Irrigation is required when a crop’s growing season cannot provide adequate rainfall to meet the water needs of the crop. According to the FAO (2021), roughly 22% of the world’s agricultural land is irrigated, although by country these percentages vary widely, from 52% in China to 4% in Africa. Irrigation water has a huge impact on yields, but it also can have a significant long-term effect on the quality of the soil receiving it. When a soil requires irrigation water to raise crops, that soil not only receives water (H2O) but also all of the elements that the irrigation water contains. There is no natural water on the planet that is pure H2O because water is so interactive with its environment. H2O the molecule is both partially positively and partially negatively charged, making it such an excellent dissolver because it is attracted to and attracts both negatively and positively charged elements in its environment. Water in an aquifer has interacted for likely millions of years with the minerals and rocks surrounding it. Water from a well or spring interacts with the minerals in the walls of the well or the earth, slowly dissolving those nutrients and carrying them within the water. Even a raindrop falling through the sky interacts with the gases it encounters on its way to the ground. For example, carbon dioxide in the atmosphere will enter a raindrop and be converted to carbonic acid (H2CO3) which causes the raindrop to be slightly acidic as well as contain elements other than just hydrogen and oxygen. So when water, particularly irrigation water, is added to a soil, that soil receives H2O along with all the minerals contained within, essentially “fertilizing” that soil with those minerals. Those minerals may be beneficial if they help alleviate specific nutrient deficiencies in that soil or help to slowly bring the soil’s pH within the optimal range of 6 to 7, or those minerals may be detrimental if they compound an existing mineral toxicity in a soil or further push the soil’s pH beyond that optimal range. The effect depends on whether the conditions of the irrigation water are complimentary or detrimental to the conditions of the soil receiving that water. In addition, the effect depends on how much irrigation water is added and how many of these elements are being leached past the root zone. But it is inevitable that when we irrigate a soil, we are fertilizing that soil, for better or for worse.
The leading theory on the origin of life is that billions of years ago, deep sea ocean water interacting with hydrothermal vents over millions of years created organic molecules, the building blocks of life. Land at the time was just cooling rock and not conducive to life taking hold; there was no soil yet. However, there was water and there were gases in the atmosphere, so when it rained, the rain drops interacted with those gases, became more than just H2O, and became acidic. When those rain drops landed on the cooled rock over many millions of years, the rock slowly started to dissolve through a process called hydrolysis, a type of chemical weathering caused when acids release nutrients from rock. These raindrops were the first fertilizers – they added elements from the atmosphere and released elements from the rock. These freed elements could then be utilized by the first simple forms of land life, like bacteria and eventually lichens which create their own organic acids to further dissolve rocks and which through living and dying created organic matter. Organic matter decomposed by bacteria and fungi, create more acids, dissolved more rocks, and over many, many iterations, slowly soil starts to be created – first topsoil, then subsoil.
Irrigation happens most often in semi-arid and arid regions of the world. These regions often have poor irrigation water quality; the water may contain high amounts of bicarbonates and carbonates that raise the pH of soils over time and cause them to be too alkaline, or high amounts of nutrients that in excess can be detrimental to soil health, like sodium and boron.
How much irrigation water should we add?
In general, enough irrigation water should be added to meet the needs of the crop and prevent excess salts from accumulating on the surface of the soil or root zone, roughly the top 30 cm (1 foot) of soil. (Note: salts, nutrients, minerals, and elements are all considered synonymous in this article and are used interchangeably. In particular, the term salts refers to all minerals, not just sodium chloride the salt we are familiar with adding to our foods). To prevent minerals from accumulating, it is best in general to irrigate with more water less frequently. If we water frequently with smaller quantities of water, much of the water can be lost through surface evaporation. When water evaporates, the H2O vaporizes, but all the nutrients contained in the water are left behind. Using irrigation water with a lot of salts carries the risk that these minerals will slowly accumulate in the soil, causing the soil to eventually become saline.
What is a saline soil?
A saline soil is a soil that has so many excess minerals that crops have a difficult time taking up enough water and become very prone to wilting. Even if there is sufficient water in the soil, crops growing in saline soils can struggle to such an extent that their vigor and yields suffer. Plants take up water through the physical property that elements at a higher concentration want to move to areas of lower concentration. This process is generally called diffusion and we are all familiar with this process. When we are cooking a delicious meal, the aromatic molecules in the kitchen are in a high concentration compared to the rest of the home. Those molecules will want to move quickly to the areas of lower concentration, causing the smells to move throughout the house and beckon any hungry inhabitants. In the case of plants, the process is called osmosis, which is the same movement of molecules from a higher concentration to a lower concentration, but this time movement is across the root’s skin or membrane. Soil water will move into the root as long as there is a higher concentration of water and a lower concentration of salts outside of the root compared to inside the root.
In saline soil, the difference in concentrations is not very large, making it difficult for water to enter the root. In fact, soil can become so saline that the concentration of water is lower outside of the root than within it, causing water to leave the root and move to the soil, the opposite of what the plant needs. Faced with this situation, the plant must expend a lot of energy simply moving water into its roots, which can take a significant toll on the plant’s health, vigor, ability to resist disease and overall yield. To learn more about saline soils, click Salinity and Sodicity
To avoid creating a saline soil, enough irrigation water must be applied to not only meet the crop needs but also to cause any excess salts in a soil to be leached past the root zone. The amount of water needed to do this depends on a variety of factors:
- the crop and its growth stage, plant density and exposure of bare soil that promotes surface evaporation,
- soil texture and structure that determines the ability of water to penetrate deeply into the soil and the ability of water in the soil to rise to the surface through capillary action as well as the soil’s water holding capacity,
- the climate (air temperature and humidity), and
- the depth of the groundwater
Since many of these factors are difficult for most farmers and gardeners to determine, one very handy tool to determine if excess salts are accumulating or being effectively leached past the root zone is an electrical conductivity meter.
An electrical conductivity meter is relatively inexpensive (roughly $30-50 US) though calibration solutions are generally needed which are a consumable that must be replaced periodically (about $25 US for a 500 mL bottle of 1413 µS/cm calibration solution). An electrical conductivity meter measures the ability of electricity to be conducted through the soil water solution. The more positively and negatively charged ions (dissolved minerals) present in a soil, the more easily electricity can pass through it. A soil is officially considered to be saline when it has an electrical conductivity of greater than 4 decisiemens per meter (dS/m). However, whenever Grow Your Soil (growyoursoil.org) gets soil test results showing 1.5 or greater dS/m, we consider that a warning sign and will discuss crop performance and possible sources of salinity with the grower to avoid the need for leaching the soil should it become saline in the future.
Using an electrical conductivity meter is relatively easy and can be done in about 10 or 15 minutes by a grower so no soil sample needs to be sent to a lab. By regularly monitoring soil salinity, one can modify their irrigation practices early on to maintain a salinity level less than 1.5 dS/m (or 1500 µS/cm). Monitoring electrical conductivity is especially important with greenhouse soils that do not receive rainfall during the year. Rainfall naturally helps to push excessive salts deeper into the soil, and without this, leaching only occurs through sufficient application of irrigation water.
Instructions for using an electrical conductivity meter will be provided with the instrument and can be found easily on the internet. It is important to calibrate the electrical conductivity meter prior to use if it is not regularly used, and to keep its probe very clean with distilled water and covered when stored (and turned off after use so the battery works when you need it next).
What other risks are present in irrigation water?
In addition to adding too many total salts to a soil, it is also possible that irrigation water contains high amounts of bicarbonates, sodium, boron, chloride, and other minerals that can be harmful to a soil. High amounts of minerals in irrigation water is common in more arid and semi-arid regions, but boron in irrigation water is common where the source of water is near an ocean due to salt water infiltration of the ground water.
If you are curious or suspicious about your irrigation water quality, it is best to submit a sample of your irrigation water to a lab for analysis. The only parameter that you can test for yourself is salinity, with an electrical conductivity meter. You can also test your irrigation water’s pH, but that does not necessarily indicate the level of bicarbonates and carbonates it may contain. To determine specific nutrient levels in your water that may be affecting crops, expensive equipment must be used under laboratory controlled and calibrated conditions, following strict training and testing procedures. When submitting a sample, be sure to request a test package for irrigation water and not drinking water, since drinking water is analyzed for different parameters. Spectrum Analytical has created an excellent reference “Guide to Interpreting Irrigation Water Analysis” to help you understand your irrigation water results and identify any potential challenges.
“Fixing” irrigation water
If you find that your irrigation water has excessive and dangerous amounts of some elements, you can invest in technology to remove those elements, but this approach is generally too expensive to be practical for agriculture, given the amount of water that would need to be treated. Rainwater catchment is a reasonable approach, but it too is expensive if you want to capture and store all of the water your garden or farm needs during the growing season. However, any amount of rainwater that can be stored means that you need to apply that much less of your lower quality irrigation water to the soil, so rainwater catchment should be considered.
The primary means of mitigating negative effects of poor quality irrigation water are:
- add enough irrigation water to meet the needs of the crops and enough to leach excess salts past the root zone by adding more water per irrigation event and irrigating less frequently,
- add organic matter to the soil so that the soil itself can store more water (a natural water storage tank, free of cost) and less irrigation water is needed to be applied. Increasing the organic matter of your soil also helps form aggregates, which improves the soil’s structure and helps water enter the soil more rapidly to prevent runoff and erosion, and
- avoid leaving the soil bare as much as possible. Bare soil loses much more water through surface evaporation than soil covered with plants or mulch. Surface evaporation drives soil water to move upward in the soil through capillary action, collecting salts, and depositing and concentrating those salts on the soil surface. Close spacing of crops and continually cropping are critical to keep the soil shaded, minimize surface evaporation and reduce the risk of soil salinity.
To learn more about salinity and sodicity, click here