Plant testing to determine the P and K status of wheat

Abstract

Practical significance

This project has increased our physiological understanding of P and K in cereal plants: chemical forms, amounts and variation with plant part and age. Ongoing LINK-funded work should result in recommendations for using a field-based colour change test.

Summary

British farmers spend about £22/ha annually on P and K fertilisers for wheat crops (equivalent to nearly £40M nationally), but have limited means of assessing how well a particular fertiliser strategy is doing or of fine tuning P and K inputs to crops.

Soil testing is the established method of assessing if P and K supplies are adequate for crop needs. The problem with soil testing is that critical concentrations of extractable soil P and K for maximum grain yield depend on soil type and few farmers, if any, know the critical values for their fields. This can lead to prophylactic applications of P and K fertiliser.

Plant testing could be used to diagnose the P and K status of wheat crops, but little use is made of this at present. This is because very little information is available on critical P and K concentrations in wheat under UK conditions, and the interpretation of conventional plant test results, that is tests based on total nutrient concentration in plant dry matter, is complicated by the effects of plant development and interactions between growth factors. The most important criteria for a plant test are that it must be quick and easy to carry out and critical concentrations should be stable, i.e. unaffected by crop growth stage or by soil type weather and management (site/season variation).

A 4-year collaborative project by IACR-Rothamsted and ADAS in the period 1992-95 investigated the use of conventional and new plant testing procedures for diagnosing the P and K status of winter wheat crops. As well as conventional dry matter testing of whole shoots, the use of the newest fully expanded leaf blade, expression of concentrations on a tissue water basis, and the use of inorganic storage pools were investigated. In all, twelve different approaches to plant testing for P and K were evaluated.

Field experiments were conducted at Rothamsted in Hertfordshire, at Ropsley in Lincolnshire and on three commercial farms in East Anglia. Winter wheat (cv. Mercia) was grown at Rothamsted and Ropsley on soils having extractable P and K levels ranging from deficient to abundant. At the East Anglian farms various wheat varieties were grown on K deficient soils to which a range of fertiliser K rates was applied.

Typical grain yield penalties as a result of soil P and K deficiency were 1-2 t/ha. Critical soil concentrations of extractable P and K for maximum yield changed little between years on the same soil, but varied with soil type. At Rothamsted, critical topsoil P and K for 95 % maximum grain yield were 10 and 80 mg/kg, respectively. At Ropsley, critical topsoil P was 16 mg/I. Topsoil K was not a reliable guide to the likelihood of a grain yield response to fresh fertiliser K on the three East Anglian farms.

Plant testing can be used to diagnose the P and K status of wheat crops. Problems associated with conventional plant testing can be reduced by specifying crop growth stage, by choosing indicator organs of a fixed physiological age, by targeting specific nutrient pools, and by expressing concentrations on a tissue water basis. The greatest variation in critical concentrations between sites and seasons was generally during tillering and at anthesis, consequently the best time to assess the P and K status of wheat is during stem elongation, GS 31-39.

For diagnosing plant P status, the recommended test is based on %P in dry matter in the newest fully expanded leaf blade (leaf(1)). Critical %P in leaf(1) was in the range 0.28-0.38% (average 0.32%) during stem elongation (GS 31-39). Alternatively, tests based on inorganic phosphate (Pi) in whole shoots, either in dry matter or tissue water, can be used. In dry matter, critical shoot %Pi was in the range 0.05-0.07% (average 0.06%) during stem elongation (GS 31-39). In tissue water, critical shoot Pi was in the range 4-6 mM (average 5 mM) during stem elongation (GS 31-39).

For diagnosing plant K status, the newest fully expanded leaf blade (leaf(1)) should be used. Critical leaf(1) K concentrations depended on yield. For crops yielding up to 8.5 t/ha, critical %K in leaf(1) dry matter was in the range 1.6-2.5% (average 1.9%) during stem elongation (OS 31-39), and critical K in leaf(1) tissue water was in the range 130-170 mM (average 150 mM) during the period GS 31-61. For a crop which yielded 9.5 t/ha, the corresponding critical values were 2.5-3.2% (average 2.9%) and 170-230 mM (average 200 mM), respectively.

Further work is required to refine tests based on leaf, tissue water and P storage pool concentrations. In particular, the effects of leaf age and position, and N and water supply on critical concentrations need further investigation. There is a good possibility of using in-situ techniques for measuring Pi and K concentrations in plant tissue water, and this also merits investigation.