Tissue testing

Soil analysis indicates the potential supply of potash to a crop. It cannot tell you if sufficient potash 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 potash can be achieved at low cost and will normally ensure that crop needs are being met without the need for tissue sampling and analysis.

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. Percent K in dry matter varies widely with season, stage of growth, part of plant, fertiliser (nitrogen or potash) 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 5.8 and 7.8 g K/litre (150 and 200 mM K) for growth stages between GS31-61. Plant concentrations below this range may result in yield penalties.

Peak potash uptake is much greater than offtake at harvest

Potash uptake during establishment is low with winter wheat typically containing around 40-50 kg K₂O/ha over winter; more forward winter barley crops may contain up to 80 kg K₂O/ha. Whilst these amounts are relatively small, a deficiency of nutrient at this early stage clearly can have critical effects and where soil potash levels are low (mid Index 1 or below) some potash should be applied to the seedbed - ideally combine drilled if the soil reserves are very low. As the crop reaches tillering and starts to make vigorous vegetative growth, potash and nitrogen uptake increase very rapidly as shown in the diagram. Potash uptake may be as high as 10 kg/ha per day and on light, low potash soils this may outpace the soil's natural capability to supply. In such circumstances spring NK top dressing makes good sense (see PDA Leaflet 22). This is also good practice on these sandy, low K-retentive soils because split autumn and spring application minimises risk of potash loss under excessive winter rainfall. Peak potash uptake with cereals occurs around late flowering stage when there may be more than 300 kg K₂O/ha in a high yielding crop. Peak uptake of potash is much greater than offtake at harvest. If this peak requirement is not available, grain number and grain fill will be affected thus prejudicing both yield and quality. This is a key reason for maintaining the adequate soil reserves which can supply these peak amounts more reliably than fresh fertiliser applications. After flowering, potash is redistributed around the plant with a general reduction from leaves to grain and stem bases. Potash is also returned to the soil with the senescence and shedding of older leaves. The rate at which this potash redistribution process occurs varies widely with different seasons and affects potash levels in the straw.

Sources and reserves of potash

Virtually all the potash supply to a cereal crop is taken up from the soil. Small quantities of potash (less than 5 kg K₂O/ha) are deposited in rainfall, but foliar uptake is not a practical route for the large requirements of this nutrient. Foliar fertiliser applications to cereals are rarely cost-effective. Potash is held in the soil as the positively charged cation, K+, which occurs in small amounts (less than 20 kg/ha) in the soil solution and in much larger amounts in readily or slowly available form attached to the clay minerals. An even larger quantity is also present within the clay minerals but this can be discounted in terms of crop supply as it is only released over long time periods by weathering. The rate and quantity of potash that a soil can supply for crop uptake depends upon soil type, texture, soil condition and previous history of manuring.

Potash in fertilisers and manures is 90-100% available, but if not taken up by crop roots, becomes held by the clay minerals and is therefore not at risk to leaching in the same way as nitrogen. Research studies have shown that for most soils only 1 kg K is lost for every 100 mm of through drainage. Except on light, shallow soils any movement of potash below plough depth is likely to be retrieved by deeper rooting crops such as winter cereals. Applications of potash in excess of crop need will therefore remain as an increase in readily and slowly available soil K reserves for the next crop.

The soil feeds the crop: fertiliser is used to feed the soil

Nutrient reserves in the soil resulting from natural fertility and previous manuring are much more effective in supplying nutrient than newly applied fertiliser. Impoverished soils will often not produce the same yields as fertile soils even if higher fertiliser rates are applied. Thus an adequate supply of potash to the crop should be ensured by maintaining an adequate reserve in the soil. Fertiliser (and manures) should be used simply to maintain this reserve by replacing what is removed in the harvested crop. For impoverished soils extra fertiliser should be used to restore fertility to a satisfactory level. Where nutrient levels are more than adequate, fertiliser usage should be reduced below removal levels, or even omitted. Unlike nitrogen, soil potash reserves are not subject to loss or major change from season to season and thus maintenance of an adequate reserve is a practical strategy on most soils.