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. 

Friday, October 20, 2017

Need a Micronutrient Review?

We’ve all been taught that plants require essential nutrients, but are you keeping up as our understanding of plant nutrition continues to increase? There has been considerable discussion the past few years about the importance of managing nitrogen, phosphorus, and potassium according to the 4R principles of nutrient stewardship of “Right Source, Right Rate, Right Time, and Right Place”, however other nutrients need your attention too.

The essential role of micronutrients is too often overlooked since the quantity required by plants is quite small. For example, did you know that nickel was added to the list of essential micronutrients? While nickel deficiencies are rather rare, a trace amount is essential for specific enzyme reactions in plants. Did you know that cobalt is essential for nitrogen fixation within the nodules of legume roots? How about knowing that silicon is now recognized as a “beneficial” nutrient for many plants?

The Nutrifact series
has great summaries
of each essential nutrient
The International Plant Nutrition Institute recently completed a series of short fact sheets that describe the role of each of the essential plant nutrients. These brief publications will help you learn the latest information on the role each essential plant nutrient and can be viewed at:

All agronomy is local” is a phrase that summarizes the approach for getting the proper nutrient conditions for each field. Accomplishing the mandate to “keep it local” challenges the skill and knowledge of each farmer and crop adviser, especially as it relates to micronutrient fertilizer decisions. Farmers must continually review yield performance along with the results of soil and tissue analysis as everchanging guides to nutrient planning.

The appearance of plant micronutrient deficiency symptoms raises immediate concerns that something critical was overlooked in the planning stage and that crop yields will likely be reduced. Deficiency symptoms appear in the plant after the internal metabolism has been sufficiently disrupted to show visible problems. Even if no micronutrient deficiency symptoms are observed in the field, many farmers are now conducting their own simple trials to see if a certain micronutrient might be holding back their push for ever-increasing yields.

When a specific micronutrient is lacking, remember that not all fertilizer sources are equivalent in meeting crop needs. Selecting a form of micronutrient that will provide a soluble form of the nutrient requires careful attention. Very little micronutrient is actually needed by plants, but supplying it in an available form is a challenge.
Iron-deficient potato

Adding a small dose of the correct form of micronutrient at planting can be very effective at meeting crop requirements. Getting micronutrients delivered to plant roots can be a challenge if the fertilizer is not uniformly applied across the field. Foliar sprays containing micronutrients can also be useful, but often require repeated application.

Biofortification (increasing the nutrient content in crops) is an often-overlooked benefit from proper fertilization. The content of trace elements in crops reflect the soil properties the plants are grown on. Crop fertilization with appropriate micronutrients offers a simple and cost effective method of improving the nutritional value of food, especially in regions where pernicious malnutrition has had devastating impacts.

It is too easy to overlook the vital role that micronutrients play in successful crop production. Take another look at the principles of 4R Nutrient Stewardship and see if micronutrients are being overlooked as part of your plan.

Tuesday, November 22, 2016

Calcium Nitrate: excellent source of nitrogen and calcium

Calcium nitrate is a highly soluble source of two plant nutrients. Its high solubility makes it popular for supplying an immediately available source of nitrate and calcium directly to soil, through irrigation water, or with foliar applications.

Phosphate rock is acidified with nitric acid to form a mixture of phosphoric acid and calcium nitrate during the nitrophosphate fertilizer manufacturing process. Ammonia is then added to neutralize excess acidity. Calcium nitrate crystals precipitate via a temperature gradient and are separated as the mixture is cooled. With the ammonia addition and crystallization, a double salt is formed [5 Ca(NO3)2•NH4NO3•10 H2O, referred to as 5:1:10 double salt] and is considered the commercial grade of calcium nitrate. Hence, small amounts of ammonical N may also be present in this grade of calcium nitrate. 
Calcium nitrate is also manufactured by reacting nitric acid with crushed limestone forming either the 5:1:10 double salt or calcium nitrate tetrahydrate (Ca(NO3)2•4 H2O). The latter product is often produced as a wet crystal or a mesh and is subject to specific regulation with respect to handling and safety. Prilling and granulating are the most common methods of making particles ready for field use.
Calcium nitrate is very hygroscopic (absorbs water from the air), so when intended for soil application, proprietary coatings are applied to minimize moisture uptake. Calcium nitrate intended for hydroponics or fertigation does not contain a conditioner, or it may be sold as a clear fluid fertilizer ready for use.

Agricultural Uses:

Calcium nitrate is popular in agronomic situations where a readily soluble source of nitrate or calcium is needed. Nitrate moves freely with soil moisture and can be immediately taken up by plant roots. Unlike many other common N fertilizers, Ca(NO3)2 application does not acidify soils since there is no acidity producing nitrification of ammonium occurring. Broadcast applications of Ca(NO3)2 are desirable in some circumstances because the risk of ammonia volatilization is eliminated with its use. In addition, some crops prefer nitrate sources of N.

Applications of Ca(NO3)2 are also used to provide supplemental Ca for plant nutrition. Some soils may contain considerable amounts of Ca, but it may not be sufficiently soluble to meet plant demands. Since Ca is not mobile in the plant it is important to apply Ca just-in-time in critical growth stages. Solutions of Ca(NO3)2 are commonly added to irrigation water and to foliar and fruit sprays to overcome such shortcomings that can affect yield and/or quality (such as apple bitter pit), or to meet peak Ca demands during critical growth periods. Part of the popularity of Ca(NO3)2 also arises from its chloride-free nature and Ca(NO3)2 can have an ameliorating effect under saline growing conditions, combating the negative effects of Na and Cl-.

Research has shown that a healthy plant with adequate Ca alleviates biotic and abiotic stresses such as fungal disease, and stresses due to drought, heat, or cold. Hence Ca(NO3)2 is widely used in intensive cropping systems that have a high focus on crop quality.
Calcium-deficient broccoli

Management Practices

There are no special practices required for the use of Ca(NO3)2 beyond the need to keep nitrate from moving below the root zone.

To avoid precipitating insoluble fertilizer salts, Ca(NO3)2 should not be mixed with soluble phosphate or sulfate fertilizers in nutrient solutions or while fertigating. The extreme hygroscopic nature of solid Ca(NO3)2 makes it important to store it in a cool and dry environment.
Calcium nitrate (double salt) is not classified as an oxidizer by government agencies, so there are no special restrictions on transport and handling as there may be for ammonium nitrate. However calcium nitrate tetrahydrate is classified as a 5.1 oxidizing agent that can, in conjunction with oxygen, cause or increase the combustion of other materials and may require special attention depending on local regulations.

Non-Agricultural Uses:

Calcium nitrate is used for waste water treatment to minimize the production of hydrogen sulfide. It is also added to concrete to accelerate setting and reduce corrosion of concrete reinforcements.

This acticle is from a IPNI Nutrient Source Specific article on fertilizer materials available here: