I ripped these photos off from a dirt grower that has cancelled his account here @ CC, so I can't give him the credit he greatly deserves.

This knowledge can really help you decide which plants to take cuttings from, so you can start clones, and know they are gonna be girls.

These are the tools you need.
The shepards crook is just a bread tie with the plastic burned from the end.


This is the size of the plants that will be sexed.


This is the area we will inspect on the 8th node.



A girl



A boy



Obvious girl



This is a boy



Look closer



Here is a closer look at the boy


My suggestion is, to take clones from the obvious girls, and hold the males until they are definately making balls.

My personal thanks to the Unkown Grower that originated this useful thread.

Harley_Farquar is a knowledgeable, accomplished, smartass grower who's presence is sorely missed.
The original thread Micro Sexing Plants 101...the pictorial continues.

Thanks for remembering it was Harley_Farquar. He deserves the credit.

oh yes I remember him from back in the day when I was allsop38. I will have to say he answered a lot of my beginner questions, he was very good at the forums I could never figure out how he went through all those forums and answered questions in each one. He was a quick one.

all those plants are still in 24hr or 18/6 veg growth.

Here are some definitions and abbreviations.

Nitrogen (N)
Phosphorous (P)
Sulfur (S)
Potassium (K)
Calcium (Ca)
Magnesium (Mg)
Hydrogen (H)
Oxygen (O)
Carbon (C)
Iron (Fe)
Chlorine (Cl)
Manganese (Mn)
Boron (B)
Zinc (Zn)
Copper (Cu)
Molybdenum (Mo)
Carbon Dioxide (CO2)
Hydrogen Peroxide (H2O2)

DO: Dissolved Oxygen, A measure of the amount of oxygen in suspension in a nutrient solution.

RO: Reverse Osmosis, Water that has been forced through a very solid membrane to remove suspended particles.

H2o2: 35% Hydrogen peroxide.
Max dose = 2.5ml/gl

HID: High Intensity Discharge, HID lights fixtures include MH and HPS

HPS: High Pressure Sodium light

MH: Metal Halide light

Conversion bulb: A bulb designed to convert a mh to hps or visa/versa

tub/bucket/tank: rez

rez: Reservoir

Rez level: the top of the nutrient solution in your rez, always maintained at a specific volume marked in your rez, or with a sight tube. It is very important to make sure your rez is it at the full level mark before you measure ph or ppm.

nutes: Chemicals designed for hydroponic growth.

nute load: The max amount of nutrients reccommended by a nute mfg,
expressed as a percentage. He is using a 50% nute load.

solution: a mixture of hydroponic nutes.

ph: a measure of the hydrogen ions available in your nute solution.

Buffer: A chemical suspended in your water that acts like a sponge to absorb acids, in tapwter it will be Calcium-carbonate. If you use RO, then you can use Potassium molecules for buffering, without the damage caused by excess Ca.

tds: Total Dissolved Solids, a measure of some the suspended chemicals in your nutrient solution.

ppm: Parts Per Million, a number representing the tds in your nute solution.

EC: Electro conductivity, a measure of the salts in your nutrient solution. tds, ppm and ec are all the same measurement, just expressed in different values.

AN: Advanced Nutrients, a company that mfg nutes.

DM: Dutch Master, another nute mfg

GH: General Hydroponics, another mfg.

CM+: cal mag plus is an additive made by Botanicare.

LL: Liquid Light, a foliar spray made by DM

MET: Mother Earth Tea,
Organic hydro nutes, grow and bloom, by AN

Conni: Connoisseur, a bloom nute by AN

Grow medium: A method to establish the sprout,
the method used to hold the plant in the water.
Ususally Coco Coir plugs and Hydroton Clay pellets,
Neopreme plugs, or Rockwool.

ml: metric measurement Milliliter.

L: Metric measurement of Liters

g/gl: US measurement, Gallons

g/gr: Metric measurement, grams

VPD: vapor pressure deficit

Power head: A pump used in aquariums to increase DO

Air pump: Another tool used for increasing DO


1. You must purchase an accurate ph meter
before you plant your first seed.

2. If you choose to grow without a ppm meter,
you must carefully follow the nute mfg directions,
to avoid nutrient burn, and you should use a reduced nute load.

3. Your rez should have a drain system with a pump,
to make nute changes fast and easy.
This will save a lot of time and spills, and pay off greatly.

4. If you grow with tap water,
you should reduce the nute load by 50%.

5. Use RO water, unless you have very low ppm tap water.

6. Use these two rules to determine when to change your rez.
a. The Add back rule.
b. The pH rule.

7. Low ppm nute solutions are easy to correct, but even slightly high nute solutions can cause irrepairable damage. Nute Mfg test their chemicals using commercial RO machines(000ppm), and very controlled temperature and humidity. Always use less nutes(chemicals) unless you are able to copy the nute mfg environment exactly.

8. 3.5-5gl/plant, is a good rule of thumb for estimating efficient rez volumes.

9. Start your seed in the medium you are going to grow in. All seed companies recommend this practice. Use the best genetics you can find.

10. Seeds can live on the stored energy they are sprouted with for 10 days. Don't feed new sprouts for at least 7 days, and then you should only use 1/4 strength nute solutions for the first week.

11. Always completely change out the solution with fresh water,
and then add new nutes.

12. Never adjust the pH of the water before you add it to the solution. Pre adjusted pH'ed water will not be stable. Add clean water to top off your rez, and then measure the pH, then adjust the pH.

13. Don't burn the flavor out of your plants with HID lights. Sure more light equals faster growing, but why would you want to quickly grow hay? Use lights close during veg and move them further away in flower. Strong evidence supports the use of UVb lights to increase potency.

14. Buy a 60 to 100 power pocket microscope. You can see bugs, their eggs and, near harvest time, you can check trichomes.

15. Use Bed lice spray from the drug store to aggressively attack spider mites. Permethrins will cause the eggs to destruct. Spray under the leaves to kill the eggs. There will be no residue from this product because it breaks down with uv light.

16. If you use RO water, AN Barricade is an excellent limited buffer with a very small dose. Do not exceed Barricade's recommended application rate. Use an eye dropper to deliver Barricade, and check the pH between each drop. Barricade will raise the pH.

17. Use metric calibrated eye droppers, syringes, and baby medicine spoons, for very accurate dosing of nutes, and for ph adjustment.

18. Too low humidity can cause VPD.

19. Insulate, shade, or isolate your rez from light, and heat. Air blowing over the surface of your rez, will reduce the temp by 7-10º.

20. A fan to move air over the plant is required.

21. Have a power head in the bottom of your rez to keep the solution moving all the time, along with your air pump.

22. Uncontrollable pH swings can indicate a diseased root system.
If your pH is falling to 3 or 4, this would be a good indicator of advanced root rot.

23. The max dose for 35% H2o2 is 2.5ml/gl.

24. Control is what gives hydro the growing edge. Control results in better flavor, faster production, and higher yields. You can control the water, the air, the temperatures(rez & air), the humidity, the nutes, the ph, and the light.
--------------------------------------------------------------
Here is the Add Back & pH rule.

It is important to follow these two rules until you have completed a full grow,
and they are especially important if you are not using a tds meter.
===========================================
1. The Add Back Rule : Once you have replenished the original rez volume with fresh water, then it is time to change out the rez with fresh water and new nutes.

Never add nutes between nute changes,
until you have experience with the nutrients you are using,
and then you must have a tds meter.

Add fresh water once or twice a day to maintain the rez volume.

Do not adjust the pH before adding water to the rez.

Most of the time you will find that adding fresh water to top off the rez, will bring the pH back to normal range.

Check the pH and adjust if necessary, after adding water to top off the rez.

If you have a 5 gl rez, and you have added 5 gl over the last few days to maintain the rez level at the full mark, then it is time for a complete change out.

Keep and re-fill your plastic jugs to help maintain your add back count.


===================================================
2. The pH Rule :
There are separate pH rules for RO and tap water.

We will discuss RO first.

With RO, you will control the buffers.
There are three chemicals
that will allow you to get a stablized nute solution
without causing any nutrient lockout.

First you need to get some AN Barricade
which is Potassium Silicate.

You should also get some Potassium Carbonate,
and Potassium Hydroxide.

I buy these two chemicals premixed
in the powdered form from GH,
and then add RO to make a solution,
which I use to increase the pH with my eye dropper.

So here is the drill.

Add the nutes and any additives you are using,
and then measure the pH. (do not add Barricade until the pH falls below 5.6)

Most of the time the ph will be around 5.6
right after you add the nutes.

In a few hours check the pH
and it will be down as the buffer is overcome.
(I let it go down until 5.2-5.4
and then add Barricade to bring the pH up to 5.6).

Add one drop of barricade per gl,
and then check the pH.
( you need an eyedropper)

Keep using Barricade to bring the pH back to 5.6
until you have reached the max dose of .5ml/gl
(5 drops/gl of barricade max)

After you have maxed out the barricade,
then the next time the pH falls,
start using the GH Potassium Carbonate/Hydroxide powder in RO solution,
to bring the pH back up to 5.6
and after a couple of days the pH will stop moving.

You will have the solution buffered
with these three potassium molecules
and can reach a perfectly stable equilibrium.

When you add nutes to RO water, the nutes alone will probably lower the ph to around 5.6
If the ph drops below 5.6 after you have added new nutes to RO water,
use .1ml/gl Barricade to buffer the ph back up to 5.6.

Solutions with RO or distilled water
will require the use of an eye dropper to maintain pH.

Measure pH between each drop,
until you are familiar with your solutions requirements.

------------------------------------------------------------------
These are the pH rules for tapwater,
or water over 50ppm and/or initial pH of near 8.

After you add nutes to your water,
the pH should be close to 5.6.

If your water is very hard, over 200ppm,
then you will need to add pH down
(Phosphoric acid, or Nitric acid)
to achieve 5.6.

In a few hours,
as the buffers absorbs the acids,
the pH will move up,
back to near the original pH of the water.

So, you will need to continue to add pH down,
at least twice a day,
to re-establish the pH back at 5.6

After doing this for a few days,
the pH will become stable for a few hours,
maybe even for a day or so,
and then the pH will fall below 5.6 without adding any pH down.

When the pH falls without you adding pH down,
you have met the pH rule.

This is when it is time to change the nutes,
even if you have not met the Add Back Rule

For some nutes,
it is best if you always let the pH move from low to high,
and then you adjust it back down,
while other nutes will react in the opposite way,
depending on whether the mfg decided to buffer the formula or not.

Try to never allow the pH lower than 5.2 or higher than 6.2,
ideally you will maintain 5.6,
but in reality you will probably drift between 5.2 and 6.0

The time that this sequence takes will vary with the plant size,
the amount of water the plant is consuming/transpiring,
and the concentrtation of the nute solution (ppm/nute load).

If you are growing with tapwater,
the pH rule is the best method to determine when to change the nutes.
It can be as long as 10 days, and as short as 4 days.

If you must use tapwater,
It is highly recommended to use a Hard Water nutrient.

Your rez volume, the phase of your grow, and the type of nutes you have
will determine how often you need to adjust the pH.

Your job is to maintain the pH,
and change out the nutes at the appropriate time.
==============================================

Here is some info about water.
This is "good to know" info since this is our main growing medium,
and will help you understand why the pH changes.

Water has very unique density qualities.
Most liquids become denser as they become cooler.
Water, however, gets denser as it cools
until it reaches a temperature of approximately 39ºF.
As it cools below this point, it becomes lighter until it freezes (32ºF).
As ice develops, water increases in volume by 11 percent.
The increase in volume allows ice to float rather than sink,
a characteristic that prevents ponds from freezing solid.

Being a "universal solvent," as it is sometimes called,
water can dissolve more substances than any other liquid.
Over 50 percent of the known chemical elements have been found in natural waters,
and it is probable that traces of most other elements can be found in lakes, streams, estuaries, or oceans.

Dissolved Gases

Dissolved gases are those which are in a water solution.
An example of gas dissolved in solution is soda water
which has large quantities of dissolved carbon dioxide.
The most common gases are oxygen, carbon dioxide,
nitrogen, and ammonia.
Concentrations are measured in parts per million (ppm)
or milligrams per liter (mg/1), both units of measure are the same.

Oxygen

Dissolved oxygen (DO) is by far the most important chemical parameter.

The amount of oxygen that can be dissolved in water decreases at higher altiitudes.

At sea level and zero salinity,
that means no nutes, 68ºF water can hold 9.2 ppm DO,
while at 86.0F, saturation is at 7.6 ppm DO,
and unable to support "useful" life.(you can grow bacteria)

In combining this relationship of decreased solubility with increasing temperatures,
it can be seen why oxygen depletion are so common when higher water temperatures occur.
If you live in Denver, then you will really suffer low DO at high water temps.
After you add nutes(salts), then you further decrease DO.
This is another reason why lower nutes loads are better, most of the time.
===================================================
Now let's take a look at pH.

Pure water is neutral and has a pH of 7.0.
Pure water consists of H20 molecules
surrounded by a relatively small number of Hydrogen ions
and Hydroxide ions (H+ and
OH-).

Pure water is considered neutral because it has an equal number of
H+ and OH- that are freely available for chemical reaction.

Pure water has a pH of 7.0 because it contains 10-7 moles of H+ per liter and the
negative logarithm of 10-7 is 7.0.

The water coming from my RO machine is 6.5pH and 022ppm

Adding acids or bases to water changes its pH.

The pH of a sample of water is a measure of the concentration of hydrogen ions. The term pH was derived from the manner in which the hydrogen ion concentration is calculated -
it is the negative logarithm of the hydrogen ion (H+) concentration.
What this means to those of us who are not mathematicians is
that at higher pH, there are fewer free hydrogen ions,
and that a change of one pH unit reflects a tenfold change in the concentrations of the hydrogen ion.
For example, there are 10 times as many hydrogen ions available at a pH of 7 than at a pH of 8.

This means a pH value below 7 is ten times more acidic than the next higher value.

For example, a pH of 4 is ten times more acidic than a pH of 5
and a hundred times (10 X 10) more acidic than a pH of 6.
This holds true for pH values above 7,
each of which is ten times more basic (also called alkaline) than the next lower whole value.
An example would be, a pH of 10 is ten times more alkaline than a pH of 9.

The pH Scale...



The pH of water determines the solubility (amount of salts that can be dissolved in the water) and biological availability (amount that can be utilized by aquatic life) of chemical constituents such as nutrients (phosphorus, nitrogen, and carbon) and heavy metals (lead, copper, cadmium, etc.). For example, in addition to affecting how much and what form of phosphorus is most abundant in the water, pH may also determine whether aquatic life can use it. In the case of heavy metals, the degree to which they are soluble determines their toxicity. Metals tend to be more toxic at lower pH because they are more soluble.

How temperature effects pH.

When temperature changes, the actual pH of the solution being measured changes. This change is not an error caused by the change in temperature. It is the true pH of the solution at the new temperature. Since this is not an error, there is no need to correct or compensate for this temperature effect.
This temperature error is very close to 0.003 pH/oC/pH unit away from pH7. In a perfect pH electrode - one that is zeroed at exactly pH 7 - there is no temperature effect on the electrode sensitivity at pH 7 no matter how much the temperature changes. Most pH electrodes are not perfect, but the errors from changes in temperature are still very minute when near pH 7, plus or minus one-tenths of a pH, and can be disregarded. However, the further from pH 7 the solution is and the greater the temperature changes, the greater the measurement error due to changes in the electrode's sensitivity. These errors come from changes in electrode sensitivity due to changes in temperature, and can be corrected by meters with temperature compensation. Most pH pens are not temperature compensated.

Odd as it may seem, despite being one of the most well known substances in the world, even today it's still widely studied by scientists, and there are many new properties still being discovered. One of its well known, but very interesting properties, is water's ability to dissolve into itself.

What?!?!? That's right...just as when you add common table salt (NaCl) to pure water, which quickly breaks the Na-Cl bond and dissolves it into Na+ and Cl- (called ions), when you "add pure water" (H2O) to pure water, part of it dissolves into H+ and OH-. The main difference is that, while with salt you can add several spoons into a glass and virtually all of it gets dissolved, only a very small amount of pure water gets dissolved into water.How much? Well, at room temperature, about 1 molecule in every 10 million (10to the7th power) is dissolved. This means that, in a typical swimming pool full of pure water, only a few teaspoons of water would be dissolved. Now, that little number 7 up there near the 10 looks familiar, doesn't it? That's because it's exactly the number used to define "neutral pH". Note that, since each dissolved molecule of H2O results in 1 ion H+ and 1 ion OH-, these two ions are in equal amounts in pure water. The term "neutral" here means just that: the same amount of H+ and OH- ions. As mentioned above, at room temperature there's about 1 of each for every 107 molecules of water, and therefore we say that neutral water has pH=7.

And what about non-neutral water? If for any reason, the relative amount of H+ and OH- ions changes, then the water begins to drift from neutrality. If the amount of H+ ions increases, the water becomes acid, if the amount of OH- ions increases, the water becomes alkaline. For instance, suppose that the amount of H+ becomes 10 times greater than in pure water. Then there'll be about 1 H+ ion for every 1 million molecules of water (106) and therefore this water will have pH=6. Note that a change in 1 point in pH represents an increase of 10 times in the amount of H+ ions (in math this is known as a logarithmic scale). Since the amount of H+ never goes below 1 in 107 (at room temperature), the pH value for acid water will always be between 0 and 7. The value pH=0 means that there's 1 H+ ion for every molecule of water (1=100).

The same idea is used to represent increases in OH- ions. There's another scale used for this ion, called pOH, which works exactly the same: if the amount of OH- becomes 10 times greater than in pure water, then the new water will have pOH=6. For the same reasons explained above, the pOH values will always be between 0 and 7.

But using 2 scales complicates things unnecessarily, so it's more common to put both of them together in a single scale - pH. Now, instead of going only from 0 to 7, it goes from 0 to 14. The first half (0 to 7, or more accurately 7 to 0) represents increases in H+ (acid water). The second half (7 to 14) represents the increases in OH- (alkaline water). So, if you take pure water and increase the amount of OH- 10 times, the pH will raise from 7 to 8.

"Alkalinity and pH are distinctly different from each other, although their definitions and functions can be easily confused.

Alkalinity is essentially a measurement of water's ability to neutralize acids. It is a measure of the buffering capacity of a system while pH is basically the measurement of the concentration of hydrogen ions in water, in terms of acidity or alkalinity.

The alkalinity of water regarding pH issues merely refers to the basic end of a pH scale (alkaline) in contrast to the acidic end of the scale and does not reflect the buffering capacity of a system.

It is easy to believe that water with alkaline pH is likely to be high in alkalinity (buffering capacity). However, this is not necessarily true. Water with a high pH, but a low alkalinity is regarded as unstable.

Buffering capacity refers to water's ability to keep the pH stable as acids or bases are added.

pH and buffering capacity are intertwined with one another; although one might think that adding equal volumes of an acid and neutral water would result in a pH halfway in between, this rarely happens in practice.

If the water has sufficient buffering capacity, the buffering capacity can absorb and neutralize the added acid without significantly changing the pH. Conceptually, a buffer acts somewhat like a large sponge. As more acid is added, the ``sponge'' absorbs the acid without changing the pH much. The "sponge's'' capacity is limited however; once the buffering capacity is used up, the pH changes more rapidly as acids are added. Using the pH rule, we will change out the nute solution when the buffering capacity is used up.

The presence of calcium carbonate or other compounds such as magnesium carbonate contribute carbonate ions to the buffering system. Alkalinity is often related to hardness because the main source of alkalinity is usually from carbonate rocks (limestone) which are mostly CaCO3. Since hard water contains metal carbonates (mostly CaCO3) it is high in alkalinity. Conversely, unless carbonate is associated with sodium or potassium which don't contribute to hardness, soft water usually has low alkalinity and little buffering capacity. So, generally, soft water is much more susceptible to fluctuations in pH.

The big problem with using tap water is the calcium/magesium ratio will be out of balance because there will always be more calcium carbonate in tap water than there is magnesium.
You can use hard water nutes, or add CAL MAG Plus to try to increase the ratio.

Dutch Master is supposedly making a new nute calculator
that will factor in your tap water ppm.

General Hydroponics makes a "Hard Water micro"
designed to be used with tap water.
If you must use tap water,
then you should ony use a 50% nute load.

Buffering has both positive and negative consequences. On the plus side, the nitrogen cycle produces nitric acid (nitrate). Without buffering, your tank's pH would drop over time (a bad thing). With sufficient buffering, the pH stays stable (a good thing). On the negative side, hard tap water often almost always has a large buffering capacity. If the pH of the water is too high for your plants, the buffering capacity makes it difficult to lower the pH to a more appropriate value. Attempts to stabilize the pH of tap water usually fail, because the buffering effects are ignored.

Bicarbonate (HCO3-) and carbonate (CO32-) are the most important ions that determine alkalinity.

When the carbonates accumulate in a growing medium, the growing medium solution pH reaches levels that cause plant growth inhibition, which is caused primarily by the transformation of soluble forms of Fe(iron) into insoluble forms.

Early investigations in plant nutrition demonstrated that normal plant growth can be achieved by immersing the roots of a plant in a water solution containing salts of Nitrogen (N), Phosphorous (P), Sulfur (S), Potassium (K), Calcium (Ca), and Magnesium (Mg). Hydrogen (H), Oxygen (O), and Carbon (C) are all derived from the air and water. These nine elements are defined as the macronutrients.

With further refinement in laboratory techniques, scientists established seven elements required by plants in relatively small quantities- the micronutrients or trace elements. These include Iron (Fe), Chlorine (Cl), Manganese (Mn), Boron (B), Zinc (Zn), Copper (Cu), and Molybdenum (Mo).

All plants require the same 16 chemical elements to grow

People use the word "chemical" as if they should be cautious about these materials.
The fact is, water is a chemical.
Everything has chemical properties.
The materials used for hydroponics
are no different from what you have used traditionally in your outdoor garden,
just in a different original(chelated) form,
and separated from the ground.

Organic growing requires living microbes
to turn the fertilizer into the chemicals that the plant can use.

In the end,
the chemicals that the plant can use use at the root zone,
are exactly the same,
whether they come from a microbes digestive tract,
or a test tube.

See how easy this is going to be now!

Show your grow,
Earl

My disclaimer,
these are just my opinions, from my experience to date. My opinions have been known to change. There may be some tips that will not apply to the way you are growing Hydro since my only experience is with DWC and AERO. Hempy buckets and Bio buckets may use different pH.
Most hydro nute mfg reccomend 5.6pH

This is the way I start seeds and is the reccomended method by all seed dealers.

I like the coco coir starter plugs. These are four years old and still good as new. I do not re-use them.


Use Hydroton clay pellets. I do re-use these.



Place the plug in the bottom of a three inch net pot and hold it like this so you can scoop the clay pellets into it.


Notice that with the plug on the bottom, this will give us room to add clay pellets once the plant is larger. This will help block light from the stem.

Flush/Rinse the net pot with clay pellets, until the water runs clear out the bottom.

Next, fill your tub with tap water to the bottom of the net pots, with the lid placed like the picture. Mark the water level inside the tub, with a led pencil. Poor the tap water out, and replace with RO or Distilled water. Chlorine is very bad for sprouts root development. Avoid using tap water for sprouting seeds.


With the tub full of water at the appropriate level, put the seed in the coco coir plug. I use the screw driver that came with my ph pen. First, I make a new "hole" with the screw driver, and then push the seed in the smaller hole until it just barely disappears below the surface.

If you use the hole in the center of the plug,
the seed will fall too low,
and have a harder time sprouting.

I am modifiying these directions
since I belive that the best way to plant is with the seed laying on it's side.

Getting the seeds planted correctly
will greatly increase your chances of getting sprouts.

If your net pots are just submerged in the water, they will stay wet, and your beans should sprout in three to four days. After a week they should look like this and be ready to move into your growing system.


Growing more plants? Use a larger tub with more holes. Move the plants into your growing system after they are a week old, and start feeding 1/4 strenght nutes.


With DWC, lower the water level to the bottom of the net pot, after the sprout has roots showing from the bottom.

Whew!
Sweet

Thanks for the sticky frmrgrl, I am deeply honored.

I have gone into my grow room and been puzzled by a phenomena that I didn't understand. There would be water standing on a leaf that was under HID light and I was very bewildered why this was happening. I knew I had not allowed any water to spill on the plant, and I knew that my roof wasn't leaking, hell it wasn't even raining outside. So, where did this mystery water come from? Obviously the plant was transpiring water through the leaf and puddling on the top. But why? The answer is due to low vapor pressure deficit in my grow room.

If your interested in increasing your plants ability to grow in an enclosed environment, and you want to avoid diseases, and provide the best growing environment for your girls, then you need to be concerned about VPD.

So what is VPD?

Vapor pressure deficit, in ecology, is the difference between the actual water vapor pressure and the saturation of water vapor pressure at a particular temperature.

Unlike relative humidity, vapor pressure deficit has a simple nearly straight-line relationship to the rate of evapotranspiration and other measures of evaporation.

Plants lose moisture by transpiration from their leaves into the surrounding atmosphere.

The less moisture they lose, the more they like it.

We tend to think that the higher the relative humidity, the moister the air, the better it is for our plants, but that is only true up to a certain point.

What I am trying to show here is that relative humidity does NOT relate directly to the rate at which transpiration of water from the plant occurs.

Changes in relative humidity are not proportional to the rate of plant moisture loss.

How come?

The moisture holding capacity of air is measured in units of pressure, and there are two important measurements concerned with figuring out how much moisture a given block of air can potentially absorb.

First is the saturation vapor pressure (SVP): think of this as the maximum amount of water vapor a given block of air can hold.

Second measurement is the difference between the amount of water vapor actually in a given block of air and its SVP (i.e., the maximum amount of water it could absorb).

This difference is called the vapor pressure deficit, or VPD. Think of VPD as the water sucking power of the air, because it is actually the VPD that interests your plants, not the relative humidity.

At face value, VPD (sucking power) seems to be the same as relative humidity - because relative humidity is the ratio of the actual vapor pressure in the air to the SVP.

Its not the same, because the SVP of a given block of air increases exponentially as the air temperature rises - the higher the temperature, the greater the amount of water vapor that air can hold.

Rather than giving a physical explanation of why humidity and VPD are different measurements, because I'll get out of my depth in about two seconds, just look at how the VPD (sucking power) changes at various temperatures if the relative humidity stays the same at 75%:

Cpa represents how much water the atmosphere can absorb

VPD calculation is an improvement over relative humidity (RH) measurement alone because VPD takes into account the effect of temperature on the water holding capacity of the air, which roughly doubles with every 20°F increase in temperature.

Rather than giving a relative measure of the water content of the air, VPD gives an absolute measure of how much more water the air can hold, and how close it is to saturation.

For example, a typical 100' long x 30' wide x 10' high greenhouse with 80% relative humidity has about 14 lb of water in the air at 50°F, while 70°F air holds about 28 lb of water at the same RH.

This is reflected in the VPD values of 0.036 psi (0.25 kPa) and 0.072 psi (0.50 kPa), for the lower and higher temperature conditions, respectively (see Figure 2). Thus, VPD can be used to identify healthy air moisture conditions for plant production, while taking into account different temperature levels.

How does VPD compare to relative humidity?

Figure 1 shows how VPD relates to the customary thinking about humidity. Higher VPD means that the air has a higher capacity to hold water, stimulating water vapor transfer (transpiration) into the air in this low humidity condition. Lower VPD, on the other hand, means the air is at or near saturation, so the air cannot accept moisture from the leaf in this high humidity condition.




Figure 1. Vapor Pressure Deficit (VPD) enhances or inhibits the crop’s ability to transpire.
Therefore, vapor pressure deficit is a useful way to express the vapor flow in the system, both for condensation and transpiration.

Higher VPD increases the transpirational demand, influencing how much moisture from plant tissues is transferred into the greenhouse air. Consequently, VPD is being used to predict crop water needs in some commercial irrigation systems.

In contrast, very low VPD indicates closer proximity to the dew point, meaning harmful condensation can begin to develop.

Using the canopy temperature to determine VPD gives the best indication of condensation risk, showing particularly how close the canopy is to the dew point.

Looking at the temperature and vpd on a graph, you can see how the vpd is increasing exponentially as the temperature rises, while the relative humidity remains constant:

Here is another good article with a graph. VPD

VPD values run in the opposite way to RH values so when RH is high VPD is low.

If humidity is too low (i.e. high VPD), the stomata on the leaves tend to close in order to limit transpiration and prevent wilting. This closing of the stomata will also limit the rate of CO2 uptake and hence limit photosynthesis and consequently plant growth. Low humidity also reduces turgidity (water pressure within the plant cells) and this in turn also restricts growth. Blossom end rot in tomatoes and capsicum can also be attributed to low humidity (high VPD).

Conversely, if humidity is too high (i.e. low VPD) the stomata will fully open but even so the plants will be unable to evaporate enough water to carry minerals into the plant and so again, growth will be impeded and mineral deficiencies (particularly calcium) may occur. In addition, the plants may exhibit soft growth, fungal problems and mineral deficiency symptoms.

It is frequently stated that VPD more closely matches what the plant "feels" in relation to temperature and humidity and therefore forms a better basis for environment control. Unfortunately, VPD is extremely difficult to determine accurately as it is necessary to know the leaf tissue temperature. Attempts to measure leaf temperature reliably on an ongoing basis have often ended in disaster. One of the problems is that the plants leaves are in differing amounts of sun with some leaves in full sun, some in partial sun and others in full shade. This makes the concept of "leaf tissue temperature" particularly complex.
VPD can be used to identify disease-causing climate conditions. For example, several studies that explore disease pathogen survival at different climate levels reveal two critical values of VPD.

Studies show that fungal pathogens survive best below VPD (<0.43 kPa).

Furthermore, VPD is most damaging below (0.20 kPa).

Thus, the greenhouse climate should be kept above (0.20 kPa), to prevent disease and damage to crops.

How do we calculate VPD?

Well this is a problem. You need to know several temperature values and use a formula to calculate VPD. This is probably not something you are going to do everyday. There is a chart but I am not able to copy it into the post, so I will try to extrapolate it here.

I reached a couple of preliminary baseline conclusions.. assuming the target Kpa is
.9 and that leaf temp is 3ºC below air temp..

if the temperature is 85F, humidity should not be higher than 62%

if the temperature is 80F, humidity should not be higher than 58%

If the temperature is 75F, humidity should not be higher than 53%

If the temperature is 70F, humidity should not be higher than 47%

the rule of thumb seems to be a target temp of 80F should have just under 60% humidity.

An interesting observation is
that as temperature drops by 5 degrees F,
required humidity also drops by 5 %

so, 80F @ ~60%, or 70F @~50%, are good ballparks..

Here is a VPD calculator if you have all the temps to plug into it. VPD calc

Sometimes, when you think you have a nute deficiency, you are really just experiecning VPD which can mimick Mg deficiency.

Here are a couple of examples of what high VPD will do.




I just live with it.


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