Features Fertilizer Seed & Chemical
Assessing performance of band applied elemental sulphur

For many years, researchers have been struggling with the development of guidelines
that would increase the agronomic usefulness of elemental-S based fertilizer
products. Quite early, it became obvious that in order to maximize the benefit
to crops in the short-term, surface applications, preferably conducted in the
fall of the year, were most effective.

Presence of swelling clays
To improve their agronomic performance, all of the high analysis-S fertilizers
contained some type of swelling clay, the purpose of which was to disrupt the
granules when the clay was moistened. This disruption of the granules was essential
for creating a high degree of intimate contact of the finely divided sulphur
particles with the soil and sulphur oxidizing organisms.

Surface application most beneficial
Researchers discovered that elemental-S that was broadcast applied was most
effective, providing sufficient time was allowed for the S granules to be broken
down and dispersed before being incorporated into the soil. Early incorporation
into the soil before the granules were degraded resulted in a significant slow-down
in the rate of conversion to the plant available sulphate form of S.

Fertilizer application trends unfavourable
Unfortunately, the trend in fertilizer application for the past 20 years has
increasingly been to place fertilizer in sub-surface bands in order to achieve
better crop utilization of the main fertilizer nutrients such as P, K and especially
N. This development has made it more difficult to develop a fertilizer program
using elemental-S as a source of plant available sulphate-S.

Factors aiding the oxidation process
Sulphur in the elemental form is not available for plant uptake. It must first
be oxidized to the plant available sulphate form of S. Conditions that encourage
more rapid oxidization of the elemental-S include:

• presence of S oxidizers, which are most abundant in the top inch of soil,
• very finely divided particles of S present within the granules,
• early disruption and dispersal of the elemental-S particles,
• close contact between the soil and individual S particles,
• tillage after granules are disrupted speeds oxidation to sulphate.

Anticipated soil-building
The fact that elemental-S based fertilizers contained a relatively high content
of sulphur made them a popular choice among growers who were seeking to reduce
the physical volume of fertilizer handled at the time of seeding. In light of
this fact, it was the hope of many fertilizer agronomists that repeat applications
of band-applied elemental-S would result in a gradual, 'soil-building' of residual
plant available sulphate which would eventually be sufficient to meet plant
requirements for S.

Minimal residual S benefits observed
In fact, researchers observed that if elemental-S was broadcast applied in the
fall of the year and was not incorporated into the soil until spring tillage
occurred, the performance of these products was improved. However, if elemental-S
was broadcast applied in the spring and incorporated with tillage, performance
as a sulphur fertilizer declined significantly. Furthermore, results from longer
trials carried out in northeast Saskatchewan appear to have dimmed if not dashed
the hopes for 'soil-building' S from seedrow or side-band applied elemental-S
fertilizers.

What about no-till conditions?
It appears that even if the seedbeds are regularly cultivated to accomplish
some breakdown of the seedrow or side-band applied elemental-S, the 'soil-building'
benefits from repeated applications of elemental-S granules will be slow to
materialize, as a result of the fact that the S granules remain relatively intact
within the soil. Unfortunately, under no-till conditions, the likelihood of
obtaining any residual 'soil-building' benefit appears to be miniscule because
of the limited opportunity that exists to disrupt the soil containing S granules
applied in previous years.

No-till recommendation
In our judgement, it would appear that attempts at 'soil-building' supplies
of available soil-S using repeat applications of seedrow or side-band applications
of elemental-S would have limited success. If growers insist on using elemental-S
as a source of plant available S, they should apply it in the fall of the year
with no incorporation.

Soil encapsulation hinders breakdown
If the elemental-S granules are left on top of the soil, the swelling of the
clay is unobstructed and is therefore quite effective in fostering granule disruption
and improved soil contact with the S oxidizing bacteria present in the soil.
However, if the S granules are seedrow or band applied, they effectively become
entombed or encapsulated in the soil. The pressure exerted by the surrounding
soils will defeat the purpose of adding the swelling clay to the granules. As
a result, the rates of oxidation and the release of plant available S are drastically
reduced.

John Harapiak has approximately 40 years of western Canadian based
fertilizer related experience. He will continue to contribute stories to
Top Crop Manager. He can be contacted by e-mail at: jharapiak@shaw.ca