Using ammonium to correct pH drift

This is not actually someone’s question, but it is a problem met by many hydroponic growers both hobby and commercial. Unfortunately, it is rarely mentioned in most hydroponic books and forums.

For growers using media based systems, pH drift is a significant change in pH between the dripper solution and the run-off solution. It is usually a rise in pH. For a water–based system without pH control, it is a change in pH with time, For a water-based system with pH control, while the pH remains steady, it results in the addition of an excess amount of acid. In turn this results in the nutrients in the recirculating solution getting out of balance.

So, how and why can ammonium be used to correct thisdrift?

Answer by RICK DONNAN

Root zone pH

The important solution to consider when managing hydroponic systems is the solution around the root zone. This applies to pH as much as to other properties such as EC, aeration, etc. Provided the root zone nutrient solution is of reasonable balance, plants take up what they require from that solution. Also the roots exude a wide range of substances. The end result is that the root zone solution balance is changing. These changes are quite likely to affect its pH.

In a recirculating system, this change can be controlled by adding acid or alkali. However, this can have a down-side, in that the added ions (such as phosphate from phosphoric acid) may push the nutrient further into imbalance. Also, this approach is not feasible for “free drainage” (non-recirculating) systems.

pH drift in the root zone solution can be controlled by adjustment of the ammonium ion content of the feed.

THE PRINCIPLES BEHIND THIS ARE:

Ammonium addition mechanism

Two forms of nitrogen (N) are used in hydroponic solutions, namely, nitrate ions (NO3) which are negatively charged ions (known as an anions) and ammonium ions (NH4+) which are positively charged ions (known as a cations). In contrast to most soils, which lock up ammonium ions, any NH4+ ions added to hydroponic solutions remain available and are taken up very much faster than the NO3 ions.

Under normal hydroponic conditions, the plant is taking up NO3 ions and remaining in electrical balance. If NH4+ ions are introduced into the solution, they are rapidly taken up by the plant. Therefore the plant compensates by exuding positively charged ions to maintain the electrical balance. These are hydrogen ions, H+, the “acid” ion. Consequently the pH of the root zone solution will fall.

Therefore increasing the proportion of ammonium in the feed will result in a relative lowering of the pH. Reducing the proportion of ammonium in the feed will result in a relative raising of the pH. This applies to all systems whether recirculating or not. For example, the pH of your recirculating solution may be over 7 even though you have been pulling your feed solution pH down to well below 6. Increasing the ammonium in the feed should allow you to feed at about pH 6 and to maintain that level with time.

Adding ammonium

Firstly, all commercial grade calcium nitrate contains a small proportion of ammonium. Its approximate formula is:

5[Ca(NO3)2.2H2O].NH4NO3

Typical analysis is 14.4% N as nitrate ion and 1.1% N as ammonium ion, but check your label. Put another way, commercial calcium nitrate contains about 7% of its total N in the ammonium form.

Often more ammonium than this is needed. The only method if you use premixed fertiliser, and also the simplest method, is to add some ammonium nitrate. This is now only available as a solution because the solid is a security banned chemical used in explosives. A 50% concentrate contains about 9% N as NO3 and 9 % N as NH4+. Adding ammonium nitrate will obviously increase the relative proportion of nitrogen in the feed.

Don’t overdo the change! Try adding 4 ml of ammonium nitrate concentrate for every 100 grams of total fertiliser. Allow up to several days for the change to take effect. If this change is not sufficient, increase in steps up to a total of 12 ml per 100 grams total fertiliser. While it is possible to go higher you need to take care if you do this. An alternative is to add increments of 4 grams of ammonium sulphate per 100 grams total fertiliser.

Note that this is not a once-off dosage. The change remains permanent until you need to make any further adjustment due to trends in the system pH. It is most commonly needed in the early vegetative stages of growth when there is a high uptake of nitrate anion. When the crop gets into the fruiting stage the uptake swings more to potassium cation and the need to add ammonium reduces.

Not too high

High levels of ammonium can be toxic to plants, especially causing root death. Also too high a level of added nitrogen will lead to soft growth. Up to 10% of the total nitrogen in the ammonium form is safe for most plants, dependent upon their stage of growth. It is very risky to go above 20%. Your calculated figure should include the ammonium that comes in the calcium nitrate.  RD

PH&G January 2017 / Issue 175


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