What is the value of testing soil for both nitrate and ammonium nitrogen when many of the interpretations, like PSNT for manured fields, only utilize nitrate values? Why test for ammonium when we can assume that all of the available nitrogen in the soil has converted to nitrate? The answer depends on the way the data is being utilized.
Nitrogen in the soil can take several forms, but plants can only utilize ammonium and nitrate. Ammonium is stable in the soil until it converts to nitrate which is mobile in the soil, and subject to loss via leaching and denitrification.
Nitrogen in manure starts as organic nitrogen, that material mineralizes to become ammonium, and then degrades to nitrate. Manures at application time will be a mix of organic nitrogen and ammonium depending on species and manure handling. Poultry and beef manure tend to contain more organic nitrogen, while dairy and swine liquid manures tend to have a higher ammonium content. While this can be measured, we cannot accurately predict the rate of conversion from organic nitrogen to ammonium, then to nitrate in the soil. When testing soil after a manure application, nitrate can be used for traditional PSNT interpretation assuming continued future release of nitrate from ammonium. By measuring the ammonium in the soil that assumption can validate or disprove this assumption. Natural soil ammonium levels are below 4 to 6 ppm. Soil test ammonium results at sidedress near, or below these levels, indicate that future release nitrogen release from the manure maybe limited and increased nitrogen rates may be warranted.
Inorganic fertilizers convert from urea to ammonium and then to nitrate. Commercial fertilizers can start at any three of these levels, UAN starts at all three. Testing for nitrate only can miss a large fraction of the available nitrogen in the soil. There are commercially available nitrogen conversion inhibitors to reduce the rate of conversion from urea to ammonium and ammonium to nitrate in the soil. Without inhibitors the conversion of urea to ammonium and ammonium to nitrate occurs quickly depending on soil moisture and temperature. Over a matter of few days to weeks. More information on using nitrate and ammonium soil tests to manage in season nitrogen applications can found on in the ALGL Newsletter. (link to article)
The conversion of ammonium to nitrate can also be inhibited by soil conditions. As soil moisture increases in warm soils, the rate of conversion from ammonium to nitrate increases. However, at soil saturation the conversion of ammonium to nitrate can stop while nitrate continues to leach and denitrify. Extremely dry or cold soil conditions can prevent both the conversion of ammonium to nitrate and the loss of nitrate.
As an example, following a recent wet spring, the ALGL agronomy staff worked with a grower that tested for nitrate only. The grower was trying to determine how much of the anhydrous ammonium they had applied spring preplant had been lost. The resulting nitrate values were very low indicating that nearly all of the applied nitrogen was lost. When the ammonium test was performed on these same samples the data values were high, high enough to indicate that all of the nitrogen remined in the ammonium form. The saturated soils prevented the conversion of ammonium to nitrate for 10 weeks, the ammonium test data defied the assumptions made in this situation and avoided an unnecessary nitrogen application.
Rather than assuming how much ammonium has converted to nitrate or assuming that there is more ammonium to convert to nitrate in the future, the most accurate way to manage these situations is to test the soil for both ammonium and nitrate.
Some places in our trade area experienced heavy rainfall in the weeks following planting and it is likely that nitrogen was lost in these saturated areas. Plant color has improved overall but some of the stressed areas might benefit from additional nitrogen applied later in the season to help finish the corn crop. Extended dry weather after crop establishment in some of our trade will improve the soil retention of nitrogen and may reduce plant availability.
Plant tissue sampling up until 2 weeks past tasseling can help monitor the condition of the crop and indicate when plants are struggling to access needed nutrients to support the rapid growth phase of development. When interpreting plant tissue data, it is important to remember this reveals a snapshot in time and does not offer predictive value of future condition of the plants. Soil nutrient levels and future weather conditions have a large impact on late season nitrogen availability.
When paired with soil nitrate and ammonium levels, a better picture of the overall conditions will become apparent. Given the current cost of Y-drop UAN applications, UAN fertigation, and urea top dress treatments these two testing methods will help guide these in-season decisions. For more information on using soil test nitrate and ammonium to determine late season nitrogen applications see our blog post.
Please contact your ALGL agronomist for assistance with mid-season nitrogen testing.
Early season field scouting is essential to ensure good emergence, detect potential weed and insect pressure, and monitor the effectiveness of your fertility program. The earlier an issue is detected, the better chance there is to correct the issue. One tool to help detect potential fertility problems is plant tissue analysis. However, to get useful results back from the lab, the proper plant part must be collected for the current growth stage of the crop.
The proper method for collecting early season corn tissue samples is to collect 15 or more whole plants to comprise a single sample. This is only referring to the above ground portion of the plant. This method is appropriate for corn up to 12 inches tall, or V4. However, corn can be sampled too early to provide useful data for making decisions. During the first three weeks after emergence, much of the nutrient content of the young plant is not coming from the soil, but from the embryotic tissues within the seed. The young plants also have a very small seminal root system during the first few weeks which are not able to access the nutrients in a large volume of soil, the main role of this root system is to get water to the seed and new leaf tissues. Nutrient deficiencies during the first few weeks are likely caused by environmental conditions and do not necessary reflect low nutrient levels in the soil. An example of this situation is young corn plants turning purple. Purpling of young corn plants can be a symptom of a phosphorus deficiency but can also occur on soils with adequate phosphorus levels when nighttime temperatures are low. Cool spring nights slow metabolic processes in the plant resulting in the buildup of anthocyanins, which appear purple. Tissue sampling in corn should be delayed until 3 to 4 weeks after emergence, or until the plants have developed a functional nodal root system to ensure that the tissue analysis is representative of the nutrients that are available to the plant.
Image: V4 corn plant showing purpling. Source: Purdue.edu
Like corn, soybeans can also be sampled too early. Proper tissue sampling for all growth stages for soybeans is collecting 25 or more of the most recently mature trifoliates without petioles. The first leaves to appear on a recently emerged soybean plant are unifoliates, or cotyledons. The nutrient content of the cotyledons, sometimes referred to as seed leaves, does not accurately represent the nutrients available in the soil. Tissue sampling in soybeans should be delayed until the V2 growth stage. The V2 growth stage is reached when the second trifoliate has completely unrolled. It will generally take a minimum of 3 to 4 weeks after emergence to reach this stage. At this point, the first trifoliate is considered mature, and can be collected for tissue analysis.
Image: V2 soybean plant. Source: Clemson.edu
Spring tissue sampling of winter wheat can be a very useful management tool. The timing of wheat sampling does not correspond to a specific growth stage though. The important factor when determining the appropriate time to sample wheat is that the wheat has broken dormancy and is actively growing again. Generally, wheat will be at a growth stage of Feekes 3 or 4 when this occurs. The appropriate method for collecting wheat samples at this stage is to collect 25 or more whole plants from ½ inch above the soil surface. One of the benefits of early season wheat sampling is to fine tuning a “green-up” nitrogen applications based on the nitrogen content of the plant at Feekes 5 (please visit the Purdue Extension News Release for more information).
Image: Feekes 5 wheat. Source: Kansas State University
Accurate plant tissue testing begins with proper sample collection and handling. Make sure to collect the proper plant part for the current growth stage of the crop and collect the proper number to make the sample. This information can be found on the plant analysis page at algreatlakes.com. Always avoid soil contamination in your plant samples. Package samples in paper bags. If shipping is delayed, store samples in a cool location, but do not freeze. Never include roots with a plant sample. If you have any questions on proper plant tissue sampling, please contact the lab for assistance.
Many acres of corn across our area are established with good stands and it is on its way into the summer growing season. Soon it will be time to apply side-dress nitrogen and concentrate on the nutrient needs of the crop and prepare it for the rapid growth phase that is soon approaching.
Given the increased price of nitrogen and the strong crop prices that are available some of our customers are considering a more intensive pre-sidedress nitrogen testing program to help them better place their fertilizer dollars where it has the best chance of making the greatest impact on harvested yield. Based on university data across the midwest it is strongly recommended that sample cores for PSNT be collected at a 12” depth to capture more of the mobile nitrogen as it begins to move downward through the soil profile. For those interested in increasing the intensity of their sampling program it may be useful to collect PSNT samples on a 5-10 acre grid or zone pattern of 8-12 cores per sample.
It may be most useful to concentrate sampling efforts on fields with historically medium to high yield potential with good plant stands and fields with likely variable levels of nitrate availability such as areas that have had applications of manure, municipal waste and other organic forms of nitrogen.
It may be advantageous to adjust nitrogen application rates based on actual measured nitrate levels in the soil at side-dress time. If you have questions or would like assistance with your pre-sidedress nitrate testing plan please contact your agronomist at A&L Great Lakes Laboratory and we would be happy to assist you.
Potassium (K) soil test levels have the greatest potential to vary between fall and spring. Soil test K levels are usually lowest in the dry conditions of late summer prior to harvest and peak in the early spring. Potassium is released from crop residues as the crop matures and crop residues degrade, and this release continues through the winter months. In addition, potassium can be released from the soil through prolonged saturation and freeze/thaw cycles in the winter months. With these processes combined, the question becomes how much higher soil test potassium levels will be in spring- versus fall- collected samples.
A simple approach is to select a location in a field such as a grid or zone sample location and physically mark it with a flag. Collect 6-8 cores at a depth of 6-8”. It is important to pull the same number of cores at the same depth each time samples are collected. Collect the cores as close as possible to each other and the flag. Do this in the fall and in the spring.In most cases, spring and fall potassium levels are usually within 5-10%. Spring soil sampling does not always result in higher potassium soil test level; this depends on crop residue release and weather patterns. Below is an example of some actual field data collected in northeast Indiana in a clay loam soil with a corn/beans/wheat crop rotation with fertilizer applied after the spring sample collection. The key to proper soil sampling is consistency. Sample the at the same depth, following the same crop at the same time of year.
As producers work through the various budgets for the 2022 crop year there is one calculation coming up in a few weeks that shouldn’t be overlooked. Wheat grain prices have been very strong recently, but if you are faced with the decision of whether to remove the straw or leave it in the field, take a few minutes to update the value of the nutrients that would be removed with the straw and make sure you are adequately compensated for the replacement costs.
The Ohio State University tells us that a well grown wheat crop will yield about 2.5 to 2.8 tons of straw. Data shows that a ton of wheat straw contains about 11 lbs of nitrogen, 3 lbs of phosphorous and 20 lbs of potassium. When we apply today’s fertilizer prices to the nutrients removed we can quickly approach $95.00/acre.
Compared to past fertilizer prices that placed the value of straw at around $17.00/ton the current value of the nutrients places the price closer to $35-40/ton. Several factors can affect the actual removal rates such as rainfall following harvest and prior to bailing that will leach a portion of the potassium back to the soil.
If you would like to submit a straw sample to the lab for testing we can more accurately estimate the nutrients removed and assist you with your calculations.
With nitrogen prices hovering around $0.80 per unit, deciding when and how much to apply is becoming increasingly difficult. Below are some tips to help make the most of your nitrogen program.
Do not skip the starter. Research has shown the benefits of applying 20-30 pounds of N at planting. Starter should even be considered on manured fields. Cold soil temperatures early in the season prevent the mineralization of N from organic forms. A corn crop that starts behind due to lack of N is not likely to maximize yield at the end of the season.
Split applications of N into as many passes as feasible. The more frequently N is applied, the more efficiently it can be used. Explore late season options for application. A corn crop takes of 60-70% of its N requirement by tasseling. If 70% of the N is applied between starter and sidedress, any additional applications can be fine tuned based on the performance of the crop at that time.
Soil test for N (both nitrate and ammonium), even on non-manured fields. Using a PSNT to adjust sidedress rates is common practice but using soil tests later in the season to evaluate a nitrogen program is less common because there is no clear interpretation of the results. However, this data especially in the hands of a trained crop advisor, can be a useful piece of the puzzle that is N management.
Use tissue tests to help fine tune an appropriate late season application rate or possibly determine if the application is needed at all. Collecting tissue samples 2 to 4 times during early growing season can help to monitor how efficiently the crop is utilizing the N that has already applied. If N was applied at a rate assuming an average growing season and yield, but conditions are favorable for a higher yield, a tissue test can help determine if a higher rate of N is needed to produce that higher yield.
Most importantly chose an appropriate yield goal based on the history of the field. Applying additional N to a field that has other limitations such as drainage problems or low fertility will likely result in financial loss. Also remember that maximizing yield does not mean maximizing profitability. To help fine tune an appropriate N rate for your area and N prices, use the Maximum Return to Nitrogen calculator available at http://cnrc.agron.iastate.edu/.
When working with plant tissue data you can either react to the data with a nutrient application, or you can use the data to continue to develop your overall nutrient management strategy. Either way, routine monitoring of plant tissue test results is the starting point. While taking one tissue test at R2-R3 soybeans to identify a nutrient deficiency so that a foliar product can be added to the fungicide pass is common, it does not identify how long the deficiency endured. While on the other extreme, weekly tissue samples can be a pile of data to wade though. A good tissue testing monitoring program starts with a tissue test once the crop is well established, one or two while the crop is rapidly growing, one at the start of reproduction, and the final sample during pollination or grain fill.
When tracking tissue test results, weather and crop observation notes throughout the season, especially the week or two leading up to a sampling event, are critical. The main nutrient uptake routes for plants, diffusion and mass flow, require water. If water is limited in the root zone, short term nutrient deficiencies can appear until moisture improves. If the soil is saturated, nutrients that are taken up by mass flow will be diluted in the water and will have limited availability to the plants.
Tissue testing provides an opportunity to identify and correct micronutrient deficiencies. If you can spray every week or two in a higher value crop, collecting tissue tests ahead of application will allow for management of low trending secondary and micronutrients. In commodity grains, weekly application of nutrients is not as practical, so being reactive to every change in a weekly collected tissue test is not practical. It is possible though with season long tissue testing to identify weak areas within the overall crop fertility plan and adjust the plan for future growing seasons.
For macronutrients like phosphorus and potassium, foliar applications in grain crops are often not enough to have a significant impact. The value to repeatedly collected tissue samples for management of phosphorus and potassium are the adjustments made to the timing and frequency of nutrient applications, target soil test levels, and rate of fertilizers in subsequent seasons. The goal for phosphorus and potassium to correct common issues that appear in tissue tests proactively in future years. To validate the effectiveness of the management changes, continue tissue testing after the implementation of the management changes.
Is it a soil? Is it a growing media? Is it a manure? Is it a compost? Is it a fertilizer?
Requests to analyze uncommon materials that cannot be completely classified as one of the above categories is becoming increasingly common. Some of these materials are industrial biproducts looking for a beneficial reuse. Some materials are novel products proposed to be used as “soil amendments”. Others are being evaluated to be a feedstock for composting or a component of a soilless or greenhouse media. To choose the correct testing methods, you need to be able answer a couple of questions about the material. What are your trying to learn about this product? How is this product intended to be used? Are there any regulatory testing requirements? If you can answer these questions, the following summarization of the different testing methods should help to select the appropriate analytical methods.
Soil testing methods for nutrients are done using different extracting solutions to estimate the fraction of the nutrients that are available or soon will be available for plant uptake to make fertilizer recommendations. These methods do not give any information about the total nutrient content and are generally intended to be used on soils that have not been heavily altered with other non-soil materials.
Growing media nutrient analysis is done through a process called a saturated media extraction. This process uses deionized water to extract the fraction of nutrients that a growing plant has immediate access to utilize. These methods are intended for materials with little or no natural soil in them and will be used to grow plants in directly.
Manure testing methods use a complete digestion to measure the total nutrient content of the material. Using the moisture content of the material, the nutrient levels can easily be converted into units of pounds per ton or pounds per 1000 gallons which is useful for calculating appropriate application rates. In addition to animal wastes, these methods can be utilized on many other materials that are being land applied for disposal.
Compost testing methods will produce very similar results to manure testing but are done using slightly different methods that are approved by the U.S. Composting Council. These methods are intended finished compost products that will be sold commercially. However, if a material is being evaluated as a potential feedstock for composting, it should also be analyzed using compost methods to allow for equal comparison to the final product.
Fertilizer testing utilizes many different methods depending on what nutrients or other components are requested. These methods are very accurate and intended to be used for products that require a Guaranteed Analysis for the sale of a product based on its nutrient content. Also, if a product is being tested as a potential liming material, it should be analyzed using fertilizer methods.
Please be aware that materials being tested as a beneficial reuse of a waste product, a liming material, or a fertilizer may fall under different state regulations and require additional testing such as heavy metals that are not included in ALGL’s routine test packages. If you have any questions regarding proper testing of a “gray area” material, contact your ALGL representative.
At ALGL we are seeing an increase in spring soil sampling. Spring soil samples have increased from 15% to 28% of annual soil samples in the past 10 years. Perceived concerns of seasonality impacting soil test data are being overshadowed by increased management flexibility.
Tradition has held soil samples to the fall sampling season. This works very well if the plan is to apply fertilizer in the spring, thus allowing time to make management plans over the winter months. More commonly the plan has been to collect the soil samples and turn the resulting data into fertilizer recommendations as fast as possible. This plan leaves no time to make key management decisions. The growing trend in our market is to separate soil sample collection, and fertilizer application, into separate seasons.
Soil test results will vary through the year. Often, nutrient levels are highest early in the crop growing season and decline through the growing season, with a recovery as nutrients begin leaving crop residues at harvest time. It is often argued that fall soil sampling shows the seasonally lowest soil test levels to ensure adequate crop fertility. A similar argument can be made for spring soil sampling in that it directly reflects what the starting point for crop fertility at the beginning of the growing season.
The greatest concern is with potassium soil test levels as it has the greatest chance of variability through the growing season. Demonstration samples collected over the last few years by ALGL shows that potassium levels vary less than ± 6-7% between spring and fall soil samples. This is often less than the cumulative sampling error between sampling events. The difference will have little to no impact in the resulting fertilizer application rates in most cases. It is assumed that soil test potassium will be higher in the spring, this same demonstration data set shows that is often not correct.
Figure 1 - Management Cycle
True management of anything is a cyclic pattern. We often start with a soil test in the analysis phase, then make decisions on how and what to correct/improve. Followed by planning on how to implement those decisions. Then implementing the plan takes place. The real value in this model is on the next sampling cycle, taking the time to review the new data and determining if your goals of the previous plan were met, and if not, determining why. When soil samples are collected repeatedly at the same time of year, location, after the same crop, etc. to reduce the sampling variation, the changes in soil test values based on management become clearer.
When soil samples are collected in the same season as fertilizer is applied, all the management steps need to take place in 7 to 10 days during one of two busiest and stressful seasons, with no time to critically evaluate the data. The risk for mistakes, or less than ideal management decisions, increases using this short time frame. Sampling a season ahead of fertilizer application provides more time to evaluate the overall fertility management plan. For example, soil sampling in the spring provides the entire growing season to evaluate/refine the plan, make fertilizer purchases, prepare prescriptions ahead of time (yield data crop removal can still be added at harvest time), evaluate the crop through the growing season, and adjust based on crop prices, yields, and fertilizer prices. This leaves the only management step taking place during fall harvest is implementation of the plan.
When we let go of the notion that we need to collect soil samples in the fall and start looking to collect soil samples a season in advance of fertilizer application, the opportunities for increased and advanced management of soil fertility becomes possible.