Why? That is a great question, let’s dig right in! When we grow plants in pots for any other similar containers, we’re limiting the ability of the roots to explore the ground looking for water and nutrients. Thus, if our intent is to grow plants inside of a pot, we must provide a suitable environment and all the necessary nutrients for crop growth ourselves.
But simply putting all of the nutrients in the irrigation water isn’t enough. Here’s where controlling pH comes in. The pH of the nutrient solution indicates whether it is alkaline (anything above 7), acidic (anything below 7) or neutral (like most tap waters), which is pH 7. pH is a major determining factor of nutrient uptake by the plant. If the nutritive solution is under or above optimal pH, the plant isn’t being fed properly.
Most plants will grow well having their roots within a slightly acidic medium: a pH between 6~6.4 for substrate cultures. In hydroponic systems without substrate, it could be a bit lower (pH 5.8). In the graph below, you can see how nutrients are more or less available for the plants to grow, depending on the pH: the sweet spot is around pH 6. This chart is for soil and substrate; soilless cultures need a slightly more acidic medium, around 5.8.
If say one nutrient is being taken in more than another, it could cause a nutrient excess situation, which would block the proper entry flow of other nutrients. This can cause nutritional deficiencies and/or toxicity issues. For example, if the solution is too much towards alkaline, some micronutrients like iron, manganese and boron may be blocked and deficiencies will occur. If it is too acidic, the solution could become toxic for the plants.
One mistake we tend to make is thinking that the more fertilizer, the better. This is false, and over–fertilizing can also lead to improper nutrient consumption. The fertilizers in the water are dissolved salts. The amount of salts that are in the nutrient solution can be easily known by measuring the electrical conductivity (or EC). Growers typically measure it in dS/m (deciSiemens per meter) Using this units, conductivities of some common solutions are:
Another aspect of this is that plants have different tolerances to salinity by species and age. Excess nutrients in very young plants can “burn” them. The same happens with species with high salt sensitivity. Each species has a maximum EC, above which the nutrients added to water will hurt more than help our culture. Some crops like strawberry or carrot are especially sensitive to salinity: over 0.7 dS/m strawberry will start producing less fruits. Other semi-resistant ones like tomato do not lose yield until 2.2 dS/m or more. Beyond that point, tomato start to lose a little bit (although it does gain firmness and flavor).
One thing to note is that the water we use to irrigate always has, in itself, some dissolved salts before we add the fertilizer. When the tap water is too saline, like in some coastal areas, it is not appropriate for irrigation, and you will need a reverse-osmosis device or (a lot of) distilled water
Other approach could be adding less calcium, magnesium, or whichever elements are already present. The downside is that you need a chemical analysis of the water to be able to make such corrections; then you will have to mix your own fertilizer solutions using raw chemicals… This is of course not for the casual grower; you need some expertise or the will to learn.
If the irrigation water has an excess of sodium chloride (common salt), the issue is more complicated. Sodium can displace other ions, like calcium, producing “blossom end rot” in crops such as tomato and pepper. These ugly marks are a symptom of calcium deficiency. Sodium chloride can also be toxic to plants in high enough quantities. In this case, we should choose crops that resist salinity well. Another interesting fact is that sharp fluctuations in conductivity can also cause disorders such as cracking or rot on fruits.
Cracking in tomato, due to watering with water with no fertilizer
As you can see, it’s incredibly important to monitor the pH and know the optimum EC for your plants. Keeping them stable within the limits is the surest way to success.