Friday, April 13, 2018

Soil Nutrient Mining... Good or Bad?

I will frequently be asked when talking about issues of plant nutrient management, “so, is that good or bad?” Experience tells me that most complex issues do not have a clear distinction between good and bad, but require a little more exploring on how to make the difference clear.

Rob Mikkelsen in cranberry field
Nutrient mining is an agronomic concept that gets discussed as either good or bad, but the real answer lies somewhere in between depending on the circumstances and the specific nutrient. The discussion here mainly refers to the more immobile nutrients, such as phosphorus (P) and potassium (K), but some of the same concepts apply to the more mobile nutrients.

Nutrient balances are measured by the difference between nutrient additions and removals. As nutrients are removed more quickly than they are replaced, a negative balance results in nutrient depletion (a.k.a. mining). When more nutrients are added to the soil than are removed, a positive balance results in accumulation or buildup.

The Bad

Nutrient mining and depletion of low-fertility soils exhausts the crop-producing potential of the soil, harms soil health, and degrades the valuable natural resource. On soils that are already low in crop nutrients, further depletion results in lost economic opportunities too. Continual nutrient depletion is a major soil-degrading practice that persists in many parts of the world.
Nutrients are not really “lost” from soil, but they are harvested and transported off of the field, eroded, leached beneath the root zone, or sometimes burned as crop residues. At some point, it is necessary to replenish the nutrients to avoid excessive soil mining and severe nutrient depletion. 

Recycling on-farm residues (e.g., crop residues, compost, green manures, animal manures) will help return nutrients to the production fields. But these organic additions will not eliminate soil nutrient mining if they are produced on the same farm where they are used.

On fields where all of the crop residue is removed, the extent of soil nutrient mining is accelerated, compared with fields where the residue remains on the soil and the nutrients are recycled. If the residues are removed for animal feed, returning the manure to the field would slow the process of nutrient mining to some degree.

The Good
When a soil testing program is carefully followed, there may be fields that have an adequate nutrient concentration for healthy plant growth and fertilization can be temporarily halted. 

There are two major philosophies concerning interpretation of soil test results. There is no simple answer of which approach is superior, since there are many factors that determine what is the appropriate route for you.

Great potato crop in Idaho
Nutrient Sufficiency:
Apply only the minimum amount of nutrient required to maximize profits in the year of application.

Build and Maintenance:
Build the concentration of nutrients to a non-limiting range and then apply sufficient nutrient to maintain that desired concentration.

The Build and Maintenance approach allows farmers to take a break in fertilizer application if economic circumstances change. An investment has previously been made to boost the soil nutrient concentrations and soil mining can be allowed to occur for a period of time without devastating results.
Many fields that receive repeated applications of animal manure eventually accumulate high concentrations of P in the soil. When manure is applied to meet the nitrogen requirement of crops, the amount of added P far exceeds crop uptake and removal, resulting in soil P build up. This accumulated P, often termed “legacy phosphorus” can be viewed as a valuable resource, but may also become a source of pollution to nearby water bodies through runoff and erosion. In this case, P application may need to cease and soil mining adopted to lower the P concentration and envir
The key to managing soil nutrient mining is to understand the balance between inputs and outputs. Where available, a comprehensive soil testing program should be used to maintain nutrient concentrations above their critical value. When nutrient concentrations are less than recommended, a phase of nutrient build up is needed to avoid loss of crop yield and quality. At soil test concentrations far greater than recommended, fertilizer applications less than crop nutrient removal may be appropriate.

Soil testing services are not available in many parts of the world. IPNI has developed easy-to-use  software (Nutrient Expert) that allows farmers to make fertilizer recommendations in the absence of local soil testing information.

So, is soil nutrient mining good or bad? It can have devastating effects, leading to soil degradation, or it can have significant economic and environmental benefits. Begin by understanding the nutrient budget for each field and then adopt specific practices appropriate for your conditions
onmental risk.

Agriculture has advanced a long ways!

Monday, March 19, 2018

Nitrification Inhibitors

Some compounds added to nitrogen (N) fertilizers can reduce the rate at which ammonium is converted to nitrate. Under appropriate conditions, this can help reduce N losses through denitrification and leaching. Nitrification in Soil Nitrification is a natural process in soils that converts ammonium to nitrite and then to nitrate. The soil bacteria Nitrosomonas spp. extract energy from ammonium by converting it to nitrite. A second group of bacteria, Nitrobacter spp. then convert nitrite to nitrate. Both types of bacteria are common in soil and it is the first reaction that limits the overall rate of nitrate production. 

With moderate temperature and soil water content nitrification occurs on most soils within a few days or weeks after application of ammonium sources. The ammonium can come from urine, manures, composts, decomposing crop residues, or fertilizer containing ammonium or urea.

Nitrate is the dominant form of plant available N in soil and unless taken up by roots, it can be transferred to either water or the atmosphere. Nitrate can leach below the root zone with the potential to be transferred to surface or sub-surface waters. Under waterlogged conditions, nitrate can be denitrified to form nitrous oxides and dinitrogen by other soil bacteria. While dinitrogen is the most common gas in the atmosphere and inert, nitrous oxide is a powerful greenhouse gas. Some nitrous oxide can also be produced during nitrite decomposition during nitrification. 

Nitrification is rapid in warm (>25o C) soils, and it largely ceases below 5o C. It occurs most rapidly where the soils are well aerated and near field capacity, and decreases with higher or lower moisture contents. In saturated soils nitrification nearly stops because of the lack of oxygen. 
The nitrogen cycle involves many biological processes

Regulating Nitrification

The rate of nitrification can be controlled by preserving N as ammonium, which can be held on soil colloids rather than leached. Nitrification inhibitors are compounds that delay nitrate production by depressing the activity of Nitrosomonas bacteria.

There are at least eight compounds recognized commercially as nitrification inhibitors although the most commonly used and best understood are 2-chloro-6-(trichloromethyl)-pyridine (Nitrapyrin), dicycandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP). These compounds suppress microbial activity for a few days to weeks depending on soil moisture and soil type although there are differences between them in the way they are deployed. In general, nitrification inhibitors are more effective in sandy soils, or soil low in organic matter and exposed to low temperatures. 

Management Practices 

Nitrapyrin can be injected directly into the soil with anhydrous ammonia or coated onto solid N fertilizers or mixed with manures. Because nitrapyrin is volatile it needs to be incorporated into the soil. Nitrapyrin is usually broken down within 30 days in warm soils. 

DMPP is usually supplied pre-blended with fertilizers. It is considered a highly specific nitrification inhibitor active for 25 to 70 days, but its effect is reduced at higher temperatures. It is relatively immobile in the soil. 

DCD can be coated on solid fertilizers, and is also used where manures are surface applied, and can be used post-grazing to reduce nitrate leaching from urine patches. The inhibitory effect of DCD usually lasts between 25 to 55 days, and it is readily leached, lowering its effectiveness.

A study in New Zealand showed that DCD applied to grazed pastures reduced nitrate leaching from urine patches by 59% and nitrous oxide emissions by 82%, as well as increasing herbage production by 30%. 

Because there are many factors that control both the rate of nitrification and the activity of the inhibitor, there is considerable variability in the reduction in the amount of nitrate leached, the reduction in nitrous oxide produced, economic return and the potential benefits associated with their use. So, the yield benefits occur when sufficient N is preserved to provide additional growth.
Feel free to contact me for additional information

This overview of nitrification inhibitors originally appeared as part of the IPNI series "Nutrient Source Specifics".  They can be viewed at

Monday, March 12, 2018

Phosphate Rock

Phosphorus (P) additions are needed in most areas of the world to improve soil fertility and crop production. Direct application of unprocessed phosphate rock (PR) to soil may provide a valuable source of plant nutrients in specific conditions, but there are several factors and limitations to consider. 
Rock phosphate production and final product
Production Phosphate rock is obtained from geologic deposits located around the world. Apatite, a calcium phosphate mineral, is the primary constituent of PR. It is primarily extracted from sedimentary marine deposits, with a small amount obtained from igneous sources. Most PR is recovered through surface mining, although some is extracted from underground mines. 

North Carolina, USA phosphate mining
The ore is first screened and some of the impurities removed near the mine site. Most PR is used to produce soluble phosphate (P) fertilizers, but some is used for direct application to soil. While PR can be a valuable source of P for plants, it is not always appropriate for direct application. Its suitability depends partly on naturally occurring mineral impurities, such as clay, carbonate, iron, and aluminum (Al). The effectiveness of PR for direct application is estimated in the laboratory by dissolving rock in a solution containing a dilute acid to simulate soil conditions. Sources classified as “highly reactive” are the most suitable for direct soil application.
Florida, USA phosphate mining
Direct use of PR avoids the extra processing associated with converting apatite to a soluble form. The minimal processing may result in a lower-cost nutrient source and make it acceptable for organic crop production systems. 

Agricultural Use When a water-soluble P fertilizer is added to soil, it quickly dissolves and reacts to form low solubility compounds. When PR is added to soil, it slowly dissolves to gradually release nutrients, but the rate of dissolution may be too slow to support healthy plant growth in some soils. To optimize the effectiveness of PR, these factors should be considered:
 •Soil pH: PR requires acid soil conditions to be an effective nutrient source. Use of PR is not usually recommended when the soil pH exceeds 5.5. Adding lime to raise soil pH and decrease Al toxicity may slow PR dissolution. 

• Soil P-fixing capacity: The dissolution of PR increases with a greater P-fixing capacity of soil (such as high clay content). 
• Soil properties: Low calcium and high organic matter in the soil tend to speed PR dissolution. 
• Placement: Broadcasting PR and incorporation with tillage speeds the reaction with the soil. 
• Species: Some plant species can better utilize PR due to their excretion of organic acids from the roots into the surrounding soil. 
• Timing: The time required for the dissolution of PR necessitates its application in advance of the plant demand. 
Global P Resources Map
Management Practices Not all sources of unprocessed PR are suitable for direct application to soil. Additionally, many soils are not suitable for PR use. The total P content of a material is not a good predictor of the potential reactivity in the soil. For example, many igneous PR sources are high in total P, but are of low reactivity and provide minimal plant nutrition because they dissolve so slowly. However, mycorrhizal fungi may aid in the acquisition of P from low-solubility materials in some environments. Over 90% of PR is converted into soluble P fertilizer through reaction with acid. This is similar to the chemical reaction that PR undergoes when it reacts with soil acidity. The agronomic and economic effectiveness of PR can be equivalent to water-soluble P fertilizers in some circumstances, but the specific conditions should be considered when making this choice.

Rob Mikkelsen examines phosphorus fertilizer response of wheat

Monday, March 5, 2018

Good Nutrition: Key to Plant Health

Getting crops off to a good start is critical for achieving high yields. During this early stage of growth, seedlings are especially vulnerable to many environmental and biological stresses. Protecting plants from stress  and disease begins with providing balanced nutrition from planting through harvest. The critical link between plant nutrition and disease resistance has become apparent
as the frontiers of plant health are better understood. A few of these examples are explained here:
How potassium protects plants; Wang, M. et al. 2013.

Potassium plays an essential role in many well-recognized metabolic processes for plants. Potassium’s contribution to sustaining high yielding crops with top quality is well understood. However, the role of potassium in plant stress resistance is less known and appreciated. Potassium is unique among the essential mineral nutrients in its role for plant survival against environmental stress, pests, and diseases.

Supplying adequate potassium to crops through proper fertilization is a simple way to lower the requirement for pest-control treatments that may be costly, time-consuming, and troublesome. The frequently observed benefits of potassium on plant health were reviewed by Wang et al. (2013), which summarizes many recent scientific studies.

Potassium deficiency symptoms on squash (normal on left, severe deficiency on right. Photo by D. Pitchay
When there is a lack of sufficient potassium in plants, low molecular weight compounds begin to accumulate. This build-up of soluble nitrogen-containing compounds (such as amino acids and asparagine) and sugars (such as sucrose) makes a particularly favorable environment for numerous pathogens and insects. For example, aphids are severely nitrogen limited, making potassium-stressed plants an attractive host as an abundant nitrogen source. The presence of sufficient potassium also promotes the production of defensive compounds (such as phenols) which are an important component in plant pest resistance.

An adequate potassium concentration within the plant decreases the internal competition with various pests and pathogens for resources. This results in more resources available for hardening cell walls and tissues to better resist penetration of pathogens and insect pests, and to repair any damaged tissue. Air-borne pathogens are more rapidly shut out from stomatal invasion when adequate potassium is present.
The link between adequate phosphate and plant health is also well known, but perhaps less understood than the association with potassium. Phosphorus is involved in the synthesis of many organic molecules and complex metabolic functions within plants. Crop growth and yields will be significantly reduced when phosphorus is deficient in soil or when plant roots cannot access it.

Pythium root rot on melon
A shortage of phosphorus frequently leads to more disease for many crops. Some of the protective response occurs because healthy plants with sufficient phosphorus have vigorous root growth which allows them to outgrow and escape disease. More specifically, an adequate phosphorus supply has been linked with decreased incidence of Pythium root rot for wheat, leaf blight for rice, numerous tobacco diseases, blight in soybean, and many other diseases. Foliar application of phosphorus-containing sprays is reported to induce protection against powdery mildew.

Severe wheat chloride deficiency (L) and moderate (R)
The important role of chloride as a nutrient is often overlooked, especially in regions where soil salinity is a  concern. However in many areas, the addition of chloride results in increased plant vigor and disease resistance. The occurrence and severity of a number of plant diseases have been documented to be reduced following the application of chloride. This includes take-all, stripe rust, and Septoria in wheat, and stalk rot in corn. Promoting plant health clearly includes a solid foundation in proper nutrition. Strong and vigorous crops are able to produce abundant yields of high quality, while better resisting diseases and pests.
Rob Mikkelsen, IPNI, discusses plant health

Wang, M. et al. 2013. Internat. J. Molec. Sci. 14:7370-7390.
Available at:

Monday, January 8, 2018

Avoid Growing Pains for your Crop... The "Right Time" for 4R Nutrient Management

Preteen children sometimes wake up at night complaining of sore legs and an uncomfortable ache that is slow to go away. These growing pains will be different for every child, ranging from no pain to a lot of discomfort.

While you won’t hear crops complain during their growth spurts, they still need the right supply of nutrients to match their demands for growth and development.

Most grain or fruiting crops have distinct peak periods of nutrient uptake that correspond to their growth pattern. Crops such as grasses and forage have a more consistent pattern of nutrient uptake through the growing season.

One of the fundamental principles of 4R Nutrient Stewardship is making certain that the proper nutrients are in the root zone at the time plants need them. This “Right Time” approach needs to be adjusted for different nutrients and individual crops.
Rob Mikkelsen

In general, it is best to apply nutrients as close to the time of uptake as practical. Nutrients such as P, K, Ca, and Mg are slow to move in the soil and can be applied several months ahead of when the plants will need them. For nutrients that readily move in the soil (such as nitrate and sulfate), it is best to apply them as close to the time for uptake as possible. This reduces the potential for nutrient loss from leaching or denitrification.

When the nutrient supply in the soil cannot match the plant’s demand, crop growth slows and a portion of the final yield is lost each day. Nutrient shortages often cause plant stress and yield loss far earlier than when deficiency symptoms become visible, a deficit known as “hidden hunger.”

It is important to understand when the peak demand periods occur for the specific crops you work with. This allows the “Right Time” component of the 4R’s to support crop yields to their full potential. A few examples of various crops are shown here to highlight some of the differences in total seasonal accumulation or daily nutrient uptake rate.

Maize: Nitrogen and K uptake follow a traditional S-shaped pattern during the growing season, with two-thirds of the total nutrient requirement acquired by the silking stage (VT/R1). However more than half of the total P uptake occurs after the VT/R1 stage, indicating the need for a continual supply of P from the soil throughout the season until crop maturity.
 N and P accumulation in maize (Better Crops, 2013).

Accumulation patterns of soybeans
Better Crops, 2013
 Soybean: For most of the crop nutrients, uptake continues during the entire growing season. The exception is K, where nearly 75% of the total K is taken up before the late vegetative and early reproductive growth stage. The season-long accumulation of most nutrients is a reminder of the importance of maintaining an adequate nutrient supply all the way to maturity.

Daily nutrient accumulation rates of potatoes,
Hermiston, Oregon (Better Crops, 2008)
Potatoes: Approximately two-thirds of the total N is accumulated in the first few months following planting. The rate of plant P uptake generally peaks during the middle of the growing season, with a daily demand of between 0.4 and 0.9 lb P/A/day depending on the variety and location. Potassium uptake reached a peak of 14 lb K/A/day during the midseason of this study from Oregon.

Sugar beets: High yielding sugar beets produce very large amount of biomass. During the growing season, the total nutrient accumulation in Idaho was approximately 240 lb N/A, 60 lb P/A, and 470 lb K/A. In addition to large amount of nutrients taken up, there were distinct peak periods where the daily N accumulation exceeded 4 lb N/A and the daily K accumulation reached 10 lb K/A.
Accumulation patterns of N, P,
and K in sugar beets
(Mriganka et al. 2018)

Link to the pdf version of this article is here:

Sunday, November 12, 2017

Gypsum: A valuable soil amendment and plant nutrient

Gypsum is a common mineral obtained from surface and underground deposits. It can be a valuable source of both calcium (Ca) and sulfur (S) for plants and may provide benefits for soil properties in specific conditions.

Gypsum is found in both crystal and rock forms. It generally results from the evaporation of saline water and is one of the more common minerals in sedimentary conditions. The white or gray-colored rocks are mined from open-pit or underground deposits, then crushed, screened, and used for a variety of purposes without further processing. Agricultural gypsum generally consists of CaSO4·2H2O (dihydrate). Under geological conditions of high temperature and pressure, gypsum is converted to anhydrite (CaSO4 with no water).

By-product gypsum comes from fossil-fuel power stations where S is scrubbed from exhaust gas. Gypsum is also a byproduct from processing phosphate rock into phosphoric acid. Gypsum from recycled wall board is finely ground and used for soil application.

Agricultural Use

Gypsum (sometimes called landplaster) is generally added to soils either as a source of nutrients or to modify and improve soil properties. Gypsum is somewhat soluble in water, but more than 100 times more soluble than limestone in neutral pH soils. When applied to soil, its solubility depends on several factors, including particle size, soil moisture, and soil properties.  Gypsum dissolves in water to release Ca and SO4, with no significant direct impact on soil pH. In contrast, limestone will neutralize acidity in low pH soils. In regions with acid subsoils, gypsum is sometimes used as a relatively soluble source of Ca for alleviation of aluminum toxicity. Some soils benefit from application of gypsum as a source of Ca. In soils with excess sodium (Na), the Ca released from gypsum will tend to bind with greater affinity than Na on soil exchange sites, thus releasing the Na to be leached from the rootzone. Where gypsum is used in the remediation of high Na soils, it generally results in the enhancement of soil physical properties – such as reducing bulk density, increasing permeability and water infiltration, and decreasing soil crusting. In most conditions, adding gypsum by itself will not loosen compacted or heavy clay soils.

 Management Practices
A well-known use of gypsum is to supply Ca for peanuts, which have a unique growth pattern. Gypsum is most commonly spread on the soil surface and mixed in the rootzone. Equipment exists that allows finely ground gypsum to be distributed through an irrigation system. Gypsum is sometimes prilled to make application more convenient for home and turf use.

Non Agricultural Uses
The primary use of gypsum is for building materials (such as plaster and wallboard). For construction purposes, gypsum is ground and heated (calcined) to remove most of the bound water, resulting in hemi-hydrate plaster (plaster of Paris). When water is later added, the powder reverts to gypsum and dries in a rock-hard state. Gypsum is  extensively used in many other applications, such as for water conditioning, in the food and pharmaceutical industries, and as a setting retardant in cement.

The original IPNI document is available HERE, as part of the Nutrient Source Specifics series.

Monday, October 23, 2017

A Check-Up Using Nutrient Deficiency Symptoms

Manganese-deficient soybeans
When the uptake of any of the essential nutrients is inadequate, the plant metabolism becomes disrupted and distinctive symptoms often begin to appear. Since nutrients are involved in specific growth processes, deficiency symptoms provide clues to what nutrient might be lacking. However, most nutrient deficiencies begin to interfere with plant productivity long before the symptoms become visible.

Plant tissue testing is needed to verify that a visual symptom is caused by a specific nutrient deficiency. This differs from soil analysis, which verifies a sufficient reserve of nutrients in the soil, but does not account for conditions that may be interfering with nutrient uptake by roots (such as cold, dry, or compacted soils).

Iron-deficient sorghum
When the cause of deficiency symptoms is known, it still must be determined if a prompt nutrient application will correct the problem. There may be economic constraints or difficulties getting equipment into the field to alleviate the deficiency. Foliar sprays of soluble nutrients are often useful to treat deficiencies as they appear during the growing season. Some nutrients may be added to irrigation water and applied via fertigation to correct plant shortages. However, nutrient deficiencies result in permanent loss of growth and plants may fail to recover from severe deficiencies even after corrective measures.
Foliar sprays can be used to correct deficiency

In general, leaf nutrient deficiency symptoms fall into general categories:
• Chlorosis (yellowing) may appear between the leaf veins or impact the entire leaf
• Necrosis (leaf death) usually begins at the leaf tip or edges, or appears between the leaf veins
• Lack of new plant growth as a result of the growing points dying and failure of new leaves to develop
• Accumulation of plant pigments (especially purple-colored anthocyanin)
• Overall plant stunting with normal or abnormal coloring

Boron-deficient citrus fruit
 A shortage of a nutrient does not immediately result in visible deficiency symptoms. Overall plant growth and metabolism is usually hindered for some time before visual symptoms are present. This so-called “hidden hunger” occurs with low levels of chronic nutrient deficiency, and is far more common than visible deficiency symptoms. By the time obvious visual symptoms first appear, the plant can no longer function properly.

Nutrient deficiency symptoms are most useful for diagnostic purposes (and correction) when they are identified as early as possible. Even when supplemental nutrients are applied to correct deficiencies, irreversible damage to yield or crop quality has likely already occurred.

Phosphorus-deficient guava
Environmental stresses also cause abnormal symptoms to appear on plant leaves that may not be directly related to nutrient deficiency. Additionally, plant disease, insect damage, herbicide impacts, or excessive salinity are examples of non-nutrient factors that cause leaf disorders and stunting.

Nutrient deficiency can cause secondary plant damage that is not readily visible. For example, potassium shortages have been shown to reduce plant resistance to various diseases and insects. Many turfgrass diseases are more common under nitrogen-deficient conditions. Maintaining an adequate supply of phosphorus reduces the severity of diseases such as root rot in wheat and barley, and minimizes various infections of corn and soybean.

E-book published by IPNI
 IPNI is developing guides to help growers identify nutrient deficiencies symptoms of important horticultural and agronomic crops. The first e-book was written by Drs. Pitchay and Mikkelsen (on broccoli) and is available for free for download from iTunes® or for small fee from Amazon®. A collection of outstanding images of deficiency symptoms of important world crops is available for purchase at the IPNI store.