John Lee

Starter Fertilizer Display: How low can YOU go?

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When profits are squeezed, more farmers are asking about optimal starter fertilizer rates and how low starter fertilizer rates can be. These questions are the result of wanting to keep fertilizer costs down, to plant as many acres per day as possible, and to take advantage of more efficient, lower rates of banded phosphorus fertilizer compared to higher rates of broadcast phosphorus fertilizer.

To illustrate the role of starter fertilizer rates and seed placement, we put together displays showing the distance between fertilizer granules or droplets at various rates and row spacings. You can see several pictures with canola, corn, soybean, sugar beet, and wheat. We greatly thank John Heard with Manitoba Agriculture for helping with the displays.

The displays show the normal seed spacing for several crops with different dry or liquid fertilizer rates alongside the seed. These displays help visualize the distance between the seed and fertilizer at several rates. University research shows that to achieve the full starter effect, a fertilizer granule or droplet must be within 1.5-2.0 inches of each seed. If the fertilizer granule or droplet is more than 1.5-2.0 inches away from the seed, the starter effect is lost. Some people wonder about these displays, but you can prove it to yourself pretty easily. Just run the planter partially down on a hard surface at normal planting speed. You will see what you imagine as a constant stream of liquid fertilizer, ends up being individual droplets at normal speed, especially with narrow row spacings and lower fertilizer rates.

These displays help illustrate the minimum starter fertilizer rate to maintain fertilizer placement within 1.5-2.0 inches of each seed for the full starter effect. In addition to an adequate starter fertilizer rate, additional phosphorus and potassium should be applied to prevent nutrient mining, causing soil test levels to decline in years when minimum fertilizer rates are applied.

John Lee

Adjusting low soil pH with sugar beet-processing spent lime

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The sugar beet processing industry uses large quantities of fine-ground, high-grade calcium carbonate (lime) to purify sucrose in the sugar extraction process. The by-product spent lime retains high reactivity and purity, making an attractive liming material for acidic soils. Application of spent lime is a common practice through the sugar beet producing areas of the upper Midwest and northern Great Plains, where its primary function is the suppression of the soil-borne disease Aphanomyces root rot of sugar beet. The spent lime also contains about 20 lb P2O5 per ton, mostly as organic phosphorus impurities gained from sugar refining.

AGVISE Laboratories installed a long-term demonstration project in 2014 to evaluate adjusting low soil pH with spent lime. The project site was located near our Northwood Laboratory. Northwood lies along the beachline of glacial Lake Agassiz, where well-drained coarse-textured soils with low pH are common. We located a very acidic soil with soil pH 4.7 (0-6 inch), which was the perfect site to evaluate lime application. In May 2014, spent lime was applied and incorporated with rototiller. The spent lime quality was very high at 1,500 lb ENP/ton. In Minnesota, lime quality is measured as effective neutralizing power (ENP), which measures lime purity and fineness. Soil pH was tracked over three years (Table 1).

The lowest spent lime rate (2,500 lb ENP/acre) increased soil pH above 5.5. This soil pH reduced aluminum toxicity risk, but it did not reach the target pH 6.0, appropriate for corn-soybean rotation. The highest spent lime rate (10,000 lb ENP/acre) increased soil pH above 7.0 and maintained soil pH for several years. Spent lime is a fine-ground material with high reactivity, so its full effects were seen in the first application year. The project showed that spent lime is an effective liming material for low pH soils.

Table 1. Soil pH (1:1) following sugar beet-processing spent lime application on low pH soil.
Spent Lime Year
May 2014 Sept 2014 July 2015 June 2016
lb ENP/acre
0 4.8 4.8 4.7 5.1
2,500 4.8 5.5 5.2 5.4
5,000 4.8 5.6 5.7 5.5
10,000 4.8 7.4 7.0 7.4
Spent lime applied and incorporated September 2008. Soil sampled in fall.
John Breker

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

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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 concept is extremely misleading on soils with high pH.

John Lee

Adjusting high soil pH with elemental sulfur

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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.

John Lee

Fertilizing grass lawn

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A productive and lush lawn requires some fertilizer every now and then. The major plant nutrients required for grass lawn are nitrogen (N), phosphorus (P), and potassium (K). Nitrogen is the nutrient required in the largest amount, although too much nitrogen can create other problems. A general rate of one (1) pound nitrogen per 1,000 square feet is adequate for most grass lawns, but some more intensively managed lawns may require more nitrogen. The total annual nitrogen budget should be split through the year according to season (Table 1). Common cool-season grasses in lawn mixtures include Kentucky bluegrass, ryegrass, and fescues.

Table 1. Nitrogen fertilizer guidelines for established cool-season grass lawn.
Maintenance Intensity Early Spring

Mar – Apr

 Spring

May – June

 Summer

July – Aug

 Early Autumn

Sept

 Total Annual N
————————- lb nitrogen per 1000 square feet ————————-
Low,

no irrigation

 0.5 0.5 0 0.5 1.5
Medium,

with irrigation

 0.5 1.0 0.5 1.0 2.0
High,

with irrigation

 0.5 1.0 1.0 2.0 4.5
Source: Bigelow, C. A., J. J. Camberato, and A. J. Patton. 2013. Fertilizing established cool-season lawns: Maximizing turf health with environmentally responsible programs. Purdue Univ. Ext. Circ. AY-22-W. Purdue Univ., West Lafayette, IN.

The nutrient application rates given in Table 1 are the actual nutrient rates. To calculate how much fertilizer product you require, you will convert the nutrient rate to fertilizer rate, using the labelled fertilizer analysis. The fertilizer analysis label reports the nitrogen-phosphorus-potassium concentration of the fertilizer product. A product with 12-4-8 analysis contains 12% N, 4% P2O5, and 8% K2O. To convert 1.0 lb N/1000 sq. ft, you divide the nutrient requirement by the fertilizer analysis (12% N), thus 1.0/0.12 equals 8.3 lb fertilizer/1000 sq. ft. The application rate of 12-4-8 fertilizer is 8.3 lb/1000 sq. ft.

A soil containing ample nitrogen may require less nitrogen fertilizer. If soil test nitrogen is more than 50 lb/acre nitrate-N (0 to 6 inch soil depth), the next nitrogen fertilization may be skipped. The soil test nitrogen value of 50 lb/acre nitrate-N is equal to 1.0 lb/1000 sq. ft nitrate-N.

Late fall is an optimal time to fertilize lawn, when grass growth has nearly stopped but before winter dormancy. Avoid fertilizing during hot summer months (July and August), unless you have ample irrigation. Controlled-release nitrogen fertilizer products applied in May and September help prolong nitrogen release to grass during critical growth periods in spring and fall.

John Breker

Split the Risk with In-season Nitrogen

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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.

John Lee

AGVISE Potato Petiole Analysis: Informative, Accessible, and Easy-to-Understand Reports

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For potato petiole analysis article

Irrigated potato production is an intensive cropping system. It requires proactive labor, critical decision-making tools, and well-timed nutrient management. There is a fine line between supplying adequate plant nutrition and applying too much, which could cause potato tuber defects like mishappen tubers or hollow heart, reducing the marketable potato yield.

Before seed potatoes go in the ground, potato agronomists begin with a good soil fertility plan based on precision soil sampling (grid or zone). Once potatoes have emerged, the next step is monitoring the soil and plant nutrient status to ensure the potato crop has no deficient or excess nutritional problems. The in-season monitoring is done with paired potato petiole and soil samples. The petiole and soil sampling starts about 30 days after emergence, then taken every week during the growing season.

A successful in-season potato monitoring program requires fast turnaround and reliable service on petiole and soil samples. This is where AGVISE Laboratories has excelled in serving the potato industry because we know the petiole and soil test results will be used immediately to make fertilizer and irrigation decisions on the fly. To make the data immediately available, the petiole and soil test results are posted online to the AGVISE website with next-day turnaround after the samples arrive at the laboratory.

It is also critical that the petiole and soil test results are easy to interpret and understandable to everyone on the agronomy staff. The AGVISE petiole and soil test report displays results in a graphic format, enabling agronomists to quickly evaluate plant nutrient levels and watch trends over the growing season. An example potato petiole and soil nutrient report is shown below. The report includes a weekly graph of petiole nitrate, phosphorus, and potassium alongside with soil ammonium- and nitrate-nitrogen.

For most irrigated potato producers, weekly potato petiole sampling is a given. But, an increasing number are also including soil samples for ammonium- and nitrate-nitrogen analysis each week. The soil nitrogen data is critical for timing an in-season nitrogen application. There are periods where very fast potato vegetative growth can cause unusually low petiole nitrate-nitrogen levels. The soil nitrogen data prevents overreaction to low petiole nitrate-nitrogen levels and avoids application of extra nitrogen, which could create potential tuber quality issues down the road.

AGVISE Laboratories has provided potato petiole and soil analysis services to the potato industry in the United States and Canada for over 40 years. In 2020, we analyzed over 12,000 potato petiole samples for potato growers at our Northwood, ND and Benson, MN laboratories. We know that timely information is important to our customers, and we are always making improvements to our service and support. If you have any questions, please talk with one of our agronomists or soil scientists about getting started with potato petiole analysis.

For potato petiole article

John Breker

Soil Science Review: Soil pH, Acidity, and Alkalinity

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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.

John Breker

Phosphorus and the 4Rs: The progress we have made

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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.

John Breker

Molybdenum: The Micro-est of Micronutrients

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Molybdenum (Mo) is an essential plant nutrient, necessary for nitrate assimilation and biological nitrogen fixation. Legumes, relying on symbiotic nitrogen fixation, have greater Mo requirement than non-legumes. Nevertheless, the Mo requirement of plants is the lowest among all micronutrients, with critical deficiency concentrations ranging from 0.1 to 1.0 ppm in plant leaves. The very low Mo concentration lies near the detection limit for most laboratory instruments used in commercial soil and plant analysis, so you may see Mo concentration reported as “below instrument detection limit.”

Plant-available Mo in soil is present as molybdate (MoO42-). Unlike most other micronutrients, molybdate availability in soil increases with soil pH. On soils with pH greater than 6.0, Mo deficiency is exceptionally rare. In the northern Great Plains and Canadian Prairies where most soils have high pH, Mo deficiency is virtually unknown, and background plant Mo concentration in legumes ranges from 4 to 8 ppm, indicating that plants obtain sufficient Mo from soil naturally. In the upper Midwest where low pH soils are more common, crop response to Mo fertilization has been limited to legume crops grown on strongly acidic, sandy or peat soils.

Since Mo deficiency is so uncommon and most soils are limed above pH 6.0, no reliable plant-available soil test method for Mo has been developed in the region. The acid ammonium oxalate method was infrequently used in the southeast United States, but the prediction of crop response to Mo fertilization aligns more closely with soil pH than soil test Mo. If soil pH is less than 6.0 and Mo fertilization is necessary, a molybdate fertilizer seed treatment or foliar application is usually sufficient. Overapplication of Mo fertilizer is not a concern for grain production. In forage production however, overapplication is a serious concern because excessive Mo in forages can cause Mo-induced copper deficiency (molybdenosis) in ruminant livestock.