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Potash and sulphur for silage yield and quality

May 2019

Unlike nitrogen and sulphur, potash moves very slowly in the soil. Failure to replenish the potash removed by grass cut for silage can lead to short-term yield penalties. The advice to keep soil potassium levels at Index 2- will maintain a fertile and productive soil throughout the horizon where roots can access nutrients. Fresh potash, whether from organic manures or inorganic fertilisers will not all be used to support grass growth straight after application; because potassium moves slowly in the soil, the grass roots will not be able to access all the potash from fresh applications. Add to this the fact that fresh applications will also be partly used to replenish soil reserves where potassium soil levels have been allowed to fall and it is clear that maintaining soil reserves at Index 2- is critical to supporting yield throughout the growing season.

Potash is also important for achieving good grass quality. In plant cells, potassium has two main functions. Firstly, it plays an irreplaceable part in the activation of enzymes, which are fundamental to metabolic processes, especially the production of proteins and sugars. A potash deficiency reduces not just growth, but quality also, with true and crude protein levels and soluble sugars concentrations at risk.

The second role for potassium in plants is as the ‘plant preferred’ cation for maintaining the water content and hence the rigidity and shape (turgor) of each cell. The concentration of potassium in the cell sap (the liquid inside a plant cell) creates conditions that cause water to move into the cell (osmosis) through the semi-porous cell wall. Turgid, well-shaped plant cells maintain the leaf’s vigour so that photosynthesis can proceed efficiently. Large amounts of potash are needed for this function and a lack of potash will lead to inefficient photosynthesis, compromising nitrogen uptake causing low protein levels as well as yield reductions.

Remember sulphur as well

According to figures from the latest British Survey of Fertiliser Practice, there are still only around 16% of grass silage swards receiving fertiliser sulphur. This is a concern when considering that even grass grown on medium soils, let alone lighter soils, will be responsive to sulphur, with grass growth becoming more responsive to sulphur as the season progresses.

It could be argued that the apparent lack of sulphur fertiliser use is partly due to the assumption that slurry applications will provide enough readily available sulphur. Considering that a 30 m3/ha application of 6% DM cattle slurry will supply 21 kg SO3/ha this comes close to the 25–30 kg SO3/ha that the grass crop needs for the next cut of silage. However, the availability of this sulphur is variable, averaging 45% for a spring application.

It is impossible to visually identify a 10–15% yield response in grass from sulphur, but this level of response it typical and highly desirable. Therefore, using a sulphate fertiliser to supply the modest rate of 25 kg SO3/ha for a silage cut is a very low-cost insurance policy against losing grass yield and quality.

Sulphur for quality

Good yield responses to sulphur applications are common and proven by experimentation. However, just like potash, we tend to neglect the quality angle when considering sulphur applications. One of the roles of sulphur is in protein production, as a core element in two of the essential amino acids that make up the building blocks of protein. But we must also consider that these proteins are used in the biochemistry of accumulating sugars in the plant through photosynthesis, so it should not be forgotten that sulphur deficient grass also tends to have lower levels of water-soluble carbohydrates (WSC), which are important in the fermentation of good quality silage.

A shortage of sulphur also has an effect on the efficient use of forage. The rumen microbes require both nitrogen and sulphur to produce their own protein, and a shortage of sulphur will mean that they stop once it’s used up, even if there is still nitrogen and carbohydrate left. Thus, the actual ‘D-value’ of the forage is reduced by a lack of sulphur.