Gypsum Blocks FAQ
On this page, you will find answers to a number of commonly asked questions. If you cannot find the answer to your problem here, then send us an e-mail and we will try to help, (support@mea.com.au). We'll even post the answer here.
Gypsum Block Soil Moisture Sensors
- How do gypsum blocks work?
- Which gypsum blocks should I use?
- How long will my gypsum blocks last? And how will I know when to replace them?
- Are gypsum blocks affected by salinity?
- My gypsum blocks have stopped reading yet my plants continue to grow, what is happening?
- How many sensors should I install? And at what depths should the sensor go?
- Why have my Watermark / GBLite sensors stopped responding in my sandy soil?
How do gypsum blocks work?
Electrical Properties
The Gypsum Block is a cylindrical block of gypsum (CaSO4) into which two electrodes are inserted. The gypsum is porous and allows water to move in and out of the block as the soil wets (under irrigation or rainfall) and dries (as the plant draws water away). In the presence of moisture the CaSO4 goes into solution allowing ion movement between the two electrodes. When the block is excited electrically, ions move to the respective electrodes establishing an effective block resistance. A greater amount of water in the block equals more ions and a lower resistance to electrical current flow.
Tension Properties
The characteristic of gypsum blocks that makes them particularly useful in soil moisture monitoring is the variable pore size structure. A gypsum block dries by releasing water progressively - from large pores first (at low soil tension) and small pores last (at high soil tension). The tension range measured by the block is highly soil type dependent. Heavy clays hold on very tightly to water particles and hence a plant may need to exert a large amount of tension to the soil (in the order of 100's of kPa) to extract available water. Alternatively, very sandy soils drain freely under gravity and have little capacity to hold onto water. As a result, the highest achievable tension may only be 10's of kPa before the soil is dry.
Calibration
Gypsum is a uniform material and gypsum blocks are manufactured under carefully controlled conditions. As such, gypsum blocks have a single calibration which translates resistance directly to kPa.
#Note that specialist hardware is required to read gypsum blocks as the electrical excitation must be AC and must be small enough in magnitude to prevent long term damage to the block through measurements. MEA can supply appropriate devices for reliable gypsum block measurement. (Back to Top)
Which gypsum blocks should I use?
The traditional gypsum blocks, (e.g. MEA's GBHeavy), operate over a tension range of 60 to 500 kPa, (the range found in soils with a clay content). The granular matrix blocks, (e.g. the Watermark or GBLite), operate over a tension range of 10 to 200 kPa, (as found in sandy soils).
Although the blocks read out to this range, in practice soil moisture levels are kept at the lower end of the scale - in the range where water is readily available to the plant. How much moisture is held in this range is a function of soil type.
The choice of which type of block to use is simple: if moisture levels are to be kept in the range where it is readily available to the plants, use the Watermark style block regardless of soil type. Where the crop is going to be deliberately pushed into deficit, (e.g. with Partial Root Zone Drying and Regulated Deficit Irrigation), use the GBHeavy style block in soils with a clay content and the GBLite style block in sandy soils.
In duplex soils (i.e. sand over clay) a combination of GBLite and GBHeavy sensors may be used to measure soil moisture at that site. When using GBLites to monitor moisture levels in the root zone, a GBHeavy can be used below the root zone to provide an economical drainage sensor. (Back to Top)
How long will my gypsum blocks last? How will I know when to replace them?
Gypsum block lifetime is somewhat variable and site/soil type dependent. Ultimately the gypsum block will dissolve into the surrounding soil and this process can occur over a period of between 1 year (in the very worst case) and 5 or more years (in the best case).
Note that the Watermark is a composite block and due to its construction has a much longer operational lifetime than the GBHeavy - a 10 year life span is typical.
Factors which affect gypsum block lifetime include -
- Soil wetness: a wetter long term installation will result in greater ion liberation (i.e. gypsum block dissolves more quickly)
- Erosion: preferential water flow past the block can cause block erosion
- Soil pH/Salinity: saline soils will liberate ions from the gypsum block more quickly than for neutral soils.
When a block is getting to the end of its life, it will start to give intermittently high readings when you know the soil is wet. (Back to Top)
Is the Gypsum Block affected by soil salinity ?
Whilst the lifetime of the gypsum block is affected by soil salinity, the measurement is not. The gypsum acts as an ion buffer preventing infiltration of salt ions into the block.
My gypsum blocks have stopped reading yet my plants continue to grow, what is happening?
When conditions are very dry or vines are being dried down as part of an RDI strategy, the gypsum blocks may start to flat line at the maximum end of the range, yet the vines continue to grow. Although the gypsum block calibration stops at 500 kPa, soil still holds water until around 1200 kPa (although plants cannot extract it). One option here is to extend the calibration range of the blocks - this has the adverse effect or robbing the blocks of sensitivity in the wet end. The second option is to consider where else the plants may be drawing water from. If the sensors are installed down to say 80cm and are located in vine row, it is possible that the vines are drawing water from deeper down and from in the mid row. Extra sensors can be installed to check if this is the case. (Back to Top)
How many sensors should I install? At what depths should the sensor go?
Enough sensors should be installed to give a representative picture of what is happening across the root zone. In vines and citrus, this means 3 sensors in the root zone. An extra sensor is added just below the root zone to detect drainage. In annual crops and shallow rooted crops, use 1 or 2 sensors in the root zone and another for drainage. If there is a hard pan or impervious layer, a sensor should be placed just above it, to detect the presence of perched water. If there are discrete soil layers, each should be monitored. (Back to Top)
Why do Watermark / GBLite sensors sometimes stop responding in sandy soils
The Watermark sensors are constructed using a small plug of gypsum at the top of the sensor to which the electrodes which measure resistance are connected. The rest of the sensor is filled with crushed quartz. In very sandy soils the sand can dry out faster than the sensor can respond, causing a break in tension between the sensor and the soil - the result is a sensor which reads wet long after the soil has dried down. The sensor performance can be improved by installing them in a slurry of either : clay loam soil, silica flour (200 to 400 grade) or a 50/50 mix of silica flour and diatomaceous earth - the latter being the preferred option. The slurry acts as a buffer and slows the rate of draw down of moisture allowing the sensor to keep up. Either pour a small quantity of slurry into the hole prior to installing the sensor or dip the sensor into a container in which the slurry has been mixed.(Back to Top)
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