Springs
12-25-2011, 09:34 PM
From: www.back-to-basics.net/agrifacts/pdf/b2b29a.pdf
Sulfate -vs- Elemental Sulfur Part I: There Is A Difference.
• Sulfate-Sulfur is the only form of S the plant can utilize.
• Elemental S is dependent upon time, temperature and moisture to be available to the plant.
• Sulfate-Sulfur will not acidify the soil.
For various reasons, sulfur (S) deficiencies are increasing in many areas of the country. The use ofthis nutrient in fertility programs is there fore becoming more routine. The most common chemical forms of Sused in fertilizers are sulfate S (SO4) and elemental S(Sº). But these two forms of S react quite differently in soils. It’s very important to understand the differences between SO4 and Sº in order to use these two forms inthe most effective manner possible.
Sulfate Sulfur
Although S exists in many different chemical forms innature, plants can only absorb S through their root systems in the SO4 form. Small amounts of sulfur dioxide gas can be absorbed through leaves, but this isof little consequence in the overall S nutrition of plants.This obviously means that all soil S must be converted to SO4 in order to be utilized by plants.
The SO4 anion carries two negative charges, so it is not absorbed by soil colloids to any extent. As soil acidity increases, some positive charge sites developon clay particles and organic matter, which allows for a limited amount of SO4 absorption. But for the mostpart, SO4 moves freely with soil moisture, especially inthe upper levels of the soil profile. It thus reacts muchlike nitrate nitrogen in soils. As a result, SO4 levels frequently increase with increasing depth in the soil profile, at least in humid climates. Table A shows the increase of SO4 with depth in several Southeastern U.S. soils.
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Table A:
www.back-to-basics.net/agrifacts/pdf/b2b29a.pdf
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For this reason, small amounts of SO4 applied in a starter fertilizer is sometimes all that’s needed to get young roots off to a fast start, quickly growing down through the profile to greater depths where supplies of SO4 are more plentiful. Work in Arkansas has shown that wheat yields were increased from 15 to 44 bushels per acre by the application of just 5 lb/A of S in the potassium sulfate form. However, a more typical recommendation for sulfur would be in the range of 25-30 lb/A of S
Elemental Sulfur Elemental S is totally unavailable to plants. Plants simply cannot absorb Sº through the root system.Elemental S is inert and water insoluble. Commercially,it is stored in the open, and it can remain in place foryears, unaffected by moisture or temperature. However, when Sº is added to soil, it’s an entirely different matter. In the soil, Sº is converted (oxidized)to the plant-available SO4 form and the rate at which this conversion takes place is the determining factor regarding the effectiveness of Sº as a fertilizer sourceof S. (See Part II.)
The following are the most important considerations regarding the use of these two chemical forms of S infertility programs.
• When relying on Sº for the total sulfur needs, best results are usually incorporation is preferable to band placement. Surface applications of Sº are not recommended.
• If fertilizer is applied at the time of planting of Spring-seeded crops, SO4 fertilizers will usually give bestresults. This is especially true if conditions are suchthat Sº oxidation rates are depressed; i.e., cool temperatures, excessive moisture. In starter or row fertilizers, SO4 forms of S generally give better results.
• If fertilizer is applied in the Fall for Spring-seededcrops, there is less likely to be any difference between Sº and SO4 sources. Elemental sulfur sources usually have better residual effects.
• Elemental sulfur sources are highly acidifying. This can be beneficial under alkaline soil conditions, but detrimental under acid conditions. Sulfate sources can be either acidifying or neutral in reaction. Ammonium sulfate is an acid-forming material; K-Mag, potassium sulfate and calcium sulfate areneutral materials and have no effect on soil pH observed if application is made prior to planting the crop.
From: www.back-to-basics.net/agrifacts/pdf/b2b29b.pdf
Sulfate -vs- Elemental Sulfur Part II: Characteristics Of SOxidation To SO4
The oxidation of S° to SO4 in soil is a biologicalprocess and is carried out by several kinds of micro-organisms. The rate at which this conversion takesplace is determined by three main factors:
• The microbiological population of the soil.
• The physical properties of the Sº source.
• The environmental conditions in the soil.
Microbiological Population In The Soil
Most agricultural soils contain some microorganisms that are able to oxidize Sº. However, the most important organisms in this respect are a group of bacteria belonging to the genus Thiobacillus. It is thenumbers of these bacteria that generally determines the degree to which Sº is converted to SO4 in soils, and there can be large differences between soils inthe population density of Thiobacillus. Under laboratory conditions, the rate of Sº oxidation in some soils can be markedly increased by inoculation with Thiobacillus. However, under field conditions,inoculation has not been found very effective.
When a source of Sº is added to a soil, it generally stimulates the growth of S-oxidizing bacteria, and the population of these organisms increases.
Physical Properties Of The Sº Source
The physical property that has by far the greatesteffect on the rate of Sº oxidation is particle size. Thefiner the particle size, the larger the surface areaexposed to soil microorganisms and the more rapidthe oxidation process. Table 1 clearly shows thiseffect of particle size.
Table 1. Particle Size Affects Rate Of S Oxidation.
Particle Size % S Oxidized
(Meshes/Inch) 2 Weeks 4 Weeks
5 -10 1 2
10-20 2 5
20-40 5 14
40-80 15 36
80-120 36 68
120-170 61 81
230 80 82
A mesh size of 5-15 is about the size range of bulk blended fertilizers and it can be seen that an S particle of this size is oxidized to SO4 very slowly. In order for Sº to be oxidized to the plant-available SO4 form at even moderate rates, it must be of a very fine particle size. But finely divided S is very difficult to handle, in addition to posing a fire hazard undersome conditions. All this would seem to largely ruleout the use of Sº as a fertilizer material. However,fertilizer manufacturers have developed techniques toimprove the handling characteristics and agronomic effectiveness of Sº. Elemental sulfur is first ground to a very small particle size range and is then agglomerated to a particle size compatible with granular fertilizer materials. About 10-15% of an expandable clay is added during the agglomeration process. The resulting material is more easily handled than finely divided S. In theory, when such a particle is applied to a soil, it comes in contact with soil moisture. As this moisture is absorbed by the particle, the clay expands, which in effect breaks the particle down into a much finer size range. The rate of oxidation to SO4 is increased accordingly. Throughout this Agri-Facts publication, when discussing Sº fertilizers, it is assumed the material has been manufactured in such a way that it does indeed break down rapidly on contact with soil moisture.
Environmental Conditions In The Soil
Since the oxidation of Sº to SO4 is a biologicalprocess, conditions must be favorable for growth ofthe organisms in order for oxidation to proceed atoptimum rates. The following environmentalconditions have been shown to have an influence onthe rate of Sº oxidation:
• Temperature
• Soil moisture and aeration
• Soil pH
• Fertility status of the soil
Temperature
Like most biological processes, Sº oxidation isreduced at both low and high temperatures. Moststudies have shown relatively low rates of oxidationbelow 55-60ºF, and steady increases in oxidationrates up to 100ºF. At temperatures above 130-140º,oxidizing bacteria are killed. All things considered, theoptimum temperature range for oxidation is 75-105ºF. Figure 1 shows the marked effect of temperature onoxidation rates of Sº.
Figure 1. Effect Of Temperature On Oxidation OfSº After 74 Days Incubation
Soil Moisture and Aeration
Most Sº-oxidizing bacteria require oxygen: any condition that restricts the oxygen supply in a soil will reduce the activity of Sº-oxidizing bacteria. Oxidation of Sº is most efficient at moisture levels close to field moisture capacity. Both water logging and excessively dry conditions greatly reduce the rate of Sº oxidation.
Soil pH.
Most Thiobacillus organisms thrive best under acid soil conditions. When a fertilizer source of Sº isapplied to a soil, oxidation occurs most rapidly underacid conditions.
Fertility Status of the Soil
Sulfur-oxidizing bacteria require most — if not all —of the nutrients required by plants. It’s not surprising, therefore, that oxidation of Sº proceeds more rapidlyin fertile soils. There is competition between thebacteria and plant roots for nutrients and this hasbeen found to cause temporary nitrogen deficienciesin plants under high Sº oxidation rates. Thiobacillusrequire ammonium rather than nitrate nitrogen. Highsoil nitrate levels can be toxic to the bacteria.
What does all this mean with respect to the use of these two forms of Sº in fertility programs? Since all S absorbed by plants is in the SO4 form, SO4 fertilizer sources are immediately available for plant uptake. And since all Sº sources must undergo conversion to SO4 in the soil, there is a certain amount of time lag between application and absorption through the root system. The extent of this time lag is obviously increased by any soil or climatic condition that suppresses the oxidation process. For this reason, when differences are observed between the effectiveness of these two chemical forms of S, the SO4 forms usually out perform Sº forms. The trials with wheat in Arkansas mentioned previously are a case in point. Table 2 compares the effectiveness of SO4 -vs- Sº at various application rates in these trials.
Table 2.Effects Of Sulfate As (K2SO4) -Vs-Elemental S On Wheat.
www.back-to-basics.net/agrifacts/pdf/b2b29b.pdf
Treatments were applied to wheat exhibiting S deficiency symptoms in the Spring (May) and plant analysis samples were collected three weeks later. According to the S levels in plant tissue, this was obviously not enough time for Sº oxidation under the existing conditions.
Sulfate -vs- Elemental Sulfur Part I: There Is A Difference.
• Sulfate-Sulfur is the only form of S the plant can utilize.
• Elemental S is dependent upon time, temperature and moisture to be available to the plant.
• Sulfate-Sulfur will not acidify the soil.
For various reasons, sulfur (S) deficiencies are increasing in many areas of the country. The use ofthis nutrient in fertility programs is there fore becoming more routine. The most common chemical forms of Sused in fertilizers are sulfate S (SO4) and elemental S(Sº). But these two forms of S react quite differently in soils. It’s very important to understand the differences between SO4 and Sº in order to use these two forms inthe most effective manner possible.
Sulfate Sulfur
Although S exists in many different chemical forms innature, plants can only absorb S through their root systems in the SO4 form. Small amounts of sulfur dioxide gas can be absorbed through leaves, but this isof little consequence in the overall S nutrition of plants.This obviously means that all soil S must be converted to SO4 in order to be utilized by plants.
The SO4 anion carries two negative charges, so it is not absorbed by soil colloids to any extent. As soil acidity increases, some positive charge sites developon clay particles and organic matter, which allows for a limited amount of SO4 absorption. But for the mostpart, SO4 moves freely with soil moisture, especially inthe upper levels of the soil profile. It thus reacts muchlike nitrate nitrogen in soils. As a result, SO4 levels frequently increase with increasing depth in the soil profile, at least in humid climates. Table A shows the increase of SO4 with depth in several Southeastern U.S. soils.
-
Table A:
www.back-to-basics.net/agrifacts/pdf/b2b29a.pdf
-
For this reason, small amounts of SO4 applied in a starter fertilizer is sometimes all that’s needed to get young roots off to a fast start, quickly growing down through the profile to greater depths where supplies of SO4 are more plentiful. Work in Arkansas has shown that wheat yields were increased from 15 to 44 bushels per acre by the application of just 5 lb/A of S in the potassium sulfate form. However, a more typical recommendation for sulfur would be in the range of 25-30 lb/A of S
Elemental Sulfur Elemental S is totally unavailable to plants. Plants simply cannot absorb Sº through the root system.Elemental S is inert and water insoluble. Commercially,it is stored in the open, and it can remain in place foryears, unaffected by moisture or temperature. However, when Sº is added to soil, it’s an entirely different matter. In the soil, Sº is converted (oxidized)to the plant-available SO4 form and the rate at which this conversion takes place is the determining factor regarding the effectiveness of Sº as a fertilizer sourceof S. (See Part II.)
The following are the most important considerations regarding the use of these two chemical forms of S infertility programs.
• When relying on Sº for the total sulfur needs, best results are usually incorporation is preferable to band placement. Surface applications of Sº are not recommended.
• If fertilizer is applied at the time of planting of Spring-seeded crops, SO4 fertilizers will usually give bestresults. This is especially true if conditions are suchthat Sº oxidation rates are depressed; i.e., cool temperatures, excessive moisture. In starter or row fertilizers, SO4 forms of S generally give better results.
• If fertilizer is applied in the Fall for Spring-seededcrops, there is less likely to be any difference between Sº and SO4 sources. Elemental sulfur sources usually have better residual effects.
• Elemental sulfur sources are highly acidifying. This can be beneficial under alkaline soil conditions, but detrimental under acid conditions. Sulfate sources can be either acidifying or neutral in reaction. Ammonium sulfate is an acid-forming material; K-Mag, potassium sulfate and calcium sulfate areneutral materials and have no effect on soil pH observed if application is made prior to planting the crop.
From: www.back-to-basics.net/agrifacts/pdf/b2b29b.pdf
Sulfate -vs- Elemental Sulfur Part II: Characteristics Of SOxidation To SO4
The oxidation of S° to SO4 in soil is a biologicalprocess and is carried out by several kinds of micro-organisms. The rate at which this conversion takesplace is determined by three main factors:
• The microbiological population of the soil.
• The physical properties of the Sº source.
• The environmental conditions in the soil.
Microbiological Population In The Soil
Most agricultural soils contain some microorganisms that are able to oxidize Sº. However, the most important organisms in this respect are a group of bacteria belonging to the genus Thiobacillus. It is thenumbers of these bacteria that generally determines the degree to which Sº is converted to SO4 in soils, and there can be large differences between soils inthe population density of Thiobacillus. Under laboratory conditions, the rate of Sº oxidation in some soils can be markedly increased by inoculation with Thiobacillus. However, under field conditions,inoculation has not been found very effective.
When a source of Sº is added to a soil, it generally stimulates the growth of S-oxidizing bacteria, and the population of these organisms increases.
Physical Properties Of The Sº Source
The physical property that has by far the greatesteffect on the rate of Sº oxidation is particle size. Thefiner the particle size, the larger the surface areaexposed to soil microorganisms and the more rapidthe oxidation process. Table 1 clearly shows thiseffect of particle size.
Table 1. Particle Size Affects Rate Of S Oxidation.
Particle Size % S Oxidized
(Meshes/Inch) 2 Weeks 4 Weeks
5 -10 1 2
10-20 2 5
20-40 5 14
40-80 15 36
80-120 36 68
120-170 61 81
230 80 82
A mesh size of 5-15 is about the size range of bulk blended fertilizers and it can be seen that an S particle of this size is oxidized to SO4 very slowly. In order for Sº to be oxidized to the plant-available SO4 form at even moderate rates, it must be of a very fine particle size. But finely divided S is very difficult to handle, in addition to posing a fire hazard undersome conditions. All this would seem to largely ruleout the use of Sº as a fertilizer material. However,fertilizer manufacturers have developed techniques toimprove the handling characteristics and agronomic effectiveness of Sº. Elemental sulfur is first ground to a very small particle size range and is then agglomerated to a particle size compatible with granular fertilizer materials. About 10-15% of an expandable clay is added during the agglomeration process. The resulting material is more easily handled than finely divided S. In theory, when such a particle is applied to a soil, it comes in contact with soil moisture. As this moisture is absorbed by the particle, the clay expands, which in effect breaks the particle down into a much finer size range. The rate of oxidation to SO4 is increased accordingly. Throughout this Agri-Facts publication, when discussing Sº fertilizers, it is assumed the material has been manufactured in such a way that it does indeed break down rapidly on contact with soil moisture.
Environmental Conditions In The Soil
Since the oxidation of Sº to SO4 is a biologicalprocess, conditions must be favorable for growth ofthe organisms in order for oxidation to proceed atoptimum rates. The following environmentalconditions have been shown to have an influence onthe rate of Sº oxidation:
• Temperature
• Soil moisture and aeration
• Soil pH
• Fertility status of the soil
Temperature
Like most biological processes, Sº oxidation isreduced at both low and high temperatures. Moststudies have shown relatively low rates of oxidationbelow 55-60ºF, and steady increases in oxidationrates up to 100ºF. At temperatures above 130-140º,oxidizing bacteria are killed. All things considered, theoptimum temperature range for oxidation is 75-105ºF. Figure 1 shows the marked effect of temperature onoxidation rates of Sº.
Figure 1. Effect Of Temperature On Oxidation OfSº After 74 Days Incubation
Soil Moisture and Aeration
Most Sº-oxidizing bacteria require oxygen: any condition that restricts the oxygen supply in a soil will reduce the activity of Sº-oxidizing bacteria. Oxidation of Sº is most efficient at moisture levels close to field moisture capacity. Both water logging and excessively dry conditions greatly reduce the rate of Sº oxidation.
Soil pH.
Most Thiobacillus organisms thrive best under acid soil conditions. When a fertilizer source of Sº isapplied to a soil, oxidation occurs most rapidly underacid conditions.
Fertility Status of the Soil
Sulfur-oxidizing bacteria require most — if not all —of the nutrients required by plants. It’s not surprising, therefore, that oxidation of Sº proceeds more rapidlyin fertile soils. There is competition between thebacteria and plant roots for nutrients and this hasbeen found to cause temporary nitrogen deficienciesin plants under high Sº oxidation rates. Thiobacillusrequire ammonium rather than nitrate nitrogen. Highsoil nitrate levels can be toxic to the bacteria.
What does all this mean with respect to the use of these two forms of Sº in fertility programs? Since all S absorbed by plants is in the SO4 form, SO4 fertilizer sources are immediately available for plant uptake. And since all Sº sources must undergo conversion to SO4 in the soil, there is a certain amount of time lag between application and absorption through the root system. The extent of this time lag is obviously increased by any soil or climatic condition that suppresses the oxidation process. For this reason, when differences are observed between the effectiveness of these two chemical forms of S, the SO4 forms usually out perform Sº forms. The trials with wheat in Arkansas mentioned previously are a case in point. Table 2 compares the effectiveness of SO4 -vs- Sº at various application rates in these trials.
Table 2.Effects Of Sulfate As (K2SO4) -Vs-Elemental S On Wheat.
www.back-to-basics.net/agrifacts/pdf/b2b29b.pdf
Treatments were applied to wheat exhibiting S deficiency symptoms in the Spring (May) and plant analysis samples were collected three weeks later. According to the S levels in plant tissue, this was obviously not enough time for Sº oxidation under the existing conditions.