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prankster1590
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Evapotranspiration is the combined loss of water from the soil by vaporization and transpiration from the plant. Since these two parameters are dependant on the waterholding capacity of the soil and the water needs of that specific plant type in that specific growstage, a reference ET value is used which is the evaporation from 1 m^2 (10,8 ft^2) of a certain type of grass. To correlate this to the waterneeds and growstage of a certain plant species, this reference ET is multiplied by the Kc value.
The reference ET changes with the weather and the sunlight power. When its hot, more water evaporates and transpires. Since plant x transpires 1.2 time more water on 1 m^2 than the reference grass, it has 1,2 * the reference ET as waterloss.
To keep the watercontent of a soil at a certain level you need to know the evapotranspiration. The water lost by ET is the volume of water that needs to be added back to the soil. If I want to keep the watercontent of the soil at 30% and it drops to 29% in a day. Than I need to add water back to make it 30% again. It dropped to 29% because of ET.
So ET(ref) is given by weatherstation for farmers to irrigate there crops. They just multiply it by the Kc value based on the growstage and plant type. But in a growtent ET(ref) should be fairly constant since there is only two climates for out favorite plant. The Vegetative climate and the flowering climate.
So first i need to know what the drying properties are of the soil and in what growstage the plant is in.
The following image is a drying curve of a Peat:Perlite = 1:1 mixture
So the water content is given in cm water/cm soil. So 0.30 cm/cm = 30% water (V/V). And the suction by the soil (suction roots experience when drinking water) is given in mH20. 1 mH2O =10 kilopascal.
I wanna keep the watercontent between the 31,64-30,39% (V/V) which corresponds to a soil suction pressure between 0,8 and 1 mH2O. This way the roots can easily suck water from the soil. Plants can suck water up to 15 mH2O but it will take alot of energie impairing growth. At about 30% (V/V) the soil contains enough oxygen for the plants to easily breath.
So 32% (V/V) is what I want after watering and 30% (V/V) is when I start watering.
So I have 15 Liter square pot with the following dimensions: LxBxh = 24x24x28. The difference between 31,64- and 0,39% = 0.19 liters.
So how long does it take for the pot and plant to lose 0.19 Liters of water? For this I need the reference ET for the climate conditions in the tent. I can experiment but that would be a waste of good plants. Im not rich. Im gonna assume a reference ET of 5,5 mm/day = 5.5 L/m^2/day. It chose this number since its a value fairly common value in outside climates where marijuana thrives like in the blue mountains of Jamaica.
This value accounts for the reference grass. To convert is for marijuana you need to multiply it with a Kc value. So there is almost no data for this. Marijuana farmers have secrets i guess. So I based these values on a paper describing kc values for Hemp. But I see no change in this curve when going from vegetative to flowering. Its the best I could find.
If I multiplied the reference ET of 5,5 mm/day with the Kc values. It would look something like this in a 84 days grow.
After 21 days the Kc is about 0.432. So ET = 5.5 mm x 0.432 = 2.38 mm or 2.38 L/M^2.
My pot is 0,24 m x 0.24 m = 0.058 m^2. So 2.38 mm x 0.058 m^2 = 0.137 L/day
So with a soil tensiometer you can measure the soil suction. If this sensor hits 1 mH2O, 0.137 L of water is added. Since it takes 0.19 Liters of water to go from 32 to 30% (V/V) it will take approximately 0.19 L/0.137 L = 1.4 days to go from 0,8 - 1 mH2O
This is useful information when you want to automate a grow with drippers. A dripper able to drip 2 L/hour should drip every 11 hours for 1.9 minutes (3 times).
But after all of this, my main question is:
What reference ET value can I best use for a grow tent and does anybody know a good reference paper for Kc values for marijuana plants. It this even applicable for container grows or is this only useful for crops outside? What better method is there to estimate the watering of container plants for automation?
The reference ET changes with the weather and the sunlight power. When its hot, more water evaporates and transpires. Since plant x transpires 1.2 time more water on 1 m^2 than the reference grass, it has 1,2 * the reference ET as waterloss.
To keep the watercontent of a soil at a certain level you need to know the evapotranspiration. The water lost by ET is the volume of water that needs to be added back to the soil. If I want to keep the watercontent of the soil at 30% and it drops to 29% in a day. Than I need to add water back to make it 30% again. It dropped to 29% because of ET.
So ET(ref) is given by weatherstation for farmers to irrigate there crops. They just multiply it by the Kc value based on the growstage and plant type. But in a growtent ET(ref) should be fairly constant since there is only two climates for out favorite plant. The Vegetative climate and the flowering climate.
So first i need to know what the drying properties are of the soil and in what growstage the plant is in.
The following image is a drying curve of a Peat:Perlite = 1:1 mixture
So the water content is given in cm water/cm soil. So 0.30 cm/cm = 30% water (V/V). And the suction by the soil (suction roots experience when drinking water) is given in mH20. 1 mH2O =10 kilopascal.
I wanna keep the watercontent between the 31,64-30,39% (V/V) which corresponds to a soil suction pressure between 0,8 and 1 mH2O. This way the roots can easily suck water from the soil. Plants can suck water up to 15 mH2O but it will take alot of energie impairing growth. At about 30% (V/V) the soil contains enough oxygen for the plants to easily breath.
So 32% (V/V) is what I want after watering and 30% (V/V) is when I start watering.
So I have 15 Liter square pot with the following dimensions: LxBxh = 24x24x28. The difference between 31,64- and 0,39% = 0.19 liters.
So how long does it take for the pot and plant to lose 0.19 Liters of water? For this I need the reference ET for the climate conditions in the tent. I can experiment but that would be a waste of good plants. Im not rich. Im gonna assume a reference ET of 5,5 mm/day = 5.5 L/m^2/day. It chose this number since its a value fairly common value in outside climates where marijuana thrives like in the blue mountains of Jamaica.
This value accounts for the reference grass. To convert is for marijuana you need to multiply it with a Kc value. So there is almost no data for this. Marijuana farmers have secrets i guess. So I based these values on a paper describing kc values for Hemp. But I see no change in this curve when going from vegetative to flowering. Its the best I could find.
If I multiplied the reference ET of 5,5 mm/day with the Kc values. It would look something like this in a 84 days grow.
After 21 days the Kc is about 0.432. So ET = 5.5 mm x 0.432 = 2.38 mm or 2.38 L/M^2.
My pot is 0,24 m x 0.24 m = 0.058 m^2. So 2.38 mm x 0.058 m^2 = 0.137 L/day
So with a soil tensiometer you can measure the soil suction. If this sensor hits 1 mH2O, 0.137 L of water is added. Since it takes 0.19 Liters of water to go from 32 to 30% (V/V) it will take approximately 0.19 L/0.137 L = 1.4 days to go from 0,8 - 1 mH2O
This is useful information when you want to automate a grow with drippers. A dripper able to drip 2 L/hour should drip every 11 hours for 1.9 minutes (3 times).
But after all of this, my main question is:
What reference ET value can I best use for a grow tent and does anybody know a good reference paper for Kc values for marijuana plants. It this even applicable for container grows or is this only useful for crops outside? What better method is there to estimate the watering of container plants for automation?
