pH and nutrition

29 march 2021

A series of five webinars for RHP-certified companies took place in February and March. Last month, we have highlighted the theme of the first webinar on safety. The second webinar was about pH and nutrition. An important point of attention when we switch to new growing media.

With new raw materials there are a lot of changes, compared to peat. Aspects that can change when using non-peat raw materials are:

• have a different pH
• have a smaller pH buffer
• have a different, often smaller, nutritional buffer
• are more unstable
• sometimes already contain (many) nutritional elements
• fix nitrogen (N) (= nitrogen immobilisation)

Plants can only absorb nutrients from the soil moisture. These must be available there. And preferably in the right composition. A deficiency but also an excess of certain nutritional elements, can lead to plant damage. With the exception of, for example, mineral wool and perlite, most raw materials have an adsorption complex (Cation Exchange Capacity; CEC) on which cations and anions are exchanged. What remains in the soil moisture (= the solution) is available for the plant to absorb. In growing media, this mainly involves the binding of cations (H⁺, K⁺, NH^₄+, Ca²⁺, Mg²⁺, Na⁺, Fe²⁺, Mn²+, Zn²+, Cu²⁺) on the adsorption complex.

The concentration of H⁺ in the solution determines the pH value. The more H⁺, the lower the pH. A plant absorbs nutritional elements by first excreting H⁺. Due to a too high concentration of H⁺ in the solution (so a low pH), a plant can excrete H⁺ more difficult and therefore it is (more) difficult to absorb nutrients. The plant grows worse because of this, also because of the energy that this costs. This generally happens at values towards pH 4. In order to be able to absorb anions (SO₄²-, HPO₄^-, NO₃^-) , the plant secretes OH^-. This causes the pH in the solution to increase. By fertilising very specifically during the culture, the pH can be adjusted. The form in which nitrogen is given (ammonium or nitrate) is particularly important here.

Raw materials with a large adsorption complex can bind many cations and thus buffer nutrient elements and the pH better. This ensures balance in the pH and nutrition in the solution. The size of the pH buffer is also determined by the type of binding (humic acid, vulvic acid and clay mineral). RHP can measure the pH buffer for (new) raw materials with a pH buffer determination. The value is expressed as: meq H⁺/pH unit. For example, peat and compost have a large pH buffer, coir pith and wood fibre a small one. In addition to H⁺, the same buffer also binds the other cations. Cations are constantly looking for balance. An example: if calcium (Ca) is increased in fertilisation, it will largely bind to the complex and other elements, including H⁺, are then pushed off the complex. For example, only a part of the given Ca is immediately available in the solution for the plant and the levels of other cations increase.

This smaller buffer means that new raw materials sometimes also have to be fertilised and limed differently. New raw materials usually already have certain nutritional elements with them naturally, sometimes in high concentrations. It is important to take this into account with the fertilisation. RHP can use the laboratory analysis EN 1:5 extraction with water and CAT to determine how much nutrition a raw material carries. Sometimes liming is not necessary, because the pH is already sufficiently high. In that case, adapted fertilisation may be necessary to get the calcium and magnesium, which are normally introduced by the liming, to the desired level.

New raw materials including bark, wood fibre and rice hulls as well as future raw materials are often still relatively fresh and therefore less stable. Bacteria and fungi can easily break down some of the organic matter. They consume nitrogen that is actually intended for the plant. This causes a temporary nitrogen deficiency for the plant and this can sometimes only be partly compensated by fertilisation. Such a nitrogen deficiency can be a great risk for a crop. RHP has developed a nitrogen immobilisation test to analyse how raw materials behave over a longer period. For example peat, coir products and compost with the RHP quality mark are stable in the test.

In short, pH and nutrition also deserve attention when it comes to new growing media. In the coming year, RHP will focus on modelling the nutritional balance and further develop knowledge on the analyses mentioned in this summary. Next month we will highlight physical characteristics.

For RHP certified companies: you can download the handouts of the webinar presentations at My RHP.