Plant and soil testing to assess the adequacy of phosphorus supply to winter oilseed rape

Abstract

Winter oilseed rape is a common cereal break crop in farm rotations, yet relatively little is known about the phosphorus (P) requirements of this crop under UK conditions. With farm profit margins being squeezed and continuing concern for the environment, P fertilisers need to be used more precisely than in the past to ensure that soil supply and crop demand are closely matched at all times.

Soil testing is the usual way of assessing the adequacy of P supply to arable crops and for making fertiliser recommendations, but there is no published information on the response of rape yield to soil P or on critical soil P for maximum seed yield in the UK. Plant testing could also be used to help with fertiliser planning, but again information is lacking on critical plant concentrations for maximum yield under UK conditions.

Maximum rape seed yields (4-5 t ha-1) were attained with a soil Olsen-P of 9 ppm (top of MAFF Index 0). Yield reductions of 50% were observed where soil P was below 5 ppm. Crops are likely to respond to P fertiliser on Index 0 soils. A much higher soil P level of 27 ppm (bottom of Index 3) was needed for maximum canopy growth. Large autumn canopies may be beneficial where pigeon damage is a recurring problem, but lush canopies were not essential for maximum seed yield. If soils have luxury P levels (Index 3 and above), growers can save money by withholding P in the short-term. But if pigeon damage is a recurring problem, growers may wish to maintain soil P at high Index 2 to encourage autumn canopy growth.

Tissue testing can be used to confirm the adequacy of soil P supply to rape crops. Traditionally, %P in leaf dry matter is used for assessing plant P status, a service offered by commercial testing laboratories. At the rosette stage in the autumn, leaf-blade samples taken from the middle of the plant canopy need a P content of 0.45% for maximum seed yield. Alternatively, growers can do their own tissue tests by measuring how much P has been stored as phosphate in leaf tissue water (the Pi test). Kits are available for carrying out this test. Between the rosette stage in autumn and the yellow bud stage in spring, leaf-blade samples taken from the middle of the plant canopy need Pi values greater than 4 mM for maximum seed yield.

If a plant test indicates P deficiency, even if it is carried out in the autumn, a grower may still not be able to take remedial action for that crop. Crops are unlikely to respond to top-dressings of P in the way that they do to N and S because of the poor mobility of P in soil. Equally, it is uncertain if foliar applications of P can be beneficial on deficient rape. At least the test will alert the grower to the need for action in subsequent crops.

Critical soil P values for high yielding crops of winter wheat (10 t ha-1 ) and oilseed rape (5 t ha-1) are very similar at 9 ppm (Olsen method), and the crops remove similar amounts of P in grain and seed (26 kg ha-1 P). Therefore, wheat and rape can be considered as identical crops in the rotation when it comes to planning a P fertiliser strategy. If soils are adequately fertilised for wheat, they should also be adequate for rape.