One steering law: coco, rockwool, soil and water
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Flowering · Crop steering

One steering law: coco, rockwool, soil and water

Coco, rockwool, soil and a tank of water use the same method of steering. This paper gives the method one time, with diagrams. Then you can use the method with each substrate, because the substrate changes only the numbers.

Flowering31 diagrams9 sources~22 min to read
In this paper
01 Purpose and scope02 Definitions03 Evidence and limitations04 Substrate water storage05 Water content and dryback06 Feed strength and root-zone EC07 Field capacity and dryback limits08 Irrigation phases in each day09 Substrate moisture as the control variable10 Sensor accuracy and readings11 Decisions of the automatic irrigation controller12 The steering law for all substrates13 References
Start here

Purpose and scope

Weak

Limit of the data: Do not use the electrical conductivity (EC) values of high-intensity rockwool steering (approximately 3.5 to 6) in coco procedures for new growers. In deep water culture (DWC), the risk of disease increases when the water is hot and the dissolved oxygen is low. There is no time limit that is correct for all systems, such as ‘at a temperature of more than 23 °C (73 °F), the heat kills the plant in one day’.

A cannabis plant can use its energy to make leaves. A plant with more leaves is larger and has more green material. The plant can also use its energy to make flower. Flower is the group of buds with resin that you harvest. ‘Steering’ is the control of the plant with water. The water causes a small change in the plant in the direction of more leaves or more flower.

One method for each substrate

Coco, rockwool, soil and a tank of water are four substrates. A substrate is the material that contains the roots. You use the same method of steering in all four. This method is the steering law. A different substrate does not change the method. It changes only the numbers.

The Two Directions of Steering Leaves make a larger plant. Flower is the group of buds that you harvest. Steering changes the plant in the direction of leaves or of flower. The Two Directions of Steering LEAVES make leaves and stems (vegetative) a larger green plant FLOWER make the buds that you harvest (generative) buds with resin direction of steering
Figure 1. Leaves make a larger plant with more green material. Flower is the group of buds that you harvest. Steering changes the plant in the direction of leaves or of flower.

The three controls do not change. You use the same three controls for each substrate:

  • Water content is the quantity of water in the substrate at this time. The grower term is volumetric water content (VWC). Use this one number to control the plant.
  • Dryback is the quantity by which the water content decreases each day. You control the quantity of water that the plant removes from the substrate. This control has the largest effect.
  • Feed strength is the concentration of the feed around the roots. The grower term is EC. You select the strength of the feed.

The substrate (coco, rockwool, soil or a tank of water) does not change the three controls. It changes only the numbers.

Same Controls for Each Substrate You use the same three controls for each substrate. water content VWC dryback water content decreases feed strength EC substrate change it The controls do not change. Only the substrate changes.
Figure 2. You use the same three controls for each substrate. The substrate changes only the numbers.

After you read this paper, you will know the full procedure. Each morning, at lights-on, let the water content decrease by a small quantity. Then fill the substrate again with small shots. A shot is a small quantity of water that the dripper supplies in a short time.

Keep the water content stable during the day. During the night, let the substrate dry. The next day, start again. The other sections of this paper give more information about each part of this loop.

The Steering Law in Five StepsThe figure shows a cycle with arrows in the clockwise direction: FILL, DRYBACK, STABLE and DRY. After DRY, the cycle starts again. The symbol on the left side shows lights-on and the symbol on the right side shows lights-off. In the middle: one cycle each day. At the bottom: other sections give more information.The Steering Law in Five Stepsfill · dryback · stable · dry · start againFILLSHOTSDRY-BACKSTABLEDRYone cycleeach daylights-onlights-offother sections give more information
Figure 3. The full task is one cycle each day. You fill the substrate, and the water content decreases (dryback). You keep the water content stable, and the substrate dries. Then the cycle starts again.
Optional sections

Sections 1 to 6 and the last section give the information for manual steering. The two optional sections are about moisture sensors and automatic irrigation controllers. If you irrigate manually, these two sections are not necessary.

In short

One method of steering with water controls the plant in the direction of leaves or of flower. Coco, rockwool, soil and water change only the numbers. They do not change the steering.

Glossary

Definitions

If a term in this paper is new to you, find it in this section. Each item gives the term and then the definition.

Crop steeringCrop steering is the control of the time and the quantity of the water that you apply to the plant. It causes a small change in the plant in the direction of leaves (a larger plant with more green material) or flower (the buds that you harvest). The change occurs during some days. The plant does not change in one night.
Substrate (medium)The material that contains the roots of the plant: coco, rockwool, soil or a tank of water. This paper uses the term ‘substrate’ for all four.
Water content (VWC)The quantity of water in the substrate, as a percentage of the space in the substrate. At 60%, water fills 60 of each 100 pores. VWC is the volumetric water content. It is the one number that you use to control the plant.
DrybackThe quantity by which the water content decreases between two shots. It is the high value minus the low value. Measure it in percentage points. When the water content decreases from 78% to 58%, the dryback is 20 points. The dryback shows the quantity of water that the plant removes from the substrate. It is the most important control for steering.
Percentage points and percentWe always measure the dryback in points. When the water content decreases from 78 to 58, the dryback is 20 points. It is not possible to know if the value ‘20%’ is a quantity in points or in percent. Thus this paper gives each dryback in points.
Feed strength (EC)The concentration of the fertilizer feed around the roots. A higher value is a stronger feed with more salt. When the substrate dries, the quantity of water decreases and the nutrients stay. Thus the EC increases, and you do not add feed. EC is the electrical conductivity, in mS/cm. You can use a scale of strength from 1 to 10 for the EC.
Field capacityThe highest water content that the substrate has after the drainage of water stops. It is the maximum water content for each day. The value is different for each pot and each substrate. It is not a reference value. The value decreases when the roots fill the pot.
Recovery floorThe minimum water content, approximately 25 to 30% in rockwool. The dryback must not make the water content less than this value. If the water content is less than this value, the substrate does not absorb water again, and the water flows straight through the substrate. The dripper cannot correct this condition.
Vegetative modeSteering of the plant in the direction of leaves, stems and size. The substrate is wetter, the dryback is small, the feed is weaker, and the shots are small and many. In this mode, the plant makes leaves and stems and becomes larger.
Generative modeSteering of the plant in the direction of flower, density and resin. The substrate is drier, the dryback is larger, the feed is stronger, the number of shots is smaller and each shot is larger. In this mode, the plant makes the buds that you harvest.
Abscisic acid (ABA)The stress hormone of the plant. A small, controlled water deficit causes the plant to release ABA. ABA causes a small change in the plant, away from leaves and in the direction of flower and resin. Thus a dryback controls the plant. You do not measure ABA.
The four phases of each day (P0–P3)The four phases of each day of steering. In the first phase, at lights-on, the water content decreases by a small quantity. In the second phase, you fill the substrate to field capacity with small shots. In the third phase, you keep the water content in a range during the day. In the fourth phase, you stop the shots, and the large night dryback sets the substrate to the start condition for the next day.
ShotA small quantity of water that the dripper supplies in a short time. The size of a shot is a percentage of the volume of the substrate. Steering uses some small shots each day, and not one large quantity of water. You calculate the time of each shot from the size of the pot, the flow rate of the dripper and the number of drippers.
SawtoothThe shape of a correct graph of the water content. The line decreases slowly (the dryback) and then increases quickly (a shot). The two parts occur again and again. A line that does not change, or a line that only decreases, shows that irrigation does not occur.
Buffer (CEC)The quantity by which a substrate decreases the effect of an error in the feed. The substrate holds nutrients and releases them again. CEC is the cation exchange capacity. Coco has a large buffer, and living soil corrects errors in the feed. Rockwool has almost no buffer, and water has no buffer.
RunoffThe small fraction of the feed that drains from the bottom of the pot, approximately 10 to 20%. Runoff is not waste. It removes the salt that collects in the substrate. When you measure the runoff, you find the correct feed strength of the substrate.
ChannelingThe condition in which the water flows straight down along one line and out of the bottom of the pot. The water does not go to the roots, and the middle of the substrate stays dry. To find channeling, examine the flow rate of the water from the pot, and not the quantity of water.
PermittivityThe size of the effect that a material has on the weak electric field of the probe. Water has a large effect, dry substrate has a small effect, and air has almost no effect. This difference lets a probe in the substrate find water that you cannot see.
Resolution and accuracyResolution is the number of decimal places that the sensor shows. Accuracy is the size of the error of the reading. A sensor with a small error has a high accuracy. A sensor can have a high resolution and an error of a small number of points until you calibrate it.
Second checkA check with a different method to make sure that the probe reading is correct. The check can be the water that drains from the pot or the weight of the pot. Do this check before you apply water.
Water balanceThe quantity of water that you apply to the pot, minus the water that the plant transpires and the water that drains. The controller calculates this value all the time. The water that you apply is the water from the shots of the dripper. The water balance gives the quantity of water in the pot. Thus you do not use only one sensor.
ConfidenceA value that shows if the reading is correct. The confidence is high when the data agree and low when the data do not agree. When the confidence is high, the controller can apply a full shot. When the confidence is low, it applies a very small shot, waits, or gives the decision to a person.
Safe irrigationThe one safety instruction for all automatic irrigation: apply more water only when you know that there is space in the substrate. If you are not sure, do the safe step. A sensor with an incorrect reading can only make the controller more careful. It cannot cause the controller to apply too much water.
Transfer functionThe change that the substrate makes to your irrigation before the roots receive it. The substrate changes the irrigation by constant quantities. The same irrigation gives a different effect on the roots for each substrate.
Gauge that shows the dryback in points
Example. Measure the dryback in percentage points. When the water content decreases from 78% to 58%, the dryback is 20 points.Diagram for education
Quality of the information

Evidence and limitations

Three types of information

We want each paper to be correct. Thus we examine each paper for information that is not solid. This information comes from the view of one grower, from a small number of sources, or from grower methods that have no controlled test. We identify this information, and we do not show it as solid.

Frequently there is no paper for the decision that you must make. Then we use the reports of other growers and the methods that work in the rooms that we know. This information can help you, but it is not the result of a laboratory test. Use the methods that work for your plants, for your room and for your meters. If a table does not agree with your crop, use the data from your crop and record the difference.

Solid
Tests of plants, specifications or many sources that agree show that the information is correct
  • The wet and dry cycle and the strength of the feed are the same steering controls in all media.
  • Soil is a buffer. Coco, rockwool and water do not have the same buffer.
Grower method
Many growers use this method in their rooms. Start with it, then change it for your room.
  • One model for all substrates, with a drive and a brake.
Weak
The view of one grower, a small number of sources, one test, or a method that works in one room
  • Coco procedures for a new grower that use the high-intensity substrate EC values of rockwool.
  • Root rot in DWC in one day or less at more than 23 °C as a time for all systems. DO and heat change the time.

If you find an error, make a report. We will correct it. Make the report on GitHub, with the name of the paper and the information that is incorrect. If you have a source, include it: Make a report of an accuracy problem. The regulations, labels and licenses of your area always override each procedure in these papers. The label weak in a paper shows the points where the risk of an error is highest.

One model

Substrate water storage

Each substrate has pores

Each substrate (coco, rockwool, soil, and also roots in water) has a constant number of small pores. Each pore contains water or air. From here, this paper uses this one model for all the substrates.

The substrate is the material that contains the roots. The total space in the substrate is constant, and each pore contains water or air. When water goes in, the water pushes air out. When there is more water, there is always less air.

The quantity of air is important because the roots must have air and water. If the substrate stays too wet, the roots do not get sufficient air. If it stays too dry, the growth of the roots stops. Each substrate has a different usual ratio of water to air.

Water and Air in the Substrate more water less air DRIER total space (constant) water air roots use air add water WETTER total space (constant) water air air is low Water pushes air out. The roots use this air.
Figure 4. The space in a substrate is constant. When you add water, the water pushes air out of the pores. The roots use this air.

Different substrates keep very different quantities of air, also when the substrate is fully wet[7]. This one fact causes the difference between substrates in the risk that you apply too much water.

Air in Each Wet Substrate Coco keeps the most air when it is fully wet. Thus you cannot easily apply too much water to coco. Air in Each Wet Substrate All four are fully wet. The bubbles on top show the air for the roots. The values are approximate. 22% air wet COCO keeps the most air low risk of too much water 18–25% air wet PEAT 10% air wet ROCKWOOL pump roots + air pump WATER TANK
Figure 5. Coco keeps approximately one part in five of its space as air when it is fully wet. Rockwool keeps only one part in ten. Thus you cannot easily apply too much water to coco.
Numbers
  • Coco keeps approximately 22% air when it is fully wet[5]. Rockwool keeps only approximately 10%.
  • Peat keeps approximately 18 to 25% air when it is wet. The total space in coco is approximately 94 to 96%.

A substrate has a second trait that you do not see easily. This trait is also important: the buffer. The buffer is the quantity by which the substrate decreases the effect of an error in the feed. Some substrates absorb the error and prevent damage to the plant[6].

In other substrates, the effect of the error occurs immediately. This paper gives the measurements in a subsequent section. At this time, we use only the terms ‘large buffer’ and ‘small buffer’.

The Buffer of Each Substrate The same error in the feed occurs in four substrates. Soil and coco have a large buffer and absorb the error. Rockwool has a small buffer. Water has no buffer, and the effect occurs immediately. the second trait of a substrate The Buffer of Each Substrate the same error in the feed ↓ large buffer LIVING SOIL absorbs the error moderate buffer COCO also absorbs the error small buffer ROCKWOOL the effect is fast no buffer WATER the effect occurs immediately large buffer no buffer more control = smaller buffer for your errors
Figure 6. Substrates with a large buffer (soil, coco) absorb the same error in the feed. In substrates with a small buffer (rockwool, water), the effect of the error occurs immediately.
In short

Each substrate has a constant quantity of space. Irrigation controls the part of the space that contains water and the part that contains air.

First two controls

Water content and dryback

The plant removes water from the substrate

Water is not only a material that the plant uses. Water is also a control. Each day, the plant removes some water from the substrate. You control the quantity of water that the plant removes. A small dryback causes more leaves. A large dryback causes more flowers.

Water content is the one number that you use to control the plant. It shows the quantity of water in the substrate at this time. At 60%, water fills 60 of each 100 pores. Dryback is the quantity by which the water content decreases between two shots. It is the high value after a shot, minus the low value before the next shot.

Give the dryback in points, not in ‘%’

Always measure the dryback in percentage points. When the water content decreases from 78% to 58%, the dryback is 20 points (the value decreases by 20 on the scale of 0 to 100). It is not possible to know if the value ‘20%’ is a quantity in points or in percent. Thus this paper always gives points.

Dryback: Water Content Decreases the plant removes water the water flows out of the substrate field capacity 78% 58% dryback = high − low (in POINTS) recovery floor: stop before it = 20-POINT dryback (rockwool example)
Figure 7. The dryback is the quantity by which the water content decreases before you apply water again. Here, the water content decreases from 78% to 58%, and the dryback is 20 points (an example for rockwool).

How drying controls the plant. A small water deficit each day causes a change in the plant. The same change occurs at the end of summer. As a result, the plant makes flowers quickly, and not more leaves[1].

This effect occurs through a stress hormone that the plant releases when it has a small water deficit[2]. The effect is a small change, and not damage. It is not necessary to know the name of the hormone for the steering.

Water Deficit Causes Flower Three panels. First, the plant has a small water deficit. Second, the plant releases the stress hormone ABA. The same change occurs at the end of summer. Third, the plant changes from the production of leaves to the production of flowers. The change is small and it is not damage. Water Deficit Causes Flower 1: dryback small water deficit 2: the plant releases ABA stress hormone same as at the end of summer (abscisic acid, ABA) 3: the plant changes leaves flowers the plant makes flowers quickly a small CHANGE, not damage
Figure 8. A small water deficit causes a change in the plant. The same change occurs at the end of summer. The plant changes from the production of leaves to the production of flowers. The change is small and it is not damage.

Larger drybacks cause more flower (the buds). Smaller drybacks, with a wetter substrate, cause more leaves and a larger plant. The control is the same, and only the size of the dryback is different. The effect increases during some days. It does not occur in one night. Make a small change each day.

A Small Change Each DayThe top row shows a red X: do not change the plant in one night. The bottom row shows a small change on each day for five days. The plant changes from mostly leaves (green) to mostly flower (pink).A Small Change Each DayDo not change the plant in one nightone large change from all leaves to all flowerthe effect increases during some daysDo this: make a small change each daymostly leavesmostly flowerDay 1Day 2Day 3Day 4Day 5small and accurate changes, not large changes
Figure 9. Do not change the plant in one night. Make a small change each day. During the week, the plant changes from mostly leaves to mostly flower.
A dryback is not a drought

Always apply water before wilt occurs. The difference between a dryback and a drought is the dose and the time. When you stop the dryback at the correct time, you keep the yield. When the dryback continues for too long, the yield and the quality decrease by a large quantity[3]. Use small and accurate changes, not large changes.

A Dryback Is Not a Drought water content FULL EMPTY time → WILT AND DAMAGE ZONE apply water before wilt too dry = damage stopped in time not stopped Apply water BEFORE the plant wilts — use small changes, not large changes
Figure 10. You stop a correct dryback and apply water again before wilt occurs. In a drought, the water content continues to decrease through the red zone, and the drought causes damage to the crop.
Numbers (example for rockwool, your values are different)
  • Vegetative mode: a dryback of 5 to 15 points. Generative mode: a dryback of 20 to 30 points.
  • Plants in the first stage: 5 to 10 points. The last stage of vegetative growth and bulking: 10 to 15 points. The night dryback adds 5 to 15 points more, and air goes to the roots again.
  • A test shows that a smaller night dryback, with a substrate that is approximately 10% wetter, increased the yield of medicinal cannabis. Some dryback is necessary, but too much dryback causes damage.
In short

You control the plant with the dryback. The dryback is the quantity of water that the plant removes from the substrate each day. A small dryback causes more leaves. A large dryback causes more flowers.

Diagram of a drought test and of drybacks each day
Example. One drought of some days in the last stage of the crop is not the same as dryback cycles each day for some weeks.Diagram for education
Third control

Feed strength and root-zone EC

Evaporation makes a solution stronger

When evaporation removes water from a solution, the other material in the solution stays. As a result, the solution becomes stronger. The root zone does the same with the feed.

Feed strength (the grower term is EC) is the concentration of the fertilizer feed around the roots. A higher EC is a stronger feed with more salt. The feed strength changes when the water content changes. One fact causes this change. When the substrate dries, the quantity of water decreases and the nutrients stay. Thus the feed becomes stronger, although you do not add more feed[4].

As a result, a large dryback for flower is a change in the feed strength and also a change in the water. Water and feed are the same control.

Less Water, Same Nutrients The same six nutrients in three containers. When the water decreases, the nutrients are in a smaller volume and the feed strength increases. Less Water, Same Nutrients Evaporation removes the water. The six nutrients stay. full of water 6 nutrients water removed ↑ half water same 6 nutrients more removed ↑ low water same 6, smaller volume feed strength → weak moderate strong buffer in coco absorbs nutrients, thus EC increases more slowly
Figure 11. Count the nutrients in the diagram. The number is always six. When the water decreases, the same nutrients are in a smaller volume, and the feed becomes stronger.

Read the two numbers together. When the water content decreases, the plant uses water. This effect is good until the water content decreases too much. It is usual that the feed becomes stronger when the substrate dries.

If the feed strength increases by a large quantity in a short time, the feed has too much salt. Then apply more water to decrease the concentration of salt.

If the feed strength decreases slowly during some days, the plant uses nutrients faster than the feed supplies them. Then increase the feed strength by a small quantity. A weaker feed and a wetter substrate cause more leaves. A stronger feed and a drier substrate cause more flower.

The Two Values Change Together high low morning night dryback → EC increases shot → EC down blue = water content amber = feed strength A dryback also makes the feed stronger When the pot dries, the feed strength increases. Each shot makes the two values go back.
Figure 12. When the pot dries, the feed strength increases. Each shot makes the two values go back to the start values. The water content and the feed strength change together.
Numbers (example for rockwool and water, your values are different)
  • The feed strength becomes approximately two times larger when only one part in three of the water stays in the substrate. For example, a feed that you set at 3.0 can read 5.0 in the block at the end of the afternoon.
  • Vegetative mode: approximately 3.0. Generative mode: approximately 4.5 to 6.0. A good range for each day is approximately 2 to 6.
  • The reading of the feed strength can be incorrect when the substrate is almost dry.
For coco growers

Coco has a large buffer. It absorbs some of the nutrients when the feed becomes stronger. Thus the feed strength increases more slowly than in rockwool or in water. The model in this section is for rockwool and water. In coco, the effect is smaller.

In short

When the substrate dries, the quantity of water decreases and the nutrients stay. Thus a dryback also makes the feed stronger. Water and feed are the same control.

Two limits

Field capacity and dryback limits

Keep the water content between two limits

The maximum limit is field capacity. If you continue to apply water after the water content is at this value, the water flows out of the bottom of the pot. The minimum limit is the recovery floor.

If the substrate dries to less than this value, the water flows straight through the substrate and does not go to the roots. The dripper cannot correct this condition. Keep the water content between the two limits. Do not let it become equal to one of the limits.

Field capacity is the highest water content that the substrate has after the drainage of water stops. It is the maximum value of each day, and you fill the substrate until the water content is at this value. The value is different for each pot and each substrate. It is not a reference value, and it decreases when the roots fill the pot. You find it from a small number of irrigation events.

The recovery floor is a safety limit. If the substrate dries to less than this value, it does not absorb water again, and the water flows straight through it. The dripper cannot correct this condition. You must soak the substrate manually, or use a new substrate. The dryback is your steering, and the recovery floor is your safety limit. They are two different numbers.

Field Capacity and Recovery Floor Keep the water content between the two limits. 0 25 50 75 100 water content % Field capacity approximately 92% more water flows out of the pot Recovery floor approximately 25–30% less than this, water flows straight through safe dryback range dryback to here, then fill again dryback decreases but stops before the line Example values for rockwool. Your values are different.
Figure 13. Do not let the water content become more than field capacity at the top. Do not let it become less than the recovery floor at the bottom. Control the plant in the safe range between the two limits.
Channeling: Less Than the Recovery Floor Correct water content water goes to all roots Less than the recovery floor flows straight out the middle is dry RECOVERY FLOOR The dripper cannot correct this condition.
Figure 14. If the substrate dries to less than the recovery floor, it becomes very dry and gaps open in it. The water that you apply flows down the gaps and out of the bottom. The middle stays dry.

Runoff is the small quantity of feed that drains from the bottom of the pot each day. It is approximately 10 to 20% of the feed that you apply. Runoff is not waste. It removes the salt that collects in the substrate. When you measure the runoff, you find the correct feed strength of the substrate.

Runoff Is Not WasteThe water that drains from the pot removes salt. It also lets you measure the feed strength at the roots.dripper supplies feedthe pot(substrate + roots)collected salt10–20%drains from the bottomtray ▸meterin the runoffremoves collected saltthe runoff moves the salt outshows the root-zone ECthe meter shows the correct feed strengthcompare the two strengthsinoutfeed inrunoffrunoff higher→ salt collects
Figure 15. The small quantity of water that drains out removes the salt that collects in the substrate. It also lets you read the correct feed strength at the roots.
Numbers (example for rockwool, your values are different)
  • The recovery floor of rockwool is approximately 25 to 30% water content. The range of operation is approximately 55 to 92%.
  • Frequently, a good range for each day is approximately 30 to 70% water content. The temperature of the substrate is 20 to 26 °C (68 to 79 °F).
  • The runoff of each day at field capacity is approximately 10 to 20%. Find your field capacity from approximately 5 irrigation events that agree with each other.
In short

Do not let the water content become more than field capacity or less than the recovery floor. Steering is the safe change of the water content between these two limits.

The rhythm of each day

Irrigation phases in each day

Four phases in each day

Each day has four phases. In the morning, the water content decreases by a small quantity. Then shots fill the substrate. During the day, the water content stays in a range. During the night, one large dryback lets air go to the roots again. The four phases are the same each day.

Do not apply water at random times. Apply water in four phases at the same times each day. The phases agree with the lights. The four phases have the labels P0, P1, P2 and P3. In this paper, the phases are:

  1. 1
    Small dryback
    At lights-on, let the water content decrease by a small quantity before the first shot. The dryback of this phase is the first dryback of the day, and you control it.
  2. 2
    Fill
    Fill the substrate again with small shots. Increase the size of each shot by a small quantity. Continue until the water content is at the peak of the day. Stop at this value. Frequently, you do not apply all the shots that you selected for the phase.
  3. 3
    Stable water content
    Keep the water content in a range all day. When the water content is less than the lowest value of the range, apply a small shot. Make small changes to the feed strength to control the plant.
  4. 4
    Night dryback
    Stop the shots before lights-off. During the night, apply water only in an emergency. The large night dryback sets the substrate to the start condition for the next day.
Four Phases in Each Day Each day has the same four phases, in the same sequence lights-on lights-off water content Dryback P0 Fill with shots P1 Keep stable P2 Night dryback P3 then the cycle starts again the next day
Figure 16. Each day with lights-on has the same four phases in the same sequence: small dryback, fill, stable water content and night dryback.

A shot is a small quantity of water that the dripper supplies in a short time. The size of a shot is a small percentage of the volume of the substrate. The irrigation controller calculates the time of each shot, in seconds. It uses the size of the pot, the flow rate of the dripper and the number of drippers. You give the controller these three values.

A correct graph of the water content has the shape of a sawtooth. The line decreases slowly (the dryback) and then increases quickly (a shot), and the two parts occur again and again. A line that does not change, or a line that only decreases, shows that the irrigation does not occur. There is a fault.

The Sawtooth Shows Correct SteeringWhen the water content graph is a sawtooth, the steering is correct. A flat line shows a fault.CORRECTfulldecreases = drybackincreases = shotshot, dryback, shot, dryback: regularFAULTfullflat lineno shots = no irrigation
Figure 17. If the graph of the water content is a sawtooth, the steering operates correctly. A line that only decreases and then does not change shows a fault.

In one room, the steering law is this: vegetative mode and generative mode use the same four phases. Only the target numbers change. The target numbers are the size of the dryback, the feed strength and the size of the shot. The rhythm does not change.

Same Loop, Different Numbers 1. shot 2. dryback 3. check 4. again numbers go in here the loop VEGETATIVE MODE make a larger plant dryback 5-15 points feed strength ~3.0 shots many small go in → GENERATIVE MODE make the buds dryback 20-30 points feed strength 4.5-6.0 shots not many, large ← go in The loop does not change. Only the numbers change.
Figure 18. Vegetative mode and generative mode use the same loop each day. You use a different set of numbers.
Check the values from the manufacturer before you use them

First find the field capacity and the recovery floor of your substrate. Then set the targets to agree with them. A value of ‘50% water content and 50% dryback’ from the manufacturer is not correct for your substrate.

In short

Each day of steering has the same four phases: a small dryback, a fill, a stable water content and a night dryback. The sawtooth that occurs again and again shows that the steering operates correctly.

Most important section

Substrate moisture as the control variable

The substrate changes the effect of your irrigation

The substrate is between your irrigation and the roots. It changes the effect of your irrigation. A different substrate does not cause a new method of steering. You find the numbers for the new substrate.

This section has the most important information in this paper. The substrate has a transfer function: it changes your irrigation by constant quantities before the roots receive it. The same irrigation has a different effect for each substrate. Thus a new substrate does not cause a new method of steering. You find only four numbers for the new substrate. The four numbers are the water content at field capacity, the quantity of air at field capacity, the buffer, and the recovery floor.

Same Irrigation, Different Substrate GROWER does not change your irrigation water content, EC SUBSTRATE = transfer function output to the roots THE ROOTS only receive the output, not your irrigation directly select one: ROCKWOOL fast effect of an error output = input, no buffer COCO large buffer makes changes smaller SOIL corrects errors absorbs small errors WATER no buffer the effect occurs immediately same input, different output for each substrate The irrigation is the same. A different substrate gives the roots a different output.
Figure 19. Your irrigation goes into the substrate. The substrate changes it, and the roots receive the changed irrigation. The method of steering is the same for the four different substrates.

Each substrate has the same set of items, and only the numbers are different. The table shows this:

Substrate Specifications Each substrate has the same rows. Only the numbers change. Air when fully wet Buffer Procedure and pH Recovery floor For a new grower COCO 22% air large buffer soak: calcium, magnesium pH 5.8–6.2 slow effect easy ROCKWOOL 10% air almost none start pH8 → 5.5 keep 5.5–6.0 fast change 25–30% accurate, not easy SOIL more air corrects errors sets pH 5.2–6.5 no fast change easy WATER tank none pH 5.5–6.0 18–20°C root rot: more than 23°C in one day or less high risk SAME ROWS ↓ rows align
Figure 20. Four sets of specifications with the same rows (air, buffer, procedure to prepare the substrate, recovery floor). The numbers in the rows are different.
The rows are the same, and the numbers are different. When you select a substrate, you do not select a new method of steering. You select the primary fault that can occur[7].
SpecificationCocoRockwoolLiving soilWater (tank)
Air when fully wetApproximately 22%. Coco keeps the most air, and you cannot easily apply too much water.Approximately 10%. Rockwool holds the most water, and it is easy to apply too much water.More air than rockwool. For peat, the value is approximately 18 to 25%.The roots are in the water. An air pump supplies the air.
Buffer (decreases the effect of an incorrect feed)A large buffer that is part of the coco.Almost no buffer. The plant gets the feed that you set.The soil corrects errors in the feed. Organisms and minerals make the feed stable in minutes.No buffer. The tank is the only buffer.
Recovery floor and primary faultWhen the coco dries, the effect on the plant is slow. The reading of the feed strength can be incorrect when the coco is almost dry. If you do not first soak the coco in calcium and magnesium, it removes calcium and magnesium from the feed.The limit is at approximately 25 to 30%, and the change at the limit is fast. At less than this value, channeling occurs. The plant shows the effect in the same hour.A large buffer. There is no fast change at a limit. Too much water has no effect that you can see at first, and then the problem increases.There is no fast change for dry conditions, but there is a limit for heat. Root rot occurs at a temperature of more than approximately 23 °C (73 °F), in one day or less.
Procedure to prepare the substrate, and pH at the startSoak the coco in a solution of calcium and magnesium for 8 to 24 h. The pH of the feed is 5.8 to 6.2.The pH at the start is approximately 8. Before you use the rockwool, change the pH to approximately 5.5. Then keep the pH at 5.5 to 6.0. You can use the rockwool again for approximately 3 years.The soil sets the pH at approximately 5.2 to 6.5. Usually, you do not adjust the pH of the input water.Keep the pH at 5.5 to 6.0. Keep the tank at 18 to 20 °C (64 to 68 °F). Operate the air pump all the time.
Good for a new growerYes. Soak the coco in calcium and magnesium before you put the plants in it.It gives accurate control, but it has almost no buffer. Use an easier substrate first.Yes. It has the largest buffer of the four substrates.No. Use it only after you use the easier substrates.

The table shows this: when a substrate gives more control, it has a smaller buffer for your errors. Thus a new grower starts with a substrate that has a large buffer, and then uses the substrates that have a smaller buffer.

Substrates by Size of BufferStart with living soil or coco. Use rockwool or water only after you use the easier substrates.Substrates by Size of BufferStart at the top. Use rockwool and water last.LIVING SOILlargest buffer, sets the pHCOCOlarge bufferPEATmoderate bufferROCKWOOLaccurate, small bufferWATER TANKroot rot in one day or lessstart heremore controlbut a smaller buffer
Figure 21. The substrates are in a sequence from the largest buffer (living soil, at the top) to the smallest buffer (water, at the bottom). Start at the top.
Select a Substrate First cultivation? your very first plant YES ✔ Start with LIVING SOIL or COCO large buffer · easy · good for a new grower ★ this is correct for almost all growers ★ no Maximum control and careful work? irrigate each day, no errors try ROCKWOOL accurate, but a small buffer Want speed? last alternative WATER TANK if you are not sure, select a large buffer
Figure 22. For your first cultivation, start with living soil or coco. Use rockwool and water after you use these easier substrates.
Start here: a procedure for coco
Start with these values. Monitor the plants for one week. Then change one control at a time.
Vegetative modeGenerative mode
Lights12 / 1212 / 12
Field capacityFind the value for each pot.Find the value for each pot.
Dryback5 to 15 points20 to 30 points
Feed strengthApproximately 3.04.5 to 6.0
ShotsMany small shotsA small number of large shots
In short

The substrate changes only four numbers and the type of error that causes damage to the plant. The steering does not change.

Optional: not necessary for manual irrigation

Sensor accuracy and readings

Optional section

It is not necessary to read this section unless you want to know how the sensors operate. If you irrigate manually, this section is not necessary. One piece of information is important: the moisture number is an estimate, and not an accurate value. Monitor the trend, and do a second check before you use the number.

A probe finds water because water has a much stronger effect on the weak electric field of the probe than dry substrate or air has[8]. The probe uses this large difference in coco, rockwool and soil.

How a Probe Finds Water A bar chart. Water has a much stronger effect on the electric field of the probe than dry substrate or air. How a Probe Finds Water the effect of each material on the electric field of the probe PROBE electric field more effect WATER a LARGE effect the most DRY SUBSTRATE small effect less AIR almost none Water has a much stronger effect than dry substrate or air. A probe finds water.
Figure 23. Water has a much stronger effect on the electric field of the probe than dry substrate or air. This difference lets a probe in the substrate find water that you cannot see.

A high resolution is not the same as accuracy. The probe shows many decimal places, but it can have an error of a small number of points[9]. Until you calibrate the probe for your substrate, it is possible that you control the plant in the range of the error.

High Resolution Is Not Accuracy A number with many decimal places can be incorrect until you calibrate the probe. High Resolution Is Not Accuracy 58.3724% many decimal places: high resolution shows each second incorrect the error can be a small number of points calibrate the correct value before calibration after calibration for your substrate a number with many decimal places can be incorrect until you calibrate
Figure 24. A number with many decimal places can be incorrect until you calibrate the probe.

One probe is one local measurement. It measures only the small volume of influence around the probe, and not all the pot. Do not connect one probe directly to a valve. Do a second check (the water that drains from the pot, or the weight of the pot) before you apply water. Use the shape of the dryback to control the plant, and not the value of one reading.

One Probe Is Not Sufficient Do not use only one sensor. Do a second check before you apply water. One probe measures only this volume measured no data from other parts volume of influence Do a second check Runoff from the bottom Pot weight (lift it) only then VALVE OPENS two checks agree → safe to irrigate Do not connect one probe directly to a valve.
Figure 25. One probe measures only a small volume. Make sure that a second check agrees before you apply water: the runoff or the weight of the pot.
Important, also if you do not read the other sections

If your dryback is smaller than the error of your sensor, your steering is an estimate. Calibrate the sensor, or use a larger dryback.

In short

The probe gives an estimate of the water content. The estimate can have a small error. Thus use the trend, and do a second check before you apply water.

Optional: not necessary for manual irrigation

Decisions of the automatic irrigation controller

Optional section

It is not necessary to read this section unless you assemble an automatic irrigation controller. If you irrigate manually, you are the controller, and the other sections give sufficient information. One important instruction: do not use only one sensor. Do not apply too much water. Do not let the substrate become too dry. If you are not sure, do the safe step.

A good controller calculates the water balance all the time. The controller adds the water from the shots of the dripper, and you know this quantity accurately. The controller subtracts the water that the plant transpires and the water that drains. The result is the quantity of water in the pot. The sensor gives one estimate, and the controller compares it with the water balance. The sensor is not the only source of data.

CONTROLLER it calculates the water balance The Water Balance water in the pot = water in − water out shots add water (water IN) the pot heat: more OUT plant removes water (OUT) the plant transpires water + some drains from the bottom ↓ a small quantity drains at this time water balance in the pot a sensor = one estimate only the controller compares it with the water balance
Figure 26. You can find the quantity of water in the pot. Add the water that you apply. Subtract the water that the plant transpires and the water that drains. Do not use only one sensor with readings that are not stable.

Each estimate has a confidence. The confidence is high when the data agree and low when the data do not agree. When the confidence is high, the controller applies a full shot. When the confidence is between high and low, it applies only a small, safe shot. When the confidence is low, the controller waits or gives the decision to a person.

Confidence and Shot SizeApply a full shot only when you are sure. If you are not sure, do the safe step.LOWMIDDLEHIGHNONEwait or tell a personSMALL SHOTsmall safe shotFULL SHOTa full quantitythe sensorA sensor with an incorrectreading can only make thecontroller more careful —it cannot cause too much water.always safe
Figure 27. Apply a full quantity of water only when the data agree. When the data do not agree, do the safe step. Apply a small shot, wait, or give the decision to a person.

The decision has only three possible results, and one safety instruction applies to all three. A sensor with an incorrect reading can only make the controller more careful. It cannot cause the controller to apply too much water.

Not too much water. Not too dry. Apply a shot? the controller examines the data APPLY a safe shot data agree + there is space applies one small, safe shot WAIT / small shot not sure: wait small shot at most, then examine ? TELL a person the data do not agree → stop gives the decision to you The controller operates correctly when it waits or tells a person
Figure 28. The controller can only apply a measured quantity of water, wait, or give the decision to you. One safety instruction limits all three: ‘If you are not sure, select a small water deficit and not too much water. Keep the VWC always more than the recovery floor.’

The controller uses the same method for each substrate. The method has three steps, and it is the same for coco, rockwool, soil and water. The controller examines the data, calculates the water balance, and uses the confidence to select the correct step. The substrate changes only the numbers that the controller uses. It does not change the method.

In short

An automatic irrigation controller calculates the water balance and gives a confidence to each estimate. If the controller makes an error, the error is only in the safe direction. The safety instruction is: ‘If you are not sure, select a small water deficit and not too much water. Keep the VWC always more than the recovery floor.’ The controller does this with each substrate.

All parts together

The steering law for all substrates

The full method

One grower does one task, with or without sensors and equipment. The grower monitors the substrate, selects the correct step, applies a small quantity of water, and lets the substrate dry. The three controls are the same for each substrate. You can use the method with each of the four substrates.

All the information in this paper is for one system. The system has four parts: the substrate, the water content, the dryback and the feed strength. The four parts operate in the four phases of each day. The probe and the automatic controller are optional. Add them only if you use automatic irrigation.

The same sawtooth of each day is correct for each substrate. Only the numbers are different:

One Steering LawA blue sawtooth with four phases is at the top: this is the steering law. Below it, four panels for COCO, ROCKWOOL, SOIL and WATER have the same shape with different numbers. At the bottom: the steering law is the same, and the substrate changes only the numbers.One Steering LawEach substrate has the same shape each day, with different numbers.STEERING LAWfour phases in each dayfilldrybackstablefill againsame shape, four substrates, different numbersCOCOfind field capacity for each potROCKWOOLfield capacity approximately 92recovery floor 25-30 · feed 3->6SOILlarge bufferWATERthe tank is fullThe steering law is the same. The substrate changes only the numbers.
Figure 29. The same shape of each day occurs in coco, rockwool, soil and water. Only the field capacity, the recovery floor and the feed strength of each substrate change.

During a full crop cycle, the dryback has the same shape on each substrate. In the first stage, the dryback is small and the substrate is wet. Through the middle of flowering, the dryback is larger and the substrate is drier. Near the end, the dryback becomes smaller. Vegetative mode uses small drybacks, and generative mode uses larger drybacks. The method of steering is the same.

The Same Method in a Full Crop Cycle Small dryback at the start, largest in the middle of flowering, smaller at the end higher feed strength (EC) larger dryback vegetative small dryback, wet EC ~3.0 stretch weeks 1-2 height: almost two times flower weeks 1-3 dryback 10-18 points wk4-6 20-30 points (largest) EC peak 4.5-6.0 ripening weeks 7-8 18-25 points, smaller flush less feed The same shape in coco, rockwool, soil and water. Only the numbers are different.
Figure 30. In a full crop cycle, you use the same method of steering. The dryback is small in the first stage, largest in the middle of flowering, and smaller again at the end. The feed strength increases at the same time.
One Plant From Start to End the same plant from start to end, and its controls Seedling wet, small shots Plant with leaves make leaves and stems Flowering make the buds Ripening time to harvest the controls on one plant small dryback · weak feed larger dryback larger dryback · stronger feed smaller dryback
Figure 31. The effect of the controls on one plant, from the start to the end. The stages are the seedling, the plant with leaves, the large flowers and the ripening.
Numbers
  • Sequence from the largest buffer to the smallest buffer: living soil → coco → peat → rockwool → water
  • Rockwool, full crop cycle: in vegetative growth, the dryback is small. In weeks 1 to 3 of flowering, the dryback is 10 to 18 points. In weeks 4 to 6, the dryback is largest, 20 to 30 points, and the feed is strongest. In weeks 7 to 8, the dryback decreases to 18 to 25 points.
  • A substrate that is wetter by a small quantity during the day can give a higher yield with the same quality in most of flowering. Control the plant with the time of the shots and the dryback, and not with a large water deficit.

There is no one set of numbers that is correct for all rooms. Start with a substrate that has a large buffer (living soil or coco). Put a probe in the root zone. Monitor the data of the probe for one usual day. Then change one control at a time. This method, and not a number, gives the same results each time in each substrate.

For more information, read the papers about crop steering in coco and crop steering in rockwool. The paper about the root-zone sensor shows how the sensor measures the substrate. The paper about automatic irrigation gives information about the controller.

The steering law

The steering law is the same for all substrates. The substrate changes only the numbers in the steering law.

Related papers
Coco and crop steering
Use water to control the plant
Rockwool and crop steering
Drybacks, saturation and the lowest safe water content
Substrates compared
Coco, rockwool, soil and hydro
Root-zone condition · TEROS-12
How to read the values from the sensor
Irrigation control · VRWE
How the controller makes irrigation decisions
F2 crop steering
The P0-P3 cycle for each day

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Each citation in the paper, for example [1], refers to an item in this list. Each item is a primary source or an instruction from an authority, unless the item shows a different type. The results of cannabis tissue culture change much with the genotype. Before you use the dilutions, hormone doses and local regulations, make sure that they agree with the primary sources.