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Everything about pH in horticulture

pH horticultureWritten by Tom de Haas | Last update: 09-03-2021

PH value: a popular term in horticulture. This value is not only important in water, but also in substrates or in full soil where the crops grow. Cultivators are very likely to be familiar with the definitions of pH and EC. But what exactly is pH, why is it important and are there any differences in terms of various cultivations?

In short

  • The pH value defines the acidity of water or substrates;
  • The correct pH value is important for a proper intake of nutritional elements;
  • There are several possibilities for the correction of the pH value in water, the fertilizer tank or substrates;
  • The ideal pH value differs for water, susbtrate- and full soil cultivations;
  • Remember that the pH value is different for the A- and B-fertilizer tank;
  • The right way of measuring the pH depends on the liquid or substrate of which the pH is needed.

What is pH?

The pH value indicates the water’s or substrate’s acidity. The lower the pH value, the more acid the water or substrate is. The pH value is related to a logarithmic value, which shows the concentration of H+ ions within the solution. 

The chemical aspect of pH

At a pH value of 7, the sampled solution contains 10-7mole H+ per liter = 0.0000001 mole H+. At a pH value of 3, which is considered to be acid, the solution contains 0.001 mole = 1 mmole H+/liter. In this example, the logarithmic scale shows that the concentration of H+ ions decreases, whenever the pH raises. The difference in the concentration of H+ ions between a pH value of 2 or 3, is 0.01-0.001= 0.009 mole H+. The difference at a pH of 5 or 6 is 0.00001-0.000001 = 0.000009 mole. In this example, not much acid is needed in order to lower the pH. Also, relatively there is not much lye needed to increase the pH value.

Why is pH important?

Within horticulture, the water’s or substrate’s pH is of crucial importance for the intake of nutritional elements. 
The six main elements (nitrogen, potassium, calcium, magnesium, phosphorus and sulphur) and the six trace elements (iron, manganese, zinc, boron, copper and molybdenum) all have their own pH value, when considering their intake and availability in the roots. 
For most trace elements an optimal availability is ensured at a pH between 4.5 – 5.0. This while many main elements can be taken in by the plant optimally at a pH above 5. In order to have all nutritional elements available to the plant, the right pH value is extremely important. A pH value of 5.2 – 5.4 is often seen as ideal to ensure proper intake of the nutritional elements. 

Correction based on the pH value

If the pH value is taken into consideration during the cultivation, you can easily correct the pH of both water and substrates when too acidic or alkaline. The pH value ensures a very precise correction when the pH has to be raised or lowered.
However, the correction of the pH value cannot always be done in the same manner. This is partially dependent on the liquid or substrate, in which the pH has to be corrected. 

pH in substrate or full soil

Not only in water the right pH value should be maintained. This also applies to substrates and full soil. In these growth mediums, alkaline or acidic environments can emerge, due to a reaction of the substrate or full soil. When the pH value is too high or too low, it can possibly damage the root environment. 

The ideal pH value in substrate or full soil
In crops cultivated on substrate, generally a pH value between 5.2 and 5.8 is recommended. For cultivations on full soil a pH value around 6 is seen as ideal. Because the type of soil is leading, the given pH value of this soil has to be respected and worked with. 
When cultivating in full soil, the pH value is not only important for the intake of nutritional elements, but also for its structure. At a lower pH value (lower than 5.5), a relatively low amount of chalk (bicarbonate) is present in the soil, which enlarges the risks for a bad soil structure. That is why a higher pH value than needed for optimal availability of elements, is recommended for cultivations in full soil. 

Related articles

pH in the fertilizer solution

When fertilizers are dissolved in water, the pH is crucial. Generally, use is made of an A- and B-fertilizer tank. Also read how to prepare fertilizer tanks. 

The ideal pH value within the fertilizer solution
Normally, fertilizers in the A tank can be dissolved easily. That is why the pH does not have to be monitored as closely as in the B tank. The recommended pH value for the A tank is always between 4 and 6. For the B tank the pH value should maximally be 5. When the pH is higher than 5, fertilizers will not dissolve completely and that could result in a not clear solution. 

pH in water

The pH value can vary in water. This does not need to have any consequences and is likely to cause any direct damage to the roots whenever the pH is below 4. The pH value of water should not be below 4.8, because this could lead to an undesired acidification of the root environment. 

The ideal pH value in water
On a short term, often there are no risks for a crop when the pH value swings above 4. Nonetheless, when this occurs for a longer time, this could negatively affect the availability of trace elements to the plants. Our specialist has written an article on
how to check the pH value in water.

How can the pH value be measured?

The pH value can be measured in several ways.
  • Full soil
    When measuring the pH in full soil, the following applies: add one part of soil to two parts of (demi)water. The pH of this solution can be measured with a normal pH meter. Other methods for directly measuring the pH of full soil are often used as indication, since these are perceived less precise. 

  • Fertilizer solution
    When the pH value in the fertilizer tank is not correct, possibilities for acidifying are there. Nitric acid and phosphoric acid can be used. Always keep in mind that the use of nitric acid, brings along the addition of extra nitrogen in the water. With phosphoric acid, this is not the case. However, phosphoric acid can only be added in the B tank. On the contrary, nitric acid can be added to both the A- and B-fertilizer tank. Just keep in mind the fact that the dosage should be limited in order to prevent chelates from breaking down.

    If the pH Value should be lowered, it is recommended to make use of nitric acid, despites the small amount of nitrogen that is added to the crop. Also take into account which of the two options fits the fertilizer scheme best. Feel free to contact one of our fertilizer specialist when in need of advice. 

  • Substrate
    The pH value in substrates can be measured by means of a pH meter. Within substrates, it is common to measure the pH of the water.

Influence the pH value

Fortunately, the pH can be altered in several ways. The right way to do this, depends on the fact if the pH has to be changed in water, full soil, substrate or within the fertilizer solution. 
  • Full soil
    Within full soil, the pH can be raised by adding extra chalk to the soil. Also, the pH can be lowered easily, by adding fertilizers that contain ammoniac. Another, less used option to adjust the pH value, is by adding ‘nutritional water’ with a high or low pH value. However, this method will have a limited effect. 

  • Water
    If water contains a high concentration of bicarbonate, this should be acidified until approximately 0,5 mmole HCO3. This is often the case for mineral water or ditchwater. However, a small buffer of bicarbonate is positive. This to prevent sharp decreases in the pH value when it drops below 5. 

  • Substrate
    When the pH in substrates is too high, two things can be done. The easiest way of correcting the pH is by making sure to pH of the water is lowered. Often, this is not sufficient due to its limiting possibilities. Keep in mind that lowering this pH, could lead to other problems, such as a worsened intake of elements, burning of the roots or in the worst case changing the substrate’s structure. The best option for a high pH value, is making use of acidifying fertilizers, such as fertilizers that contain ammoniac. 

  • Fertilizer solution
    Most units contain a pH regulator, which can dose more of less (nitric) acid when needed. When you set a higher or lower EC, more or less acid will be dosed. If the pH of the water is too low, due to acidification of the drain water, this can be raised with lye. Some units can both ‘acidify’ and ‘lye’. However, most units only have one of these options. When this is the case, the product can be changed. Make use of potassium hydroxide or potassium bicarbonate if you need to ‘lye’ the fertilizer tank. 

Questions about shading screens

Do you have questions about ph in horticulture or would you like tailor-made advice? Then please contact our chalk and screen specialist Han van der Kooij using the form below. We will contact you as soon as possible - on weekdays even within 24 hours.

Han van der KooijAbout Han van der Kooij

Han van der Kooij, fertiliser product specialist at Royal Brinkman, understands the customer like no one else. After being a grower himself, for 26 years, he knows the ropes and is aware of what is involved in being a grower. Converting his own knowledge and experience into recommendations that are useful to growers, is what he likes best about his job. "Finding that this leads to good results, makes me quite satisfied."

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