Fert Options with Robin Boom Agronomic Advisory Services

Sulphur is an essential element for all plants. Legumes (clovers, beans, peas, lucerne) have a higher requirement than grasses, although symptoms of sulphur deficiency are often seen in grasses and cereal crops like maize which can look yellowy and often is confused with nitrogen deficiency, and it can also exacerbate nitrogen deficiency.

In many horticultural crops, sulphur deficiency is often seen in the yellowing of younger leaves in particular.

Soil testing for sulphur can sometimes give conflicting results, where sulphate sulphur, organic sulphur and total sulphur levels are measured, and soil conditions at time of sampling and time of year is important when interpreting lab results.

Leaf tissue testing can also be a reliable indicator for diagnosing sulphur deficiencies.

Allophanic soils

Sulphur deficiencies are common throughout all of New Zealand with the exception of the volcanic ash (allophanic) soils found in parts of the Coromandel, Waikato, Bay Of Plenty, King Country and Taranaki regions.

Allophanic soils cover about 12 per cent of the North Island and are so named because of the tiny nanocrystalline clay mineral formed in them, allophane.

These allophanic soils are made up of very fine grained interfingering tephra sequences from rhyolitic and andesitic volcanoes over the past 10,000-50,000 years.

They are very responsive to phosphatic fertilisers because of their high aluminium content and have developed dark, carbon-rich top soils that have built up over the years from pastoral agriculture. Allophanic soils retain sulphur much better than pumice, peat, clay, sandy and silty soils, which readily leach it out, particularly during periods of high rainfall.

Deficiencies

On most soils, the majority of sulphur is held in the soil organic matter and this is released and mineralised during microbial breakdown of the organic matter.

Therefore, sulphur deficiency in pasture is unlikely to be seen over the summer and autumn due to the build up of biologically mineralised sulphur, and with less rain, it remains available for plants to utilise.

Sulphur deficiency is more likely to occur over the winter and spring, after rain has flushed this mineralised sulphur beyond the root zone and microbial activity naturally slows down because of wetter and cooler conditions.

Getting value

I had a recent discussion with a hill country farmer comparing the immediately available sulphate sulphur found in single superphosphate vs slow release elemental sulphur.

Sulphate sulphur is prone to leaching out the same as biologically mineralised sulphur and therefore its application in the autumn is unlikely to benefit as much as spring application, since there is plenty of microbially oxidised sulphur present in the soil in the autumn.

For this farmer, it was better value for him to consider a blend of Triple Super or RPR and Sulphur 90, which has slow release elemental sulphur that would break down and become available over the winter and spring when it was most needed.

If he was to apply a single superphosphate based fertiliser in autumn, it should be fortified with elemental sulphur such as found in Sulphur Super 20 or Sulphur Super 30, which would last through the winter and spring.

Best time to apply

The same can be said for sulphur containing nitrogen fertilisers. The sulphate sulphur found in Sulphate of Ammonia is going to be of little benefit in the autumn and is better applied in the spring, whereas the elemental sulphur found in the Ballance product Phased N is a better option for autumn application as the sulphur it contains will become available over the winter and spring period.

Superphosphate and sulphate of ammonia based fertilisers are therefore better to apply in the late winter and spring period when the sulphate sulphur will give an immediate response, whereas elemental sulphur applied in the spring will be less effective when oxidising over the summer and autumn.

Other sulphate sulphur containing fertilisers like kieserite (magnesium sulphate), gypsum (calcium sulphate) and potassium sulphate are also likely to be of greater benefit applied in the spring than the autumn.

Compared to the other major elements needed by plants, sulphur is relatively cheap to correct in the soil, and therefore should be one of the first elements to consider on a limited budget.

On most soils, 30-35 kg/ha of elemental sulphur is all that is required, and at $450-600/tonne, depending on supplier, works out at $14-20/ha.

Too much sulphur can be a bad thing for animal health as it reduces copper and selenium availability in livestock and if overdone in brassica crops like kale, it can cause red-water.