Development of on-farm plant tests for phosphate and potassium in wheat
Critical phosphorus (P) and potassium (K) concentrations in winter wheat plants and soils for maximum grain yield were determined in a three year field study by IACR-Rothamsted and ADAS-Gleadthorpe (1996-98). The work was a continuation of an earlier project funded by HGCA on PK diagnostics of winter wheat (1992-95). In the earlier project, plant tests recommended for field use were leaf %P, leaf %K, leaf Kw, and shoot %Pi and Piw (Pi is inorganic orthophosphate, the stored form of P, whilst Kw and Piw are Pi and K concentrations in tissue water). The main aims of the present project were to develop on-farm testing procedures for Piw and Kw in wheat, and to investigate the effects of N and water supply on plant P, Pi and K concentrations.
On-farm methodologies were developed in the laboratory at Rothamsted and validated under field conditions at Rothamsted and ADAS sites. Field experiments on the PK responses of winter wheat were conducted at Rothamsted in Hertfordshire, Gleadthorpe in Nottinghamshire, Ropsley in Lincolnshire and Sedge Fen in Suffolk. At Rothamsted, crops were grown on soils having plant-available P and K levels ranging from deficient to abundant. At Ropsley, soil P levels ranged from deficient to sufficient. The effects of water supply were studied in irrigation experiments on the light sandy soil at Gleadthorpe. Sedge Fen, a K-deficient site, was used to study responses to K fertiliser. The effects of N were studied in field experiments at Rothamsted, Gleadthorpe and Ropsley.
Critical plant-available soil P and K for maximum yield changed little between years, but varied with soil type. Critical topsoil P for 95% maximum grain yield averaged 8 mg/kg and 17 mg/l at Rothamsted and Ropsley, respectively, with yield responses of 2.1 and 1.6 t/ha relative to Index 0 and 1 soils at Rothamsted and Ropsley, respectively. Critical topsoil K at Rothamsted averaged 88 mg/kg with a yield response of 4.2 t/ha relative to the Index 0 soil. Topsoil K was not a reliable guide to the likelihood of a grain yield response at Sedge Fen.
Critical plant concentrations, for 95% maximum grain yield, generally agreed well with those determined in 1992-95. Over all sites and seasons (1992-98), critical leaf(1) %P was in the range 0.23-0.38% (GS 31-39). Similarly, critical shoot %Pi was in the range 0.028-0.071% (equivalent Piw range 2.7-6.7 mM). Critical leaf(1) %K was in the range 1.61-3.21% (equivalent Kw range 126-227 mM).
Applying N to N-sufficient crops had no effect on plant P, Pi and K concentrations. Differences in N supply in the normal working range, 100-250 kg/ha, are unlikely to have any effects. In N-deficient crops, shoot and leaf(1) %P and %K were lower, Kw was the same, and Pw, %Pi and Piw were higher than in N-sufficient crops. It is likely therefore that critical %P and %K, for 95% maximum grain yield, will be lower where N is limiting, but that critical %Pi, Piw, and Kw will be unaffected.
Dry matter concentrations of %P, %Pi, and %K in shoots and leaf(1) were reduced by drought, and critical dry matter concentrations for 95% maximum grain yield also appeared to be reduced. In the case of tissue water concentrations (Pw, Piw, Kw), the effects of drought were not so consistent, with responses depending on the particular concentration parameter and the plant organ. Critical values in two of the recommended plant tests however, namely leaf(1) Kw and shoot Piw, appeared to be largely unaffected by drought.
Critical tissue water concentrations were determined by standard laboratory methods in this and the previous project, consequently it is important that on-farm methods give the same results as the laboratory methods if the previously determined critical values are to be used as the basis for on-farm tests. On-farm tests involve extraction of tissue water, dilution, addition of reagents and analysis with portable analytical equipment. We assessed two portable analytical instruments suitable for on-farm use, the RQflex meter for Pi and K which measures reflectance from test strips, and the Cardy-K meter which is a miniaturised ion-specific electrode.
The RQflex meter generally performed well for Pi but was not thoroughly assessed for K. The meter gave a linear response up to 60 mg PO4/l (0.63 mM Pi) with standards. Tissue water would typically need to be diluted 30 times to bring Pi concentrations into this range. There was good agreement between the RQflex and standard laboratory instruments for analysing Pi in plant extracts.
The Cardy-K meter gave a linear response up to 3900 ppm K (100 mM K) with standards. Tissue water would typically need to be diluted 3 times to bring K concentrations into this range. Cardy underestimated the standard laboratory instrument (Flame Photometer) with plant extracts possibly due to interference by other ions.
Extracting representative samples of tissue water from fresh plant material was not easy, especially for wheat leaves. It was necessary to rupture plant cells by freezing and thawing them to achieve this. Tissue water was then readily extracted from fully thawed material by squeezing it in a plastic syringe.
For Pi analysis, it was essential to analyse tissue water within 30 minutes of the start of thawing to minimise conversion of organic to inorganic phosphates. If this was done there was good agreement between the on-farm and laboratory methods for shoot Piw (a protocol for this test is given in the Appendix).
In the case of K, tissue water concentrations obtained by the 'freeze/thaw' method were invariably less than those obtained by extracting oven-dried material with water, irrespective of the pressure used. This may be due to the difficulty of extracting K from cell walls in frozen/thawed material. The Cardy meter also underestimated K compared with the Flame Photometer for all types of extract. Consequently, more development work is required on the on-farm method for K before it can be recommended.
About this project
Cereal growers face increasing economic and environmental pressures to use fertilisers as efficiently as possible. Most arable soils in England and Wales are unlikely to be responsive to P and K, but uncertainties over soil tests in some situations means that many growers are still far from confident about the P and K supply to their crops. Plant testing is the only way to tell if crop P and K requirements are actually being met.
Critical concentrations of P, Pi and K in plants and soils for maximum yield of winter wheat were determined in a previous HGCA project (Project Report No. 137). In the present LINK project, plant testing procedures suitable for the on-farm diagnosis of Pi and K status were investigated. On-farm tests are potentially attractive to growers and consultants because of the ease and speed with which they can be carried out.
When cereals are over-supplied with P, plants store the excess as inorganic orthophosphate (Pi) which is a better diagnostic indicator of plant P status than total-P. Maintaining Pi above 6 mM in wheat shoots ensures maximum grain yield. This can be quantitatively monitored on-farm by sampling the crop, squeezing out sap and analysing for Pi using test-strips and a portable hand-held reflectometer. Plants must be frozen and thawed to obtain sufficient amounts of the correct pool of sap. A protocol for the shoot Pi test is included in this report.
Cereal K requirements are completely satisfied when K in leaf tissue water attains a concentration of 170-230 mM as determined by standard laboratory methods. On-farm procedures consistently under-estimated plant K concentrations compared with standard laboratory methods. This was due to a combination of under-extraction of K by the freeze/thaw/squeeze process, and under-analysis of K by portable instruments. More development work is required before an on-farm method can be recommended for plant K.
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