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.

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  K₂SO_4.  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   K₂SO_4.   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  K₂SO₄  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