Thiosulfate, an excellent fluid sulfur source

Thiosulfate (S2O3^2-) fertilizers are clear liquids that provide a source of sulfur (S) and can be used in a variety of situa­tions. They also contain other nutrients including nitrogen (N) as ammonium (ATS), potassium (KTS), calcium (CaTS), or magnesium (MgTS).

 

Thiosulfate fertilizer

 Production

ATS is the most commonly used S-containing fluid fertilizer. It is made by reaction of sulfur dioxide, elemental S, and aque­ous ammonia. Other common fluid thiosulfate fertilizers are similarly produced.

Thiosulfates are highly soluble in water and are compatible with many other fluid fertilizers. ATS is commonly mixed with urea ammonium nitrate (UAN) to produce a widely used fertilizer with the analysis 28-0-0-5 (5% S).

Chemical Properties

Formula Common             Nutrient                Density,               pH

name                              content                     kg/L                                        

(NH4)2S2O3 ATS         12% N; 26% S           1.34                  7 to 8.5

K2S2O3        KTS         25% K2O; 17% S       1.46                7.5 to 8

CaS2O3       CaTS          6% Ca; 10% S          1.25                6.5 to 8

MgS2O3      MgTS          4% Mg; 10% S          1.23               6.5 to 7.5

Agricultural Use

After application to soil, most of the thiosulfate quickly reacts to form tetrathionate, which is subsequently converted to sulfate. Thiosulfate is not generally available for plant uptake until it is converted to sulfate. In warm soils, this process is largely complete within one to two weeks.

Molecular structure of
thiosulfate and tetrathionate

Thiosulfate is a chemical “reducing agent” and it also produces acidity after oxidation of the S. Due to these properties, thio­sulfate molecules have unique effects on soil chemistry and biology. For example, a band application of ATS has been shown to improve the solubility of some micronutrients. Local guidelines should be followed for maximum rates for placement in the seed row.

Thiosulfate can slow the rate of urea hydrolysis…the conversion of urea to ammonium (NH4+)…and reduce loss of ammonia (NH3) as a gas when ATS is mixed with UAN. This inhibiting effect is likely due to the formation and presence of the intermedi­ate tetrathionate, rather than the thiosulfate itself. Nitrification...the conversion of NH4+ to nitrate...is also slowed in the presence of ATS. Although the initial pH of thiosulfate fertilizers is near neutral, thiosulfate oxidizes to form sulfuric acid and the NH4+ in ATS will form nitric acid, thus resulting in slight soil acidification in the application zone.

Sulfur-deficient corn

Thiosulfates may be applied through surface and overhead irrigation systems, sprinklers, and drip irrigation. Many of them are used in foliar sprays to provide a rapid source of plant nutrition (not recommended with ATS).

Management Practices

Sulfur deficiencies are noted in crops throughout the world. Thiosulfates are valuable fertilizer materials because they are easy to handle and apply, require minimal safety precautions, and are compatible with many other common fertilizers. However, these fertilizers should not be mixed with highly acidic solutions since this will cause the decomposition of the thiosulfate molecule and subsequent release of harmful sulfur dioxide gas.

Non-Agricultural Use

Thiosulfate materials are used in a variety of industrial applications. In photographic processing, they are used to bind silver atoms present in film or paper. Sodium thiosulfate is used in water treatment systems to remove chlorine. It is also used for gold extraction, since it forms a strong complex with this metal in a non-toxic process.

Diammonium Phosphate (DAP)

Diammonium phosphate
(DAP)

Diammonium phosphate (DAP) is the world’s most widely used phosphorus (P) fertilizer. It is made from two common constituents in the fertilizer industry and it is popular because of its relatively high nutrient content and its excellent physical properties.

Production

Ammonium phosphate fertilizers first became available in the 1960s and DAP rapidly became the most popular in this class of products. It is formulated in a controlled reaction of phosphoric acid with ammonia, where the hot slurry is then cooled, granulated, and sieved. DAP has excellent handling and storage properties. The standard grade of DAP is 18-46-0 and fertilizer products with a lower nutrient content may not be labeled as DAP.

The inputs required to produce one ton of DAP fertilizer are approximately 1.5 to 2 tons of phosphate rock, 0.4 tons of sulfur (S), to dissolve the rock, and 0.2 tons of ammonia. Changes in the supply or price of any of these inputs will impact DAP prices and availability. The high nutrient content of DAP is helpful in reducing handling, freight, and application costs. DAP is produced in many locations in the world and is a widely traded fertilizer commodity.

 Chemical Properties

Chemical formula:           (NH4)2HPO4

Composition:                   18% N  and 46% P2O5 (20% P)

Water solubility (20 ºC):  588 g/L

Solution pH:                    7.5 to 8

Agricultural Use

DAP fertilizer is an excellent source of P and nitrogen (N) for plant nutrition. It is highly soluble and thus dissolves quickly in soil to release plant-available phosphate and ammonium. A notable property of DAP is the alkaline pH that develops around the dissolving granule.

As ammonium is released from dissolving DAP granules, volatile ammonia can be harmful to seedlings and plant roots in immediate proximity. This potential damage is more common when the soil pH is greater than 7, a condition that commonly exists around the dissolving DAP granule. To prevent the possibility of seedling damage, care should be taken to avoid placing high concentrations of DAP near germinating seeds.

The ammonium present in DAP is an excellent N source and will be gradually converted to nitrate by soil bacteria, resulting in a subsequent drop in pH. Therefore, the rise in soil pH surrounding DAP granules is a temporary effect. This initial rise in soil pH neighboring DAP can influence the micro-site reactions of phosphate and soil organic matter.

DAP is made of
two molecules of ammonia
reacted with one molecule
of phosphate

Management Practices

There are differences in the initial chemical reaction between various commercial P fertilizers in soil, but these dissimilarities become minor over time (within weeks or months) and are minimal as far as plant nutrition is concerned. Most field compari­sons between DAP and monoammonium phosphate (MAP) show only minor or no differences in plant growth and yield due to P source with proper management.

Non Agricultural Uses

DAP is used in many applications as a fire retardant. For example, a mixture of DAP and other ingredients can be spread in advance of the fire to prevent a forest from burning. It then becomes a nutrient source after the danger of fire has passed. DAP is used in various industrial processes, such as metal finishing. It is commonly added to wine to sustain yeast fermenta­tion and to cheese to support cheese cultures.

A pdf version of this is available from the IPNI website here:

Compound Fertilizer: Mixing several nutrients in each granule

Compound fertilizers contain
several nutrients in each granule

Many soils require the addition of several essential nutrients to alleviate plant deficiencies. Farmers may have the option of selecting a combination of single-nutrient fertilizers or using a fertilizer that has several nutrients combined into each particle. These combination (compound or complex) fertilizers can offer advantages of convenience in the field, economic savings, and ease in meeting crop nutritional needs.

Production

Compound fertilizers are made using basic fertilizer materials, such as NH3, ammonium phosphate, urea, S, and K salts. There are many methods used for making these fertilizers, with the specific manufacturing processes determined by the available basic compo­nents and the desired nutrient content of the finished product. Here are four brief examples.

Compaction methods (agglomeration) involve binding small fertilizer particles together using compaction, a cementing agent, or a chemical bond. Various nutrient ratios can be combined using undersized particles that may not be suitable for other applications.

Accretion-based fertilizers are made by repeatedly adding a thin film of nutrient slurry which is continually dried, building up mul­tiple layers until the desired granule size is reached.

Pipe-cross reactors are used to chemically melt NH3, acids containing S or P, and other nutrients—such as K sources and micronutrients—into a solid fertilizer with the desired nutri­ent content.

The nitrophosphate process involves reacting phosphate rock with nitric acid to form a mixture of compounds containing N and P. If a K source is added during the process, a solid fertilizer with N, P, and K will result.

Three different techniques for making compound fertilizers
(l) Compaction, (c) accretion, (r) pipe-cross reactor

Agricultural Use

Compound fertilizer contains multiple nutrients in each individual granule. This differs from a blend of fertilizers mixed together to achieve a desired average nutrient composition. This difference allows compound fertilizer to be spread so that each granule delivers a mixture of nutrients as it dissolves in the soil and eliminates the potential for segregation of nutrient sources during transport or application. A uniform distribution of micronutrients throughout the rootzone can be achieved when included in compound fertilizers.

These fertilizers are especially effective for applying an initial nutrient dose in advance of planting. There are certain ratios of nutrients available from a fertilizer dealer for specific soil and crop conditions. This approach offers advantages of simplicity in making complex fertilizer decisions, but does not allow the flexibility to blend fertilizers to meet specific crop requirements. Turf managers and homeowners often find compound fertilizers desirable.

Management Practices

Compound fertilizers are sometimes more expensive than a physical combination or blend of the primary nutrient sources since they require additional processing. However, when a consideration is made of all the factors involved with nutrient handling and use, compound fertilizers may offer considerable advantages.

Nitrogen is the nutrient that most commonly needs to be carefully managed and reapplied during the growing season. It may not be feasible to supply sufficient N in advance of planting to meet the entire demand (using only compound fertilizer) without overapplying some of the other nutrients. It may be advisable to use a compound fertilizer early in the growing season and then later apply only N fertilizer as needed.

Compound fertilizers are usually produced regionally to meet local crop needs. There is a wide range of chemical and physical properties that can be adjusted to meet these needs. For example, a desire to minimize P in urban stormwater runoff has led some communities to restrict the addition of P to compound fertilizers sold for turf and ornamen­tal purposes. Soils of a region that are typically low in a specific nutrient may have this element boosted in the compound fertilizer. 

A pdf version of this post can be found at the IPNI website here: 

Ten Laws of Sustainable Soil Management (Lal)

Dr. Rattan Lal developed 10 laws for sustainably meeting the demands of the growing world population.  They are worthy of some discussion and thought by soil scientists.

(1) Soil degradation and poverty: The biophysical process

of soil degradation is driven by economic, social, and political

forces.

(2) Stewardship and desperateness: The stewardship concept

is relevant only when the basic necessities are met. Desperate

people do not care about the stewardship.

(3) The soil bank: The nutrient and C pools in soil bank can

only be maintained if all outputs are balanced by the inputs.

(4) The law of marginality: Marginal soils cultivated with

marginal inputs produce marginal yields and support marginal

living.

(5) The organic dilemma: Plants cannot differentiate the

nutrients supplied through organic or inorganic sources. It is

a question of logistics and availability.

(6) Soil as a source or sink of greenhouse gases: Agricultural

soils can be a major sink for CO2 and CH4, depending

on land use and management.

(7) Extractive farming and the environment: Extractive

farming and mining soil fertility adversely impact soil quality,

perpetuate hunger and poverty, exacerbate CO2 emissions,

and reduce ecosystem services.

(8) Synergism between soil management and improved

germplasm: The yield potential of improved germplasm can

be realized only if grown under optimal soils and agronomic

conditions.

(9) Agriculture as a solution to environmental issues:

Rather than a problem, agriculture must always be integral to

any solution towards environmental development. Humans

will always depend on agriculture, and it must be the engine

of economic development.

(10) Modern innovations: Yesterday’s technology cannot

resolve today’s problems.

The entire article can be downloaded here:

Lal. 2010. Managing sols for a warming earth in a food-insecure and energy-starved world. J. Plant Nutr. Soil Sci. 173:4-15.