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Category - Water Quality

How to Balance Nutrient Solutions

Author: Guy Sela

 

Before we learn how to balance a nutrient solution, we must first understand what "A balanced nutrient solution" means.

 

Absorption of mineral nutrients by plants

Plants absorb mineral nutrients from aqueous solutions, whether it is a hydroponic nutrient solution or soil solution. Mineral nutrients are present in the nutrient solution as dissolved ions, which means they carry an electric charge.

 

Some mineral nutrients carry a positive charge (cations) and some carry a negative charge (anions).

 

There are specific ionic forms in which the plant can absorb nutrients. For example, Nitrogen can be absorbed either as NO3- or as NH4+.

 

 

 

The Cation-Anion balance

Many refer to "balance" as the cation-anion balance. Is this the balance we are looking for in the nutrient solution?

 

In order to be able to calculate the cation-anion balance we need to be able to measure the number of charges. To do so, a unique unit must be used, a unit that integrates both the molecular weight of the ion and its charge.

 

This unit is the Equivalent.

 

The equivalent is calculated simply by multiplying the number of molecules of each ion (measured in moles) by the charge it carries.

 

For example, how many equivalents are in 80 grams of Calcium (Ca+2)?

 

The molecular weight of calcium is 40 grams/mole (1 mole = 6X1023 molecules).

 

Therefore, 80 grams of calcium are 2 moles.

 

Calcium carries a positive charge of +2.

 

Using the above definition of equivalent, we get 80 grams of Ca+2 = 2X2=4 equivalents.

 

In the same way, 248 grams of NO3- are also 4 equivalents. This means that 80 grams of calcium carry the same amount of charges as 248 NO3- does.

 

The cation-anion balance is calculated by comparing the number of equivalents of cations with the number of equivalents of anions.

 

 

 

Checking the cation-anion balance of a water analysis

Consider the following water analysis.

 

If we calculate the total anions and total cations in units of ppm, we get 311.69 ppm anions and 118.21 ppm cations.

 

 N-NO3-  1.5    N-NH4+  7.3
 SO42-  81.48    K+  9.62
 Cl- 36    Ca+2  47.55
 HCO3- 192.71    Mg+2  11.22
       Na+   42.52
 Total   311.69      Total   118.21

 

 

However, when we convert each anion and cation to meq/l and make the balance, the result is 5.99 meq/l anions and 5.94 meq/l cations (1 meq= 1/1000 of an equivalent).

 

 

N-NO3- 0.11       N-NH4+ 0.52
SO42- 1.7   K+ 0.25
Cl- 1.02   Ca+2 2.38
HCO3- 3.16   Mg+2 0.94
      Na+ 1.85
Total 5.99   Total 5.94

 

 

Very close to a complete balance!

 

The difference is within the range of an acceptable measuring error.

 

Was this result achieved by chance?

 

Actually not…

 

According to the law of electroneutrality, the total charge of an aqueous solution must be zero.

 

∑ negatively charged ions = ∑ positively charged ions

 

This is a law of nature.

 

It brings us to conclusion # 1 - Water is ALWAYS balanced, with respect to cation-anion balance.

 

Can the cation-anion balance be affected by fertilizers?

To find an answer to this question, let's first look at the composition of one fertilizer. We will use Calcium Nitrate as an example.

 

Composition in 100 grams in meq/l
N-NO3-  14.4% 14.4 1.03

N-NH4+

1.1% 1.1 0.08
Ca+2  19% 19 0.95

 

We can see that the fertilizer is balanced with respect to cation-anion balance.

 

A surprise? Not exactly...

 

Being a salt, ANY fertilizer will always contain the same amounts of cations an anions, measured in equivalents (sum of electrical charges is zero).

 

So when we dissolve fertilizers to water, no matter which fertilizers and in which amounts, the resulted nutrient solution will always be balanced, with respect to cation-anion balance!

 

If so, why should we calculate this balance?

 

We do so in order to validate the water analysis / nutrient solution test results.

 

An error of more than 5% in the cation-anion balance might imply that the analysis is not accurate.

 

However, if the laboratory did not test for one of the major cations or anions, then a correct balance cannot be calculated.

 

What about the soil solution?

All of the above conclusions are valid for the soil solution as well.