A substrate has to do two things at once that seem to contradict each other: hold water and let air through. Roots take up water and nutrients, but they also breathe. Without oxygen in the root zone, root growth stops, nutrient uptake is disrupted and pathogens such as Pythium have free rein. The ratio between water and air in the pot is therefore one of the most important properties of a substrate, and at the same time one of the least visible.
Large and small pores
Most of a substrate consists of empty space: with peat and coir, often 85 to 95 per cent of the volume. That space is made up of pores of different sizes. Small pores hold water through capillary action; large pores drain after irrigation and fill with air. The distribution between small and large pores therefore determines how much water and how much air remain in the pot once the excess water has drained away.
Fine raw materials such as fine peat and coir pith have many small pores and hold a lot of water. Coarse materials such as bark, coarse coir fibre and perlite mainly have large pores and bring in air. By combining them in the right ratio, you set the balance.
The pot plays its part
The same substrate behaves differently in a shallow pot than in a tall container. After irrigation, a saturated zone remains at the bottom of every pot from which the water does not drain. In a shallow pot or tray, this wet zone takes up a large part of the volume; in a tall container, it is relatively small. That is why small pots and trays usually need a more open substrate than large containers, even though intuition may suggest the opposite.
The way you pot up also counts. A substrate that is handled too wet or under too much pressure loses large pores. The air content in the pot then ends up considerably lower than the specification would lead you to expect.
What determines the balance?
- Raw materials: peat and fine coir for water; bark, coarse coir, wood fibre and perlite for air.
- Fraction: the coarser the particles, the more large pores. A 0-5 mm fraction behaves very differently from 0-25 mm.
- Pot size and height: shallower pots hold relatively more water.
- Handling: pressing, vibrating and potting too wet all compact the substrate.
- Time: organic raw materials break down. Wood fibre does so faster than peat or coir; in long crops, the air content gradually decreases.
- Irrigation: ebb and flood, drip irrigation and overhead irrigation each distribute water differently through the pot.
How much air is enough?
There is no fixed standard. The air content of substrates is usually measured at a low suction (pF 1), which roughly corresponds to the situation in a pot just after it has drained. For many pot crops, an air-filled porosity somewhere between 10 and 25 per cent of the volume is heading in the right direction. Sensitive crops, long crops and wet conditions tend to call for the upper end of that range; fast crops with an accurate irrigation system can manage with less.
More important than any number is whether the substrate suits your crop and the way you irrigate. An open substrate needs watering more often; a water-retentive substrate calls for more restraint.
Fine-tuning in practice
- Choose the structure based on pot size, crop duration and irrigation system, not just on the crop.
- Do not pot up too wet or too firmly. A substrate that feels like a wrung-out sponge is usually about right.
- Pots staying too wet? Consider a slightly coarser fraction or a higher proportion of coarse coir or perlite.
- Pots drying out too quickly? A higher proportion of fine coir or peat, or a wetting agent, helps the substrate take up water again more quickly.
- With peat-reduced and peat-free mixes, keep an eye on the top layer: it can look dry while the bottom of the pot is still wet. Lift or weigh pots to get a better picture.
Measure instead of guessing
Not sure whether your substrate has the right balance? An extended substrate analysis determines, among other things, the air and water content. Compare the results with your growing experience and you will quickly see whether the cause lies in the substrate or in the irrigation. With a bespoke mix, we can then adjust the ratio of raw materials and the fraction in a targeted way.
