Potash for Cereals |
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| Effect of potash on nitrogen response |
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Cereals require a balance of nitrogen and potash to obtain full yield response to applied nitrogen. Careful optimisation of nitrogen is a waste of time if potash supplies are not adequate. As shown in the diagram, both the level of yield and the shape of the N response curve are totally altered by potash limitations.
If potash supply is limiting, the uptake and utilisation of nitrogen will be restricted. If soluble forms of nitrogen remain in the soil and are not taken up there is increased risk of leaching when through-drainage occurs. Ready availability of both nutrients at peak crop demand helps the uptake of the large requirements of N and K. During rapid vegetative growth, the rapid uptake of nitrogen as negatively charged nitrate ions (NO3-) is normally balanced by a similar uptake of positively charged potash ions (K+) which maintains the electrical neutrality of the plant. Adequate potash is clearly important in the production of quality wheat as it assists the conversion of nitrate to protein.
Deficiency symptoms
Potassium deficiency causes yellowing and chlorosis to
the edge and tip of older leaves, with progressive
senescence. Plants may be stunted and exhibit excessive
basal tillering. Symptoms are not easily distinguished
from those resulting from many other causes especially
from physiological stress - drought, water-logging, wind,
etc.
By the time such symptoms are seen it is often too late to remedy the situation and avoid crop loss. Potassium deficiency is sometimes referred to as 'hidden hunger' because losses occur despite any recognisable visual symptoms. Visual diagnosis is thus a totally unreliable means of detecting potassium shortage. Soil analysis is the most cost-effective approach providing an indication of adequacy or deficiency for future crops.
Tissue testing
Soil analysis indicates the potential supply of K to a crop. It cannot tell you if sufficient K is actually getting into the crop, only tissue testing can do that. Tissue testing can be used to assess the potassium status of a growing crop but can come too late in the season to remedy a deficiency. Tissue testing can be used as an adjunct to soil analysis or as an extra diagnostic tool in specific circumstances. The maintenance of an adequate level of soil K can be achieved at low cost and will normally ensure that crop needs are being met without the need for tissue sampling and analysis.
| Potassium in leaf sap of winter wheat |
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| Source: P Barraclough, IACR Rothamsted |
Where tissue testing is used, measurement of tissue water (cell sap) is recommended. This indicates whether K concentrations are at or below optimum levels and is more reliable and meaningful than measurement of K content in the dry matter. %K in dry matter varies widely with season, stage of growth, part of plant, fertiliser (N or K) application and weather or other factors which affect nutrient uptake and rate of growth.
Work at Rothamsted Research funded by the HGCA has indicated that adequacy of potash in winter wheat is represented by tissue water concentration in leaf 1 (the youngest mature leaf blade) between 150 and 200 mM K for growth stages between GS31-61. Plant concentrations below this range may result in yield penalties.