Are you putting back what you take away?
Across the UK more potash and phosphate is being removed in the main arable crops than is being applied in fertilisers. This will lead to a steady decline in soil fertility if not corrected.
Great Britain is unique in having an annual survey of fertiliser use. Information from this survey, crop yields available from Defra and standard potash and phosphate contents in crop produce were used to calculate annual potash and phosphate balances since 1974. The proportions of cereal areas where straw is baled and removed were taken into account.
Up to 1992, potash balances tended to be positive - a little more fertiliser potash was applied than was removed in crops. Since 1992, balances have become strongly negative and for the past few years have averaged around -15 kg K2O/ha/year. In 2002/2003, 40% of the winter wheat area received no fertiliser potash. Most soils cannot sustain this level of net potash removal for many years without crops suffering. Potash is not an expensive nutrient and it doesn't pay to risk yield by not replacing removals.
Phosphate balances also have been negative for the main arable crops since 1995. This too is not sustainable in the medium term and the costs of recovery from low soil indices are likely to be greater than those of annual applications according to soil analysis.
The two charts show the balances for potash (upper) and phosphate (lower) for the total area of wheat, barley, oilseed rape, sugar beet and potatoes in England and Wales (such long-term data are not available for Scotland). No account is taken of inputs from manures on these arable crops, which are applied to about 16% of all arable land (including forage crops etc), but for example to only 12% of winter wheat, and at unknown rates.
Most of the potash in a harvested crop of cereals is in the straw. Typically, wheat and barley straw contains around 0.8 - 1.25% K (1.0 - 1.5% K2O). Lower concentrations are found where rainfall has leached potassium from the standing crop as it senesces. Potassium concentration tends to be lower in the straw of winter wheat with its longer growing period than in that of winter barley that is harvested earlier. The potassium concentration in oat straw tends to be greater than that in wheat or barley straw and can exceed 2% K (2.4% K2O). Typically, around 50 kg K2O/ha is removed in wheat straw where 4 t straw/ha is baled and in barley straw where 3.5 t/ha is baled. This removal must be taken into account when decisions are made on fertiliser use.
A survey of 455 farms in England, Scotland and Wales carried out for the PDA in 2005 showed that the straw was baled and removed in 50% of the wheat area, 95% of the winter barley area and 80% of the spring barley area during the 2004 harvest.
A few simple rules will help avoid any unexpected costs due to yield loss or to the need for corrective fertiliser applications:
- Soil sample fields regularly every 3 - 4 years. Information is in PDA Leaflet No. 24:Soil Analysis: Key to Nutrient Management Planning.
- Calculate potash and phosphate balances each year. This can be done easily for arable crops either manually or in a spreadsheet. Standard removal figures for a range of crops are in PDA Leaflet Phosphate and Potash Removal by Crops or can be found in Defra RB209 Fertiliser Recommendations.
- Ensure the removal of potash in cereal straw is taken fully into account.
For further information please contact:
PDA info@pda.org.uk
Potash Development Association
update: July 2005
