Lessons (Ghosts) of Droughts Past

From Alberta to Iowa, the region has experienced everything from abnormally dry soil conditions to exceptional drought. In some places, the drought started in 2020 and has continued through 2021. Considering lower than expected crop yields, we expect that residual soil nitrate-nitrogen levels will be much higher than normal in many wheat, canola, and corn fields this fall. There was reduced crop nitrogen uptake and little to no soil nitrogen losses to leaching or denitrification through the growing season, which should result in higher soil test nitrate-N remaining in the soil profile.

In major drought years, high residual nitrate levels are a normal phenomenon. In 1988, the average soil nitrate test following wheat across the region was a staggering 107 lb/acre nitrate-N (0-24 inch soil profile). This is considerably higher than the long-term average around 30-45 lb/acre nitrate-N (0-24 inch soil profile). The 1988 drought was extreme, and 2021 has rivaled that in some locations. Based on previous drought years, it will be no surprise to find wheat fields with 80-100 lb/acre nitrate-N (0-24 inch soil profile) or even higher.

Past experience also shows us that drought can create greater crop yield variability across fields. Some zones in the field with better water holding capacity and soil organic matter may have produced a decent crop yield, and these will have lower residual soil nitrate-N. Yet, other zones may have had very poor crop growth and yield, leaving very high amounts of soil nitrate-N remaining.

Zone soil sampling is always a good idea, but it is especially important in drought years. Soil sampling based on productivity zones is the only way to determine the correct amount of nitrogen fertilizer in each zone across the field. To create good productivity zones for soil sampling, it is best to use multiple data layers such as satellite imagery, crop yield maps, topography, or electrical conductivity (Veris or EM38).

This fall, we expect residual soil nitrate-N to be higher than normal, but there will be exceptions to the rule. Last spring, there was a lot of broadcast urea fertilizer applied without incorporation. If no rain was received for several weeks after application, much of the nitrogen could have been lost to ammonia volatilization. This means some fields will seem out of place with lower residual soil nitrate-nitrogen because fertilizer nitrogen was lost last spring.

For fields with more than 150 lb/acre nitrate-N (0-24 inch soil profile), the crop nitrogen requirement for next year may not call for much, if any, nitrogen fertilizer. We must remember that drought creates variability within a field and even within large productivity zones. This is why we always suggest applying a base amount of nitrogen fertilizer to address the variability, even if the soil nitrate test is more than 150 lb/acre nitrate-N. A base nitrogen fertilizer rate (maybe 20 to 40 lb/acre N) should address most of the field variability and provide a fast start to the next year’s crop. In 1988, we learned the tough lesson that applying no nitrogen fertilizer on fields testing very high for nitrate-N was a mistake, and the best producing parts of fields had early-season nitrogen deficiencies. A modest base nitrogen fertilizer rate was the right decision to cover field variability.

Three Simple Lessons from Droughts Past

  1. Soil test all fields for residual soil nitrate-N. There will be considerable variability from field to field and even zone to zone.
  2. The residual soil nitrate-N test allows you to reduce nitrogen fertilizer rates for next year, saving money on crop inputs for 2022.
  3. Remember to apply a modest base nitrogen fertilizer rate on fields testing very high in nitrate-N to address field variability. You will want to get next year’s crop started right.

Update: Feed Nitrate Testing in a Drought Year

Drought continues to stress crops across the upper Midwest and the Canadian Prairies. As crop conditions continue to deteriorate in some places, we have received more phone calls about salvaging the drought-stressed crop as livestock feed and the need for feed nitrate testing. As you consider what to do with your standing crop, whether to harvest for grain or cut for hay, an important part of that consideration will be the nitrate concentration of the crop.

When drought-stressed annual crops (e.g., wheat, barley, oat, corn) are cut or grazed, producers must exercise caution about livestock nitrate poisoning when feeding these forages. Drought-stressed crops often accumulate nitrate because plant uptake of nitrate exceeds plant growth and nitrogen utilization. Nitrate is usually concentrated in lower plant parts (lower stem or stalk). When livestock, particularly sheep and cattle, ingest forages with a high nitrate concentration, nitrate poisoning can occur.

Instructions for collecting and submitting a feed nitrate test

1. Collect the plant part that livestock will consume, which may be the whole aboveground plant. If grazing, be mindful of the grazing height because the plant nitrate concentration will be lower near the base of the plant. If baling for hay or chopping for silage, cut at the intended cutter bar height.

Picture used for feed nitrate email - corn collage

2. Cut plant material with sturdy garden shears into 1- to 2-inch pieces. Mix the chopped plant parts together and take one quart-sized subsample for analysis (about four good handfuls).

3. Place subsample in AGVISE Plant Sample Bag. Write “Feed Nitrate” as the crop choice and select “Nitrate-nitrogen” as the analysis option.

    • If you are considering chopping corn for silage, also write “%Moisture” as an additional analysis because you will need to know if the moisture content is still adequate for silage fermentation. You may be surprised how much water will still be in drought-stressed corn stalks.

4. Ship plant sample to AGVISE Laboratories. If you cannot ship the sample right away, store it in a refrigerator until you can ship it.

IMPORTANT: Resample the hay or silage before feeding to any livestock. You need to know what is actually being fed to livestock, and you may need to blend it with other feed sources to dilute the nitrate concentration. For dry hay in bales, the nitrate concentration will not change in storage; use a hay probe to obtain the best possible feed sample. For silage, the nitrate concentration may decrease 20 to 50% during fermentation, so a fresh sample is necessary before feeding.

IMPORTANT: Many crop protection products have grazing restrictions on their labels that dictate if or when a crop treated with a product can be fed to livestock. Before using or selling a crop for livestock feed, check all labels of crop protection products that have been used on the crop this season. This includes seed treatments, herbicide applications, fungicide applications, and insecticide applications.

AGVISE Laboratories offers next-day turnaround for feed nitrate analysis. Rapid turnaround on nitrate analysis is important for producers debating to cut and bale or graze small grains or corn as livestock feed.  We also provide livestock water analysis, which includes total dissolved solids, nitrate, and sulfate, to assess livestock drinking water quality. Please call AGVISE staff in Northwood, ND (701-587- 6010) or Benson, MN (320-843-4109) with questions about nitrate, feed and hay quality, or water analysis. We can send you sampling supplies if needed.

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Helpful resources on using drought-stressed crops for livestock feed:

Nitrate Poisoning of Livestock (NDSU)

Using Drought-Stressed Corn as Forage (SDSU)

Drought-Related Issues in Forage, Silage and Baleage (Univ. of Missouri)

Potassium and Drought: A Two-fold Water Uptake Problem

Potassium is back on the radar for many farmers and agronomists across the upper Midwest and northern Great Plains. In the past two weeks, corn growth and development have reached the stage where potassium deficiencies are becoming quite apparent, and widespread dry soil conditions during the 2021 drought have worsened the problem. In some instances, corn is displaying potassium deficiency symptoms on soils with medium to high soil test K (120 to 180 ppm) in spite of potassium fertilizer application.

Potassium is required in large quantities for plant growth and development. The plant tissue K range in normal corn plants is 3-5% K, which is similar to nitrogen. A 200-bushel/acre corn crop will typically uptake 200 lb N, 108 lb P2O5, and 280 lb K2O per acre through the growing season (IPNI, 2014). In other words, an actively growing corn crop takes a lot of potassium! Luckily, you do not have to apply all that potassium as fertilizer, and much will come from the plant-available K pool in the soil.

Potassium deficiency in corn. Symptoms are leaf chlorosis (yellowing) and necrosis (death) beginning at the leaf tip and outer leaf margin and progressing toward the midrib, often with wavy leaf edges. Potassium is mobile in the plant, so symptoms appear on the lower leaves first as the plant remobilizes potassium from lower leaves to support new plant growth. 

Drought reduces potassium availability

The plant-available K pool becomes less available when soil water is limited. This has become the top story as the 2021 drought has continued. Plant roots acquire potassium mostly through a process called diffusion. Diffusion is the slow movement of ions through water around soil particles to the plant root for uptake. As soil becomes drier, the thickness of the water film around soil particles becomes thinner and thinner, thus the diffusion path for potassium ions becomes longer and longer. The soil pore space becomes mostly air with little water remaining. This ultimately slows the rate at which potassium from soil or fertilizer can reach the plant root, and potassium deficiency may occur.

The consequence of the drought-induced potassium deficiency is two-fold because potassium also plays an essential role in plant water regulation. Potassium-stressed plants experience reduced photosynthesis and transpiration rates, resulting in poor water use efficiency of the already limited soil water that is available. In a nutshell, low soil water content reduces potassium availability from soil and fertilizer, and then the soil water that is there is poorly utilized because of the lack of potassium. In addition to limited soil water, other factors compound to reduce potassium uptake: soil test K, soil texture, clay mineralogy, soil compaction, and even fluffy soil syndrome.

Believe it or not, fluffy soil syndrome has been a component of more than one phone call concerning potassium deficiency. Do you see greener plants near the planter wheel tracks or sprayer tracks? Fluffy soil syndrome occurs when soil has not completely settled since spring tillage, which results in poor soil particle-to-particle contact and slow soil-water-root diffusion routes for potassium ions. The wheel tracks adequately firmed the soil to provide good soil particle-to-particle contact, maintaining better potassium diffusion.

Potassium deficiency in corn: A case study

In June 2021, AGVISE started to receive plant and soil samples to diagnose suspected potassium deficiencies in various crops. This corn example from west central Minnesota included plant and soil samples collected in the good and poor areas of the field. The leaf K concentration was 0.59% in the good and 0.52% in the poor area. For comparison, the corn leaf K sufficiency range at this growth sage should be 2-3% K. The corresponding soil samples had soil test K at 148 ppm in the good and 140 ppm in the poor area. The soil test K critical level for corn is 150-200 ppm, and the farmer had applied 50 lb/acre K2O broadcast + incorporation, which is very close to the university sufficiency guideline for corn. Although the farmer more or less did everything right for a normal rainfall year, drought conditions have reduced potassium availability to the point where potassium deficiency symptoms were apparent and visible.

One week after the plant and soil samples were collected, the field received an inch of rain, and the potassium deficiency symptoms disappeared! The entire corn field is green now. It is amazing what a little water will fix.

Potassium deficiency in corn confirmed with plant and soil analysis. Potassium-deficient corn plant (left) displays chlorosis and necrosis of the outer leaf margin and wavy leaf edge. Plant and soil samples were collected June 2021 in west central Minnesota.

Correcting the problem

So, what do you do next? Do you try to apply an in-season rescue potassium fertilizer application? You still need rain to water in any fertilizer applied to the soil surface. If you had applied an adequate amount of potassium fertilizer before planting, then the appropriate decision is to wait for rain to improve soil and fertilizer potassium availability. However, some people may not have applied enough potassium initially. In these cases, a rescue application of 60 lb/acre K2O broadcast (100 lb/acre potash, 0-0-60) followed by some rain should correct the symptoms. Do not skimp with anything less because you are already behind the eight-ball and you will need that much material to cover the soil surface adequately and affect enough individual corn plants. In NDSU research (2014-2016), an uncorrected potassium deficiency in corn could cost 20-30 bushel/acre compared to corn receiving adequate potassium fertilizer.

For liquid materials, potassium acetate and potassium thiosulfate could be dribbled between the rows, but the potassium rate will need to be similar to the dry potassium fertilizer rate and cost will likely be greater. Remember, potassium is something required in large quantities, not something corrected with a small application of 5-10 lb/acre K2O.

There is no way we could have planned for the very dry conditions that are exacerbating potassium deficiency symptoms across the region. For the future, the best preventative strategy is precision soil sampling (grid or zone) and fertilizing accordingly. It is important to identify and address those parts of fields where potassium may be limiting crop yield potential and spend fertilizer dollars where needed.

Feed Nitrate Testing in a Drought Year

Drought is an unwelcome but well-known phenomenon on the Northern Plains and Canadian Prairies. Rainfall has been sparse and scattered across the region, and high temperatures exceeding 90 to 100° F (32 to 38° C) have already caused stress to young crops. These same stresses have also wracked pastures, prompting livestock producers to think about alternative feed options for cattle. Believe it or not, we have already received questions from farmers and ranchers about decisions to cut and bale or graze small grain fields for livestock feed.

When drought-stressed annual crops (e.g., wheat, barley, oat, corn) are cut or grazed, producers must exercise caution about livestock nitrate poisoning when feeding these forages. Drought-stressed crops often accumulate nitrate because plant uptake of nitrate exceeds plant growth and nitrogen utilization. Nitrate is usually concentrated in lower plant parts (lower stem or stalk). When livestock, particularly sheep and cattle, ingest forages with a high nitrate content, nitrate poisoning can occur if large amounts of nitrate convert to nitrite in their digestive system.

Dry soil conditions and high soil nitrate levels favor plant accumulation of nitrate. There is one upside to very dry soil conditions: Some soils may not have had enough soil water to convert all nitrogen fertilizer from the ammonium form to the nitrate form, especially if nitrogen fertilizer was applied in a concentrated band that delays nitrification. Therefore, this may limit the amount of soil nitrate available for plant uptake and accumulation. Regardless, there is still variation across the landscape, and a feed nitrate analysis is the best method to assess livestock nitrate poisoning risk.

When collecting plant material for nitrate analysis, collect the plant parts that the livestock will eat. If plant material will be grazed, recall that lower plant parts contain higher nitrate concentrations; monitor grazing height closely. If plant material will be cut and baled, you should collect plant material above the cutter bar height. Alternatively, plant material can be sampled with a hay probe after being baled.

For the fastest turnaround, submit feed materials for nitrate analysis using a plant sample bag. Write “feed nitrate” for crop choice and select “nitrate-nitrogen” as the analysis option. 

AGVISE Laboratories offers next-day turnaround for feed nitrate analysis. Rapid turnaround on nitrate analysis is important for producers debating to cut and bale or graze small grains or corn as livestock feed.  We also provide livestock water analysis, which includes total dissolved solids, nitrate, and sulfate, to assess livestock drinking water quality. Please call AGVISE staff in Northwood, ND (701-587- 6010) or Benson, MN (320-843-4109) with questions about nitrate, feed/hay quality, or water analysis. We can send you sampling supplies if needed.

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How Much AMS Does Your Spray Water Need?

AGVISE Laboratories Spray Water Analysis

Hard water is a fact of life for those of us in the northern Great Plains and Prairie Provinces. It is why our homes have water softeners, why our well water tastes funny (or delicious), and one reason we need to add AMS (ammonium sulfate) or UAN to our spray tanks to optimize weed control.

When we talk about conditioning “hard water” for herbicide applications, we are preventing dissolved salts (calcium, magnesium, sodium, potassium, and iron) in water from antagonizing, or binding, the pesticide we’re putting in the tank. Dissolved salts bind to weak-acid, salt-formulated pesticides and reduce their efficacy (e.g. glyphosate [RoundUp], growth regulators, ACCase inhibitors [Select, Axial, etc.], ALS inhibitors [Pursuit, Express, etc.], HPPD inhibitors [Callisto etc.], and glufosinate [Liberty]).

A water conditioner like AMS prevents salts in spray water from binding to pesticides. AMS is most often recommended at rates from 8.5 to 17 lb/100 gal spray volume on herbicide labels. This is a wide window, however, and handling dry AMS can be a pain. So, how do you know how much AMS you should add to the tank to overcome antagonism?

A spray water analysis!

An example of an AGVISE Laboratories spray water report

AGVISE Laboratories provides fast and convenient analysis of spray water used for pesticide applications. The Spray Water Analysis package includes calcium, magnesium, sodium, iron, pH, salt, hardness, and SAR (sodium adsorption ratio). The spray water report uses NDSU data to determine the recommended amount of AMS required per 100 gallons of water to overcome antagonism. You will want to test each water source you use for pesticide applications. This information can help avoid problems throughout the spraying season.

Give us a call in either Benson, MN, or Northwood, ND and we will send you a water sample kit. Each kit contains a water collection jar and a sample information sheet. Water sample tests are completed within a week and results are emailed to you, so you have information on your water source right away.

Don’t let salts take away from your weed control this summer – get your spray water tested!

Protect Nitrogen Fertilizer from Ammonia Volatilization

Recent rain and snow have brought much-needed precipitation to the northern Great Plains and upper Midwest regions. Some degree of drought conditions stretch from Alberta to Iowa, and agronomists and farmers are wondering the best ways to protect spring-applied nitrogen as the planting season continues. How much nitrogen might I lose if I cannot incorporate it? Does vertical tillage incorporate fertilizer enough? We have compiled some resources to help answer those questions.

There are three ways to lose fertilizer nitrogen: ammonia volatilization, denitrification, and nitrate leaching. In excessively wet soils, denitrification and nitrate leaching are a concern. However, for spring-applied nitrogen, ammonia volatilization is the main concern with dry soil conditions and unpredictable rainfall forecasts.

When you apply ammoniacal fertilizers (e.g. anhydrous ammonia, urea, UAN, ammonium sulfate) to the soil surface without sufficient incorporation, some amount of free ammonia (NH3) can escape to the atmosphere. Sufficient incorporation with tillage or precipitation is needed to safely protect that nitrogen investment below the soil surface. With dry soil conditions, this is important to remember because we must balance the need to protect nitrogen fertilizer while conserving soil water for seed germination and emergence.

Ammonia volatilization risk depends on soil and environmental factors (Table 1) and the nitrogen fertilizer source (Table 2). Typically, we are most concerned about ammonia volatilization for surface-applied urea or UAN. It is not easy to estimate how much nitrogen might be lost, and sometimes the losses can be substantial. Although you cannot change the soil type or weather forecast, you do have control over the nitrogen source and application method (Table 2) to protect your nitrogen investment.

Practices to reduce ammonia volatilization, in order of most effective: 

  • Apply urea in subsurface bands at least 3 inches below the soil surface. A shallow urea band (1 or 2 inches) acts like a slow-release anhydrous ammonia band, and nobody should ever apply anhydrous ammonia that shallow.
  • If nitrogen will be broadcast with incorporation, make sure the fertilizer is sufficiently incorporated at least 2 inches below the soil surface to ensure good soil coverage. A chisel plow or field cultivator is usually needed. The popularity of high-speed disks (vertical tillage) has led some people to think that it counts as a meaningful incorporation event. In reality, it just moves soil and crop residue around on the soil surface without really incorporating any fertilizer. Take a look after you run across the field and you will see white urea granules everywhere. There are soil-applied herbicide incorporation videos from the 1970s that show what a thorough incorporation job really requires.
  • If nitrogen will be broadcast without incorporation, try to time the fertilizer application right before rain (at least 0.3 inch of precipitation). Soils with good crop residue cover (no-till) may require more rain to sufficiently move urea or UAN into the soil surface.
  • If no rain is forecasted in the near future, consider applying a urease inhibitor on urea or UAN to provide temporary protection until rain arrives. The university research-proven urease inhibitor is NBPT, available in products like Agrotain (Koch) and its generic cousins. For generic products, make sure the active ingredient rate is 1.3 to 1.8 lb NBPT per ton of urea to ensure effective NBPT activity and protection. NBPT begins to breakdown after 7 to 14 days. In addition, it is important to remember that nitrification inhibitors like nitrapyrin and DCD do not protect against ammonia volatilization.

These practices should also be considered if you will be applying in-season nitrogen to corn or wheat later in the summer. it is always best to apply nitrogen below the soil surface, such as injected anhydrous ammonia or coulter-injected UAN, to protect nitrogen fertilizer. For surface-applied urea or UAN, you will want to time the fertilizer application just before a rainfall or consider NBPT to extend the rainfall window.

Resources on ammonia volatilization and urease inhibitors

Nitrogen extenders and additives for field crops, NDSU

How long can NBPT-treated urea remain on the soil surface without loss?, NDSU

Should you add inhibitors to your sidedress nitrogen application?, University of Minnesota

Split the risk with in-season nitrogen, AGVISE

Soil Testing and 4R Nutrient Stewardship

Each year, farmers aim to increase agricultural production and profitability while conserving our land resources for the next generation. These tandem goals drive sustainable soil fertility and crop nutrition decisions on cropland across the world.

In 2005, global fertilizer industry and environmental stakeholders began developing a standard theme to emphasize science-based stewardship in soil fertility and crop nutrition. The theme eventually became known as 4R Nutrient Stewardship, where each “R” referred to the “right” way to manage nutrients for crop production. The 4Rs are summarized as managing crop nutrition with the 1) Right Source, 2) Right Rate, 3) Right Time, and 4) Right Place.

To successfully implement 4R Nutrient Stewardship, you must start with a high-quality soil sample and an informative soil test. To begin, the fertilizer need and amount is determined through soil testing, which is based on regionally calibrated soil test levels for each crop. If you do not have a soil test, how do you know what the Right Rate is? Using crop removal rates or simply guessing without soil testing often leads to overapplication of fertilizer, cutting into profit.

A conventional whole-field composite soil sample (one soil sample per field) is certainly better than no soil sample. It gets you in the ballpark, but it does not detect variation in soil nutrient levels across the field. You might underapply fertilizer on high yielding parts and overapply fertilizer on low yielding parts. To get the Right Rate applied in the Right Place, precision soil sampling, either grid or zone, is the best way to determine the appropriate fertilizer rate and where to apply it in each field. Precision soil sampling is a proven tool to reduce over- and under-fertilization across fields, thus optimizing crop yield and profitability while reducing the potential risk of soil nutrient loss to the environment.

When you start soil sampling and making soil fertility plans for next year, keep 4R Nutrient Stewardship in mind. AGVISE Laboratories is a proud 4R Partner. To learn more about the 4Rs or become a 4R Partner, visit the 4R Nutrient Stewardship website.

Split the Risk with In-season Nitrogen

For some farmers, applying fertilizer in the fall is a standard practice. You can often take advantage of lower fertilizer prices, reduce the spring workload, and guarantee that fertilizer is applied before planting. As you work on developing your crop nutrition plan, you may want to consider saving a portion of the nitrogen budget for in-season nitrogen topdress or sidedress application.

Some farmers always include topdressing or sidedressing nitrogen as part of their crop nutrition plan. These farmers have witnessed too many years with high in-season nitrogen losses, usually on sandy or clayey soils, through nitrate leaching or denitrification. Split-applied nitrogen is one way to reduce early season nitrogen loss, but do not delay too long before rapid crop nitrogen uptake begins.

Short-season crops, like small grains or canola, develop quickly. Your window for topdress nitrogen is short, so earlier is better than later. To maximize yield in small grains, apply all topdress nitrogen before jointing (5-leaf stage). Any nitrogen applied after jointing will mostly go to grain protein. In canola, apply nitrogen during the rosette stage, before the 6-leaf stage. For topdressing, the most effective nitrogen sources are broadcast NBPT-treated urea (46-0-0) or urea-ammonium nitrate (UAN, 28-0-0) applied through streamer bar (limits leaf burn). Like any surface-applied urea or UAN, ammonia volatilization is a concern. An effective urease inhibitor (e.g. Agrotain, generic NBPT) offers about 7 to 10 days of protection before rain can hopefully incorporate the urea or UAN into soil.

Long-season crops, like corn or sunflower, offer more time. Rapid nitrogen uptake in corn does not begin until after V6 growth stage. 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. Topdress NBPT-treated urea is a quick and easy option when corn is small (before V6 growth stage). After corn reaches V10 growth stage, you should limit the topdress urea rate to less than 60 lb/acre (28 lb/acre nitrogen) to prevent whorl burn.

Sidedress nitrogen provides great flexibility in nitrogen sources and rates in row crops like corn, sugarbeet, or sunflower. Sidedress anhydrous ammonia can be safely injected between 30-inch rows. Anhydrous ammonia is not recommended in wet clay soils because the injection trenches do not seal well. Surface-dribbled or coulter-injected UAN can be applied on any soil texture. Surface-dribbled UAN is vulnerable to ammonia volatilization until you receive sufficient rain, so injecting UAN below the soil surface helps reduce ammonia loss. Injecting anhydrous ammonia or UAN below the soil surface also reduces contact with crop residue and potential nitrogen immobilization.

An effective in-season nitrogen program starts with planning. In years with substantial nitrogen loss, a planned in-season nitrogen application is usually more successful than a rescue application. If you are considering split-applied nitrogen for the first time, consider your options for nitrogen sources, application timing and workload, and application equipment. Split-applied nitrogen is another tool to reduce nitrogen loss risk and maximize yield potential.

Phosphorus and the 4Rs: The progress we have made

The year 2019 marked the 350th anniversary of discovering phosphorus, an element required for all life on Earth and an essential plant nutrient in crop production. Over the years, we have fallen in and out of love with phosphorus as a necessary crop input and an unwanted water pollutant. Through improved knowledge and technologies, we have made great progress in phosphorus management in crop production. Let’s take a look at our accomplishments!

Right Rate

Phosphorus fertilizer need and amount is determined through soil testing, based on regionally calibrated soil test levels for each crop. Soils with low soil test phosphorus require more fertilizer to optimize crop production, whereas soils with excess soil test phosphorus may only require a starter rate. Across the upper Midwest and northern Great Plains, soil testing shows that our crops generally need MORE phosphorus to optimize crop yield (Figure 1), particularly as crop yield and crop phosphorus removal in grain has increased. Since plant-available phosphorus varies across any field, precision soil sampling (grid or zone) allows us to vary fertilizer rates to better meet crop phosphorus requirements in different parts of the field.

For phosphorus and the 4Rs article

Figure 1. Soil samples with soil test phosphorus below 15 ppm critical level (Olsen P) across the upper Midwest and northern Great Plains in 2019.

Right Source

Nearly all phosphorus fertilizer materials sold in the upper Midwest and northern Great Plains are some ammoniated phosphate source, which has better plant availability in calcareous soils. Monoammonium phosphate (MAP, 11-52-0) is the most common dry source and convenient as a broadcast or seed-placed fertilizer. Some new phosphate products also include sulfur and micronutrients in the fertilizer granule, helping improve nutrient distribution and handling. The most common fluid source is ammonium polyphosphate (APP, 10-34-0), which usually contains about 75% polyphosphate and 25% orthophosphate that is available for immediate plant uptake. Liquid polyphosphate has the impressive ability to carry 2% zinc in solution, whereas pure orthophosphate can only carry 0.05% zinc. Such fertilizer product synergies help optimize phosphorus and micronutrient use efficiency.

Right Time

Soils of the northern Great Plains are often cold in spring, and early season plant phosphorus uptake can be limited to new seedlings and their small root systems. We apply phosphorus before or at planting to ensure adequate plant-available phosphorus to young plants and foster strong plant development. In-season phosphorus is rarely effective as a preventive or corrective strategy.

Right Place

Proper phosphorus placement depends on your system and goals. Broadcasting phosphorus fertilizer followed by incorporation allows quick application and uniform distribution of high phosphorus rates. This strategy works well if you are building soil test phosphorus in conventional till systems. In no-till systems, broadcast phosphorus without incorporation is not ideal because soluble phosphorus left on the surface can move with runoff to water bodies.

In no-till systems, subsurface banded phosphorus is more popular because phosphorus is placed below the soil surface, thus less vulnerable to runoff losses. In general, banded phosphorus is more efficient than broadcast phosphorus. In the concentrated fertilizer band, less soil reacts with the fertilizer granules, thus reducing phosphorus fixation, allowing improved plant phosphorus uptake. Some planting equipment configurations have the ability to place fertilizer near or with seed, which further optimizes fertilizer placement and timing for young plants.

For more information on 4R phosphorus management, please read this excellent open-access review article: Grant, C.A., and D.N. Flaten. 2019. J. Environ. Qual. 48(5):1356–1369.

Prevented Planting Acres in 2020: Maximizing Cover Crop Effectiveness

In 2020, there are again widespread acres of Prevented Planting (PP) in North Dakota and northwest Minnesota. Farmers are now making plans to plant cover crops on unplanted cropland in the next few weeks. It is important to establish cover crops on PP fields because growing plants help reduce the chance these fields will be PP fields again next year.

Let’s look at the major reasons why cover crops are valuable tools on Prevented Planting acres.

Soil Water Use

A field without any growing plants is a fallow field. Before no-till, summer fallow was a widespread soil water conservation strategy in dryland agriculture. Actively growing plants transpire (use) a lot more water than evaporation from the soil surface alone does. Cover crops help fill the water-use void by transpiring a lot of water, helping to dry the soil surface and lower the water table before the following year. This also opens space in the soil profile for summer and fall rains to leach soluble salts from the soil surface and reduce salinity in the root zone.

Soil Erosion Control

Tillage is a popular weed control tool, but it also destroys crop residue and leaves soil exposed and vulnerable to water and wind erosion. Planting cover crops protects the soil surface from rain and wind, keeping soil firmly in place. Just because you cannot grow a cash crop on the field this year, you should not let your soil blow into the next field, letting your neighbor farm it next year.

Weed Control

An established cover crop can compete with weeds, helping suppress weed growth and weed seed production. For fields with problematic broadleaf weed histories, a cover crop mix containing only grass species is preferred. In grass cover crops, you can still use selective broadleaf weed herbicides to control the problematic broadleaf weeds of conventional or no-till systems such as Canada thistle, common ragweed, kochia, volunteer canola, and waterhemp while not killing the grass cover crop. For fields with low weed pressure, a cover crop mix containing grasses, brassicas, and legumes will provide more soil health benefits.

Soil Biological Activity

Have you heard about “fallow syndrome” before? Fallow syndrome is an induced nutrient deficiency, often seen in corn following fallow, when the population of mycorrhiza fungi is insufficient to colonize plant roots and help them acquire water and nutrients. Mycorrhizae are especially important in plant uptake of phosphorus, so plants with fallow syndrome often show phosphorus deficiency symptoms. Fallow syndrome is a major concern in corn following summer fallow or Prevented Planting without cover crop.

During the Prevented Planting year, it is important to include grass species in the cover crop mix to support and maintain the mycorrhiza population through next year. Brassica species, like radish and turnip, are often included in cover crop mixes for their deep taproot architecture and high forage value for grazing livestock, but brassicas do not support mycorrhizae. You do not want a cover crop mix consisting of brassica species alone because fallow syndrome might occur next year.

 

In June 2020, excessive rainfall slammed some parts of the upper Midwest and northern Great Plains, drenching soils with 3 to 15 inches of rain over a couple days. On summer-flooded fields, cereal rye is an attractive soil management tool. You can plant or fly on cereal rye well into August or mid-September, and it will continue to use soil water through late summer and fall. Next spring, the overwintered rye will grow again, using more soil water and maintaining soil structure, providing you with a much better chance to plant the field. If soybean is the next crop, you can plant glyphosate-tolerant soybean into green cereal rye then terminate the cereal rye with glyphosate later. This practice has become more and more popular on difficult fields.

Do not forget about soil fertility and plant nutrition for cover crops. A modest application of nitrogen will help cover crop establishment, plant water use, and competition with weeds, as cover crops with adequate nitrogen will grow faster and larger than those without nitrogen. Around 46 lb/acre nitrogen (100 lb/acre urea, 46-0-0) should be enough to establish nice cover crop growth. Prevented Planting fields, being wetter than those successfully planted in spring, lost some, if not most, soil nitrogen via nitrate leaching or denitrification. Although additional nitrogen may have mineralized from soil organic matter during May and June, excess precipitation in June may have caused additional soil nitrogen loss. The best way to know is collecting 0-12 or 0-24 inch soil samples for nitrate-nitrogen analysis.

As you choose the appropriate cover crop mix on Prevented Planting fields, you must consider the pros and cons of each cover crop species and how each will help accomplish your goals. These are some helpful resources that will provide additional information on what cover crop options will work best on your fields.