Soil Science Spotlight: One of the primary functions of the GROW BIOINTENSIVE method is to allow small-scale farmers everywhere to build and maintain soil fertility levels that will allow the farmers to grow a large amount of food and compost materials in a very small area, with greatly reduced resource use, for an indefinite period of time, sustainably. Soil testing and the application of the correct type and quantity of organic soil amendments at the correct time is a fundamental part of building and maintaining sustainable soil fertility levels. To introduce the topic of soil testing and the reasoning and methodology involved in soil test analysis and making soil amendment recommendations to a wider audience, John Beeby and Ecology Action are creating a series of topics on the subject called “Soil Science Spotlight”, which is posted to growbiointensive.org in the “Protocol” section with new posts added often.
In parts 1-4 of this segment, I introduced Dr. John
Doran's USDA Soil Quality Test Kit Guide Plants largely take up nutrients in ionic form, as negatively or positively charged atoms or molecules.
Collectively, all of these agriculturally important ions can be referred to as nutrients or ions, but they can also be referred to as salts. When discussing soils, these terms are used interchangeably. A fertile soil has an abundance of nutrients in available ionic forms, typically suspended in the soil’s water, so that when plants need them, they can get them. However, as with all things, there can be too much of a good thing. Salinity is what happens when there are too many ions or salts in the soil. When most of us think of “salts”, we probably think of table salt, what we add to our foods to improve flavor; but table salt (sodium chloride or NaCl) is simply one type of salt that exists in soils. As with all salts, when dissolved in water NaCl becomes two distinct ions, sodium and chloride (Na+ and Cl-,). When we refer to salts in soils, we are referring to all salts, all ions, all nutrients, not just sodium and chloride. In fact, when a soil has a high content of sodium, based on its saturation percentage or the percentage of cation exchange sites it occupies, that soil is referred to as “sodic” not “saline”. In contrast, a saline soil's distinct property is an excessive amount of total salts or ions (not just sodium). The main trouble with too many ions in a soil is that plants growing in that soil have a more difficult time taking up water. Plants, like all living things, need water to grow and thrive, and they need the nutrients that come along with that water. The reason that plants can take up water from the soil is because of a physical force called diffusion. Diffusion is a physical property that we can see all around us. For example, when we are cooking in our kitchen, aromas spread through our living area because molecules in high concentrations (such as aromatic molecules in the kitchen) want to move into areas where there is low concentration (such as the rest of the home). As a result of this diffusion, over time everyone in the home can smell what is cooking in the kitchen! Water in the soil tends to have much fewer salts than the water inside a plant’s roots. So, water in most soils has a high concentration of water molecules relative to the number of salt molecules in the soil, and that water will naturally move into roots that have a lower concentration of water relative to its salts. However, if the concentration of water and salts is similar between soil and plant—which occurs with a saline soil or a soil with high concentration of ions/ nutrients/salts—then the plants cannot take up the water they need and become very prone to wilting even when the soil is not dry and has available water. Plants without sufficient water become less vigorous, more prone to pests and diseases, and unable to reach their yield potential. For this reason, soil salinity has been a major cause of the decline of past civilizations (Mesopotamia and the Vinu Valley of Peru) and continues to affect 25-30% of irrigated land in the world. You can determine if your soil is saline, or close to becoming saline, with an electrical conductivity (EC) meter, which measures the water's ability to conduct electricity. Pure water (H2O) is a very poor conductor of electricity; it is only when it has ions (positively or negatively charged elements) or salts in it that electricity can pass through it more easily. The more ions there are in water, the easier it is for the water to conduct electricity. An EC meter is a relatively inexpensive tool that is essential for anyone working with soils, particularly in arid or semi-arid environments. In such environments, there is often not enough natural precipitation to leach excess salts (which may have accumulated due to fertilization or simply due to capillary action bringing water to the soil surface where it evaporates and leaves the salts it carries on the soil surface) past the root zone. In more humid environments with more precipitation, salinity is much more infrequent, since rain and snow-melt naturally moves downward through the soil, carrying excess salts with it. Soil salinity is also more of a risk in clayey soils in drier climates compared to sandy soils in drier climates, since clayey soils will not leach as easily and are more prone to water movement through capillary action. To determine the electrical conductivity of a soil or water, you can either send a sample to a laboratory for testing or you can do it yourself. For instructions on taking a soil sample and submitting it to a lab for testing, see https://www.growyoursoil.org/. To measure electrical conductivity yourself, you will need an EC meter and at least one calibration solution, typically a 1.413 dS/m (decisiemens per meter) solution. EC meters are relatively easy to use, but they do require calibration to ensure they are reading accurately. Most electrical conductivity meters come with instructions on how to calibrate them with 1 or 2 calibration solutions, and how to use them once they are calibrated. In addition, their instructions will describe the care needed to maintain the probe and solutions to ensure accurate results for many years to come. To measure the electrical conductivity of irrigation water, simply calibrate your EC meter, place the probe in the water, and take the reading. To measure the electrical conductivity of a soil, you will also need a small measuring cup (something like 1/8 cup or 30 ml), a container with lid (about a ½ cup or 120 ml, or slightly larger), and a small amount (1/8 cup) of distilled water:
To evaluate your measurements and determine if irrigation water or a soil is saline:
Keep in mind also that the electrical conductivity of a solution is affected by temperature. Generally, the electrical conductivity of a solution increases with temperature at a rate of approximately 1.9% per 1°C increase. How saline is saline? As a soil consultant, if I find a soil has an EC reading equal to or higher than the values in the table (based on the dilution factor of the test performed), then I would consider ways that the farmer can reduce their soil’s salinity, as well as determining the source(s) of the excess salts. While the official classification of a saline soil is that it is above 4 dS/m when prepared as a saturated paste (1:1 soil:water dilution), in my experience, the signs and even the effects of salinity show themselves well below this limit. By lowering the limit to the values shown in the table above, we can prevent salinity at an earlier stage, which allows the farmer to remedy the soil before the problem reaches a crisis and the remediation is even more difficult. What causes excess salt in a soil? In addition to excessive use of fertilizers (in which nutrients are added that a soil does not need) and excessive capillary action and surface evaporation, a major source of excessive salts in the soil is poor irrigation water quality and irrigation practices. Water interacts constantly with its environment. Water from a well interacts with the rocks and minerals in that well, and in that process, can dissolve and accumulate an excessive quantity of minerals depending on the composition of the rocks present. In an arid or semi-arid environment, a lot of water may be required to grow a crop, so there is a potential for excess salts to be added to the soil through heavy irrigation. In addition, if the farmer irrigates often with small quantities of water, much of the water can evaporate from the soil surface, leaving behind salts, either directly or through capillary action, especially in more clayey soils with poor surface aggregation that is bare and not protected by plant coverage or mulch. In this environment, irrigating less frequently and with larger quantities of water, salts will tend to move downward into the soil with the water, and excess salts will be pushed down past the root zone where they do not pose as great a threat, rather than collecting in the root zone or at the surface and causing soil salinity. How to remediate a saline soil What is the best way to fix a saline soil if poor irrigation practices have already occurred and the quality of the irrigation water is poor? The best way to reduce salinity in a soil is by leaching the excess salts past the root zone. Apply as much water as possible, up to the limit that the soil can absorb without runoff (which can cause erosion). In general, applying 6 inches (15 cm) of water will lower salinity by 50%, 12 inches (30 cm) by 80% and 24 inches (60 cm) by 90%. I generally start with a recommendation to add 6 inches of water, as this can be challenging enough to get into a soil without running off the top. Depending on the soil’s ability to take in water (infiltration rate), borders may be required to essentially build a 6-inch (15 cm) pond above the growing bed to allow enough time for the water to move down into the soil and leach the salts past the root zone. While the water is entering the soil, the soil should be covered with shade netting or mulch to prevent evaporation. Ideally, you have good quality (non-saline) irrigation water to apply and leach excess salts, but if this is not the case (as it often is not) use the irrigation water that you have. By applying it in very large volumes as described above, it will be able to carry much of the excess salts past the root zone, including the salts contained in the water applied. If your soil is compacted, you should consider cultivating it—ideally by double-digging—to ensure good water and salt movement to the depths greater than at least 12 to 24 inches (30 to 60 cm). Soil salinity is a common threat to our agricultural soils and will likely increase with rising soil temperatures, decreasing quantity and quality of groundwater, and evaporation due to climate change. Learn to measure your soil’s electrical conductivity to prevent salinity and contact Grow Your Soil (www.growyoursoil.org) for help.
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