Potash for Potatoes | print version (pdf 273kb) |
Deficiency
Yield loss and adverse effects on quality may occur without
any visible symptoms of K deficiency being seen in the leaves.
When symptoms are identified, potential yield loss is likely
to have already occurred and it is unlikely that this can
be recovered by adding K. The visual signs of potassium
deficiency are that the leaf margins, especially on the older
leaves, become brown and leaves die prematurely. In severe
cases, growth is much retarded and the leaf canopy may not
close between the rows. Growth may be very uneven across
the field and this can have serious consequences for the yield
and quality of the crop.
Forms of potash
Potassium occurs in nature as various salts that occur in very large, mainly underground, deposits. These
deposits can be mined and the salts purified to supply the potash fertilisers used commercially. These
K fertilisers are soluble in water and when dissolved they separate into the two components, the cation K+
and the anion Cl-, SO42-, or NO3-, according to the form of the fertiliser applied. For example muriate of potash separates into K+ and Cl- ions. The plant always takes up the cation, (K+) but the chloride (Cl-), sulphate (SO42-) and nitrate (NO3-) are also required by plants. Measurement of potassium in soil or plant material is expressed as elemental K. Potassium fertilisers are commonly referred to as potash and their
content is measured as K2O.
1 kg K = 1.2 kg K2O. 1 kg K2O = 0.83 kg K.
Potassium chloride (muriate of potash, MOP) KCl
This is by far the commonest form of potash fertiliser and contains 60% K2O. The other component, chloride, is also an essential plant nutrient but is very rarely in short supply in the UK. Chloride levels in soils are maintained through fertiliser application and natural deposition in rain and it is lost from soil by leaching in drainage water. The chloride ion has no adverse effect in the soil.
Potassium sulphate (sulphate of potash, SOP) K2SO4
This provides two nutrients 50% K2O and 45% SO3. Sulphur inputs are increasingly required for UK crops as other sources decline and SOP is highly effective being readily soluble in the soil solution. However, sulphate, like nitrate, is readily lost from soil in drainage water, so SOP should be applied in the spring and not in autumn. If the sulphur content is not required, the potash content of SOP is more costly per kg K2O than that of MOP. SOP provides specific quality attributes for potatoes.
Potassium nitrate (nitrate of potash) KNO3
Again this fertiliser provides two nutrients because it contains 46% K2O and 13% N This is a speciality fertiliser normally used for high value crops where a fully soluble form of nitrogen is needed in conjunction with potassium. It is sometimes used as a top dressing for potatoes to secure quality benefits, but its relatively high price demands a high return to make it cost effective. It can be produced in very pure forms suitable for use in fertigation - where plant nutrients are added to irrigation water.
Potash uptake and removal
Potatoes take up more potash than many other arable crops. In the six weeks after plant emergence, the crop will take in at least two thirds of the total K uptake. During peak vegetative growth, potatoes may require 10 kg K2O/ha per day from the soil. Maincrop potatoes contain the maximum quantity of potash in late July - early August and this may be more than 500 kg K2O/ha for high yielding crops. As the tops die back and the plant matures, some potash is returned to the soil. By harvest more than 75% of the maximum K uptake is found in the tubers, which typically contain around 5.8 kg K2O per tonne of tubers. For the determination of nutrient removal this figure is assumed to be constant over the normal yield range. From the potash offtake figure it is possible to calculate the maximum K uptake by the crop. For example, a crop yielding 40 t/ha will remove 40 x 5.8 = approx 230 kg K2O/ha but this is only about 75% of the maximum supply needed, which would be over 300 kg K2O/ha (230 x 1.33 = 306)*. A crop yielding 60 t/ha will remove 60 x 5.8 = 350 kg/ha which will have needed a maximum supply of over 450 kg/ha (350 x 1.33 = 466).
Varietal and husbandry improvements have resulted in a continual increase in potato yields - for second earlies and maincrops, yields have increased from around 20 t/ha in 1960 to approaching 50 t/ha in recent years. This has an important implication for potassium supply because it needs to match this increased level of production.