure the NPK of a sample without needing a lab coat and chemistry set? Is TDS a reliable measurement? Is there anybody out ther doing this successfully that can give me a guide to their process and the readings I should be aiming for?

[Ray]

TDS meters give you a number, but that tells you nothing about the analysis of the solution. I would approach it like this:

- Send away a sample of your concoction and get a true analysis.

- With that in hand, make samples of various dilutions and measure the TDS. From that you can build a calibration chart for your meter - X ppm TDS = Y ppm N.

- Fill a large bucket of known volume with water, insert your venturi pickup into it, and collect all of the water discharged in other buckets of known volume. Collect that water as far downstream as possible, so that the pressure drop comes as close as possible to your normal setup and use two buckets to collect the water - dumping one while the other fills. When the known volume has been totally consumed, by knowing the volume of the total output, you will have a reasonable "calibration" for the venturi.

Put that together with your TDS calibration, and you should be able to monitor your final solutions.

There are some caveats with this, however: your use of natural ingredients could mean that your concoction may be variable, batch to batch, and if you have nozzles in the irrigation system beyond where you calibrated the venturi device, the pressure drop will be greater, so your ratio will change. You can, however, use your TDS meter to check the difference - what it ought to be versus what it is - and change the concentration in your feed tank to compensate.

Also be aware that if your incoming water pressure changes, that'll affect it, too.

[bpolky]

Thanks for the input. I do not live in a major city, and access to supplies/services is limited. Most of my plants are growing quite well, so I know what I have is not bad enough to be toxic. I also know the incoming water supply is highly variable, as are my starter ingredients. Also, as the concentrated solution level goes down, I know the pick up rate changes. Overall, this is not a situation where I can be in total control. I was hoping for a fast way to do a rough check at the nozzles so I understand things better and can maybe improve the situation. I know in hydroponic vegetable growing they usually aim for about about 1200 ppm. I was guessing that around half that figure would be good for orchids. Do you know what a reasonable target TDS/ppm value would be for orchids?

[Ray]

I'd make my estimates by the nitrogen concentration, not by the TDS. As that is likely mostly coming from your commercial fertilizer, you should be able to easily estimate that.

In reading about plant nutrition applied to epiphytes, the number often quoted is an exposure to about 10-15 ppm every time it rains, which can be several times a day. In my own experimentation, I have concluded that somewhere between 50 & 100 ppm N applied over a week is good.

Watering every one or two days, I went with 25 and now that I feed weekly, I apply 75 ppm N.

So let's use 75 as the target, and assume your fertilizer is 20%N and your siphon averages 15:1.

You'll want that concentrate to contain 15 x 75= 1125 ppm N. As a ppm is a mg/kg, 1125/0.20 = 5625, so your concentrate should contain 5.625g of the 20%N fertilizer per kg - you're probably OK making that a per-liter addition.

Then measure the TDS of the diluted solution and you'll have your control point.

[bpolky]

Thanks Ray. I can see I will need to drag out the lab coat if I want to get any further into this. At the moment, the only sensor I have is the plants, and they are growing OK. I know it could be better if I put in more effort and provided different plants with different requirements, but that is more time and effort than I can offer at the moment. I have a number of 'exotics' (exotic to here!), but 95% of my collection is local species. I live in the north of Borneo, so there are enough locals to fill ten collections. That at least means the required conditions are at least remotely similar. My biggest problem is temperature, not nutrition. Most of the species grow in the local wet mountainous rain forests, and I am down at sea level where every day, year round, is almost the same. 31-34 degrees. Up in the hills is normally closer to 25. Most plants I get are from local markets, are un-named, and the sellers know nothing about them. I never know what I am dealing with unless I can get a flower. I will try anything, but if it struggles here I usually guess it has come from high in the mountains and I know not to try it again. I will try the TDS meter for now and see if that teaches me something. Because I use mostly naturally derived fertilizers (worm tea, fermented chicken manure etc) I will never be able to produce a constant and exact outcome. Here I guess it is just working with what is available. A TDS reading might just help a little.

[Ray]

The trouble is that different materials will give you different TDS readings, even if they are the same concentration. So if you shoot for a constant TDS, but you formula changes, you don't know any more about what you're applying than you did!

[bpolky]

I will know more. I will know what the TDS reading is. :-) But you are right, that will not tell me anything that I need to know. I think the only way I could get answers would be to buy one of the NPK chemistry sets that they use in commercial hydroponic operations.

[bpolky]

Here are my outcomes on using a TDS meter. I am making some big assumptions given the lack of data and the limitations of my readings. Lets see if I have this correct.....
The water supply here is surprisingly soft (about 50ppm). A test at the nozzles of my watering system gave a reading of just over 100ppm. Assumption 1 : the salts in the tap water are not nutrients. That means my fertilizer system is adding about 50ppm. Assumption 2 : The NPK values in the fertilizer solution are similar. That means I am adding about 50/3 or 16ppm of nitrogen. I water every morning and it rains most days in the late afternoon. Based on your figures, I probably should be aiming to have the reading at the nozzles of about 20ppm nitrogen. That would mean 60ppm total nutrients, and therefore 110 ppm total given the 50 in the water supply. Have I made any blatant errors in that calculation?

[Ray]

Originally Posted by bpolky

Have I made any blatant errors in that calculation?

In the calculation, no. You have, however, probably grossly overestimated the nutrient concentration by assuming that all of the fertilizer TDS is N, P, & K.

As an example, the MSU fertilizer for RO water is 13-5-19-8Ca-2Mg, plus trace elements. At 125 ppm N, the solution has a TDS around 1000 ppm.

All cations and anions contribute to the net electrical conductivity of a solution, and a TDS meter is just a cheap EC meter with a built-in conversion to TDS (making the meters less than reliable for actual levels).

You must "calibrate" your meter with solutions of KNOWN nitrogen concentrations if you want to be reasonably correct in your estimates.

It's easy - get three, 1L containers of your plain water @ 50 ppm TDS (as you stated). Add 0.5 gram of your fertilizer to one and 1g to another. If the fertilizer is 20% N, then you've added 0.1 and 0.2 g N, respectively, or stated another way, 100 mg/kg and 200 mg/kg of solution, which is the same as ppm. {Note: my fertilizer additions were chosen arbitrarily, and I have rounded the "per kilogram", as technically it should be 100/(1kg water + 0.5g fertilizer)}

So you now have three solutions - 0, 100, & 200 ppm N. measure the TDS of each, plot it on a graph versus the N concentration, and you're all set.