Potash time; holidays should be over now

Reminder from the PDA wheat plots

The recently reprinted PDA Leaflet 5a 'Results from Wheat Demonstration Plots' provides a useful reminder of the serious consequences of allowing soil K levels to run down. Considerable quantities of potassium are carried out of fields in grain trailers, and as much again or more when the straw is sold off the field. An average 8 t/ha wheat crop plus straw removes almost 100 kg K₂O/ha from the field, and spring cereal and oat straw removes even more than winter cereal straw. Crops grown on soils with significantly depleted K reserves will never perform to expectation.

'Potash for Sugar Beet' - new PDA Leaflet No 12

The PDA has been working with the British Beet Research Organisation, Rothamsted Research and British Sugar to produce a fully revised edition of the PDA Leaflet No 12, 'Potash for Sugar Beet'. The leaflet includes the results of the most recent work undertaken to look into the potassium, sodium and magnesium requirements of the sugar beet crop, and confirms the recommendations that are valid for today's higher yields. The leaflet includes this Summary:

• Potassium (K, potash: K₂O) is essential for all crops and sugar beet requires large amounts.
• Soils should be maintained above the critical level for plant-available K, P and Mg to achieve optimum yields and quality of all the crops grown on the farm.
• For sugar beet, the critical level for potash is K Index 2-. Below this level, yield will be lost. Yields are not increased when soils are at K Index 3 and above.
• Crop need K where the roots can get it, i.e. throughout the soil explored by the roots. Sugar beet only occasionally responds to freshly applied potash, and then only on soils with very low levels of available K (K Index 0). It is wise to regard applications of potash as part of the longer-term strategy to raise or maintain soil K at a suitable level rather than as a means of immediately increasing beet yields.
• Potassium in the ploughed-in tops is returned to the soil K reserve.
• To maintain adequate levels of soil K, the K removed in the harvested beet needs to be replaced on all soils below K Index 3. The quantity removed can be seen for individual contracts from British Sugar On-line.
• Sugar beet requires a small amount of sodium (Na). In addition sodium can substitute for some of the functions of potash; applying Na will be of benefit on K Index 0 and 1 soils and on soils low in available sodium.

Large proportion of arable crops not receiving a dressing of potash

The British Survey of Fertiliser Practice 2005 has just been published, and it again highlights a potentially serious situation in mainland Britain; only 65% of arable crops received a dressing of potash in the 2004/05 season. The chart shown overleaf illustrates the significant reduction in the area of all the major arable crops which received potash in recent years compared with applications during the 1990s. All these crops remove large amounts of potash from the soil.

While the Survey shows that there was a large percentage of the arable area which did not receive potash (or phosphate), it cannot of course give the reason; less than 20% of the arable area (and only 10-12% of winter wheat or OSR) gets manure, so that does not explain it. PDA Leaflet 5a clearly shows the effects of not replacing potash removed from the soil, both in terms of yield and of soil nutrient status (fertility). If savings have had to be made in economically difficult seasons, it is very important to check the soil K index to make sure that soil reserves are not low and that sufficient is applied when better times allow. However, the serious yield and quality effects resulting from a shortage of K mean that withholding a potash application should never be among the first economies considered.

For further information please contact:
PDA info@pda.org.uk
Potash Development Association

update: August 2006