Posts Tagged: gypsum
Soil Moisture Measuring Devices
Irrigation timing can be determined more precisely using a tensiometer. These water-filled tubes with a pressure gauge accurately reflect the amount of energy a plant needs to extract water from the soil. The pressure gauge measures tension values in centibar units (cbars). For citrus, when the gauge reads 30 centibars, it is a good time to irrigate. Tensiometers must be placed in the root zone between the emitter and the tree trunk. Having two tensiometers next to each other can be helpful in deciding when to turn the system on and off. For example, a tensiometer at a depth of 1 foot would indicate when to turn the water on, and a second at 3 feet would indicate when to turn the water off. Prevent tensiometers from being damaged during harvesting and other grove operations by placing a plastic milk crate or some other structure over each device.
Other devices can also be used to measure soil moisture. Gypsum blocks are very effective; the part in the ground is inexpensive but the reading device costs about $250, so a relatively large acreage is required to spread out the cost of the system. Portable meters rely on an electrical current carried by water in the soil. Even cheap $10 meters can give a rough estimate of the soil water content, but they are not very effective in rocky ground, because their sensitive tips break easily.
Soil-based methods monitor an aspect of soil moisture that, depending on the method, requires a correlation to plant water use. Some methods are well understood and inexpensive, others are expensive, inaccurate, inappropriate, or not well researched. Some methods allow multiple site readings, while others require a device to be left in place. Some measure soil water directly (e.g., oven-drying), and others measure another parameter, such as electrical conductance. Some methods are affected by salts or soil iron content, and others have limited value in the desired soil moisture range. Some, like tensiometers and gypsum blocks, give a reading from a porous material that comes to equilibrium with soil moisture, while many others use the soil directly as the measured medium—an important distinction, since discontinuities in the soil caused by rocks or gopher holes can affect readings. Also, some of the older techniques have been improved. For example, gravimetric oven-drying can now be done by microwave, considerably speeding up the process, and tensiometers and gypsum blocks can now be found with digital readouts and connections to data loggers that make data easier to manage. Many types of monitoring devices are available; the table below describes their characteristics.
As with any tool, the value of these devices increases with use and familiarity. Even though several are stationary devices, by placing them in representative positions in the grove, they can accurately reflect the entire grove. Some types of device can be stationary or portable, depending on the model.
Method |
Cost |
Ease of use |
Accuracy |
Reliability |
Salt-affected |
Stationary |
Gypsum block |
L |
H |
H |
H |
L |
YES |
Tensiometer |
L |
M |
H |
M |
L |
YES |
Portable tensiometer |
M |
M |
H |
M |
L |
NO |
Solid-state tensiometer |
M |
H |
H |
H |
L |
YES |
Time domain reflectometer |
H |
M |
H |
H |
M |
BOTH |
Neutron probe |
H |
L |
H |
H |
L |
YES |
Feel (soil probe) |
L |
H |
H |
H |
L |
NO |
Gravimetric (oven) |
L |
M |
H |
H |
L |
NO |
Conductance |
L |
H |
M |
M |
H |
BOTH |
Capacitance |
M |
H |
M |
H |
M |
BOTH |
H – high, M – medium, L - low
tensiometer2
Another Look at Liming Acid Soils
Great article here by Dr. Robert Mikkelson from the International Plant Nutrition Institute concerning the why and how of liming a soil which is acidic.
Here's the short of it:
Several factors contribute to the acidity of a soil, one being the geologic composition of the base material, another being lots of rain which leaches out calcium and magnesium. Another contributing factor to soil acidity in agricultural soils is the continual use of nitrogen fertilizers, especially in the same spot over a longer period of time. Both urea and ammonium, when converted to nitrate by soil micro-organisms, release hydrogen ions, the higher the concentration of which raise the acidity of the soil.
A common grower solution to acidic soils is the addition of ground limestone. However, while limestone neutralizes acidity, adds calcium and enhances the solubility of phosphate (and consequently availability of phosphorous to the the plant) does not dissolve well at all at pH's above 6.5 (common for soils of the Central Coast) and one would be hard put to realize the aforementioned benefits at this pH. So if one needs to add calcium to a soil with a pH above 6.5, the better choice would be gypsum, which while it also does not dissolve well in a neutral pH soil, does supply more soluble calcium.
/span>Salt and Gypsum
With the drought our perpetual salt problems are exacerbated due to less water and often more saline water. The question keeps coming up if gypsum (calcium sulfate) can help correct the problem. And the answer is maybe, but along the coast, probably not. The problem there is confusion about what is a saline soil and what is a sodic soil. A saline soil is one that is dominated by salts, but has a pH below 8.5 and can have a white crust that will actually taste salty. A sodic soil is one dominated by sodium, has a pH above 8.5 and can be saline, as well. Often though, there is a brownish cast to the surface salt crust. This is caused by dispersion (dissolved) of soil organic matter caused by the high pH. It's like cooking with vinegar when you make ceviche out of fish. Saline soils often have a high calcium content and may have sodium, but at a very low ratio compared to calcium. Most of the sodic soils in California are found in the Central and Imperial Valleys. Along the coast, the soils, if they have a problem, are largely saline.
The way gypsum works, is that the added calcium displaces soil sodium, pushing it lower in the soil column. The process also requires a lot of water to move the sodium through the soil column.
So the answer is, along the coast, gypsum is unlikely to improve soil conditions. However, there are other instances where it might help. In the San Luis Obispo area there are lots of serpentine derived soils that have a high magnesium content relative to calcium. And they commonly aren't saline, just an imbalance between the two cations. This can lead to infiltration problems and calcium deficiency in plants. Apples are especially sensitive to this high Mg:Ca ratio and develop a condition called “bitter pit”, a surface, brown pitting in the skin. There are other crops, like celery that are especially sensitive, but even avocado can be mildly affected. In the case of magnesium imbalance, gypsum can help.
sodic-crust stutsman-co
Understanding Plant Salt Tolerance in Gypsiferous Soils
For those of you who attended Steve Grattan's presentation at UCCE Monterey last week on managing salinity in vegetable production, you may recall he mentioned that strawberries in gypsiferous soils can tolerate a higher EC reading than the salt tolerance guidelines allow. I wasn't quite clear about this, so I emailed about it and his answer concerning this is as follows:
"Crop salt tolerance is based on crops response to the electrical conductivity of the saturated soil paste (ECe). But in actuality, crops respond to the salinity in the soil water. These are different.
The field soil water content for many berries and vegetable crops is slightly above or below the field capacity. This means that about 1/2 of the pore volume in the soil is water and the other half is air. To make a saturated paste, distilled water (pure water without salts) is added to fill the extra pore space. Now if the salts in the soil water are largely sodium and chloride (very soluble), then the ECe would be about half the EC of the soil water in the field...which is what the crop is truly responding to. But if the soil contains a lot of gypsum (CaSO4), then by adding distilled water, more salts will become dissolved so that the ECe would be higher.
As an example, the literature indicates that strawberries can tolerate a maximum ECe of 1 dS/m, beyond which yields decline 33% for every 1 EC unit is increased beyond that. This was based on a chloride dominated water. Therefore, stawberries can tolerate about an EC of 2 in the field water. Gypsum has a maximum solubility of about 20 meq/l which is an EC of about 2 dS/m but this is just a generalization and can vary depending on soil chemistry. But for simplicity, if the salts were all gypsum (no sodium chloride) than if the soils had excess gypsum, the EC of the field water would be about 2 (no yield reduction). But to make a saturated paste, distilled water is added and more gypsum is dissolved so the resulting ECe is not 1 but remains at 2. Therefore an ECe of 2 dS/m is not growth limiting in this case.
The rule of thumb is that plants can typically tolerate a 1-2.5 dS/m higher ECe than the salt tolerance guidelines indicate in gypsiferous soils because of this relationship."