Which factors determine the pH in culture and how can you influence them?

At the time of delivery the pH of the potting soil has a certain prearranged value. However, during cultivation the pH can actually change. How does this happen and what can a grower influence these factors? So, before the start of the cultivation season, it is a good moment to reflect on pH.

The acidity is expressed in a pH value. A low pH is acidic and a high pH is basic. The more H+ ions are present, the lower the pH value. Every crop has an optimal pH value for the substrate in which it grows well. When drawing up a soil recipe, the desired pH specification for the crop must be known. On the basis of the calcium requirement of the raw materials used, a correct calcium feed is determined. This means that the grower with an RHP substrate can count on an optimal start of the cultivation in this respect. But once the potting soil is in the pot, the grower takes over the pH control.

Various factors influence the pH of the potting soil during cultivation. The components of the potting soil can also play a role in this. For example, peat soil contains many H+ ions, both in the solution and on the adsorption complex. This complex is like a kind of magnet for positive ions, such as H+ ions. Part of this can enter into the solution, causing the pH to drop. This does not happen automatically; the conditions during the cultivation determine this. In addition to the role of the adsorption complex, there are also some other effects that can change the pH in a cultivation situation.

For example, at the beginning of a cultivation, the pH may decrease caused by fertilization. Positively charged ions (cations) from this fertilization, such as calcium or potassium, displace H+ ions from the adsorption complex. Potting soils supplied with a low EC are more sensitive to this. At the start of a cultivation, the pH can also drop because a coarse potting soil mixture has been considerably refined during potting. In these cases it may be wise to have a potting soil produced with a higher pH value (a few tenths). Also during cultivation there are factors that influence the pH, for example ammonium from fertilizers. Many fertilizers, such as mix fertilizers, have a high ammonium percentage. But also the organic and controlled release fertilizers (CRF) can have a high ammonium content. A crop easily absorbs ammonium and releases H+ ions in its place. In addition, ammonium in potting soil is converted in nitrate, called nitrification. This also decreases the pH. Another effect occurs during the flowering or fruit phase. A crop then usually absorbs more potassium. For the positively charged potassium, the plant root again releases H+ ions with a decreasing pH value as a result. This is called selective absorption.

The cause of the increase in pH value during cultivation may be related to the irrigation water and the presence of bicarbonate therein. This may be the case when using spring or surface water. Bicarbonate (HCO3-) captures H+ ions out of the solution and thus ensures a pH increase. With a content of 2 mmol / l bicarbonate in the irrigation water, attention must be given. Below 1 mmol / l this usually does not cause any problems. Another effect that can increase the pH is a high intake of nitrogen in the form of nitrate (NO3-) with a strong vegetative growth. If, on balance, a plant absorbs more anions than cations, the pH will rise as a result of more release of OH- by the plant.

Crops can react to changes in pH in the substrate. A too low pH value usually leads to poor root development. An excess of manganese may occur that poisons the crop. A too high pH value obstructs the crop from well absorbing a number of nutrient elements, especially trace elements. A well-known example of this is iron deficiency in Rhododendron. It should be noted here that each crop has a certain tolerance around the optimum pH. A crop does not respond to a half-point deviation from the target value.

Because pH has a logarithmic scale, adjustment at a strongly deviating pH is only possible to a limited extent. For example, the step from pH 5 to 5.5 is easier to make than from pH 4 to 4.5. For this reason, it is very important to monitor the pH well from the start of the cultivation and not let it deviate too much from the desired value. During the growing season, RHP recommends taking samples every 4 to 6 weeks. Incidentally, not only for the pH, but also for the nutritional status. This also applies to potting soils in which slow-acting fertilizers are processed. In case of deviating growth, RHP recommends an even more frequent sampling. It is always useful to keep a close eye on crop growth, in relation to fertilization. A correct pH measurement requires good sampling. Measuring the pH directly in the pot is not recommended. A grower can take samples himself or have this done by a specialist. When after measuring a pH deviation is noted, the grower can make adjustments straight away. How to do this during cultivation? Hereby some important examples:

Use rain water as irrigation water instead of water containing bicarbonate.

Use ammonium fertilizers in a controlled manner.

Add acid to the nutrient solution in a controlled manner.

Stop using ammonium fertilizers.

Use carbonated water in a controlled manner.