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Justus von Liebig and the Barrel

Date Added: 10/02/2017

Blog: By Richard Elsdon, Technical Consultant, United Oilseeds

Yes the title does sound like a children’s story but it is applicable to agriculture and, in particular, winter oilseed rape.

Justus von Liebig (1803-1873) was a German chemist who made major contributions to agricultural and biological chemistry. One of his more memorable statements was the Law of the Minimum which suggests that crop output is controlled not by the total amount of resources, such as fertilizer or water availability, but by the scarcest resource.

Justus von Liebig, German Chemist

I was thinking about this statement when another statistic came to light. According to the British Survey of Fertiliser Practice 2015, only 73% of oilseed rape received sulphur in the form of applied fertiliser. This was not a “fluky” year. The previous two years had levels of 76% and 72% respectively. Other information in the same survey showed that about 20% applied animal manures to the crop. Would this go some way to explaining the gap between trial results and UK yields?

 My concern is that too many growers are under the impression that animal manures contain enough sulphur to supply the needs of the crop. Assuming an application rate of 5 mt per ha of chicken litter (which for example is not unusual in the West Midlands where there are a large number of broiler chicken producers) my calculations show that this would supply 41 kg of sulphur as SO3. This may be enough to convince some growers that they have an opportunity to miss the fertiliser sulphur application in the spring.  I beg to differ.

We need to remember that the chicken litter would have been applied in August, just before the rape crop was drilled. In addition, the sulphur is in an organic form and as such would be released slowly – over the winter. The sulphate ion is very soluble so it would not be unreasonable to expect that much of the sulphate was leached from the soil leaving little for the plant to access and utilise when it needed it, in the spring.  Work done by Mcgrath, Berry, and Sagoo showed that animal manure applied in the autumn had 5-10% of the sulphur in the fertiliser available to the plant in the spring.

My guess is that there would be some sulphur available to the plant. This would enable it to set flowers which would most likely be a little paler than crop receiving fertiliser sulphur. In addition, the plant would set pods so again it’s difficult to see any difference. When combined, the yield would be less than crops adequately fertilised but that would probably be put down to the variety or the year.  The loss of yield or quality would have occurred without visual symptoms.

Sulphur deposition from industry in 2007 had declined to 10% of that deposited in 1980s. It needs to be applied to the oilseed rape crop (and the wheat crop) in the spring, usually in late February to early March.  I suggest waiting until the crop has started to produce new spring growth and then apply it. Research backs up the sulphur recommendation from RB209 that the optimum amount to apply is50-75 kg/ha of SO3.

Average yields of oilseed rape should be higher than those seen at present. Is it possible that on many farms, yield is reducing because one of the component planks in Liebig’s barrel is too short?

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