Sampling Fields for SCN

Soybean cyst nematode (SCN) is a microscopic, parasitic worm that attacks the roots of susceptible soybean and dry edible bean, causing unseen or unexplained yield losses. Soybean and dry edible bean are naturally susceptible to SCN, but through plant breeding, most soybeans have some level of resistance, varying in level from good to poor. The most common source of resistance to SCN in soybean is PI88788, which is about 30 years old, and many soybean growing areas have SCN populations that are becoming resistant to this source. The Peking source is a very effective SCN resistance source but is only available in less than 5% of all soybean varieties.

Soybean cyst nematode cysts each harbor hundreds of eggs. Cysts and eggs of SCN can survive in the soil and remain viable for many years even without a soybean or dry bean host. Any activity that moves soil around will move SCN, meaning that areas with a history of soybean production likely have or will have this pest. Soybean cyst nematodes were first reported in Minnesota in 1978, South Dakota in 1995, North Dakota in 2003, and Manitoba in 2019.

During the growing season, the developing SCN cysts containing the eggs can be seen on susceptible plant roots, as seen in the picture below. To get an accurate assessment of the infestation level of the field, you need to collect soil samples and submit them to a laboratory to get a measure of the SCN egg count.

Photo of soybean roots with SCN cysts. Photo courtesy of NDSU.

Sampling strategies

If you have never tested for SCN before, you will want to sample fields intended for soybean or dry bean for the presence of SCN and gather a baseline SCN egg count. The best time to collect this sample is at the end of the growing season, right before harvest or just after (before any tillage). Sampling in the fall coincides with the highest egg levels in the soil and typically falls in the months of September and October. Collect 10-20 soil cores (6 to 8 inch soil depth) right in the soybean row from areas of the field that are likely to have SCN. Since SCN is a soil-borne pathogen, it moves wherever contaminated soil can enter the field. Therefore, the areas you will want to collect samples from are field entry points where soil can be transferred on equipment and tires, places where blown soil accumulates (e.g., fence lines), ditches and flooded areas, and locations in fields with consistently low soybean yields. Mix the soil cores together and take a subsample to fill a soil sample bag.

If you know you have SCN, you will want to sample soybean fields twice during the year: once in June to get an initial SCN egg count and then again in the fall to get a final SCN egg count. The early and late SCN samples allow you to measure if SCN populations are being effectively controlled (i.e., no increase in SCN egg count) or if the soybean variety SCN resistance source is failing (i.e., SCN egg count increases). Choose a single point in the soybean field and collect 8-10 soil cores (6 to 8 inch soil depth) taken within the soybean row at that spot. Mix the cores together and fill a regular paper soil sample bag. Mark that point with a flag and collect its GPS coordinates. Come back to that exact spot in the fall and collect a second sample. This will help you assess how your SCN management strategies, including the soybean variety SCN resistance source and soybean seed treatment, are working in the field.

Preparing and sending SCN samples to AGVISE Laboratories

You can submit SCN samples via paper form or online through AGVISOR. AGVISE provides special paper forms for SCN sampling and special stickers for online AGVISOR submission at no charge. The bright yellow forms and stickers help us sort samples and ensure samples submitted for SCN analysis are not dried and ground. All SCN samples analyzed by AGVISE Laboratories are analyzed at the Benson, MN laboratory. You can either send the SCN samples directly to the Benson Laboratory (see addresses below) or to the Northwood Laboratory, where they will be routed to Benson for analysis. AGVISE Laboratories reports SCN results in “eggs/100 cc” of soil and provides interpretation on our reports informed by university research.

Helpful links:

Soybean Cyst Nematode, ISU

Plant Disease Management: Soybean Cyst Nematode, NDSU

Soybean Cyst Nematode (SCN), UMN

Soybean Cyst  Nematode in South Dakota: History, Biology, and Management, SDSU

The SCN Coalition

 

Sidedress Corn Using the Pre-sidedress Soil Nitrate Test (PSNT)

As the corn crop begins to emerge, it is time to prepare for sidedress nitrogen applications. Sidedress nitrogen for corn can be applied any time after planting, but the target window is generally between growth stages V4 and V8, before rapid plant nitrogen uptake occurs. Split-applied nitrogen has become a standard practice in corn to reduce in-season nitrogen losses on vulnerable soils, such as sandy and clayey soils. More and more farmers now include topdress or sidedress nitrogen as part of their standard nitrogen management plan. These farmers have witnessed too many years with high in-season nitrogen losses through nitrate leaching or denitrification.

The target timing for PSNT sampling is when corn is 6 to 12″ tall. Twelve-inch corn is often V4 or V5 (like in the picture above). Do not hesitate in collecting soil samples for the PSNT; the target window for sidedress-nitrogen applications in corn is between the V4 and V8 stages. 

Whether your nitrogen management plan includes a planned sidedress nitrogen application or not, the Pre-Sidedress Soil Nitrate Test (PSNT) is one tool to help make decisions about in-season nitrogen. You may also hear this test called the Late-Spring Soil Nitrate Test (LSNT) in Iowa. PSNT is an in-season soil nitrate test taken during the early growing season to determine if additional nitrogen fertilizer is needed. PSNT helps assess available soil nitrate-nitrogen prior to rapid plant nitrogen uptake and the likelihood of crop yield response to additional nitrogen.

The Pre-sidedress Soil Nitrate Test (PSNT), taken when corn is 6 to 12 inches tall, can help you decide the appropriate sidedress nitrogen rate. The PSNT requires a 0-12 inch depth soil sample taken when corn plants are 6 to 12 inches tall (at the whorl), usually in late May or early June. Late-planted corn may not reach that height before mid-June, but PSNT soil samples should still be collected during the first two weeks of June. The recommend soil sampling procedure requires 16 to 24 soil cores taken randomly through the field, staggering your soil cores across the row as you go. All soil cores should be placed in the soil sample bag and submitted to the laboratory within 24 hours or stored in the refrigerator.

You can submit PSNT soil samples using the online AGVISOR program by choosing the “Corn Sidedress N” crop choice and submitting a 0-12 inch soil sample for nitrate analysis. AGVISOR will generate sidedress nitrogen fertilizer guidelines, using the PSNT critical level of 25 ppm nitrate-N (0-12 inch depth). If PSNT is greater than 25 ppm nitrate-N, then the probability of any corn yield response to additional nitrogen is low. If spring rainfall was above normal, then the PSNT critical level of 20 to 22 ppm nitrate-N (0-12 inch depth) should be used. Iowa State University provides additional PSNT interpretation criteria for excessive rainfall, manured soils, and corn after alfalfa.

If the PSNT is taken after excessive rainfall, the soil cores will be wet and difficult to mix in the field. Therefore, it is best to send all soil cores to the laboratory to be dried and ground, ensuring a well-blended soil sample for analysis. Although in-field soil nitrate analyzers have improved over the years, the difficult task of blending wet, sticky soil cores in the field still remains. The only way to get accurate, repeatable soil analysis results is to dry, grind, and blend the entire soil sample in the laboratory before analysis. AGVISE provides 24-hour turnaround on PSNT soil samples. The soil samples are analyzed and reported the next business day after arrival. Soil test results are posted on the online AGVISOR program for quick and easy access. With AGVISE, you get not only great service but also the highest quality data with four decades of soil testing experience.

Pre-Sidedress Soil Nitrate Test (PSNT) resources

Please call our technical support staff if you have any questions on PSNT and interpreting the soil test results for sidedress nitrogen application.

Active carbon (POXC): What does it measure?

Carbon is the currency of nature: the backbone of soil organic matter and the energy source for soil microorganisms. Therefore, much interest in soil health focuses on increasing carbon storage in soil. When you reduce tillage or increase crop rotation diversity, you expect soil organic matter to increase. However, soil organic matter often changes slowly for several years. In fact, less than 1% of plant biomass carbon returned to soil eventually becomes stable humus organic carbon.

Active carbon, also known as permanganate-oxidizable carbon (POXC), is a sensitive tool for measuring soil carbon change. This portion of soil organic matter is actively involved in nutrient cycling and changes more quickly when cropping systems are changed (e.g. reduced tillage, diversified crop rotation, cover crop inclusion). Active carbon is a quick, repeatable soil test that measures the easily oxidizable, biologically active carbon fraction.

Why is active carbon important?

Active carbon typically comprises about 1 to 4% of total organic carbon in soil. It represents the microbially available carbon energy sources, that is microorganism food. As one component of the total organic carbon pool, active carbon has a strong relationship with overall soil organic matter (r = 0.80, Fig. 1), but it responds more quickly to changes in crop and soil management. It helps explain why two soils with 3% soil organic matter, for example, may behave differently regarding biological activity or nutrient cycling.

​Active carbon should be utilized as a tracking tool to measure improvement in soil quality. In a 17-year tillage experiment in Mandan, ND, total organic carbon did not change much after conversion to no-till; however, active carbon increased significantly from 470 to 600 ppm (Weil et al., 2003). Recall that soil organic matter includes all forms of organic carbon (i.e. microbial biomass, recently decomposed plant material, stable humus) and requires large carbon inputs to change that total measurement. In contrast, the active carbon fraction increases much more quickly and detects improvements in soil quality sooner. This is why active carbon is considered a “leading” soil health indicator.

What do the numbers mean?

In agricultural soils of the upper Midwest and northern Great Plains, active carbon commonly ranges from less than 300 ppm to 1000 ppm (Fig. 2). As with soil organic matter, soil texture is a major factor controlling active carbon. Coarse-textured soils generally contain less active carbon than medium- or fine-textured soils. Native prairie soils may contain active carbon as high as 1500 ppm.

While these are the common active carbon values you may expect, each field and its history is different; therefore, making comparisons between fields is not advised. There are no index ranges for active carbon because different climatic regions and soil types cannot be judged by the same standard. Active carbon should be utilized as a tracking tool in improving the soil quality of that field or zone, rather than making broadscale comparisons.

Cropping systems that include reduced tillage or no-till, diversified crop rotations, and cover crops will help increase active carbon. In addition, any system with greater organic matter inputs (e.g., plant biomass, manure) helps build active carbon and soil organic matter. Under perennial grass, active carbon can easily exceed 1000 ppm.

With sensitive soil health tools like active carbon, you can more easily quantify positive changes achieved through better soil management.

How to soil sample

Active carbon can be added to any standard soil fertility analysis, simply requested as an analysis add-on. Collect the soil sample with standard soil probe. The standard soil sampling depth of 0-6 inches is most common. In reduced tillage and no-till systems, stratification may warrant 0-2 or 0-4 inch soil sampling depths. Like any soil health test, it is a tracking tool measuring soil quality improvement, so make sure you are using GPS-marked soil sampling points.

Frequently asked questions (FAQ)

Q: Does higher active carbon (POXC) indicate better crop yield?

A: Not necessarily. Crop yield is comprised of numerous determinants: genetics, climate, soil type, soil fertility, crop pests, etc. Active carbon measures the carbon sources available to microorganisms as food. This is only one factor within a wide range of various soil biological functions and yield-determining factors. Soil management practices that can improve crop production, such as no-till in water-limited environments, also work to improve soil properties like active carbon.

Q: Can I use active carbon to reduce fertilizer rates or modify plant population?

A: Active carbon has NOT been calibrated with fertilizer rate or plant population trials. So far, active carbon provides information on soil management . The relationships with crop productivity remain unclear. We must wait for more research to answer these yield-focused questions.

References

Weil, R.R., K.R. Islam, M.A. Stine, J.B. Gruver, and S.E. Samson-Liebig. 2003. Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use. Am. J. Altern. Agric. 18(1):3–17.

Soil aggregate stability: What does it measure?

Soil aggregates are the building blocks of soil structure. Soil texture is the relative percentage of sand, silt, and clay in soil, but soil structure describes how those particles are arranged in the soil profile. Soil aggregates are glued together with soil organic matter, plant root exudates, and microorganisms like fungi.

We classify soil aggregates by their size: large macroaggregates (>2000 μm), macroaggregates (250-2000 μm), microaggregates (53-250 μm), and free particles (<53 μm). A large macroaggregate is bigger than a sand particle. A microaggregate has the thickness of one or two human hairs. A macroaggregate lies in between.

Multiple soil management practices come together to improve soil aggregate stability. These include reduced tillage or no-till, greater crop rotation diversity, more plant roots, greater earthworm and microbial activity, and more soil organic matter. This is what makes soil aggregate stability such an attractive soil health indicator. Stable soil aggregates take time to form, so you should consider measuring soil aggregate stability every 3 to 5 years.

Why is soil aggregate stability important?

Soil aggregate stability is a comprehensive soil quality measurement. Soil aggregates provide numerous soil ecosystem services:

● Resistance to water and wind erosion
● More pore space for air and water movement, allowing deep root exploration
● Faster water infiltration, reduced runoff
● Less surface crusting
● Improved equipment trafficability and reduced soil compaction, especially on wet soils
● Diverse habitat for soil microorganisms

Strong soil aggregates form naturally with plant root and microbial activity. However, disturbances like tillage quickly break soil aggregates apart. If soil aggregates are broken apart by tillage, the soil pores fill with small particles that can clog and restrict air and water movement. Loose soil particles can also plug the surface pore network to reduce water infiltration and cause surface crusting.

Cropping systems that include reduced tillage or no-till are necessary to improving soil aggregate stability (Figure 2). Tillage also reduces soil organic matter that is needed to bind soil particles into larger soil aggregates. Diversified crop rotations, cover crops, and manure help improve soil aggregate formation.

Soil texture is a major factor in soil aggregate formation. Coarse-textured soils may take longer to develop soil aggregates. Fine-textured soils tend to develop soil aggregates quickly. This is important because good soil structure with large pore spaces is essential to air and water movement through fine-textured soils.

What do the numbers mean?

You can see large soil aggregates (>2000 μm) with the naked eye, but quantifying the different water-stable soil aggregate sizes must be done in the laboratory. This requires special wet sieving equipment that sorts the water-stable soil aggregates by size.

If you see the water-stable macroaggregate fractions increasing, you know that you are making real improvements in soil quality. Soils with high soil aggregate stability have less soil erosion, better equipment trafficability, faster water infiltration, and more diverse habitat for soil microorganisms. Increasing soil aggregate stability should be a long-term goal in all cropping systems.

Soil aggregate stability should be utilized as a tracking tool in improving soil quality. Each field has a different cropping history and soil type (e.g. soil texture); therefore, making broadscale comparisons between fields is not advised.

How to soil sample

A separate soil sample must be taken for soil aggregate stability. Unlike routine soil fertility analysis, this soil sample cannot be dried and ground (destroys soil aggregates). Soil aggregate stability requires special equipment and more labor than routine soil fertility analysis; ask about turnaround time when submitting soil samples.

Collect an undisturbed soil slice with tiling spade or tulip bulb planter (Figure 3); this helps prevent destruction of soil aggregates. Do not collect the soil sample with a soil probe because the soil probe will destroy or compress soil aggregates. Place the soil sample in one-quart plastic bag and clearly mark with permanent marker “SOIL AGGREGATE STABILITY. DO NOT GRIND.” Like any soil health test, it is a tracking tool measuring soil quality improvement, so make sure you are using GPS-marked soil sampling points.

References

Mikha, M.M., and C.W. Rice. 2004. Tillage and manure effects on soil and aggregate-associated carbon and nitrogen. Soil Sci. Soc. Am. J. 68(3):809–816.

High soil pH and calcium carbonate inflate base cation saturation and cation exchange capacity (CEC)

Soil pH is a soil chemical property that measures soil acidity or alkalinity, and it affects many soil chemical and biological activities. Soils of the northern Great Plains and Canadian Prairies often have high soil pH (>7.3) and contain calcium carbonate (free lime) at or near the soil surface. It is the calcium carbonate in soil that maintains high soil pH and keeps it buffered around pH 8.0. The calcium carbonate originates from soil formation processes since the latest glacial period.

Soils with high pH and calcium carbonate create analytical challenges in determining cation exchange capacity (CEC) and subsequent base cation saturation ratio (BCSR) calculations. Cation exchange capacity is the amount of positive-charged cations (e.g. ammonium, calcium, hydrogen, magnesium, potassium, sodium) held on negative-charged soil particles, like clay and organic matter. Fine-textured soils (high clay content) and organic soils (high organic matter content) have high CEC, while coarse-textured soils (low clay content) have low CEC. The BSCR is the relative proportion of base cations in soil.

The routine laboratory method to determine CEC is the summation method, where all extractable cations on soil particles are added together. The assumption is that all positive-charged cations extracted are held on negative-charged exchange sites on soil particles. The assumption falls apart in soils with pH > 7.3 because the soil test method also extracts calcium from the naturally occurring soil mineral calcium carbonate, which is not held on cation exchange sites. The resulting amount of extractable calcium is inflated. The summation procedure still sums together the inflated calcium result, producing an inaccurate and inflated CEC result. Any subsequent base cation saturation calculations are similarly flawed.

To illustrate the inflated CEC problem, AGVISE Laboratories conducted a laboratory experiment. The experiment showed that more calcium carbonate in soil increased the amount of extractable calcium (Table 1). Furthermore, the inflated amount of extracted calcium also inflated the CEC because all the extractable cations are summed together. The correct CEC is 24 cmolc/kg, yet the inflated CEC could be 150 to 200% higher with increasing calcium carbonate content. Adding calcium carbonate to soil did not increase the inherent CEC sources, i.e. clay and organic matter, yet the laboratory CEC result increased. In reality, the ability to hold more cations did not change. This highlights the analytical challenge in determining CEC via summation method on soils with high pH and calcium carbonate.

Table 1. Effect of calcium carbonate addition on extractable cations and cation exchange capacity.
Calcium carbonate equivalent (CCE) pH EC Ca Mg K Na CEC
% dS/m —————– ppm —————– cmolc/kg
0 7.5 0.43 3350 730 220 45 24
1 7.6 0.51 6150 730 220 50 37
5 7.7 0.52 7480 720 215 45 44
10 7.8 0.50 7240 650 180 40 42
Abbrev.: pH (1:1); EC, electrical conductivity (1:1); Ca, calcium; Mg, magnesium; K, potassium; Na, sodium; CEC, cation exchange capacity via summation of cations.

The subsequent base cation saturation calculations were also affected, showing much lower percent potassium saturation as calcium carbonate content increased (Table 2). This presents a major challenge in using the base cation saturation ratio (BCSR) concept to guide soil fertility and plant nutrition on soils with high pH and calcium carbonate. Without an accurate analytical method, the BCSR concept loses its grounding and any practical application.

Table 2. Effect of calcium carbonate addition on base cation saturation ratio and cation exchange capacity.
Calcium carbonate equivalent (CCE) Ca sat. Mg sat. K sat. Na sat. CEC
% ————————– % ————————– cmolc/kg
0 71 26 2.4 0.8 24
1 82 16 1.5 0.6 37
5 85 14 1.2 0.4 44
10 86 13 1.1 0.4 42
Abbrev.: Ca, calcium; Mg, magnesium; K, potassium; Na, sodium; CEC, cation exchange capacity via summation of cations.

In general, the routine CEC method via summation of cations is quite accurate on soils with pH less than 7.3 and no calcium carbonate. However, the routine method has clear challenges on soils with pH greater than 7.3. The resulting CEC and subsequent base cation saturation calculations are inflated and/or flawed. This is why the BCSR concepts is extremely misleading on soils with high pH.

The Meaning of Soil Health Testing and the Big Picture

This submission is courtesy of Dr. Caley Gasch, Assistant Professor of Soil Health, North Dakota State University, Fargo, ND. It was originally published in the AGVISE Newsletter Winter 2017.

Multiple definitions of “soil health” exist. In general, they all recognize that soil is a complex system, with interacting physical, chemical, and biological factors, which should be managed in a manner that sustains its function and integrity over the long term. Conceptually, soil health is a topic that is easy to understand and support.

Yet, it has been difficult to capture this definition with measurements. Soil scientists can choose from hundreds of field- or laboratory-based methods to quantify different properties in the soil, but there is no single best test for assessing soil health. Research and commercial labs and focus groups across the country have compiled lists of properties that are being used to assess soil health, but we still have a lot to learn about how these suites of measurements can guide management decisions.

As much as we desire hard numbers that support the concept of soil health, I argue that we should keep the big picture in mind. Last summer, I heard a perfect analogy that demonstrates the disparity between the concept of soil health and the push for soil health testing. The idea solidified for me on a recent doctor visit.

You visit your doctor for a routine check-up. The doctor will ask you about your lifestyle: How often do you exercise? Do you smoke? What do you eat? How many drinks do you have per week? Which medications are you taking? What is your family history? They’ll take your weight, pulse, blood pressure, and temperature. They may order a few more tests, and they’ll most likely give you some advice: Wear sunscreen, eat more oatmeal, and come back in a year.

The doctor learns something from your vital signs and can compare them to the past; however, the doctor learns more about your overall health after understanding your eating habits and levels of activity. After all, it wouldn’t be good practice for the doctor to prescribe blood pressure medication solely based on a single blood pressure reading, or claim to know your heart condition based on your pulse alone (what if you ran up a flight of stairs on your way to the doctor’s office?). You are not given a mathematical health score or rank. The doctor knows that your body is a complex system, and that to understand its overall condition, we need to consider how you care for yourself, any risky behaviors, and the results of some routine tests. Certainly, if you have a specific complaint, or if the doctor identifies a particular problem, appropriate tests are available to diagnose and monitor the malady.

I propose that we take a similar approach to soil health. First, understand the “lifestyle” of a soil: What is the rotation? What kind of tillage do you use and how frequently? Do you ever use cover crops? What is your fertility plan? What are the “genetic” limitations of the soil? Second, identify specific barriers to overall health: salinity, erosion, poor drainage, disease issues, etc. Third, identify areas for improvement, address them with a lifestyle adjustment, and develop a monitoring plan to track specific problem areas, realizing that it may take a few years to see change.

For humans and soils alike, we know that there are certain risk factors associated with health and condition. We know a great deal about how practices influence soil properties,
and we don’t usually need a test to detect whether or not a practice is healthy. The testing becomes meaningful only after we focus on a specific challenge area, which will differ from person to person and from field to field. Targeted testing and acute treatments are useful and necessary, but they should not be the basis for managing a complex system.

I’m grateful that soil health is a popular topic, because it is helping us all to think about soil in a way that recognizes its true value and potential. But I challenge us to remember our overall goal for soil health, rather than focusing solely on the vital signs. Approach each field with a broad view, understanding that it is a unique case that reflects its own history and management. From this perspective, we can identify and adopt practices that will protect our soil and improve its value throughout our region.

Adjusting high soil pH with elemental sulfur

Soil pH is a soil chemical property that measures soil acidity or alkalinity, and it affects many soil chemical and biological activities. Soils with high pH can reduce the availability of certain nutrients, such as phosphorus and zinc. Soils of the northern Great Plains and Canadian Prairies often have high soil pH (>7.3) and contain calcium carbonate (free lime) at or near the soil surface. It is the calcium carbonate in soil that maintains high soil pH and keeps it buffered around pH 8.0. The calcium carbonate originates from soil formation processes since the latest glacial period.

An unfounded soil management suggestion is that soil pH can be successfully reduced by applying moderate rates of elemental sulfur (about 100 to 200 lb/acre elemental S). Elemental sulfur must go through a transformation process called oxidation, converting elemental sulfur (S0) to sulfuric acid (H2SO4), a strong acid. Sulfuric acid does lower soil pH, but the problem is the amount of carbonate in the northern region, which commonly ranges from 1 to 5% CCE and sometimes over 10% CCE. Soils containing carbonate (pH >7.3) will require A LOT of elemental sulfur to neutralize carbonate before it can reduce soil pH.

To lower pH in soils containing carbonate, the naturally-occurring carbonate must first be neutralized by sulfuric acid generated from elemental sulfur. You can visualize the fizz that takes place when you pour acid on a soil with carbonate. That fizz is the acid reacting with calcium carbonate to produce carbon dioxide (CO2) gas. Once all calcium carbonate in soil has been neutralized by sulfuric acid, only then can the soil pH be lowered permanently. It is important to note that sulfate-sulfur sources, such as gypsum (calcium sulfate, CaSO4), do not create sulfuric acid when they react with soil, so they cannot neutralize calcium carbonate or change soil pH (Figure 1).

Figure 1. Soil pH following gypsum application on soil with high pH and calcium carbonate.

In 2005, AGVISE Laboratories installed a long-term demonstration project evaluating elemental sulfur and gypsum on a soil with pH 8.0 and 2.5% calcium carbonate equivalent (CCE). The highest elemental sulfur rate was 10,000 lb/acre (yes, 5 ton/acre)! We chose such a high rate because the soil would require a lot of elemental sulfur to neutralize all calcium carbonate. Good science tells us that 10,000 lb/acre elemental sulfur should decrease soil pH temporarily, but it is still not enough to lower soil pH permanently. In fact, this is exactly what we saw (Figure 2). Soil pH declined in the first year, but it returned to the initial pH over subsequent years because the amount of elemental sulfur was not enough to neutralize all calcium carbonate.

Figure 2. Soil pH following elemental sulfur application on soil with high pH and calcium carbonate.

A quick calculation showed that the soil with 2.5% CCE would require about 16,000 lb/acre elemental sulfur to neutralize all calcium carbonate in the topsoil. Such high rates of elemental sulfur are both impractical and expensive on soils in the northern Great Plains. The only thing to gained is a large bill for elemental sulfur. While high soil pH does lower availability of phosphorus and zinc, you can overcome these limitations with banded phosphorus fertilizer and chelated zinc on sensitive crops. All in all, high soil pH is manageable with the appropriate strategy. That strategy does not involve elemental sulfur.

Soil Salinity Analysis: Which method to choose?

This submission is courtesy of Dr. Heather Matthees, Research Soil Scientist, USDA-ARS, Morris, MN. It was originally published in the AGVISE Newsletter Fall 2017.

Salt-affected soils are a major problem for agricultural producers, resulting in $12 billion annual losses in crop production across the world. In the northern Great Plains and Canadian Prairies, soil salinity has always existed in some soils of the region, but the problem has become more widespread and severe since a hydrological wet period began in the 1990s.

Salinity is the overall abundance of soluble salts, which compete with plant water uptake and reduce crop productivity. The soluble salts pull soil water toward themselves in the soil solution, which leaves less soil water available for plant uptake. This causes an apparent drought stress, reducing crop productivity and sometimes may kill the plant. Soluble salts are naturally occurring and a product of regional geology in the northern Great Plains and Canadian Prairies. Since the 1990s, the hydrological wet period has raised the groundwater level and allowed saline groundwater to rise toward the soil surface, causing soil salinization. Saline soils are often called “salty,” “sour”, or “white alkali.”

The severity of soil salinity will control which plant species are suitable for crop or forage production. Some crop species like dry bean and soybean are very sensitive to salinity, whereas other crop species like barley and sunflower have good tolerance to salinity. For soils with very high salinity, the only practical forage option may be salt-tolerant perennial grasses. To assess soil salinity, there are two soil analysis methods: saturated paste extraction and routine 1:1 soil water methods.

Saturated Paste Extraction Method

The gold standard in soil salinity research is the saturated paste extraction method. The method requires a trained laboratory technician to mix soil and water into a paste, just reaching the saturation point, which is about the consistency of pudding. The saturated paste rests overnight to dissolve the soluble salts. It is then is placed under vacuum to draw the saturated paste extract. Soil salinity is then determined by measuring the electrical conductivity (EC) of the saturated paste extract.

The saturated paste extraction method is fairly straightforward, but it requires a trained technician, specialized equipment, and over 24 hours to complete the procedure. The procedure is labor intensive and difficult to automate, so it is considered a special analysis service in commercial soil testing. Therefore, it is more expensive than routine soil testing methods. Among soil salinity determination methods, it is considered the most accurate because the soil:water ratio at saturation controls for differences in soil texture and water holding capacity.

Routine 1:1 Soil:Water Method

The routine method for soil salinity assessment is the 1:1 soil:water method, which mixes standard mass of soil (10 g) and volume of water (10 mL) in a soil-water slurry. Soil salinity is then determined by measuring the electrical conductivity (EC) of the soil-water slurry. It is most commonly abbreviated EC1:1.

The method is fast and inexpensive (only 5-10% of saturated paste extraction cost). The low cost per soil sample allows a person to collect more soil samples from various soil depths and multiple locations within a field (e.g. zone soil sampling), which can create a more comprehensive and detailed soil salinity map to evaluate soil salinity presence, severity, and variability. Since soil salinity is so intimately related to soil water movement across the landscape, the soil salinity map also provides information about soil water accumulation and leaching, soil nutrient movement (e.g. chloride, nitrate-nitrogen, sulfate-sulfur), and crop productivity potential.

A general caveat about the 1:1 soil:water method is that the reported values will be lower than the saturated paste extraction method. Fortunately, the two methods are highly correlated. AGVISE Laboratories worked with soil science researchers at North Dakota State University and South Dakota State University to validate the correlation between the two methods using over 2,300 soil samples from the northern Great Plains. You can convert the two methods by multiplying the 1:1 soil:water result by 2.26 to estimate the saturated paste extraction result (Figure 1).

The simple method conversion enables you to quickly and cheaply monitor soil salinity using the 1:1 soil:water method and still utilize the historical soil salinity interpretation criteria based on the saturated paste extraction method.

Figure 1. Soil salinity method conversion between saturated paste extraction and 1:1 soil:water methods.

References

Matthees, H. L., He, Y., Owen, R. K., Hopkins, D., Deutsch, B., Lee, J., Clay, D. E., Reese, C., Malo, D. D., & DeSutter, T. M. 2017. Predicting soil electrical conductivity of the saturation extract from a 1:1 soil to water ratio. Communications in Soil Science and Plant Analysis, 48(18), 2148–2154.

Estimating soil texture with cation exchange capacity (CEC)

Soil texture is a basic physical soil property that describes the proportion of sand-, silt-, and clay-sized particles in soil. It controls the ability of soil to retain water and nutrients and the movement of water and nutrients through the soil profile. Soil texture is a fundamental soil property, but measuring soil texture requires time-consuming and expensive laboratory analysis procedures. An alternative option is estimating soil texture from cation exchange capacity (CEC), which is a rapid, routine procedure in commercial soil analysis.

Cation exchange capacity is the amount of positive-charged cations (e.g. ammonium, calcium, hydrogen, magnesium, potassium, sodium) held on negative-charged soil particles, like clay and organic matter. Since soil texture and CEC are intimately related, we can use each soil property to make predictions about the other. Soils with more large soil particles are coarse-textured (sandy), whereas soils with more small soil particles are fine-textured (clayey). Each soil particle has some surface area, which controls the CEC. Fine-textured soils have more soil particle surface area, so their CEC is greater (Table 1).

Table 1. Estimating soil texture from cation exchange capacity (CEC).
Cation exchange capacity (CEC), cmolc/kg Soil organic matter, % Particle size family Soil texture class
≤10 <20 coarse sandy
11-20 <20 medium coarse loam
21-30 <20 medium fine loam
>30 <20 fine clayey
>20 organic peat or muck

The soil texture estimation procedure is valid on soils with pH < 7.3 and no salinity. For soils with high pH or salinity, analytical challenges in routine laboratory CEC determination make soil texture estimation inaccurate.

Soil Science Review: Soil pH, Acidity, and Alkalinity

Soil pH is a basic soil property that affects many biological and chemical processes in soil. Simply knowing if a soil is acidic or alkaline can tell us a lot about how it behaves and how we can manage it. This is why soil pH is often called the master variable of biological and chemical reactions.

Soil pH is the activity of hydrogen ions (H+) in the soil solution, expressed on a logarithmic scale. A neutral soil has pH 7.0 and contains equal parts hydrogen (H+) and hydroxide (OH) ions. An acidic soil has more H+ ions. An alkaline soil has more OH ions. The relative acidity or alkalinity is shown in Table 1. In the laboratory, soil pH is analyzed using the 1:1 soil:water ratio routine method. Other soil pH methods include CaCl2, KCl, and saturated paste.

Table 1. Relative soil acidity or alkalinity range from soil pH.
pH (1:1) Relative Acidity or Alkalinity
≤ 4.4 Extremely acidic
4.5-4.9 Very strongly acidic
5.0-5.4 Strongly acidic
5.5-5.9 Moderately acidic
6.0-6.4 Slightly acidic
6.5-7.5 Neutral
7.6-7.9 Slightly alkaline
8.0-8.4 Moderately alkaline
8.5-8.9 Strongly alkaline
≥ 9.0 Very strongly alkaline

 

The optimal pH range for most plant species is near neutral or slightly acidic. In the optimal pH range, most plant nutrients are at or near their highest solubility in the soil solution. If soil pH is too low or too high, the availability of plant nutrients decreases; therefore, soil pH may be corrected with soil amendments or other strategies to mitigate reduced nutrient availability.

To demonstrate the importance of soil pH, let’s look at soil pH and aluminum. Aluminum is a natural component of soil clay particles, and it is insoluble above pH 5.5. In strongly acidic soils (pH < 5.5), aluminum solubility increases, so aluminum begins to dissolve and enter the soil solution. Soluble aluminum is very toxic to plant root growth and development, and it may cause reduced plant production or plant death. Soluble aluminum also binds with phosphate in the soil solution to create insoluble aluminum phosphate compounds, which then reduce soil phosphorus availability and plant uptake.

Soil acidity and aluminum toxicity is often the primary limitation of crop production in tropical and subtropical regions. Acidic soils are frequently amended with lime (calcium carbonate) to increase soil pH, improve nutrient availability, and increase crop production. On the glaciated plains of North America, soil acidity is not a common phenomenon. However, some localized areas of long-term no-till crop production on coarse-textured soils has produced more soils with very low pH (<5.0) and new aluminum toxicity problems.

Soil alkalinity similarly reduces the availability of plant nutrients in soil. In moderately alkaline soils (pH > 8.0), phosphorus binds with calcium to create insoluble calcium phosphate compounds, which then reduce soil phosphorus availability and plant uptake. Similarly, the micronutrients iron and zinc are less soluble. To improve nutrient availability in alkaline soils, farmers apply fertilizer in narrow bands. These bands decrease the volume of soil with which the fertilizer can react, thus keeping more nutrients available in the soil solution. It is generally uneconomical to lower alkaline soil pH to the optimal pH range in crop production.

Soil pH goes beyond inorganic soil chemistry. It also controls the biological activity of soil microorganisms that help create soil structure, cycle organic nutrients, and fix nitrogen in the nodules on legume roots. Soil pH also controls the degradation of many pesticides in soil. If there is something going on in soil, it probably starts with pH.