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Contents |
| Background |
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| Treatments |
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| Visual differences |
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| Yields |
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| Soil analysis |
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| Conclusions |
Results from Wheat Demonstration Plots |
Yields
| t/ha | 1987 | 1988 | 1989 | 1990 |
| K0 | 6.1 | 6.5 | 3.8 | 2.0 |
| K1 | 6.1 | 7.0 | 7.9 | 5.6 |
| K2 | 6.1 | 7.7 | 6.9 | 5.9 |
Total potash supply to the plant was not limiting in the first year when soil K reserves fully made up for the lack of potash fertiliser on the K0 plot with no differences in yield between the three treatments.
By 1988 soil K levels on the K0 plot had been considerably depleted on this light soil and there was some indication of a potash effect on yield. This was quite a good year for wheat on this site with adequate summer moisture and modest temperatures for grain fill resulting in higher potential yields that in 1987.
Whilst 1989 was hot and very dry, the yields from the plots receiving K fertiliser did not appear to be limited by potash. The lack of potash in the K0 plot by contrast had a major limiting effect upon yield, clearly reflecting the role of potassium in the water regulation of plants. There was no obvious explanation for the considerably higher yield from the K1 plot (compared to K2) but the effect of the greater removal of K in the larger crop shows up in the reduced soil K level at the end of that season.
The need to look at potash on a long term rather than an annual basis is clearly emphasised by the yields in 1990 when the nil potash plot was virtually a crop failure giving only 2 t/ha. This effect was solely due to K as all other inputs were identical . This also emphasises the importance of nutrient balance and particularly for N and K as the normal input of 175 kg/ha of nitrogen was largely wasted. The N response was restricted by lack of potash and there is no doubt that this un-utilised N was lost into the environment.
Although not measured for the whole period, grain from the K0 plot consistently had:
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lower protein content |
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higher % of small grains |
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lower thousand grain weight |
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poorer grain sample |
Soil analysis
| SOIL K mg/l (index) |
START | 1987 | 1988 | 1989 | 1990 |
| K0 | 93(1) | 48(0) | 39(0) | 32(0) | 34(0) |
| K1 | 93(1) | 71(1) | 64(1) | 31(0) | 36(0) |
| K2 | 93(1) | 81(1) | 72(1) | 57(0) | 57(0) |
The soil K figures clearly show the serious depletion of soil fertility even after the first season where no potash was applied. Whilst visually there were no problems at this stage, the potential problem had already been created.
It is notable that soil K levels were slowly being reduced even where the higher rate of 140 kg/ha of fertiliser potash was being applied to the K2 plot. This situation would need to be kept under review by regular soil analysis.
The surprisingly large yield from the K1 plot in 1989 obviously took more potash from the soil resulting in a large drop in soil K to 31 mg/l by the end of that season.
Conclusions
Where potash supply is limiting, yields and grain quality will be seriously affected. Nitrogen response will also be impaired with increased risk of N loss to the environment.
Potash should be treated as a long term nutrient and fertiliser use should be based on regular soil analysis and crop removal rather than apparent annual yield response or visual appearance. Potash requirements for cereals and cereal straw are detailed in PDA leaflet 7 and leaflet 11.