30 APRIL 2021
On the one hand, a plant takes up water and nutritional elements to, among other things, fill its cells and thus grow. On the other hand, a plant needs oxygen to allow the processes of uptake, for example, to run smoothly.
The pore structure of a growing medium determines the degree of water uptake (and therewith the air filled porosity) and the water binding. The pore structure is determined by the solid particles, their size and structure. Peat always has a high porosity. This is sometimes different with other raw materials, with as a consequence that the water uptake, binding and buffer and the air content are also different.
The water buffer indicates how well a growing medium retains water after irrigation. Coarse pores hold water less easily and are therefore more air filled. Fine pores retain water for a very long time, until the plant has extracted it. When raw materials change, in particular the pore size and the distribution thereof change. So, how many coarse pores are there compared to fine pores. With many raw materials mainly coarser pores are formed, which means that water is less buffered. It can also cause that the water that is present, is more strongly bound and that it therefore is more difficult for the plant to reach it. That means that the Easy Available Water (EAW) is diminished.
The pF curve (moisture retention) shows the binding of water at different pressure heads. For growing media it is often determined at -100 cm. That is the point where the uptake of water becomes more difficult. After that, it takes a lot of strength for the plant to take up water. The wilting point is at -158 meters (pF 4.2). The pF curve can be influenced by changing the pore structure, for example by adding a raw material with a coarser structure to a fine material. With this, water retaining fine pores are replaced by coarser ones, which increases the oxygen content. The water binding, water buffer and EAW will then diminish. Without the use of peat, also fine raw materials have to be applied.
Peat can dry irreversibly and then take up water poorly. The water uptake characteristic (in Dutch the abbreviation is: WOK) can be very different for other raw materials, usually much faster. It is important to know how quickly a raw material takes up water (WOK analysis), in order to have a substrate produced with a water uptake that fits the irrigation method of the grower. A grower can make adjustments in the culture with the frequency and amount of irrigation. With the same frequency as before, the culture on a growing medium with a fast WOK can “drown”, which also causes problems with the oxygen content. And if not enough water is given during irrigation, even with a fast WOK the water will not be able to distribute properly over the root ball.
For the plant, it is about the oxygen content that the roots experience in the substrate. If there are enough air-filled pores in the substrate, oxygen can enter the root ball from outside the pot. The roots can then take up the oxygen (oxygen diffusion) and the plant excretes gases such as CO2 and ethylene. The air filled porosity interacts with the irrigation. If the water content rises after irrigation, the oxygen content drops because the pores are then filled with water. Those pores become slowly air-filled again as the plant takes up the water. For optimal oxygen diffusion it is important that there are sufficient air-filled pores that are interconnected to each other. The pore size distribution of growing media determine the turning point between water and oxygen. Where there is little oxygen in the root ball, few roots will form.
In short, with new growing media a lot of things change, also in the field of physical aspects. It is usually not a question of right or wrong, but it is a question of which substrate fits the grower and his culture best. And also, how the grower can subsequently make adjustments in the culture to ensure that the crop grows optimally. RHP has developed analyses, such as the WOK analysis, sieve fraction analysis and potting reference, which can provide insight in the physical aspects of raw materials. RHP continues its research on how raw materials interact with each other. Next month we will address the composition of growing media.
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