Soil Science Spotlight:
The Dr. John Doran/ USDA Soil Quality Test Kit Guide, Part 7
by John Beeby, Ecology Action Soil Fertility Advisor
growyoursoil.org

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 1-6 of this segment, I introduced Dr. John Doran's USDA Soil Quality Test Kit Guide
(bit.ly/DoranSoilTest), and discussed tests for infiltration, physical observations, aggregate stability, earthworms, soil respiration and pH, electrical conductivity, and bulk density. In this issue, I want to talk about the last of the Guide's tests: water quality.

Water – The First Fertilizer

There is no natural water on the planet that is “pure” H₂O because water is so interactive with its environment. The
H₂O molecule is partially positively and partially negatively charged, and so attracts and is attracted to both negatively and positively charged elements in its environment, which makes it an excellent dissolver of and carrier for those elements. Consider that water in an aquifer has likely interacted for millions of years with the minerals and rocks surrounding it. Similarly, 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, interact with the individual H₂O molecules there, and be converted to carbonic acid (H₂CO₃) which causes the raindrop to be slightly acidic and to contain elements other than just hydrogen and oxygen.

This is more than an interesting bit of trivia: it’s one of the fundamental reasons life on our planet exists! 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 the process of living and dying, created organic matter. Organic matter decomposed by bacteria and fungi created more acids, dissolved more rocks, and over many, many iterations, slowly started to create the soil that makes advanced biological life possible on our planet—first topsoil, then subsoil.

Irrigation becomes necessary when your growing season does not provide adequate rainfall to meet the water requirements of your crop. According to the UN-FAO (2021), roughly 22% of the world’s agricultural land is irrigated, although these percentages vary widely by country, from 52% of cropland in China to 4% in Africa. Most of us water our gardens as a matter of course, so it may not be something that you consider as an “amendment” as you would with compost or nutrients. However, if a soil requires irrigation to raise crops, it’s important to understand that the soil receives not only water (H₂O) but also all of the minerals that the irrigation water contains, essentially "fertilizing” that soil with those minerals over time. Those minerals may be beneficial if they help alleviate specific nutrient deficiencies in that soil, or if they 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. The effect depends on how much irrigation water is added and how many dissolved elements are 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.

This is of interest because 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 [1] from accumulating on the surface of the soil or root zone, roughly the top 30 cm (1 foot) of soil. 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. To prevent this from happening, it is usually best practice to irrigate with more water, less frequently. If we water more frequently with smaller quantities of water, much of that 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 more water less frequently means that at least some of the dissolved minerals will have a chance to leach past the root zone and minimize surface accumulation.

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 referred to as diffusion, something we all encounter in daily life. 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 areas of lower concentration, which causes 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 if 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 move into 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 avoid creating saline soil, enough irrigation water must be applied in sufficient quantities 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 required and must be replaced periodically at additional expense (about $25 US for a 500 mL bottle of 1413 µS/cm calibration solution). This device 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 to assess salinity 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. 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 are provided with the instrument or can be found easily on the internet. Remember 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 specific salts – bicarbonates, sodium, boron, chloride, and other minerals that can be harmful to a soil. High amounts of minerals in irrigation water are 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 saltwater infiltration of the ground water.

If you are curious or suspicious about your irrigation water quality and want to know more about the specific salts and compounds it carries, it is best to submit a sample of your irrigation water to a lab for analysis. The only parameters that you can easily test for yourself are salinity, with an electrical conductivity meter), and pH (with a pH meter or test strips) but that does not necessarily indicate the level of bicarbonates and carbonates your water 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 Analytic Inc. 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 certain elements, you can invest in technology to remove those elements, but this 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 to sourcing water with fewer dissolved minerals, but it may be prohibitively 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 stored means that you will need to apply that much less of your lower quality irrigation water to the soil, so rainwater catchment, even in small quantities, should be considered.

The primary means of mitigating the negative effects of poor-quality irrigation water are:

1. 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;
2. 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
3. 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 rather than moving downward, past the root zone. Close spacing of crops and continual cropping are critical to keep the soil shaded, minimize surface evaporation and reduce the risk of soil salinity.

This article concludes the series that has focused on Dr. John Doran’s excellent Soil Quality Test Kit Guide (USDA 2001). If you have been following this series and practicing these tests, you are now well equipped to determine many critical parameters of your soil, with or without the help of a soil testing laboratory. So now, how can you use all these observations and data to improve your soil? To effectively improve a soil, you must first understand your soil’s true strengths and weaknesses so that you can address the root causes of these weaknesses (and maintain the strengths) rather than simply reacting to symptoms after they’re already creating challenges in your garden. Knowing how to do this requires that you know not only the fundamental needs of a soil (air, water, organic matter and biology which form a handy acronym AWOMB, which the soil is) but even more importantly, how they all interact with each other. Transferring this knowledge and skill to you, the reader, will be the focus of the next series of Soil Science Spotlight articles.

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[1]

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 (Back to text)

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