By David Jones, Broad Acre Agronomist

Poor protein quality and low grain protein, 20% yield response in Canola and improved nitrogen use efficiency. So why are we still not following the science and advice on Sulphur nutrition? This week I ask what we need to be doing in Kenya to take our Sulphur more seriously, to take yields and quality to the next level.

Why Sulphur?

We are all now aware that sulphur levels in the atmosphere are a lot lower than last century as air quality has been cleaned up, so less Sulphur is deposited on farmland. Sulphur is required in such a quantity that it is often referred to as the ‘fourth macronutrient’ along with N, P and K. It is found in most amino acids – the building blocks of proteins – and chlorophyll, and is especially important in oil production, hence the large quantities required by canola crops.

It is important to remember that Sulphur is very immobile in the plant. Whereas Nitrogen deficiency shows up as yellow older leaves on cereal plants, Sulphur shows in the new leaves as the plant cannot mobilise the nutrient towards new growth.

A sulfur deficient plant will experience yellowing or pale green coloring throughout the plant

Interestingly however, Sulphur is mobile in the soil, and can leach down through the profile with heavy rain. Crops take up sulphate rather than elemental Sulphur (SO4). To convert Sulphur into Sulphate (SO4), multiply by 2.5 – this is important when comparing the amount of sulphur in fertiliser materials.

How much Sulphur do crops require?

The sulphur content of grain can be multiplied by the expected yield to calculate the total amount removed by the crop at harvest. Whilst this is extremely useful, what this doesn’t tell us is how much is actually taken up by the crop then returned in the straw and leaf residue at harvest to be recycled – or how much has to be available to the crop during the growing period. This is why some crops respond to applications that are much greater than what they eventually remove.

In the case of Sulphur, thankfully uptake and removal are very similar. What does stand out in this table though is the enormous amount taken up by canola.

So three things jump out here; 1) this is a lot of money in nutrients – especially in Canola, 2) how can we reliably estimate sulphur requirement to optimise returns. And 3) how accurate is the crop removal method for estimating nutrient requirements?

I want to look specifically at Canola because of the importance of getting Sulphur right. A lot of work has gone into this area in other canola growing countries, so I had a look into what the advice is. As you will see, the difference in thinking between Europe and Australia is remarkable…

Being in the middle of the two hemispheres, with our own climate, cropping systems and rainfall patterns illustrates how much work we have to do on this subject, and not just in canola!!! But we can draw some conclusions from the points where there is a strong consensus:

  1. Apply Sulphur in the sulphate form, for rapid and reliable availability
  2. Canola crops with low deep sulphur levels after a wet period will require Sulphur
  3. Test the leaf tissue at the 5-6 leaf stage to check crop uptake.

We are planning to conduct several trials within CropNuts examining Sulphur, and I would encourage farmers to use low S and high S strips in the field to test nutrient response. In the meantime, my advice would be:

  1. Apply at least 20kg of Sulphur to all canola crops.
  2. Leave untreated strips to check for both yield and visual deficiency symptoms (note total N fertiliser must be equal when making comparisons).
  3. Take deep soil tests to estimate soil supply, and to compare with crop responses in untreated strips to inform future Sulphur decisions.
  4. Spread it well! I would always insist on farmers carrying out a tray test to check the accuracy of fertiliser applications. Not easy to do here yet but well worth setting the machine up properly

How Much Is Your Sulphur Really Costing?

In order to work out the most cost effective way to apply sulphur, we need to know the unit cost per Kg. This can be tricky when there are other nutrients in the fertiliser, but as a rule of thumb

This exercise in calculating the true cost of each kilo of Sulphur you are using is always extremely useful when making decisions about the true cost of the fertiliser you are purchasing. The Sulphur in all of the above is in sulphate form for will behave equally in the field.

Ultimately we have a significant amount of work to do on Sulphur before we can really refine the guidance, but it starts here with simple but effective on farm trials, and soil & leaf testing.

Nice weekend!


About David


David Jones is the Broad Acre Specialist at Crop Nutrition Laboratory Services Ltd. (CROPNUTS). David has a keen interest in soils and no till farming systems where he has undertaken work looking into weed levels and changes in soil structure, and has extensive experience in field trials and in the development of precision farming techniques. In his spare time he enjoys playing rugby.