My experiment is effects of irrigation intervals on growth and yield of rice. My treatments are continuous flooding, irrigation at field capacity,90% field capacity and 80% field capacity. Before transplanting I can determine field capacity of soil in pots but after transplanting how can I determine field capacity of soil in pot, 90%field capacity and 80% field capacity to give irrigation treatment at these levels?
FC is determined by weighting pots in some literature but after transplanting how to determine when plant grows?
Good question. You might assume that the mass of the fresh plant is (say) 90% water. Since the mass of water in the soil is usually very much greater than that in the plants, a small uncertainty in plant mass will cause very little uncertainty in soil water. Validation of this approach requires serial estimates of plant mass.
No matter how much the percentage of Field capacity you have to determine, It is a relative value which could obtained from soil water content in your plots.
Good question. You might assume that the mass of the fresh plant is (say) 90% water. Since the mass of water in the soil is usually very much greater than that in the plants, a small uncertainty in plant mass will cause very little uncertainty in soil water. Validation of this approach requires serial estimates of plant mass.
This is sort of missing the point. Don't take advice to just weigh pots
Field capacity is the moisture content at which gravitational drainage stops nominally 2 days free drainage. This is often equated to moisture when soil is eqilibrated at -100 cm or -330 cm tension. A pot does not have field capacity, as it can only undergo drainage by gravity! there is no suction from the lower soil profile so for a 10 cm high pot the suction at the top is -10 cm and the base 0 cm. in other words a pot has 10 to 30 times less suction than what field capacity should be determined at. Weighing a10 cm pot after two days free drainage will give moisture content at -5 cm, not-100 cm.
to determine field capacity of both the pot soil and real soil obtain an intact core saturate it for 2 days then drain under suction to equilibrium at -100 cm tension on a sand table or ceramic plate for up to 2 weeks then weigh and determine bulk density.
Hello everybody. I will look to the problem in another way. We should first characterize the soil used which means find the suction theta relation (on the weight basis for theta). Determine at what value of suction corresponds on weight basis the theta for 80 and 90 percent of field capacity. Finally just use a tensiometer and take a decision for irrigation at the values obtained from soil water characteristics. The increase in difference in the plant weight is not a big effect with regard to the big weight of the pot. Also the use of the tensiometer works very well in the range of 80 to !00 percent field capacity.
You have had some useful responses. The issues is what uncertainty the rice plant weight adds to your problem. Ratio of the soil mass to the plant mass will determine this. Plants are more like 80% water than 90%. I did some work on drying plants in oil a few years ago and almost all plants were around 80%. It is not field capacity but the available water you should be using as your independent variable. To get this you will need to determine the moisture characteristic (relationship between water content and matric potential). If you do this then you can use a tensiometer to determine your irrigation point. However, as Marcus pointed out the potential will vary with depth in the pot, so depth of placement to give you an average value can be determined from the pot height and the moisture characteristic. Hope this helps.
I fully agree with the answer given by Mohamed Mechergui as this is the only scientifically best technique to measure moisture at any level of Field capacity.
Field capacity can be obtained after 2 to 3 days of irrigation, or after 2 to 3 days when the soil was completely saturated. So field capacity is an approximate term, which is also defined as a most favorable condition for root growth. What you can do I think is measure soil moisture using tensiometer, decagon or some other device after 2 to 3 days of irrigation, which should correspond to the field capacity level. Then for that treatment, always try to maintain that soil moisture level. For other 2 treatments, instead of saying 90 % or 80 % field capacity, just try to maintain the lower soil moisture level, in other words, you have to stress rice plant a bit. Hope this helps.
You have got some good advice already. Just be careful, the field capacity is the value in rather large interval (in pF curve of soil water retention is the soil water content related to the interval somewhere between 2.0 to 2.7 pF in Europe. US scientist use the value related to 0.33 bar of soil water potential at the SWRC). Definitely it is the value defined many years ago by Heinrich (1886) with rather week theoretical background. If you know from the measurement the SWRC, this is the chance for good estimate.
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