Causes of pH Change and How Nitrogen Affects pH

This is the 200th post on this blog.  Thanks for all the questions and comments!
Most varieties of vegetables grow at their best in a nutrient solution having a pH between 5.5 and 7.5 and a temperature between 20 and 22C. (68F-72F). Good growers need to find the best pH for their plants but should always start in this range.  See my post on how to check the pH of soil and soiless mixes

Why does the pH change?  A simple explanation as to why the pH changes is that plants will take up nutrients from the water and when these nutrients are taken from the reservoir this changes the pH.

The light cycle also affects pH, you should check the pH at the same time of the light cycle when you check it. If you think you have pH problems, you should check it several times a day throughout the light cycle. If you have an accurate pH meter and keep notes you can actually plot and see how the pH changes for your plants throughout the day.

When you provide light to plants they can do photosynthesis which is a metabolic pathway that produces various ions. During the dark cycle, photosynthesis stops but plants still do cellular respiration and if you are using beneficials  they will be respiring too. All this respiration and don’t forget the decomposition of organic matter in the solution leads to pH changes. If you are having pH prolems cleaning out the reservoir can sometimes help, if you have leaves or plant pieces in the reservoir their decomposition can change the pH and can lead to other problems with fungus.

In low light plants take up more potassium and phosphorous from the nutrient solution so the acidity increases (pH drops). In strong intense light plants take up more nitrogen from the nutrient solution how this affects pH depends on the type of nitrogen you are giving your plants.

Nitrogen is the inorganic nutrient required in the largest quantity by plants. Most plants are able to absorb either nitrate (NO3-) or ammonium (NH4+) or both. NH4+ as the only source of nitrogen or in excess is harmful to the growth of many plant species. Some plants yield better when supplied with a mixture of NH4+ (ammonium) and NO3- (nitrate) compared to NO3- alone. Good Growers will experiment to see what ratio is best for their plants. Good growers also know a combination of NH4+ and NO3- can be used to buffer against changes in pH.

Plants grown in nutrient solution containing only NO3- as the sole nitrogen source tend to increase solution pH - hence the need to add acid.  But when approximately 10%-20% of the total nitrogen is supplied as NH4+, the nutrient solution pH is stabilized at pH 5.5. You may end up with less NH4+ than you add if you use beneficials.  It has been shown recently that micro-organisms  growing on plant root surfaces can convert the NH4+ to NO3-.
Good Growing,
Dr. E.R. Myers