Organic substrate mixtures are prepared according to customer specifications to contain the nutrients and acidity (pH) required. During cultivation the pH in the root ball is influenced by various factors, so as a result, these specifications do not always fully meet expectations. On the basis of an extensive inventory, RHP (European knowledge centre for potting soil and substrates) documented the facts. So, which factors play a role and what can growers do to keep, or achieve, the pH at the desired level?
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First of all let’s refresh our memories with some theoretical chemistry. The pH is a measure of the acidity of an aqueous solution. This can be mains water but also a mixture of water, soil and other (dissolved) substances in the root environment. The value can vary from 0 to 14, although these extremes never occur in practice. The solution is neutral at pH 7. Below that we call it acidic, above that it is alkaline.
The pH indicates, according to a reverse logarithmic scale, the amount of free hydrogen ions (H+) present in the solution, expressed in mol/l. This means that for every full point of pH the concentration of H+ ions changes by a factor of ten, higher or lower. The polar opposite of hydrogen, the hydroxide (OH-), moves to the same extent in the opposite direction.
The importance of this logarithmic scale becomes obvious when we have to adjust the pH of the solution. To lower the pH by a full point requires a ten-fold amount of acid. To go from pH 6 (0.000001 mol/l) to pH 5 (0.00001 mol/l) requires 0.000009 mol/l, from pH 5 to 4 requires 0.00009 mol/l and a drop from 4 to 3 requires 0.0009 mol/l. That is one hundred times more than for the first step.
Adsorption complex as pH buffer
Potting soils consist mainly of organic matter, for example peat, coir and fibre, wood fibre, bark etc. Organic matter – as well as clay particles – has the ability to bind cations such as Ca, Mg and K. We call this the adsorption complex. This complex also acts as a pH-buffer; when the pH rises or falls due to the uptake of nutrients by the crop, the organic matter slows down the change.
Research by RHP has found that the buffering effect of different potting soils varies widely. Potting soils that are made up largely of peat have a relatively strong buffering effect. Other materials, such as coir, coconut fibre and wood fibre, usually form a much weaker buffer. Therefore, in mixtures that contain very little peat, the pH will react more strongly to specific fertiliser components, likewise the uptake behaviour by the plant.
The adsorption complex acts as an exchange platform for many cations. What exactly is exchanged and to what extent, depends on the conditions that are created by the crop (by selective absorption of nutrients and release of H+ and OH-) and the grower (via choice of fertiliser, EC and the pH of the water).
As a result the pH in the root environment is constantly changing. This is usually subtle because the substrate always has a certain buffer around the target pH values; dependent on the crop this is usually between 4.5 and 6.5. If over the long term not enough attention is paid to the pH and therefore it is not adjusted, the pH can fall or rise to the extent that the nutritional balance and crop development is disturbed.
Factors lowering the pH
We often see a lowering of the pH early in the cultivation, especially when the mixture in the pots has a low EC value. An increase in the EC by fertilisation can in this case displace a large amount of H+ from the adsorption complex. It is therefore worth considering having the potting compost delivered with a pH that it slightly above the target value or start with an EC that is closer to the eventual target level.
A decrease in the pH can also be associated with a relatively high proportion of ammonium in the fertiliser. Plants preferably take up ammonium with the release of H+. For as long as a plant is offered ammonium, it will firstly take up that before it starts to take up (pH increasing) nitrate. As a result the pH can drop sharply.
When the crop moves into the generative phase this too often results in a lowering of the pH. The plants then take up more K+ with the release of extra H+.
Factors increasing the pH
A factor that may lead to a gradual rise in pH in the root environment is a strong vegetative growth phase. Then the crop requires a large amount of nitrogen and will take up a relatively large amount of nitrate, releasing OH-. OH- leads to a direct rise in pH. This is visible in the solution because OH- reacts with CO2 to form bicarbonate (HCO3-). Well and surface water with a high pH often have a high level of bicarbonate. Rainwater does not contain this compound.
Measuring brings the facts
Regardless of accurate control over the pH in the fertilisation and water system, a stable pH in the root is by no means a given. Based on extensive research and practical experience RHP advises growers to measure the pH in the root environment every two to four weeks, depending on the speed of growth and development phase of the crop. Never do this directly in the pot but work according to the standard 1:1.5 method and take samples in several places. Anyone not having the appropriate facilities, knowledge or expertise available can outsource the sampling and analysis to specialised laboratories. During an intermediary analysis of the nutrient solution the pH is usually measured as well.
In addition to taking regular measurements, it is still important to keep a close eye on the crop, also in relation to the fertilisation. Be especially alert during the transitions between growth stages and regularly check the root development. Also under the soil surface, visible defects can be a signal that the pH is out of line.
Adjustments and checklist
Correcting the pH in pot plant crops usually occurs by adjusting the nutritional scheme and/or the pH control. A small correction is made easily but as the necessary pH-correction becomes larger, significantly more time and heftier artillery is needed. Remember what was previously said about the logarithmic scale.
When the measured pH values deviate regularly from the norm structural changes may be needed regarding the choice of fertiliser, water source or the substrate mixture used.
RHP has been forwarding the information gained by the research to potting compost companies by means of training. Advisors from (RHP certified) potting compost companies have a checklist that makes it simple to see why the pH in a crop can change or has changed. If any questions or problems with the pH arise during production, growers are advised to work through this checklist with their supplier.
In container crops the pH in the root ball is influenced by various factors and over time this can deviate considerably from the target values. The EC, the proportion of ammonium nitrogen and the development phase of the crop (vegetative or generative) are some of these factors. The organic adsorption complex, which works as a pH-buffer, also has a major influence on the speed and the extent to which the pH may change. Peat has a stronger buffering capacity than coir and wood fibre components. Potting soils with little peat are usually more sensitive to pH fluctuations, which may impact the choice of fertiliser.
Text: Jan van Staalduinen and Hans Verhagen (RHP). Photos: RHP