A good soilless growing medium exceeds 85 percent total pore space. That is the threshold set out in the growing media chapter of the FAO guide to good practice under greenhouse conditions. Everything follows from that figure: the substrate does not feed the plant, it holds water and air around the roots, and nutrition arrives through the solution. For a cucumber house in West and Central Africa, three materials are either available locally or easy enough to import: coir, rice husk and sand. Their porosity, their pH and the volume you need to allow per plant are nothing alike, and the choice is made before you order the containers, not after.
What a substrate has to deliver
The 2013 FAO guide to good agricultural practice for greenhouse vegetable crops, in the chapter written by Nazim Gruda, Muien Qaryouti and Cherubino Leonardi, sets four measurable benchmarks.
- Total pore space. Above 85 percent for an ideal substrate. Organic materials sit between 85 and 95 percent, the others between 60 and 90 percent.
- Particle size. Water holding follows the share of particles below 1 mm: maximum water capacity approaches 95 percent with 100 percent fine particles, against roughly 70 percent without them. A substrate that is too fine holds water and starves the roots of air, one that is too coarse dries out between two applications.
- Cation exchange capacity. It runs from about 1,5 to 3,5 meq per 100 g for perlite, an inert material, up to 100 to 180 meq per 100 g for sphagnum peat. A substrate with high exchange capacity buffers fertiliser better and needs less frequent fertigation. An inert substrate demands the opposite: short, repeated applications.
- Solution pH. Nutrient availability is at its best between 5,5 and 6,5. Above 7,5, calcium and magnesium carbonates precipitate and block the drippers. Before choosing a substrate, have the borehole water analysed.
These four points decide the equipment as much as the material. A house run on an inert substrate needs a reliable control head and management by electrical conductivity, where an organic substrate forgives a missed application.
Coir
This is the easiest substrate to manage for a first cucumber house. The values published by FAO are as follows: total pore space of 86 to 94 percent for coir dust and up to 98 percent for the fibre, easily available water of around 35 percent, and a natural pH of 5,5 to 6,8, which lands straight in the useful range.
Coir also brings nutrients of its own: 6 to 60 ppm of phosphorus and 170 to 600 ppm of potassium depending on the batch. That contribution is not a fault, provided you allow for it when formulating the solution rather than discovering it on a drainage analysis.
The real point to watch is salinity. FAO records electrical conductivity ranging from 0,4 to 6,0 dS/m across commercial products. The spread is wide enough that an unwashed batch can stop emergence. So the site rule is fixed: measure the conductivity of the batch you receive, rinse with clean water until you are back in the target range, and only then fill the containers.
Rice husk
Rice husk is available around every mill in West Africa, and it is the most credible alternative to imported peat. A trial published in March 2024 in Scientific Reports by Li, Meng and co-authors tested it as rice husk ash, blended with peat, vermiculite and perlite, on cucumber and melon grown in 10 litre containers under greenhouse conditions at 22 to 28 °C by day and 16 to 18 °C at night.
Six formulations were compared. Bulk density of the blends fell between 0,17 and 0,23 g/cm³ and pH between 6,0 and 7,0, rising with the share of ash. Total pore space rises as well: the formulation with 50 percent ash shows 18,88 percent more porosity than the control, and 98,32 percent more air volume.
The best measured compromise is not the most heavily loaded one. The blend the authors settled on combines rice husk ash, peat, vermiculite and perlite in a volume ratio of 4:4:1:1. On cucumber it delivered an individual fruit weight 34,62 percent higher and a yield per plant 38,20 percent higher than the control of peat, vermiculite and perlite in equal parts.
Two points before you transfer this to your own house. The trial used carbonised husk, not raw husk: carbonising stabilises the material and raises its pH. And it was used in a blend, not on its own. Raw husk used neat settles, breaks down during the cycle and immobilises nitrogen, as FAO notes for woody substrates whose carbon to nitrogen ratio runs from 100 to 272 to 1 and which require additional nitrogen fertilisation.
Sand
Sand is everywhere and costs no more than the haulage, but its physical characteristics take it away from the specification. FAO gives it a total pore space of only 0,30 to 0,45, that is 30 to 45 percent, for a bulk density of 1,48 to 1,80 g/cm³. That is four to ten times heavier than coir, for two to three times less porosity.
It keeps one precise advantage: FAO notes that stratified profiles on sand, as managed in Almería, allow more saline water to be used without reducing the harvest. Where the borehole is loaded with salts, that is an argument. Everywhere else, sand suits a raised bed better than a suspended container, simply because of the weight to be carried.
Comparing the three on the same criteria
| Criterion | Coir | Rice husk ash in a blend | Sand |
|---|---|---|---|
| Total pore space | 86 to 94 percent (dust), up to 98 percent (fibre) | Higher than the peat control, up to 18,88 percent more | 30 to 45 percent |
| Starting pH | 5,5 to 6,8 | 6,0 to 7,0 depending on the ash rate | Varies with origin, needs analysis |
| Bulk density | Low | 0,17 to 0,23 g/cm³ for the blends tested | 1,48 to 1,80 g/cm³ |
| Mandatory preparation | Conductivity measurement and rinsing | Carbonising and blending | Washing and screening |
| Local availability | Oil palm and coconut zones, otherwise imported | All rice growing zones | Everywhere |
| Where it is the right choice | A first house, simple management | Replacing imported peat | Irrigation water loaded with salts |
How much volume per plant, and in what
Volume matters as much as material, because it sets the water reserve between two applications. Fact sheet ANR-2926 on greenhouse cucumber production, published by the extension service of Auburn University in Alabama, uses 3 to 5 gallon pots, that is roughly 11 to 19 litres per plant, or Dutch bucket containers fitted with a drainage hole. The 2024 trial cited above worked in 10 litre containers. Between 10 and 19 litres per plant, you are within the published benchmarks.
The same fact sheet gives the spacings: rows on 5 foot centres, roughly 1,50 m, and within the row 12 to 18 inches, that is 30 to 46 cm, for a single stem vertical cordon. In a double row, in row spacing goes to 18 to 24 inches, that is 46 to 61 cm.
On the management side, the Auburn sheet sets a solution conductivity of 2,0 to 2,5, a pH of 5,5 to 6,5, solutions at 150 to 200 ppm of nitrogen and leachate held between 10 and 25 percent of the volume applied. It puts the optimum temperature between 80 and 85 °F, roughly 27 to 29 °C, with growth slowing below 65 °F, that is 18 °C, and a first harvest possible seven weeks from sowing. In a tropical climate it is the ventilation of the structure that holds those temperatures, not the substrate: the design rules in figures are gathered in our file on agricultural greenhouses in tropical Africa.
Reusing a substrate from one cycle to the next
Reuse is tempting and it is done, provided you know what it changes. FAO states that a reused substrate sees its water holding increase and its air filled porosity decrease. In other words, the same volume becomes wetter and less aerated with every cycle, and the fertigation schedule has to be relaxed accordingly.
It adds a methodological warning: with closed pore materials such as perlite, pumice or expanded clay, bulk density no longer allows total pore space to be estimated correctly. On a second or third cycle substrate, trust the behaviour of the drainage rather than a calculation.
The mistakes that cost a cycle
- Adopting a local material without characterising it. The FAO recommendation is explicit: determine the physical and chemical characteristics of the substrate and correct them if necessary, rather than adopting an unstandardised local material without testing it.
- Filling containers with unrinsed coir. With conductivity that can reach 6,0 dS/m depending on the batch, the risk is not theoretical.
- Using raw, neat rice husk. Settling, breakdown and nitrogen immobilisation during the cycle.
- Ignoring the water pH. Above 7,5 it is the drippers that scale up, and the problem then shows as an irrigation symptom rather than a nutrition one. The causes of blockage and the filtration that prevents it are covered in our article on the mistakes that make small drip schemes fail.
- Under sizing the volume per plant. Below 10 litres, your margin for error on a pump outage is measured in hours.
- Transplanting weak seedlings. A correct substrate does not make up for a badly run nursery. The rules on height, ventilation and watering that apply to every vegetable species are described in our guide to setting up a nursery under shade net.
Putting it into practice
The choice comes down to three questions in this order: what is the quality of your water, which material can you source regularly, and what volume per plant can your containers carry. For the detail of managing the nutrient solution and the drainage once the containers are filled, see our article on soilless tomato, substrate and nutrient solution, whose management benchmarks apply broadly to cucumber.
JOGOO supplies substrates, containers and control heads through its range of agro-inputs and farm supplies, and designs the matching structures through its agricultural greenhouses offer. For a quote matched to the area and water quality of your site, go through the Contact page.
Sources and references
- FAO, 2013, Good Agricultural Practices for greenhouse vegetable crops, Growing Media chapter by N. Gruda, M. Qaryouti and C. Leonardi
- Full text of the Growing Media chapter, FAO AGP217
- Li, Meng and co-authors, 2024, Rice husk ash based growing media impact on cucumber and melon growth and quality, Scientific Reports 14, article 5147
- Blanchard, Pickens and Wells, 2022, Greenhouse Cucumber Production, Alabama Cooperative Extension System, publication ANR-2926
Frequently asked questions
What porosity should a soilless substrate have?
More than 85 percent total pore space, according to the Growing Media chapter of the 2013 FAO guide. Organic materials sit between 85 and 95 percent, the others between 60 and 90 percent.
Does coir need rinsing before the containers are filled?
Yes, after measuring. FAO records conductivity ranging from 0,4 to 6,0 dS/m across commercial products, a spread wide enough to stop emergence. Measure the conductivity of the batch, rinse with clean water, then check again.
Can rice husk replace peat?
As ash and in a blend, yes. The trial published in 2024 in Scientific Reports settles on a volume ratio of 4:4:1:1 between rice husk ash, peat, vermiculite and perlite, with a yield per plant on cucumber 38,20 percent higher than the control.
How much substrate per cucumber plant?
Between 10 and 19 litres. Fact sheet ANR-2926 from Auburn University uses 3 to 5 gallon pots, that is 11 to 19 litres, and the 2024 trial worked in 10 litre containers.
What conductivity and pH for the solution?
The Auburn sheet uses a conductivity of 2,0 to 2,5 and a pH of 5,5 to 6,5 on cucumber, with leachate held between 10 and 25 percent of the volume applied. FAO puts the optimum for nutrient availability between 5,5 and 6,5 pH.
Is sand suitable for soilless growing?
It stays useful where the irrigation water is loaded with salts, since FAO notes that profiles on sand allow more saline water to be used without losing yield. Its total pore space of 30 to 45 percent and bulk density of 1,48 to 1,80 g/cm³ do make it a heavy, poorly aerated material.
Further reading
- Soilless tomato: substrate, nutrient solution and greenhouse management: conductivity, pH and drainage once the containers are in place.
- Agricultural greenhouses in tropical Africa: ventilation, height and orientation, the parameters that hold the temperature around the containers.
- Setting up a nursery under shade net: producing healthy seedlings before transplanting into containers.
- Late blight of tomato in a tropical climate: why a humid canopy under cover remains the first disease risk factor.
