# Relationship between TDS and unused ferts



## papa_c (9 Jan 2021)

Something I have wondered about and never found the answer.

After the weekly water change, the TDS of my tank is around 220. Dosing salts at 1.5 x EI levels, by the end of the week my TDS rises to around 375. 

Does that mean that I have excess ferts that are not consumed by the plants?

Is there a directly relationship N P and K and TDS?

If I dosed at exactly the level that are consumed by the plants would me TDS remain consistent through the week?


----------



## sparkyweasel (9 Jan 2021)

papa_c said:


> Does that mean that I have excess ferts that are not consumed by the plants?


That's the EI principle, there are always excess ferts, therefore never any shortage.


----------



## sparkyweasel (9 Jan 2021)

papa_c said:


> If I dosed at exactly the level that are consumed by the plants would me TDS remain consistent through the week?


You would get some increase from fish and plant waste, uneaten food etc.


----------



## dw1305 (10 Jan 2021)

Hi all, 


papa_c said:


> the TDS of my tank is around 220. Dosing salts at 1.5 x EI levels, by the end of the week my TDS rises to around 375.
> 
> Does that mean that I have excess ferts that are not consumed by the plants?


It just means that there are more ions, but when you add a salt like MgSO4.7H2O, you are adding sulphur (S)  (the sulphate ion SO4--) as well as Mg++.  The amount you add will be many times the sulphur requirement of your plants. 

We can limit the excess of "spare ions" by adding salts like potassium nitrate (KNO3), where both anion (NO3-) and cation (K+) are nutrients, same would apply for di-potassium phosphate etc. but you will always have ions in excess. 


papa_c said:


> Is there a directly relationship N P and K and TDS?


Unfortunately  not.

cheers Darrel


----------



## papa_c (10 Jan 2021)

Thanks guys for the comments.



dw1305 said:


> Unfortunately not.



I'm trying to look at unnecessary overdosing and was hoping on a simple option of TDS.


----------



## PARAGUAY (10 Jan 2021)

That were the necessary 50% water weekly change is advised with EI


----------



## dw1305 (10 Jan 2021)

Hi all,


papa_c said:


> I'm trying to look at unnecessary overdosing and was hoping on a simple option of TDS.


I use a <"conductivity datum range">,  in conjunction with the <"Duckweed Index">, but I'm adding probably <"an order of magnitude less nutrients"> than I would be if I used EI. Because I'm not adding many nutrients, and I'm using a 90 : 10 mix of rainwater : hard tap water in the tanks, the ions that I'm measuring are probably <"mainly bicarbonate (HCO3-) and calcium (Ca++)">.

cheers Darrel


----------



## Basviola (11 Jan 2021)

So what can I actually use my TDS meter for?!


----------



## dw1305 (30 Jan 2021)

Hi all, 


Basviola said:


> So what can I actually use my TDS meter for?!


Checking whether your tank water has continually rising TDS. If it does you probably need to add less fertiliser or change a bit more water.

cheers Darrel


----------

