Category: Soil Fertility and Plant Nutrition - Page 4

Fertilizing grass lawn

in Lawn and Garden, Nitrogen/by John Lee

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.

Split the Risk with In-season Nitrogen

in Canola, Corn, In-Season Fertilizer, Nitrogen, Sunflower, Water Quality, Wheat/by John Breker

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.

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

in Nitrogen, Plant Analysis, Potato/by John Lee

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.

Soil Science Review: Soil pH, Acidity, and Alkalinity

in Soil Chemical Analysis, Soil pH/by John Breker

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.

Phosphorus and the 4Rs: The progress we have made

in Fertilizer Placement, Phosphorus, Regional Data, Water Quality/by John Breker

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.

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.

Molybdenum: The Micro-est of Micronutrients

in Molybdenum, Plant Analysis, Soil Chemical Analysis, Soil pH/by John Breker

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 (MoO₄^2-). 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.

Fertilizing soybean

in Iron, Nitrogen, Phosphorus, Potassium, Soybean, Sulfur, Zinc/by Richard Jenny

Soybean acres expanded greatly across the northern Great Plains and into Manitoba through the 1990s and 2000s. Today, soybean occupies a large portion of planted acres and makes a desirable rotation crop in canola, corn, and small grain production systems. As soybean has advanced northward and westward, soybean is often billed as a low maintenance crop, requiring no fertilizer or even seed inoculation. The fact is, if you expect soybean to be a low maintenance crop, you can expect low yield results. Achieving high soybean yields starts with a good, long-term soil fertility plan.

Nitrogen

Soybean yielding 40 bu/acre requires about 200 lb/acre nitrogen, but luckily you do not have to provide all the nitrogen! Soybean relies on nitrogen-fixing bacteria to meet its nitrogen requirements. Legumes, like soybean, form a symbiotic relationship with N-fixing bacteria, housed in root nodules, to provide sufficient nitrogen. Each legume species requires a unique N-fixing bacterium, thus an inoculant for lentil or pea does not work on soybean. Soybean seed must be inoculated with the N-fixing bacteria Bradyrhizobia japonicum. Ensure you have the proper soybean-specific seed inoculant. You can count the number of nodules on soybean roots and verify the presence of active N-fixing bacteria in the nodules with bright pink centers. These soybean plants have enough active N-fixing bacteria to meet soybean nitrogen requirements.

For new soybean growers, the N-fixing bacteria Bradyrhizobia japonicum is not naturally present in soil and seed inoculation is required. During the first few years of soybean establishment, supplemental nitrogen may be required to achieve good soybean yield while the N-fixing bacteria population builds. University of Minnesota researchers in the northern Red River Valley showed that soils with less than 75 lb/acre nitrate-N (0-24 inch) required 40-50 lb/acre additional preplant nitrogen. If successful inoculation and good nodule counts are observed in the first year, then no additional nitrogen should be required in subsequent years.

Plant soybean on soils with less than 100 lb/acre nitrate-N (0-24 inch), if possible. High residual soil nitrate may delay root nodulation with N-fixing bacteria and increase the severity of iron deficiency chlorosis (IDC). Because soybean can fix its own nitrogen, you may recoup better economic return on soils with high residual nitrate with crops that do not fix their own nitrogen like corn or wheat.

Phosphorus

Soybean does not respond to phosphorus as dramatically as grass crops like corn or wheat do. Nevertheless, medium to high soil test P are required to achieve good soybean yields. Soybean responds to broadcast P placement better than seed-placed or sideband P. In dryland regions where soybean is planted with air drills, seed-placed P or sideband P is often the only opportunity to apply phosphorus. You must pay special attention to seed-placed fertilizer safety with soybean. An air drill with narrow row spacing (6 inch) should not exceed 20 lb/acre P₂O₅ (40 lb/acre monoammonium phosphate, MAP, 11-52-0). Fertilizer rates exceeding the seed safety limit may delay seedling emergence and reduce plant population. For wider row spacings, no fertilizer should be placed with seed.

Potassium

Soybean removes far more potassium in harvested seed than canola or wheat. Soybean yielding 40 bu/acre removes about 60 lb/acre K₂O, while wheat yielding 60 bu/acre removes only 20 lb/acre K₂O. Pay close attention to potassium removal across the crop rotation. After soybean is added to the crop rotation, cumulative potassium removal greatly increases, and declining soil test K is observed over time.

Do not place potassium with soybean seed; delayed seedling emergence and reduced plant population can occur. Any potassium fertilizer should be broadcasted or banded away from seed.

Sulfur

Sulfur deficiency in soybean is uncommon, yet sometimes observed on coarse-textured soils with low organic matter (< 3.0%). Soybean response to sulfur is usually confined to certain zones within fields. With additional sulfur, soybean can produce more vegetative growth, but more vegetative growth may increase soybean disease severity, such as white mold. The residual sulfur remaining after sulfur-fertilized canola, corn, or small grain is often sufficient to meet soybean sulfur requirements.

Iron

Soybean is very susceptible to iron deficiency chlorosis (IDC). Soybean IDC is not caused by low soil iron but instead by soil conditions that decrease iron uptake by soybean roots. Soybean IDC risk and severity are primarily related to soil carbonate content (calcium carbonate equivalent, CCE) and worsened by salinity (electrical conductivity, EC).

Soybean IDC is common in the upper Midwest, northern Great Plains, and Canadian Prairies, where soils frequently have high carbonate and/or salinity. Within a field, IDC symptoms are usually confined to soybean IDC hotspots with high carbonate and salinity; however, symptoms may appear across a field if high carbonate and salinity are present throughout the field. Soybean IDC severity is made worse in cool, wet soils and soils with high residual nitrate. Soil pH is not a good indicator of soybean IDC risk because some high pH soils lack high carbonate and salinity, which are the two principal risk factors.

Guidelines for managing soybean IDC:

Soil test each field, zone, or grid for soil carbonate and salinity. This may require soil sampling prior to soybean (possibly outside of your usual soil sampling rotation) or consulting previous soil sampling records.
Plant soybean in fields with low carbonate and salinity (principal soybean IDC risk factors).
Choose an IDC tolerant soybean variety on fields with moderate to high carbonate and salinity. This is your most practical option to reduce soybean IDC risk. Consult seed dealers, university soybean IDC ratings, and neighbor experiences when searching for IDC tolerant soybean varieties.
Plant soybean in wider rows. Soybean IDC tends to be less severe in wide-row spacings (more plants per row, plants are closer together) than narrow-row spacings or solid-seeded spacings.
Apply chelated iron fertilizer (e.g., high quality FeEDDHA) in-furrow at planting. In-furrow FeEDDHA application may not be enough to help an IDC susceptible variety in high IDC risk soils (see points #2 and #3).
Avoid planting soybean on soils with very high IDC risk.

Zinc

Zinc deficiency in soybean is rare, even on soils with low soil test Zn. Soybean seed yield response to zinc is limited on soils with less than 0.5 ppm Zn. More zinc sensitive crops like corn, dry bean, flax, and potato will respond to zinc on soils with less than 1.0 ppm Zn. If zinc sensitive crops also exist in the crop rotation, you may apply zinc with broadcast phosphorus or potassium during the soybean year as another opportunity to build soil test Zn across the crop rotation.

Copper for Small Grains

in Copper, Disease, Wheat/by John Lee

Among crops grown in the northern Great Plains, small grains (cereals) are the most susceptible to copper deficiency. Copper (Cu) is an essential micronutrient required in small concentrations for plant growth and reproduction. Copper deficiency symptoms in cereals include pale yellowing, wilted and twisted leaf tips, and malformed seed heads. Severe copper deficiency will stop plant growth and kill plants during tiller formation. During pollination, copper deficiency will cause florets to remain partially open. This creates a vulnerable period for diseases, such as Fusarium head blight (head scab) and ergot, to infect the seed head and reduce grain yield.

Small grains sensitive to copper deficiency include barley, oat, rye, triticale, and wheat (including durum, spring, and winter types). Copper deficiency is most common on soils with less than 0.5 ppm Cu. Soils with low soil test Cu frequently include sandy soils with low organic matter (<3.0%) and organic soils (peat) with very high organic matter (>10%). Between soil and plant analysis, diagnosing copper deficiency with soil analysis is the most predictive. Plant analysis is less helpful because the plant Cu concentrations in sufficient and deficient plants are very close.

The most effective strategy to build soil test Cu on mineral soils is to broadcast-incorporate copper sulfate (25% Cu). building soil test Cu for many years. Do not mix copper sulfate with seed-placed dry fertilizer blends for air drills; copper sulfate is a hygroscopic (water holding) material that makes blending difficult and bridging is a concern. For seed-placed copper, use a liquid copper source injected in furrow. Liquid copper sources include dissolved copper sulfate and various chelated Cu products.

On organic soils, soil test Cu is difficult to build as copper readily forms complexes with soil organic matter. To reduce copper complexation, apply seed-placed liquid copper at planting and follow with foliar copper in the first herbicide application. Liquid copper sources include dissolved copper sulfate and various chelated Cu products.

Caution: Ammonium Sulfate with Seed

in Canola, Fertilizer Placement, Phosphorus, Starter Fertilizer, Sulfur/by John Lee

Seed-placed fertilizer is a common practice to increase seedling vigor and optimize fertilizer placement and crop response. This is a popular strategy to apply phosphorus for canola, corn, and wheat. However, the seed-placed fertilizer rate cannot exceed seed safety limits, otherwise seedling germination and plant population may be reduced. Sulfur is very important in canola growth and development, so farmers often try placing ammonium sulfate (AMS) with canola seed as well! This can create big problems.

A team of agronomists and soil scientists at the University of Manitoba conducted greenhouse and field studies, examining the effect of seed-placed ammonium sulfate on canola plant population and seed yield. The plant population loss was much greater on soils with pH > 7.5 (Figure 1). The high pH soils contained calcium carbonate (CaCO₃), which reacts with ammonium sulfate to create calcium sulfate (gypsum) and ammonium carbonate. The higher reaction pH of ammonium carbonate produces free ammonia (NH₃). Free ammonia (NH₃) in soil is toxic to living organisms and kills germinating seeds. Acute ammonia toxicity is a major concern with fertilizer materials that liberate free ammonia (NH₃) in soil, such as anhydrous ammonia (82-0-0) or urea (46-0-0), ultimately reducing plant population if you are not careful with fertilizer rate and placement.

Figure 1. Ammonium sulfate (AMS, 21-0-0-24S) included with seed-placed monoammonium phosphate (MAP, 10-52-0) reduced canola plant population. Soil carbonate content is 21% CCE and 0.5% CCE in knoll soil and hollow soil, respectively. Brandon, Manitoba.

Across the landscape, soil pH and carbonate content will vary. The well-drained lower landscape positions (swales, hollows) often have acidic to neutral pH and little carbonate. The upper landscape positions (knobs, knolls), suffering decades of soil erosion, often have high pH and ample carbonate (Figure 1). The risk of plant population loss is greater on eroded knobs where adding ammonium sulfate can create ammonia toxicity concern.

Considerable yield loss will occur if canola plant population is less than 70 plants per square meter. Even with low fertilizer rates, the interaction of seed-placed ammonium sulfate and phosphorus can greatly reduce canola plant population. In Manitoba, 25% plant population loss was observed with only 8 lb/acre S and 18 lb/acre P₂O₅ (Figure 2).

Figure 2. Ammonium sulfate (AMS, 21-0-0-24S) included with seed-placed monoammonium phosphate (MAP, 10-52-0) reduced canola plant population. Carman, Manitoba, 2011.

Sulfur is vital for successful canola production, but it must be applied safely. There are new air drill configurations with innovative seed and fertilizer placement options. Seed safety is paramount with seed-placed fertilizer. Ammonium sulfate should be broadcasted or banded away from seed (mid-row). Keeping ammonium sulfate away from seed will also allow you to maximize seed-placed phosphorus rates and efficiency without jeopardizing seed safety.

Placing ammonium sulfate with seed should be an emergency option only. Canola plant population loss should be expected, even at low ammonium sulfate rates, on soils with pH greater than 7.5 and calcium carbonate.

5 Things You Should Know About Phosphorus

in Phosphorus, Regional Data, Soil Chemical Analysis/by John Lee

1. The two accepted soil phosphorus tests in the North Central Region are the Olsen and Bray-P1 methods

The Olsen (bicarbonate) method is the standard soil P test in the North Central region. This method was developed to work on soils with low and high pH. The Olsen method works well in precision soil sampling, where the same field may have zones with acidic and calcareous soils. The Bray P-1 method is another accepted method in our region, but not always recommended. This method was developed in the U.S. Corn Belt, has a long history of soil test calibration studies and works well on acidic soils. The Bray P-1 method fails on soils with pH greater than 7, producing results with false low soil test P. Therefore, it has remained limited to the U.S. Corn Belt proper. The Mehlich-3 method was introduced as a multi-nutrient soil extractant. But like the Bray P-1 method, the acidic Mehlich-3 method does not perform well on calcareous soils; therefore, it has not gained approval by universities in the northern Great Plains and Canadian Prairies.

All soil P test methods are designed to predict the probability of crop response to P fertilization. The methods measure the plant-available P pool. Since the soil test method is an index of availability, the units are reported in parts per million (ppm) and ranked low, medium, or high based on university soil test calibration research. No soil P test method measures the actual pounds of available P in soil, they are only indexes of crop response.

2. Most soils in the Northern Plains/Canadian Prairies region could use more phosphorus

Soils in the region are naturally low in P and historical P fertilizer use has been low, relative to crop P removal. As a result, many areas in the region still have low soil test P (below soil test critical level of 15 ppm Olsen P) after many decades of crop production. In other words, most farmers are not over-applying P. In fact, soils with low soil test P should receive moderate to high rates of fertilizer P each year to achieve good crop yield and maximize profitability.

Figure 1. Map developed using AGVISE soil test data. AGVISE has created regional summaries like this for the past 40 years. Check out the summary data for Montana and Canada and summaries of other nutrients and soil properties here.

3. You should use starter phosphorus fertilizer

Starter fertilizer placed near, or with the seed, is critical for crops like corn and wheat, regardless of soil test P level. A P fertilizer band placed near the seed will ensure soluble P near developing plant roots and results in vigorous early season growth, which is important in cold, wet soil conditions. Placing P fertilizer in bands also improves P use efficiency, especially in soils with relatively low or high pH. Phosphorus availability is greatest near soil pH 6.5. Since changing soil pH is difficult and costly, fertilizer P use efficiency is more easily improved with application in fertilizer bands to reduce the volume of soil involved in P fixation reactions.

4. Phosphorus source doesn’t really matter

No matter the starting material, all P fertilizers go through the same chemical reactions in the soil. It does not matter if the fertilizer starts as a poly-phosphate or ortho-phosphate. Within about one week in the soil, all P fertilizer sources react to form lower solubility compounds. What is more important than source is the placement of the fertilizer to increase availability (banding) and the rate of actual P fertilizer applied.

5. Phosphorus can be an environmental concern

Phosphorus entering surface waters can create algae blooms and fish kills. Since P is not mobile in soil, the P leaching risk is very low. However, P does move to surface waters with soil particles when erosion occurs. In cold climates like those on the northern Great Plains and Canadian Prairies, dissolved P released from vegetation can move with snow melt to surface water.

For more information about phosphorus and its reactions in soil, explore the links below:

Understanding Phosphorus in Minnesota Soils (Univ. Minnesota)

Understanding Plant Nutrients: Soil and Applied Phosphorus (Univ. Wisconsin)

Phosphorus Facts: Soil, plant, and fertilizer (Kansas State Univ.)