Technical Note (TN718): Phosphate and Potash Recommendations for Crops Grown in Fife, Lothian and Scottish Borders

TECHNICAL NOTE TN718 February 2020 • ELEC Phosphate and potash recommendations for crops grown National Advice Hub T: 0300 323 0161 E: [email protected] in Fife, Lothian and Scottish Borders W: www.fas.scot Summary • Regular soil analysis with accompanying field information is essential to manage soil PK inputs for optimum yields, profitability and minimising pollution. • A new soil specific approach to P management is presented that takes into account the relationship between differing soils capacity to regulate P availability for plant uptake. • Soils of SE Scotland have been designated and mapped as index 1, 2 and 3 to reflect inherent soil phosphorus sorption capacity (PSC) which impacts how soils must be managed. • For cereal-based arable rotations the target soil P Status has now been lowered to M- on PSC 1 and 2 soils but stays at M+ on PSC 3 soils. The target soil P Status is M+ for rotations including potatoes and responsive vegetable crops. • Adjustments to P and K fertiliser recommendations are made taking account of PSC indices to build up or run down to the new target soil PK Status. • AHDB-funded research on P nutrition has shown that the critical level of grain P in winter wheat is 0.32%. Grain P contents repeatedly below this level indicate crop P uptake was deficient enough to reduce grain yield. Introduction Soils should be sampled every 4 to 5 years and analysed for soil pH, applications. Particular care should be taken to avoid building up extractable P, K and Mg. The value of accompanying field information crop-available phosphate above the target Status. Recent research such as FID, and previous and next crop increases the value of any has shown that phosphate transfer from soil to surface water subsequent analysis of data including use of PLANET Scotland, a increases as soil P Status increases. Much of this transfer is due to software tool designed for Scottish farmers and consultants to plan the loss of crop-available phosphate through land runoff. and manage nutrient use on individual fields (http://www.planet4farmers.co.uk). Regional differences in soil extractable P concentrations exist primarily in response to variations in land use and particularly the Fields are best sampled before potatoes which often give a yield greater role of livestock in the west compared to arable cropping in response to PK fertiliser in the year of application. Cereals and the east. No obvious or consistent temporal trend in extractable P oilseed rape do not generally respond on moderate PK status soils is apparent, but a very wide range of soil properties and extractable and sampling time is therefore not so crucial, but the application of P on concentrations is evident. In this technical note an attempt fertilisers and manures may influence results for up to 12 weeks after is made to link advisory P and K data from SE Scotland with application. Results may be lower than expected if fields are sampled information from the National Soil Database of Scotland; and during periods of maximum nutrient uptake, particularly for K after to explore the value of introducing ‘soil type’ into the P fertiliser cuts of silage, but higher (or more variable) than expected in spring recommendation system, initially published in TN668 (2015) due to decomposition of crop residues. Fields should be sampled (www.fas.scot/publications/technical-notes/). between September and February, depending on timing of fertiliser AD18.2.2020 Trends in soil PK advisory data from 1993-2017 Classification of extractable soil PK into PK Status SAC Consultancy classifies soil PK test values extracted by the Modified Morgan’s method into 6 categories (Table A). Table A. Classification of extractable soil PK (mg/l) into PK Status PK Status Very Low (VL) Low (L) Moderate (M-) Moderate (M+) High (H) Very High (VH) Range (mg P/l) <1.8 1.8-4.4 4.5-9.4 9.5-13.4 13.5-30 >30 Range (mg K/l) <40 40-75 76-140 141-200 201-400 >400 Nationally 225,396 samples have been analysed from 1993 to 2017 for the advisory services. PK extractable values are classified into P and K status expressed as % of the total number of samples (Table B). The data show that about 33% of samples required more than maintenance P applications i.e. VL and L samples; compared with 19% required more than maintenance K applications. Table B. Percentage of national samples in each P and K Status PK Status VL L M- M+ H VH P 6.5 26.1 40.5 13.7 11.0 2.3 K 2.4 16.8 40.9 22.2 16.2 1.5 Effect of 5-year periods on distribution of P and K Status SE Scotland data from 52,661 samples, divided into 5-year periods, are shown in Table C for P Status and Table D for K Status. There has been a large drop in the number of samples for PK analysis from 17,154 (1993-1997) to less than half during the last 10 years. Changes over time were apparent for extractable soil P. The data showed that between1993-1997 about 31% of samples required more than maintenance P applications i.e. VL and L samples, and 20% of samples require less than maintenance i.e. H and VH samples. In comparison during the last 5 years about 42% of samples require more than maintenance P applications i.e. VL and L samples, and 12% of samples require less than maintenance i.e. H and VH samples. The distribution of samples across K status remained very similar over the entire time period with about 13% of samples requiring more than maintenance K applications i.e. VL and L samples, and about 20% requiring less than maintenance i.e. H and VH samples. Table C. Percentage of samples in each P Status Years Total VL L M- M+ H VH 1993-1997 17,154 7 24 34 15 15 5 1998-2002 12,181 7 23 34 15 16 4 2003-2007 9,086 6 27 34 14 14 5 2008-2012 7,858 9 30 35 12 11 3 2013-2017 6,382 8 34 35 11 9 3 Table D. Percentage of samples in each K Status Years Total VL L M- M+ H VH 1993-1997 17,154 1 11 44 25 18 1 1998-2002 12,181 1 14 43 24 16 1 2003-2007 9,086 1 12 41 24 20 2 2008-2012 7,858 1 15 41 24 17 2 2013-2017 6,382 1 12 40 25 20 2 Effect of distribution of P and K status by crop Nearly 70% of samples had additional information recorded such as previous and next crop. This provides a mechanism for assessing the possible influence of different fertiliser strategies for individual crops. About 45% of samples were taken in grass and 22% of samples were taken before a spring arable crop; 20% before winter arable crop and 10% before potatoes. Of samples in grass a large proportion of these (75%) continued in grass and 50% in vegetables continued in vegetables. The range in soil P and K status reflects a wide range in intensity of farming. The distribution of soil P status is summarised for the next crops in Table E and K status in Table F. These differences are clearly evident with P Status have 58% of samples in grass and fodder crops with VL and L status compared with only 6% in VL and L status going into vegetables and 50% of samples with H or VH status (Table E). In comparison K Status have 14% of samples in grass and 10% in fodder and vegetable crops with VL and L status compared with 23% of samples in grass, 24% in fodder and 20% in vegetables with H or VH status (Table F). 2 Table E. Percentage of samples of each P Status for next crops P (%) VL L M- M+ H VH Grass 15.5 42.0 28.9 7.2 5.0 1.1 Spring arable 3.9 23.1 38.6 16.4 15.3 2.3 Winter arable 2.8 22.3 39.7 15.9 15.0 4.2 Potato 1.2 15.5 38.6 19.2 20.4 5.2 Vegetables 0.4 5.7 22.8 21.6 33.7 15.9 Fodder 15.5 42.9 27.0 6.6 7.4 0.3 Table F. Percentage of samples of each K Status for next crops K (%) VL L M- M+ H VH Grass 1.2 13.2 39.0 23.8 20.9 2.0 Spring arable 1.2 13.4 44.1 24.0 16.2 1.1 Winter arable 0.5 10.9 44.8 25.8 17.3 0.8 Potato 1.2 11.5 44.9 27.0 14.3 1.1 Vegetables 0.7 9.8 43.3 26.6 17.7 2.0 Fodder 0.5 9.8 36.9 28.8 22.1 2.0 Estimated yield loss Large responses to P and K in the year of fertiliser application are only expected where the soil P and K Status is very low, although some vegetable crops respond to fresh additions of P and K at low soil Status. Low PK Status soils do not necessarily have a low yield potential given adequate fertiliser P and K but site/season and other aspects of husbandry are the major determinant of yield potential. Season factors are not predictable so soil PK Status is the main variable determining fertiliser requirements.

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