Potting soil in the pot? Grower at the pH wheel!

29 january 2018

At the time of delivery, the pH of a potting soil has a certain determined value. But during the culture, that pH can change. How does this happen and what can the grower do about it? With the start of the culture season approaching, now is a good time to talk about pH.

The acidity is expressed in pH value. A low pH is acidic, a high pH is alkaline. The more H+ ions are present, the lower the pH value. Every crop knows an optimum pH value of the substrate that it grows in. When drawing up a potting soil recipe, the desired pH specification of the crop needs to be known. Based on the demand for lime of the used substrate components, the right amount of lime is determined. That way the grower can count on an optimum start of the culture with an RHP substrate. But as soon as the potting soil is in the pot, the grower takes the pH wheel.

Why does pH change?

Several factors influence the pH of the potting soil during the culture. The components of the potting soil can also play a role in this. For example, peat contains many H+ ions in the solution as well as on the adsorption complex. This complex is like a kind of magnet for positive ions, like H+ ions. A part of it can mix into the solution, causing the pH to decrease. This doesn’t happen randomly; the circumstances during the culture make it happen. Apart from the role that the adsorption complex plays, there are a couple of other effects that can change the pH during a culture situation.

Decreasing pH
At the start of a culture, the pH can decrease for example because of fertilizing. Positively charged ions (cations) from this fertilizer, like calcium or potassium, suppress the H+ ions from the adsorption complex. Potting soils delivered with a low EC are more sensitive to this. At the start of the culture, the pH can also decrease because a coarse potting soil mixture is refined during potting. In these cases, it may be wise to produce the potting soil with a higher pH value, just a few tenths. During the culture there are also factors that influence the pH, for example ammonium from fertilizers. Many fertilizers, like mixed fertilizers, have a high ammonium percentage. But the organic and controlled release fertilizers (CRF) can also have a high level of ammonium. A crop easily takes up ammonium and releases H+ ions in its place. Apart from that ammonium in potting soil is converted to nitrate, a process called nitrification. This causes the pH to decrease as well. Another effect shows during the flowering or fruiting phase. A crop then usually takes up more potassium. The plant root yields H+ ions in favour of the positively charged potassium, with a decreasing pH as a result. This is called selective uptake.

Increasing pH
An increasing pH during the culture can have to do with the irrigation water and the presence of bicarbonate in it. This can be the case when spring or surface water is used. Bicarbonate (HCO3-) catches H+ ions from the solution and causes an increase in pH. Be careful with a level of bicarbonate in the irrigation water of 2 mmol/l and up. A level of 1 mmol/l or less usually doesn’t lead to any problems. Another effect that can increase the pH, is a high intake of nitrogen in the form of nitrate (NO3-) with strong vegetative growth. If, on balance, a plant absorbs more anions than cations, the pH will increase as a result of more release of OH- by the plant.

What are the effects of pH change?

Crops can react to pH changes in the substrate. A pH that is too low usually results in bad rooting. A manganese surplus may develop that poisons the crop. A pH that is too high prevents a crop from properly taking up nutrient elements, especially trace elements. A well-known example of this is iron deficit in Rhododendron. It must be noted however, that every crop has a certain tolerance when it comes to the optimum pH. A crop won’t respond to a half-point deviation from the target value.

How to adjust during the culture?

Adjusting a strongly deviating pH is only possible within limits, because pH uses a logarithmic scale. For example, it is easier to go from a pH of 5 to 5,5 than it is from 4 to 4,5. Therefore, it is very important to monitor the pH from the start of the culture and not deviate too much from the desired value. During the growing season, RHP advises to take a sample every 4 to 6 weeks. Not only for the pH, but also for the nutrient situation. This also counts for potting soils in which slowly functioning fertilizers are incorporated. In case there is a deviating growth, RHP advises an even more frequent sample taking. It is always useful to keep a close eye on the crop growth, in relation to fertilization. A proper pH measurement requires a properly taken sample. Measuring the pH directly in the pot is advised against. The grower can take the samples himself or let a specialist do it. If after measuring a pH deviation is found, the grower can immediately adjust it. How to adjust during a culture? Several important examples:

Increasing pH

  • Quit using fertilizers that contain ammonium
  • Use water that contains carbonate in a controlled manner

Several factors that cause the pH in the pot to decrease or increase.

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