Development of the malate:sulphate ratio method for routine testing for sulphur deficiency in winter wheat and oilseed rape
About this project
In 1999, it was found that the malate:sulphate ratio in youngest leaves of plants was potentially a reliable and practical indicator of S deficiency in agricultural crops. The aim of this project was to develop this method further and to make it commercially available to all levy-payers.
Malate is an organic acid present in all plants, responsible for the sour taste of unripe apples. One function in cells is to counterbalance the uptake of inorganic anions, such as nitrate and sulphate. When sulphate accumulates in cells due to a sufficient external supply, malate decreases; when sulphate decreases as a result of S deficiency, malate increases.
Both compounds can be extracted simultaneously and measured by ion chromatography. The malate:sulphate ratio is determined by dividing the malate peak by the sulphate peak from the ion chromatograph. A previous project indicated that when the ratio is larger than 1.5 the plants are S deficient at the time of sampling; when the ratio is lower than 1.5 the plants have sufficient S supply at the time of sampling.
Using field trials with oilseed rape and wheat, it was shown in this project that application of increasing amounts of N fertiliser had no direct effect on the malate:sulphate ratio, but that N increased the growth rate of plants which in turn increased their S requirement.
S deficient plants could recover from S deficiency when S fertiliser was applied: for winter wheat, applications up to the beginning of May (approximately growth stage 32) were effective for a full yield recovery, but application in June (approximately growth stage 55) was too late. However, when S was applied in June the concentrations of protein S increased to normal levels indicating that a late application of S might still be beneficial to grain quality. For oilseed rape, full yield recovery was observed with S applications up to the beginning of April (approximately growth stage 3.7). The application of S in May (approximately growth stage 4.6) resulted in a partial recovery.
The performance of the malate:sulphate ratio in identifying deficient samples was consistent throughout the 3 years of trials and on average only 4% of samples were misdiagnosed as being sufficient when they were not. In contrast, using the N:S ratio in grain on average 13% of samples were misdiagnosed as being sufficient when they were not; this increased to 20% when S deficiency was moderate. Using the N:S ratio in leaves, on average 16% of samples were misdiagnosed; this rose to 37% when the deficiency was moderate.
Collaboration with a commercial laboratory showed that crops should not be tested too early: for oilseed rape not before the end of February and for wheat not before the beginning of April. Early in the season, plants are small and have a low S requirement. Testing at this time might give a "not deficient" result, but this is no guarantee that the plants will not be deficient later in the season, when they are starting to grow rapidly. In this case it is recommended to sample again when they are growing rapidly.
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