Nursery growers, plant propagators and the humble gardener all have one thing in common: having to find the most suitable substrate for their growing requirements. Currently the market offers potting mixes made from compost, peat, coco coir, vermiculite, perlite and rockwool.

How, with such a vast selection of options, do we make the correct choice? Primarily, we purchase according to tried and tested methods, or perhaps we pay attention to the day’s marketing messages. Either way, in terms of long-term costs, selecting the most suitable substrate might be the most important decision a grower makes, particularly on a commercial basis.

A growing medium is composed of three distinct phases: the solid phase, the liquid phase and the air phase. The relationship between these phases determines the suitability of a media for the grower. The solid phase covers the stable component, and is the one that gives substance to the media, while the liquid and air phases provide the ebb and flow of resources. The ability of media to retain water, facilitate gas exchange and provide sufficient anchorage for root growth are critical in determining the suitability of a medium for horticultural purposes.

A substrate’s physical properties help explain the unique relationship that occurs between solids, liquids and air. Properties of interest that control water retention and gas exchange are water-holding capacity (liquid) and air-filled porosity (air).

For plants growing in containers, roots tend to grow downwards, possibly forming a dense layer of roots at the bottom of the container. If water remains perched, the free flow of oxygen is restricted.

Although aeration may be sufficient, reduction in plant performance might be due to the perched water table that exists at the bottom of the container. The ratio of air to water will not only depend on the type of growing media, but also the distribution of particle sizes within the container.

Easily available water refers to the amount of water available in a profile easily accessed by plant roots. Measuring easily available water gives an overall picture of how media retains or drains liquid in the early stages of water availability.

Media with high retention will gradually lose moisture over a period of time while a highly drained media will almost immediately drain any available water, leaving the remainder difficult to access. For example, a sample of coco coir was graded to include four relatively different particle sizes with the smallest being 0.5 mm and the largest 2.36 mm.

Results indicate that the largest grade drained liquid at a much faster rate compared to the smallest particle, the reason being that the forces that hold water molecules to the particles are greater as particle size decreases. The relationship between moisture retention and release is fundamental in understanding the importance of irrigation management.

During the research, the solid particles, whether they are small, medium or large, occupied only a small percentage in the three distinct phases, but appeared to exert a major influence on the root zone environment. The question we wished to answer was, does size matter?

Research undertaken using coco coir as the medium and tomato as the crop shed some interesting results on whether or not size really matters. A trial was set up using different sizes of coco coir containing significantly different physical properties.

Each plant bag used in the trial consisted of identical media weight. As such, the volume of coco coir in each bag increased with particle size due to bulk density (i.e., the smallest size was the densest, occupying less volume).

Yield data was used to explain the effect of particle size. Tomato yield ranged between 14 to 19 lbs. per plant while average fruit weight ranged between 3.88 and 4.23 oz. Results suggest that as air-filled porosity increased and available soil moisture decreased, so the yield per plant decreased, but the mean weight per fruit increased.

This was attributed to the fact that the better growing conditions of the higher moisture provided for a better fruit set per truss, and therefore a slightly lower mean fruit weight per fruit. Note: no truss thinning was carried out. The significant difference between small and large size particles appear to largely be affected by the amount of moisture in the root system.

It is also worth noting that a growth analysis trial was undertaken using the same size graded coco coir to compare air-filled porosity against relative growth rate. Growth was compared over a two-week period after first true leaf emergence. Trends follow that of marketable yield per plant. As particle size increased, relative growth rate decreased. These results suggest that a short-term bioassay may enable us to determine the potential of different soilless media without going through the expense of a full yield production trial.

There are a number of comparisons that can be drawn between media choices. Peat provides easily available water for a longer period, compared to rockwool and coco coir. Even though peat and coco coir are similar in appearance, huge differences occur when comparing internal structure.

For example, ungraded coco coir consists of fine fibrous strands and spongy mesophyll cells. Forces retaining water molecules to coco coir particles are weaker compared to peat, largely due to the smaller particle size of peat.

Peat also contains decomposed materials, found in boglands through centuries of structure breakdown. As such, the physical structure is different, having adapted to retention of moisture, while coco coir undergoes limited decomposition. It’s possible that if coco coir is decomposed at a rate similar to peat, similar results may occur.

Regardless of growing media used, it may well be important to consider relative size when grading, in terms of physical properties, for specific crop specifications. For example, tree saplings may require a substrate with low air-filled porosity, while a highly drained substrate is preferred for propagation.

Furthermore, irrigation strategy may play an equally important role in moisture retention when considering particle size. For example, highly porous media may require a more frequent application of irrigation compared to a moisture retentive media, which may undergo anoxia if left in a state of constant saturation.

In conclusion, size matters, but how particle size is used is equally as important. Substrate particle size governs a grow medium’s ability to retain or release moisture while irrigation strategies can take advantage of a medium’s moisture and release properties to better control growth and timing. Miss part 1 of this series? Check it out here!

Source: This article has been posted with permission from Practical Hydroponics & Greenhouses.