I want to grow plant under water stress (30% field capacity). But I don't know how to calculate amount of water needed to maintain soil moisture at 30% field capacity in pot with growing plant. Isn't the plant weight will affect the total weight? Then, how can we measure the amount water needed to maintain the field capacity?
Let say 500 g of water is needed for 2 kg dry soil to reach 100% capacity. To have 30% field capacity, 150 g water is needed (30% x 500 g). Then, when the plant is growing and plant weight is increasing, how can we calculate the amount of water needed to maintain soil moisture at 30% field capacity?
Please help, thank you.
Generally for scheduling irrigation we are using following formula for calculating water on area basis.i.i.
Q=pan coefficient x crop coefficient x CPE x Area. CPE stands for cumulative pan evaporation (mm) This will give you quantity (Q) of water in liters.
Firstly, calculate the water content in given soil at FC.
To bring the soil at FC, saturate tha soil column and let the pressure come to 0.33. It can be examined with the help of pressure plate apparatus.
If you don't have pressure plate apparatus, then it happens after 24-48 hours after total soil saturation when flow out of water stop.
Now calculate the water content at FC.
Suppose the weight of soil is 2 kg (2000 g)
MC % at FC = 40%
Amount of water (g) at FC= 2000×40÷100
= 800 g
Water required (g) at 30% FC = 800×30÷100
= 240 g
You first you need to know he equivalent of the Moisture content percentage to field capacity of the soil. Secondly the depth of the total available water between field capacity and PWP, then calculate the 30% of the depth of total available (I.e from your 2nd calculation. You then know the 30 MC% equivalent of of your soil
What is the purpose of setting water to 30% field capacity? I imagine you are simulating drought conditions to some extent.
Now consider a natural environment with rainfall, sunlight, wind etc. 30% field capacity is hardly going to be uniform across a crop, pasture or prairie. It is a theoretical value that you are choosing to set for the purposes of your experiment. It really won't matter if it is 28% or 32%, the plants will be water stressed compared to 100% field capacity.
If you are really worried about how much plant mass will affect your calculations, grow some sacrificial plants and harvest them at weekly/monthly intervals to estimate the biomass accumulation and adjust your mass balance accordingly. Remember that the plants could be 80-90% water depending on species and age, so you will have to adjust for that too.
On the other hand, if your pots are not sealed, they will be losing water through evaporation (and transpiration) as soon as you have finished adding the water, so 30% is really only the maximum field capacity you are setting your pots to. Unless you are growing high-biomass woody species for a very long time, I don't think the mass of plants will make that much impact.
I suppose you can compensate by watering more often, then you can record daily water use.
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