Library › Part C · Article 25 in the series · Level 2 · 12 min read

Drip irrigation and substrate culture: slabs, bags and Dutch buckets

Standfirst: After reading this article you will be able to explain why a crop in a rockwool, coir or perlite substrate is watered many times a day in small doses, choose between draining the surplus to waste and recirculating it, set a drain fraction and read the drain EC, and size the daily irrigation of a tomato crop from its transpiration and its slab.

Before you read: Anatomy of a hydroponic system, Managing a recirculating solution over time, EC and concentration.

What drip and substrate culture is

In substrate culture the roots grow in a porous material, not in a tank of solution. The material is a wrapped slab of rockwool or coconut fibre, a bag of perlite or coir, or a rigid bucket of perlite or expanded clay, and each plant receives nutrient solution from its own drip emitter^1,3. The solution is supplied in excess from above, and the surplus leaves the bottom of the substrate as drainage³.

Between two irrigations the substrate holds a reserve of water and a reserve of air in its pores. That reserve, not a pump running without pause, is what the roots live in. The grower’s job is to refill it before the water runs low and before the air is squeezed out, which is why the irrigation schedule is the central decision in this family of systems^2,3. The physics of the reserve belongs to Growing media I: air, water and particle size; here we use only its consequences.

Open or closed: where the drain water goes

Every drip system drains. In an open system, also called free drainage or drain-to-waste, the surplus is discarded. In a closed system it is collected, treated and pumped back to the plants^3,5. Resh reports that almost all commercial growers now set their slabs in collection channels that return the drain water to a central treatment area¹, and Dutch growers are legally bound to recirculate⁹.

The case for closing the loop is water and fertiliser. The drain water carries the full strength of the nutrient solution, so an open system throws away fertiliser with every irrigation and can pollute ground and surface water⁵. Bergstrand, whose thesis studied closed systems with slow filters, states that closing the system is central to cutting the use of fertiliser and water and the discarding of nutrient solution¹¹. Bar-Yosef adds that an open system is limited in how much it can apply each day, so salts build up at the root surface between irrigations, a limit a closed system does not have⁴.

The case against is disease and chemistry. Drainage from one infected plant can carry Fusarium, Pythium or Phytophthora to every plant on the same return line, which is why closed systems disinfect the returning water⁵. In a recirculating solution the pH can rise with root activity, and a high pH precipitates calcium carbonate and calcium phosphate that clog emitters⁴. The returning water also accumulates sodium and chloride, which Managing a recirculating solution over time covers. Castilla adds that an open system must dose precisely, because the surplus is lost, while a closed system needs less precision but must correct the solution as uptake and salinity change it³.

The substrate is a small buffer, so irrigation is frequent

A slab or bag holds very little water compared with a raft tank. Castilla puts the mechanism in one sentence: the limited storage of water in substrates, and the difficulty of rehydrating them once they dry, forces a much higher irrigation frequency than soil needs³. In rockwool the water content falls steeply as suction rises, and almost no water is held above a suction of 5 kPa, so the buffer is small¹⁰. Hydraulic conductivity, the ease with which water moves through the material, also collapses as rockwool dries, so a late irrigation brings on water stress quickly¹⁰.

Slab height matters because water pools at the bottom. After free drainage a rockwool slab holds only 4% air at 1 cm above its base while its upper layers are dry, and the recommended height for a good water-to-air ratio is 7.5 to 10 cm¹⁰. A well-designed substrate drains to an air content of about 20% to 30% by volume, and a shallow container holds less air than a tall one of the same material³.

There is a second reason for frequent, generous irrigation. As the roots drink, the film of solution around them becomes more concentrated, and in soilless culture plants suffer salt stress before water stress; frequent irrigation with a surplus refreshes that film³. So a drip crop is watered many times a day, each time with a small dose that refills the reserve and pushes a little old solution out of the bottom.

Scheduling: when and how much

Transpiration follows intercepted solar radiation and the dryness of the air, so the water demand of the crop rises and falls with the sun³. A fixed timer cannot follow it, so growers use one of several triggers^2,3.

Radiation sum. The controller adds up the solar radiation received and starts an irrigation each time the sum passes a threshold. For rockwool, Castilla reports a recommendation of one to four irrigations per megajoule per square metre of global radiation, and another author’s threshold of 2 MJ/m² per irrigation, roughly equivalent under Mediterranean conditions to applying 0.5 litres per square metre at each event³.

Start tray. A representative slab sits in a tray with an electrode at its low end; while solution touches the electrode no irrigation starts, and when the slab dries and the level drops the circuit breaks and a cycle runs for a preset time^1,3.

Tension. In substrates the available water falls sharply between 0 and 3 kPa of suction, so the advised tensiometer threshold is 1.5 kPa, which needs a pressure-transducer instrument rather than a soil tensiometer³. Lieth and Oki warn against the finger test: skin only feels dryness at a point already too dry for good growth, so irrigation by feel keeps the crop under stress².

How much per event is set by the substrate. Resh cites Smith’s rule that slabs should be watered before they have lost more than 5% to 10% of their water-holding capacity, and reports five to eight irrigations a day for an established crop, up to about 20 on a bright day under radiation control¹. The Florida guide expects six to eight events a day in fine coir and a dozen or more in perlite, which holds about half as much water⁶.

Timing in the day is deliberate. One rockwool grower stops the last irrigation 2.5 to 3 hours before sunset so that the slabs dry to 65% of capacity overnight, which improves root oxygenation, and starts the first about 2 hours after sunrise¹. The Florida guide agrees: first event one or two hours after dawn, and no night irrigation, because transpiration is low at night and warm media with active roots under cool night air raise the risk of fruit cracking in tomato and pepper⁶. Castilla records the opposite for radiation-based control under Mediterranean conditions, at least one night irrigation in winter and two in summer³. The climates differ; the mechanism does not: enough water in the slab for the morning, enough air for the night.

The drain fraction and the drain EC

Each irrigation applies more than the plants have used. The surplus that leaves the substrate, as a share of what was applied, is the drain fraction, which Lieth and Oki call the leaching fraction². It keeps dissolved salts from climbing in the substrate and makes sure that every plant, including the last on a long drip line, receives enough solution^2,6.

How large should it be? Lieth and Oki give 0% to 20% under well-controlled conditions, and higher only rarely². Resh gives 20% to 25% runoff for rockwool slabs, up to 25% to 30% for perlite slabs, and reports one large grower running at 40% with up to 60% in the midday peak¹. The Florida guide, written for low-concentration solutions irrigated often, asks for about 10% and not more than 20%^6,7. All agree that poorer water needs more drainage, because the salts it brings must be carried out^2,3.

Lieth and Oki, citing Marshall and co-workers, give a formula: divide the EC of the irrigation water by the highest EC you will accept in the drain, so if the water is one fifth of the limit, drain one fifth of what you apply². For drip-irrigated soil, Castilla, citing Ayers and Westcot, divides the water EC by twice the highest EC the crop tolerates in the saturated soil extract³. Both concern salts carried by the raw water, so treat them as a starting point.

Measure both the volume and the EC of the drain every day from a representative slab set in a collection tray, and the volume applied with a jug fed by one extra emitter^1,6. The Florida guides give the rule: the drain EC should not vary more than 1.0 dS/m above or below the EC of the solution applied^6,7. A drain more than a unit above the feed means the plants are using water faster than salts and the salts are accumulating, so irrigate more or feed less; a drain more than a unit below means too little fertiliser is being injected⁶. If the drain runs 3 to 4 dS/m above the feed, flush the substrate with water, then return to a corrected solution rather than continuing with plain water⁶. A drain EC that rises over several days is the slab telling you the schedule has fallen behind the weather.

Emitters, filters and clogging

Most drip emitters deliver 2 to 11 litres per hour depending on line pressure¹; Resh recommends designing so that each emitter applies 4 to 6 L/h, and a pressure-compensating emitter of 2 L/h per plant is the common choice in rockwool and Dutch-bucket culture¹. In a large circuit the last plant receives water tens of seconds after the valve opens, and if distribution is poor the plants nearest the valve get the most, so the plants furthest from the valve are the ones to watch².

Clogging has two sources. Particles are stopped by in-line strainers of at least 100 mesh, installed downstream of the fertiliser injector and preferably upstream of it too¹. Precipitates form in the line when the pH rises, as calcium carbonate and calcium phosphate, the chemistry that Fertiliser salts describes⁴. Emitters punched into the lateral at plant spacing dry out less between cycles than emitters at the end of a long spaghetti line and plug less often; spitter emitters that wet the stem base promote disease and are avoided; pipes and emitters are black so that algae cannot grow inside them¹. Because the water reserve is so small, Castilla lists dripper and filter inspection, drain control and power-failure alarms as essential precautions³.

Dutch buckets

A Dutch bucket, or Bato bucket, is a rigid plastic pot of about 16 litres, filled with coarse perlite or lava rock and set on a drainpipe that returns the surplus to the tank^1,9. A double elbow in the base forms a siphon that holds about 1.2 cm of solution at the bottom of the pot as a reserve while everything above it drains, and that reserve is why the system suits perlite and not sawdust, peat mixes or coir, which would stay waterlogged¹. One emitter of 2 L/h feeds each bucket¹; buckets sit at about 40 cm centres with two tomato, pepper or eggplant plants or one cucumber each¹. The perlite must be coarse and loose, because fine or compacted perlite plugs the siphon and the bucket fills with solution¹.

Conditioning the substrate before planting

New substrate is not ready to plant. Rockwool used to leave the factory at pH 7.0 to 8.0¹⁰; today 6 to 6.5 is the industry standard, and below pH 5.0 the fibres begin to dissolve¹⁰. Slabs are soaked for 24 to 48 hours with nutrient solution at EC 2.5 to 3.0 mS/cm and pH 6.0 before transplanting, which sets the pH and wets the slab evenly, and the drainage slits are cut only after the soak¹. Coir from palms near the sea can carry sodium chloride; most coir sold for hydroponics has been treated, and untreated coir is soaked in good water and flushed several times¹. Coir also holds cations on its cellulose and exchanges them with the solution, so suppliers sell “buffered” slabs¹. Growing media II: chemistry and biology explains the exchange and Choosing a medium compares the products.

Worked example: a summer tomato crop in rockwool slabs

Each input has a source; the results are this article’s own arithmetic.

1. Water use per plant. The maximum transpiration of a greenhouse tomato crop under Mediterranean conditions is 6 mm per day, which is 6 litres per square metre³. At a standard density of 2.5 plants per square metre⁸, that is 6 ÷ 2.5 = 2.4 L per plant per day on a peak day, our estimate^3,8. The Florida guide expects a mature fruiting tomato to use 2 to 2.5 US quarts a day at peak harvest⁶, 1.9 to 2.4 L in our conversion⁶, so the sources agree.

2. Daily irrigation. With a drain fraction of 25%, the upper end of Resh’s 20% to 25% for rockwool¹, the volume applied is 2.4 ÷ (1 − 0.25) = 3.2 L per plant per day and the drain is 0.8 L, our figures^1,3,8.

3. The slab buffer. A slab of 90 cm × 15 cm × 7.5 cm¹ has a volume of 10.1 L, our calculation¹. Grodan’s data sheet for its GT Master Dry slab gives a practical water-content control range of 50% to 80% by volume¹²; at the top of that range the slab holds 0.80 × 10.1 = 8.1 L of water, our figure^1,12. Smith’s rule, as cited by Resh, is to irrigate before 10% of the water-holding capacity is gone¹, so an irrigation is due before 0.1 × 8.1 = 0.8 L per slab has gone, and we take 0.8 L per slab as the dose per event. A quarter of each dose drains (step 2), so a dose restores 0.6 L, a swing of 6 percentage points, from 80% to 74%, inside the control range and inside Smith’s rule^1,12.

4. Events per day. With four plants on the slab, inside Resh’s four to six¹, the slab receives 4 × 3.2 = 12.8 L per day and is refilled 12.8 ÷ 0.8 = 16 times, our figures^1,3,8. Each plant gets 0.2 L per event, which a 2 L/h emitter¹ delivers in 6 minutes. The drain is 4 × 0.8 = 3.2 L per slab per day, the volume to expect in the collection tray^1,3,8.

5. Cross-check. Castilla’s 0.5 L per square metre per event³ at 2.5 plants per square metre⁸ is 0.2 L per plant, our figure exactly, and Resh’s 20 irrigations on a bright day¹ is close to our 16.

Key numbers

Common mistakes

1. Irrigating on a fixed timer all day. Transpiration follows radiation, so a timer under-waters at noon and over-waters at dusk; a radiation sum or a start tray follows the demand^1,3.
2. Running no drain to save water. The salts left behind by uptake accumulate in the slab, and in substrate culture plants show salt stress before water stress^2,3.
3. Reading only the feed EC. The root zone is the drain, and a drain EC more than 1 dS/m above the feed means the schedule has fallen behind the crop^6,7.
4. Watering into the evening. A slab at capacity all night holds little air at its base, and warm media with active roots under cool night air raise the risk of fruit cracking^1,6,10.
5. Packing fine perlite into a Dutch bucket. It plugs the siphon and the bucket fills with solution¹.

Summary

Substrate culture puts the roots in a small reservoir of water and air that a drip emitter refills many times a day. Because the reservoir is small, dries unevenly and concentrates between irrigations, the schedule follows the weather, through a radiation sum, a start tray or a tensiometer, not a clock. Each irrigation applies a surplus, the drain fraction, which carries salts out of the slab; published values run from 0% to 30%^1,2,6. The drain EC, kept within about 1 dS/m of the feed, is the daily check on the root zone^6,7. The surplus is either discarded, which wastes water and fertiliser, or disinfected and returned, which saves both but needs disease control and chemical correction. For a summer tomato at 2.4 L per plant per day with a 25% drain, our example gives 3.2 L per plant in about 16 events of 0.2 L^1,3,8,12.

Next in the series: Aeroponics and fogponics.

References

1. Resh, H. M. (2013). Hydroponic Food Production: A Definitive Guidebook for the Advanced Home Gardener and the Commercial Hydroponic Grower, 7th ed. CRC Press, Boca Raton. ISBN 978-1-4398-7869-9.
2. Lieth, J. H. and Oki, L. R. (2008). Irrigation in soilless production. In: Raviv, M. and Lieth, J. H. (eds.), Soilless Culture: Theory and Practice, Chapter 4. Elsevier, Amsterdam, pp. 117–156. PII B978-0-444-52975-6.50006-X.
3. Castilla, N. (2013). Greenhouse Technology and Management, 2nd ed. CABI, Wallingford.
4. Bar-Yosef, B. (2008). Fertigation management and crops response to solution recycling in semi-closed greenhouses. In: Raviv, M. and Lieth, J. H. (eds.), Soilless Culture: Theory and Practice, Chapter 9. Elsevier, Amsterdam, pp. 341–424. PII B978-0-444-52975-6.50011-3.
5. Postma, J., van Os, E. and Bonants, P. J. M. (2008). Pathogen detection and management strategies in soilless plant growing systems. In: Raviv, M. and Lieth, J. H. (eds.), Soilless Culture: Theory and Practice, Chapter 10. Elsevier, Amsterdam, pp. 425–457. PII B978-0-444-52975-6.50012-5.
6. Hochmuth, R. C., Parkell, N. B., Laughlin, W. L. and Rider, S. C. Water and Nutrient Management Guidelines for Greenhouse Hydroponic Vegetable Production in Florida. University of Florida IFAS Extension, publication HS1274. https://edis.ifas.ufl.edu/publication/HS1274
7. Hochmuth, G. J. (1990, revised 2001, reviewed 2018 and 2021). Irrigation of Greenhouse Vegetables. Florida Greenhouse Vegetable Production Handbook, Vol. 3. University of Florida IFAS Extension, publication HS786 (CV264). https://edis.ifas.ufl.edu/publication/CV264
8. Jones, J. B. Jr. (2005). Hydroponics: A Practical Guide for the Soilless Grower, 2nd ed. CRC Press, Boca Raton.
9. Mason, J. Commercial Hydroponics, 3rd ed. ACS Distance Education.
10. Papadopoulos, A. P., Bar-Tal, A., Silber, A., Saha, U. K. and Raviv, M. (2008). Inorganic and synthetic organic components of soilless culture and potting mixes. In: Raviv, M. and Lieth, J. H. (eds.), Soilless Culture: Theory and Practice, Chapter 12. Elsevier, Amsterdam, pp. 505–543. PII B978-0-444-52975-6.50014-9.
11. Bergstrand, K.-J. (2010). Approaches for Mitigating the Environmental Impact of Greenhouse Horticulture. Doctoral thesis, Acta Universitatis Agriculturae Sueciae 2010:93, Swedish University of Agricultural Sciences, Alnarp. ISBN 978-91-576-7538-5.
12. Grodan (Rockwool B.V.). Product data sheet: Grodan GT Master Dry slab, and brochure: Grotop Master Dry. https://editserver.grodan.com/siteassets/downloads/downloads-en/product-data-sheets-en/Grodan-Product-data-sheet-GT-Master-Dry-slab.pdf and https://editserver.grodan.com/siteassets/downloads/downloads-en/brochures-grodan-en/grodan-brochure-grotop-master-dry.pdf

This article was drafted with the help of AI tools and reviewed and edited by PonicLabs. Sources are listed above; tell us if you find an error.