Authors: Natalie Hoidal, Carl Rosen
Lately we’ve been hearing more growers express interest in saturated media testing instead of doing a regular soil test in high tunnels, and on farms with high organic matter. In this article, we’ll explore what this test is, when it might make sense, and when it’s not the best fit for making nutrient management decisions.
Soilless mixtures tend to be made up of some organic materials like peat, coconut coir, or bark, along with inorganic materials like perlite. Soil on the other hand is made up of mostly mineral matter (sand, silt, or clay), plus air, water, and a small amount (generally around 1.5 - 6%) of organic matter. Clay particles and organic matter in soil provide “buffering capacity”, which is the ability to hold on to nutrients, and slowly release them over time. Even compost has some buffering capacity. Since potting media generally has a low buffering capacity, it is not able to release nutrients over the course of a growing season, and so greenhouse growers tend to rely on weekly fertigation of nutrients via their irrigation water. In greenhouses, growers use the saturated media extract to gauge nutrients, since the media itself does not hold nutrients well. They usually do this test weekly, and then fine-tune their fertigation accordingly.
Because vegetable farms often have high organic matter, and high tunnels are protected environments, some people have started to claim that a saturated media extract is more appropriate for a high tunnel than a standard soil test.
How similar are high tunnel soils to regular field soils vs. potting media? Well, they are probably somewhere in between, but generally more like field soils. They do tend to have high organic matter, and sometimes have low bulk density as a result. Drip irrigation can minimize the rooting zone, and so the roots are more constrained than they would be in the field. But, the root zone is also less constrained than a pot: most growers use 3-4 drip lines per bed in a high tunnel, which allows for a much larger rooting zone than a pot.
While some sources do advise using a saturated media extract test to fine tune early applications of phosphorus, potassium, or micronutrients, we advise growers to do this cautiously. A saturated media test may suggest adding calcium or magnesium even if soil levels are very high: High tunnels in Minnesota already tend to have excessive levels of calcium and magnesium, and adding too much can lead to long term problems.
Cations: SME does not capture the buffering capacity of the soil (organic matter and clay). Buffering capacity is critical to understanding cations, which bind tightly to clay and organic matter particles.
Phosphorus: Phosphorus is insoluble in soil water solution, a water extract cannot effectively determine how much phosphorus is available. Plants have evolved mechanisms to access phosphorus through root hairs, excreting organic acids and other compounds, and through relationships with mycorrhizal fungi. None of this is represented by a SME test.
Lately we’ve been hearing more growers express interest in saturated media testing instead of doing a regular soil test in high tunnels, and on farms with high organic matter. In this article, we’ll explore what this test is, when it might make sense, and when it’s not the best fit for making nutrient management decisions.
What is a saturated media extract?
Saturated media extract is a test that was developed for potted media or soilless mixtures (e.g. greenhouse potted plants). In a standard soil test, a variety of chemical compounds are used to extract nutrients from the soil. The specific compounds used are specific to each region based on the soils most commonly found in the area, with the goal of approximating how much of a given nutrient a soil can provide over the course of a growing season (here’s an example of how plant available phosphorus is determined in Minnesota soils with a pH below 7.4). With saturated media testing, water is simply added to a soil to make a slurry, then filtered out and the nutrients found in the water are measured.Soilless mixtures tend to be made up of some organic materials like peat, coconut coir, or bark, along with inorganic materials like perlite. Soil on the other hand is made up of mostly mineral matter (sand, silt, or clay), plus air, water, and a small amount (generally around 1.5 - 6%) of organic matter. Clay particles and organic matter in soil provide “buffering capacity”, which is the ability to hold on to nutrients, and slowly release them over time. Even compost has some buffering capacity. Since potting media generally has a low buffering capacity, it is not able to release nutrients over the course of a growing season, and so greenhouse growers tend to rely on weekly fertigation of nutrients via their irrigation water. In greenhouses, growers use the saturated media extract to gauge nutrients, since the media itself does not hold nutrients well. They usually do this test weekly, and then fine-tune their fertigation accordingly.
Because vegetable farms often have high organic matter, and high tunnels are protected environments, some people have started to claim that a saturated media extract is more appropriate for a high tunnel than a standard soil test.
How similar are high tunnel soils to regular field soils vs. potting media? Well, they are probably somewhere in between, but generally more like field soils. They do tend to have high organic matter, and sometimes have low bulk density as a result. Drip irrigation can minimize the rooting zone, and so the roots are more constrained than they would be in the field. But, the root zone is also less constrained than a pot: most growers use 3-4 drip lines per bed in a high tunnel, which allows for a much larger rooting zone than a pot.
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| SME is often used in potted plants in greenhouses. Photo: Karl Hakanson |
When might a SME test be appropriate?
A saturated media extract test is a good snapshot of what is available right now, not so much for what will become available over the season. This is useful for diagnosing excess nutrients like ammonium and boron, which can cause plant toxicity. It is also useful for diagnosing salts (which plenty of high tunnels have), determining pH, and it can be an effective snapshot of the amount of nitrate available in the soil at a given moment. But, a regular soil test is just as effective at determining pH and nitrate. Some growers may use it in addition to a regular soil test to fine tune early season fertilizer applications.While some sources do advise using a saturated media extract test to fine tune early applications of phosphorus, potassium, or micronutrients, we advise growers to do this cautiously. A saturated media test may suggest adding calcium or magnesium even if soil levels are very high: High tunnels in Minnesota already tend to have excessive levels of calcium and magnesium, and adding too much can lead to long term problems.
When is a SME test not appropriate?
A saturated media extract test is not effective for predicting nutrient availability throughout the season (i.e. not the best tool for making season-long fertility decisions). Why not?Cations: SME does not capture the buffering capacity of the soil (organic matter and clay). Buffering capacity is critical to understanding cations, which bind tightly to clay and organic matter particles.
Phosphorus: Phosphorus is insoluble in soil water solution, a water extract cannot effectively determine how much phosphorus is available. Plants have evolved mechanisms to access phosphorus through root hairs, excreting organic acids and other compounds, and through relationships with mycorrhizal fungi. None of this is represented by a SME test.
What is the right test for high tunnel soils?
It is true that high tunnel soils are unique, and that standard soil tests may not always give us the best picture of how to manage them. Here are a few other tools for addressing the discrepancy between regular soil tests, and how high tunnel soils with high organic matter and a limited rooting zone might behave:- We can still use regular soil tests, but make sure to use high-tunnel specific fertility recommendations. For crops like beets, carrots, or lettuce, we don’t expect yields to be different in the high tunnel compared to the field, we just expect those crops to grow faster. Therefore, you can use field-based recommendations. However, for crops that stay in the tunnel for a long time and keep producing, we expect much much higher yields in high tunnels than we do in the field. Therefore, we have higher recommended fertility rates.
- If your bulk density is less than 1, consider a bulk density adjustment for potassium. Vegetable educators have seen that even when a soil test shows a high value for potassium, we often see potassium deficiency symptoms when the soil bulk density is low. This is especially true for long-season crops that produce fruit like tomatoes. If your organic matter is above 6 or 7, it’s safe to say that your bulk density could be less than 1.
- If you were to use SME testing instead of soil testing, it should really be done frequently, every week or two. Soil labs will provide ideal ranges for each nutrient. Essentially, you would see whether your soil is within the ideal range, and then fertigate accordingly for each nutrient. It’s important to note that these ranges have not been calibrated for vegetable crops.
- Instead, if you’d like the certainty of regular testing, consider doing foliar testing. Foliar tests give you a good snapshot of what your plant has taken up, which is a more accurate way to gauge how your plants are accessing soil nutrients than simply testing the soil nutrients.
- Treat your soil more like soil: use more drip lines so your root zone isn’t drying out, and use soil moisture sensing to make sure you’re irrigating adequately. Additionally, if you’re already sitting at 7%+ organic matter, consider what goals you’re trying to achieve by adding more compost, manure, or organic matter before doing so.
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