Nutrient content of straws.
Straw has always been an important by-product of cereal production, with its main use historically being as feed and bedding for livestock. This remains the primary use, but other markets have developed over recent years, particularly as a result of the low profitability of arable farming and the need to maximise returns through the sale of the straw.
Currently two factors combine to focus attention on cereal straw. The improved price of grain has lessened the pressure to look for income from by-products, leading to a review of whether the removal and sale of straw is in the best interests of soil structure and fertility. The second driver of interest in straw has been the rise in the value of the fertiliser nutrients which straw contains, principally potash but also some phosphate.
This lessening of pressure to sell the straw presents a choice. Some pros and cons can be considered.
Incorporating straw |
|
| Some benefits | Some disadvantages |
Adds organic matter to the soil, and can help to improve soil structure. Returns the nutrients in the straw to the soil. No soil structural damage from baling and carting in wet conditions. Potential reduction in loss of N from the soil in the autumn. No delay caused by baling and carting. Lower labour requirement, unless contracted. |
Extra diesel used by the combine when chopping straw. Potential for slug problems. Competition with crop for available soil nitrogen in the autumn. May be difficulties with incorporation. No additional direct income. |
Removal and sale of straw |
|
| Some benefits | Some disadvantages |
Income from the sale of the straw. Potentially easier establishment of the following crop. Reduced potential for slug problems. |
Costs of baling and carting, unless contracted. Significant extra removal of fertiliser nutrients from the field. Possible delay in baling and carting, leading to delayed establishment of following crop. Possible soil structural damage if soils are wet during baling and carting. Income from the sale of straw may not be spent on replacing the nutrients removed! |
None of these issues is new, although changed circumstances lead to different priorities. The recent rise in the price of fertiliser nutrients has led to a detailed consideration of these pros and cons of chopping or selling straw, but other factors such as the price of diesel will also have a significant impact on the decision.
In order to factor the changed price of fertiliser nutrients into the decision, an estimate of the nutrient content of straw must be available. This can usefully be calculated on a per hectare basis, but when straw is sold and bought it is also important to know the probable quantity of fertiliser in each tonne of straw. Analysis shows that the nutrient content of spring cereal straw is higher than of winter straw, and that the potash content of straw is much higher than the phosphate content.
On average the amount of straw produced per hectare is in proportion to the grain yield, so the nutrient content per hectare of straw can be estimated by multiplying the grain yield by standard factors for phosphate (P2O5) and for potash (K2O) which take into account the ratio of straw to grain yield.
Winter cereal straw nutrient content (wheat and barley):
8 x 6.2 = 49.6 kg K2O/ha
9.5 kg K2O per tonne of straw.
Spring cereal straw nutrient content (wheat and barley):
6 x 8.1 = 48.6 kg K2O/ha
12.5 kg K2O per tonne of straw.
Potash in different straws - kg per tonne of straw
The table shown below gives the best estimates of the phosphate and potash contents of a number of straws as harvested, but the values for those marked with an asterisk are from relatively few samples. These figures provide a useful guide, but the actual values for specific crops may vary. For example the potash (but not the phosphate) contained in the straw may be reduced, by up to 40%, if there is appreciably more rain than average between the onset of senescence and baling.
| Straw type | Guide kg nutrient / tonne of fresh straw | |
| Phosphate - P2O5 | Potash - K2O | |
Winter wheat/barley straw |
1.2 |
9.5 |
Spring wheat/barley straw |
1.5 |
12.5 |
Oat straw |
1.6 |
16.7 |
Oilseed rape straw |
2.2 |
13.0 |
Rye straw |
2.1* |
10.0* |
Pea straw/haulm |
3.9* |
10.0* |
Bean straw/haulm |
2.5* |
15.9* |
Linseed straw |
1.6* |
9.2* |
* best estimates from few samples. |
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For further information please contact:
PDA info@pda.org.uk
Potash Development Association
update: August 2008
