Saturday, August 23, 2014

A Closer Look at Phosphorus Uptake by Plants

ALMOST ALL PLANT PROCESSES require phosphorus (P) to operate. Phosphorus is essential for life sustaining reactions including energy transfer, activation of proteins, and regulation of metabolic processes. While the primary source of P for plants is inorganic phosphate, there are very few soils that naturally contain a sufficient P supply to allow unrestricted plant growth.

Compared with the other major nutrients, P is the least mobile and available to plants in most soil conditions and therefore it is commonly a major limiting factor for plant growth. Even in well-fertilized soils, the P concentration rarely exceeds a few hundred parts per billion (ppb) and is commonly less than 50 ppb in the soil solution.

Up to half of the soil P is commonly found in organic forms, derived from materials such as plant residues and soil organisms. However, organic P must be mineralized to inorganic phosphate (the form found in most fertilizers) before it can be taken up and used by plants for growth.

Because of its strong reactions with soil components, P is principally supplied to plant roots by diffusion rather than mass flow. Phosphorus uptake occurs primarily at the young root tip, into the epidermal cells with root hairs, and into cells in the outer layer of the root cortex.
The P depletion zone in soil

The young root tips, continually expanding into fresh soil, are exposed to P concentrations found in the bulk soil solution (Figure 1). While P is rapidly taken up along the root surface, a P depletion zone of 0.2 to 1.0 mm (about the thickness of a dime) develops surrounding the root. Root hairs help expand the surface area available for P absorption. Mycorrhizal fungi, growing in association with root cells and extending up to several centimeters into the soil, can also transfer P to the root. Various crops have been shown to have different abilities to extract P from the soil and to make these beneficial associations with mycorrhizal fungi that extend their effective root system.

Rhizosphere is the soil zone (within 2 mm of the root) that is influenced by heightened biological activity, nutrient uptake, and chemical changes that result from root activity.

Getting P into the Plant: Apoplastic and Symplastic Transport
 The apoplasm comprises the root walls, the cortical cells and the open spaces between these tissues (Figure 2 ). This “dead space” consists of interlaced fibers that form an open latticework in roots that serves to filter the soil solution. The soil solution moves through these spaces and pores until it reaches the tough “Casparian strip” that surrounds the core (stele) of the root. A net negative charge associated with the cell wall fibers repels anions...such as phosphate and nitrate (NO3 - ) in solution and confines their transport to larger pores within the apoplasm. Movement of nutrients through the apoplasm into the root is greatest near the root tip. Mucilage, a complex mixture of organic materials excreted around the root, also carries negatively charged hydroxyl groups which can further repel the movement of anions towards the root.
Root cross section: pathways of uptake

The symplast , by contrast, is a living system within the plant that connects each growing organ. It consists of thin walled cells called parenchyma, which have small openings acting as “tunnels” where the content of one cell connects with the contents (protoplasm) of the adjacent cell. The symplast is very fragile and requires the rigid framework of the apoplast to hold it in place.

Although the soil solution P concentration may be less than 50 ppb, the concentration in plant cells is much higher…50 to 500 parts per million (ppm). For plants to boost their P concentration by over a thousand times, active transport (requiring energy) across the root membranes (the plasmalemma) is required.

The movement of P from the apoplast across root cell membranes is the critical step in the transport of nutrients into the plant, requiring an energy-driven transport mechanism to move P through the membranes into the plant root cells (phosphate transporters).  Researchers are currently working on ways to increase the uptake of P by stimulating these nutrient transport proteins found in the root.

Other changes take place in the rhizosphere to improve P uptake and recovery. Many roots exude organic acids  (primarily citrate and oxalate) into the rhizosphere, which enhance P availability. While this response has been frequently measured, the major effect may be due to the organic compounds displacing P held by the soil (ligand exchange), rather than a direct effect of the organic acids on the rhizosphere pH. Some microorganisms living in the rhizosphere have been shown to help solubilize soil P.
These mechanisms can help improve P recovery, but they all require that P be present in the soil to begin with. No amount of excreted organic acid, root-zone microorganisms, or mycorrhizal fungi can allow a plant to recover P from a soil that does not contain P to begin with.
Nitrate uptake increases pH

Plant roots can also have a considerable effect on rhizosphere pH. This pH change comes from the release of H+ or OH-  (as HCO3 - ) as plants balance their uptake of excess cations or anions. Nitrogen nutrition plays the most important part in this ion balance since it is the mineral taken up in the largest quantity by most plants and it can occur as either an anion (NO3 - ) or a cation (NH4 + ). Research suggests that greater P uptake that occurs as plants are fertilized with a source of NH4+ may be the result of rhizosphere acidification that occurs with this N source. However, depending on the buffering capacity of the soil, only little change in pH may be observed in some soils. 
Ammonium uptake decreases soil pH

There are many complex reactions occurring under our feet as roots invisibly work to meet the nutritional needs of the growing plants. Research is underway in many areas to improve the efficiency of P fertilizer use. Advances in improving root function may someday lead to better ways of meeting the nutrient demands of crops. For now, the focus remains on maintaining adequate soil fertility by using regular plant and soil testing, proper fertilization techniques, and appropriate management to keep the nutrients where they are needed for the growing plant.

This article originally appeared as a newsletter of the International Plant Nutrition Institute.  The pdf is available here.



Tuesday, July 1, 2014

Kieserite... naturally occurring magnesium sulfate

Kieserite is a naturally occurring mineral that is chemically known as magnesium sulfate monohydrate (MgSO4·H2O). It is mined from geologic marine deposits and provides a soluble source of both Mg and S for plant nutrition.
Magnesium deficient grapes

Production

Kieserite is primarily obtained from deep underground deposits of minerals in Germany. It is present in the remnants of ancient oceans that were evaporated and are now buried beneath the earth’s surface.
Mining in Germany
These mineral resources contain a variety of valuable plant nutrients. The ore is brought to the surface where the magnesium salts are separated from potassium and sodium salts using a unique, dry electrostatic (ESTA) process.

The fine crystalline kieserite is sold for direct application to soil, or it is granulated to a larger particle size that is better suited for mechanical fertilizer spreading or for bulk blending with other fertilizers.

Chemical Properties

Chemical formula:   MgSO4·H2O
Mg content:             16% (kieserite fine); 15% (kieserite granular)
S content:                22% (kieserite fine); 20% (kieserite granular)
Solubility:                 417 g/L (20°C)
Solution pH:             9
Granular kieserite

Agricultural Use

Kieserite provides a highly concentrated form of two essential plant nutrients—Mg and S. Since kieserite applications have no major effect on soil pH, it can be supplied to all kinds of soil, irrespective of soil pH. It is commonly used prior to or during the growing season to meet the nutrient require­ment of crops. Due to its high solubility it can be used to supply both Mg and S during peak periods of crop demand. Since kieserite is an earth mineral mined from naturally occurring deposits, it is permitted as an organic nutrient source by some organic certi­fying agencies.

Kieserite itself is not used as foliar fertilizer or in fertigation systems, but it serves as raw material for the production of Epsom salt (MgSO4·7 H2O), which is totally soluble and suitable for both fertigation and foliar application.
Fine kieserite particles

 Management Practices

Many soils are low in Mg and require supplemental nutrients to support crop yield and quality. Sandy-textured soils and soils with a low pH (such as highly weathered tropical soils) are frequently characterized by a low Mg supply for plants. Under these conditions, it is a prerequisite to raise the Mg content in the soil by adequate fertilization.

Splitting Mg applications into two or more doses is recommended in areas with high precipitation in order to avoid leaching loss­es. Soils in temperate climates with higher clay content may have higher Mg contents and are often less prone to leaching losses.

Fertilizer Mg application rates vary depending on factors such as the specific crop requirement, the quantity removed during harvest, and the ability of soil minerals to release adequate Mg in a timely manner to support crop yield and quality. Kieserite application rates are typically in the range of 200 to 300 kg/ha for many crops. Additional Mg and S demands during peak growth periods demand can be met by foliar application of materials such as Epsom salt or a variety of soluble nutrient sources.
Magnesium deficient soybeans


This post originally appeared as one in a series on specific nutrient sources on the IPNI website.

Thursday, June 12, 2014

Nutrient Deficiency Symptoms... Don't wait until you see them


 Plant nutrient deficiency symptoms begin to appear when one of the essential nutrients is lacking.
Iron-deficient cowpeas
 
Some­times deficiencies appear early in the growing season when soils are cold or wet, and when root activity is low. Deficien­cies are also commonly observed later in the season when the soil cannot satisfy the high nutrient demand of a rapidly growing crop. Whether the deficiency is caused by poor root uptake or low nutrient-supplying power of the soil, proper management practices can help alleviate these problems.

Deficient plants do not initially show any obvious symptoms of nutrient shortage other than slower growth, which can also be due to many factors. In the case of a mild deficiency, plants may never show a visual symptom ex­cept slow growth and reduced yield.

Nutrient deficiency causes a disruption in any number of essential metabolic processes within the plant. Crops mature unevenly because deficiencies rarely occur uniformly across entire fields. This leads to lower yield, harvest­ing difficulties and poorer crop quality. And as previously stated, this can all occur without diagnostic symptoms appearing.
Potassium-deficient cucumber

When deficiency symptoms become noticeable, severe stress is already occurring and steps should be considered to overcome the problem, if it is practical and economical to do. The effects of other stresses such as drought and pests can complicate diagnoses. Another problem is that not all deficiencies produce clear-cut symptoms. Then there is the possibility of multiple deficiencies. The most severe deficiency may be manifested first. Knowing which nutrients are mobile or immobile within the plant is helpful in pinpointing the cause of the deficiency symptom. Diagnosing symptoms also requires understanding of specific crop colors and markers. It is worth noting that some crops are more susceptible to visible symptoms than others.

Plant analysis (tissue testing) is useful for diagnosing specific nutrient deficiencies as they arise. It is best when nutrient concentrations in deficient plants growing in problem areas are compared with healthy plants to identify the differences. It is also helpful to collect soil samples for analysis from the two areas at the time the plant samples are col­lected.
 
Zinc-deficient potato
Tissue testing also is valuable for monitoring plant health during the season to verify that nutrient concentrations do not drop below nor exceed established critical values. Guidelines have been developed for many crops for what the ap­propriate nutrient concentrations should be during various growth stages. Supplemental fertilization should be considered if the concentrations fall below these established thresholds.

Pre-season soil testing should also be part of a strategy for preventing nutrient shortages. In addition to helping avoid plant stress, soil analysis will allow decisions to be made that will avoid over or under application of fertilizer and resulting economic inefficiency.

The International Plant Nutrition Institute (IPNI) has a large database of nutrient deficiency images that is continually growing. Visit the website at: http://media.ipni.net. Additionally, a collection of over 500 of our best plant nutrient deficiency photos is available for purchase at http://ipni.info/nutrientimagecollection. A condensed version of this collection is available as an app for iPhones and iPads at http://www.ipni.net/article/IPNI-3273. When nutrient deficiency symptoms appear, first act quickly to diagnose the problem and then make plans to correct it and to avoid having them reoccur in the future.  
 
Potassium-deficient cotton
This blog posting originally appeared as part of the Plant Nutrition Institute quarterly newsletters "Plant Nutrition Today".  The entire series can be viewed here.



Tuesday, June 3, 2014

Using potash fertilizer to kill zebra mussels in Lake Winnipeg


An interesting project is currently underway in Lake Winnipeg. 
Lake Winnipeg in Manitoba, Canada
The Manitoba government is trying to stop the growth of the aggressive zebra mussel.  Zebra mussels, native to Eastern Europe and Western Asia, are extremely invasive and latch onto boats, buoys, rocks, and other structures.  

Zebra mussels are very difficult to control and previous attempts to halt their spread in the U.S. and Canada have largely been ineffective.  However mussels are sensitive to high potassium concentrations and it is hoped that this may be the key to slowing down their invasion.
 
Zebra mussels (Wikipedia)
A concentrated potassium chloride solution was recently added to the Winnipeg Beach Harbor and all of the zebra mussels were killed within ten days.  Potassium chloride does not have a negative impact on fish, other mussels, humans, or water quality as it gradually dissipates into the lake.
Zebra mussels (Wikipedia)
 
You will recall that Canada has the largest geologic reserves of potassium chloride in the world.  These mines harvest naturally occurring potassium minerals from the ground, wash away any impurities, and then sell valuable potassium fertilizer (potash) to agricultural regions around the world where soil reserves of potassium are too low to support healthy plant growth.  
Potash mine in Saskatchewan, Canada








You can read more about this in these news outlets:

www.cbc.ca/news/canada/manitoba/manitoba-hopes-potash-will-kill-zebra-mussels-in-lake-winnipeg-harbours-1.2627645 




Wednesday, May 14, 2014

So Many Choices - Selecting the Right Nutrient Source



The 4R principles of nutrient stewardship involve selecting the “Right Source” of nutrients to meet plant demands. This fundamental decision of nutrient source influences the process of choosing the Right Place, Right Time, and Right Rate for each field.

4R: Right Source, Right Rate, Right Time & Right Place

 A misconception persists that using manufactured fertilizers means opposing the use of organic nutrient sources. Most agronomists agree that selecting the right source of nutrients begins with first considering the supply of on-farm nutrients and then supplementing them with commercial fertilizers.

Integrated Plant Nutrient Management is the term used by agronomists to describe the appropriate use of both fertilizer and organic sources of nutrients. Every farming operation will differ in its access to various nutrient sources and there is a range in specific crop requirements, but all farmers have the goal of maximum crop output and harvest quality from the right nutrient application.

Organic nutrient sources can include soil organic matter, a small portion of which decomposes and releases nutrients each year. Crop residues vary greatly in nutrient content, but can be a contributing nutrient source in many situations. Animal manures are commonly used as a valuable source of plant nutrients. Manures and composts can have a wide range in nutrient
Compost can be a useful nutrient source
composition, so it is useful to have them chemically analyzed to assess their fertilizer-replacement value. Cover crops can also be a useful nutrient source. Legume cover crops have the benefit of providing extra nutrients by hosting N-fixing bacteria. Grass cover crops can capture and retain nutrients that might have otherwise leached past the root zone, then release their nutrients again as they decompose.


Many excellent commercial fertilizers can be used to deliver nutrients that are lacking for successful crop production. Commercial fertilizers are most commonly used as bulk blends of popular granular fertilizers; compound fertilizers, which are a mixture of multiple nutrients within a single fertilizer particle;  fluid fertilizers, homogeneous clear liquids which can be blended with materials such as micronutrients, herbicides, and pesticides, or diluted for foliar application; and suspension fertilizers which use a suspending clay or gelling agent to keep small fertilizer particles from settling out of the liquid.
Bulk blends

Fluid fertilizers














Additional considerations in selecting the Right Nutrient Source might include:

   The soil chemical and physical properties (such as avoiding nitrate application in flooded soil, or surface application of urea on high pH soils).

Preparing soil for rice
   Availability of fertilizer application equipment to get the nutrients delivered properly.

   Blends of multiple fertilizer materials must account for their chemical properties and compatibilities.

   Recognize sensitivities and secondary benefits of specific fertilizer materials (such as chloride additions that may be beneficial for small grains, but possibly detrimental for the yield and quality of other crops in excessive concentrations).


Selecting the Right Source of nutrients is too often overlooked due to tradition and the ease of doing the same thing every year. Remember that crop production is very complex and that successful farmers need to be both
artists and scientists with an understanding of all the 4R’s to meet their goals.

This post originally appeared as part of a series of quarterly newsletters (Plant Nutrition Today) published by the International Plant Nutrition Institute.


Wednesday, May 7, 2014

Potassium Sulfate - A potash fertilizer with many benefits




Soluble potassium sulfate
Potassium fertilizer is commonly added to improve the yield and quality of plants growing in soils that are lacking an adequate supply of this essential nutrient. Most fertilizer K comes from ancient salt deposits located throughout the world. The word “potash” is a general term that most frequently refers to potassium chloride (KCl), but it also applies to all other K-containing fertilizers, such as potassium sulfate (K2SO4, commonly referred to as sulfate of potash or SOP).
Chemical formula; K2SO4

 Production
Potassium sulfate, Great Salt Lake, Utah


Potassium is a relatively abundant element in the Earth’s crust and production of potash fertilizer occurs in every inhabited continent. However, K2SO4 is rarely found in a pure form in nature. Instead it is naturally mixed with salts containing Mg, Na, and Cl. These minerals require additional processing to separate their components. Historically, K2SO4 was made by react­ing KCl with sulfuric acid. However, it was later discovered that a number of earth minerals could be manipulated to produce K2SO4 and this is now the most common method of production. For example, natural K-containing minerals (such as kainite and schoenite) are mined and carefully rinsed with water and salt solutions to remove byproducts and produce  K2SO4.  A simi­lar process is used to harvest  K2SO4 from the Great Salt Lake in Utah, and from underground mineral deposits.

In New Mexico (USA), K2SO4 is separated from langbeinite minerals by reacting it with a solution of KCl, which removes the byproducts (such as Mg) and leaves   K2SO4.   Similar processing techniques are used in many parts of the world, depending on the raw materials accessible.
Potassium sulfate


Chemical Properties
Chemical Formula:  K2SO4
K content: 40 to 44% (48 to 53% K2O)
S content: 17 to 18%
Solubility (25 ºC) 120 g/L
Solution pH approx. 7

 Agricultural Use
Concentrations of K in soil are often too low to support healthy plant growth. Potassium is needed to complete many es­sential functions in plants, such as activating enzyme reactions, synthesizing proteins, forming starch and sugars, and regulating water flow in cells and leaves.
Potassium-deficient cotton

Potassium sulfate is an excellent source of nutrition for plants. The K portion of the  K2SO4 is no different than other com­mon potash fertilizers. However, it also supplies a valuable source of S, which is sometimes deficient for plant growth. Sulfur is required for protein synthesis and enzyme function. There are certain soils and crops where the addition of Cl- should be avoided. In these cases, K2SO4 makes a very suitable K source. Potassium sulfate is only one-third as soluble as KCl, so it is not as commonly dissolved for addition through irrigation water unless there is a need for additional S.
Potassium-deficient almonds

Several particle sizes are commonly available. Fine particles (<0.015 mm) are used for making solutions for irrigation or foliar sprays since it is more rapid to dissolve. Foliar sprays of  K2SO4 are a convenient way to apply additional K and S to plants, supplementing the nutrients taken up from the soil. Leaf damage can occur if the concentration is too high.

Management Practices
K2SO4 is frequently used for crops where additional Cl- from more common KCl fertilizer is undesirable. The partial salt index of  K2SO4 is lower than some other common K fertilizers, so less total salinity is added per unit of K. The salt measurement (EC) from a  K2SO solution is less than a third of a similar concentration of a KCl solution (10 mmol/L). Where high rates of K2SO4 are needed, it is generally recommended to divide the application into multiple doses. This helps avoid surplus K accumulation by the plant and also minimizes any potential salt damage.


Potassium-deficient soybeans

A pdf version of this information is available
at the IPNI website here