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Relationship between TDS and unused ferts

papa_c

Member
Joined
22 Jan 2013
Messages
334
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?
 

dw1305

Expert
Staff member
Joined
7 Apr 2008
Messages
11,493
Location
nr Bath
Hi all,
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.
Is there a directly relationship N P and K and TDS?
Unfortunately not.

cheers Darrel
 

dw1305

Expert
Staff member
Joined
7 Apr 2008
Messages
11,493
Location
nr Bath
Hi all,
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
 

dw1305

Expert
Staff member
Joined
7 Apr 2008
Messages
11,493
Location
nr Bath
Hi all,
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
 
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