Crop Production in the East of Scotland - SASA

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Climateware potatoes to winter wheat can poseproblems if the potato harvest is late, as isoften the case; and there is a limited choiceof crops to precede winter oilseed rape,which is normally drilled in August. (Theimplications of winter cereals for diseaseepidemiology are discussed in Chapter 5).The minimum temperature for growth ofmany temperate crop species is around 0°Cbut growth rates are low below 6°C. Thegrowing season for grassland species is,therefore, commonly defined as the numberof days when the mean daily soiltemperature at a depth of 0.30 m is above6°C, giving typical values around 210 days,rising to 225 days in coastal parts of theLothians (Table 2.1). Even at 6ºC,mineralisation of nitrogen in the soil isinhibited, such that the onset of rapidgrowth is delayed in crops not supplied withfertiliser nitrogen. Mean air temperaturedeclines by 0.6 to 1.0°C for each 100 mincrease in altitude, and, because of the lowrate of increase in temperature with time inspring, the potential growing season fallsoff rapidly with altitudeAgriculturally, the temperature of the top5 cm of soil is more important than the airtemperature, because, for example, theapical meristems of cereal and grass plantsremain below the soil surface duringvegetative growth; nevertheless, air andsoil-surface temperatures are normally wellcorrelated(Hay, 1976, 1977; Hay et al., 1978).The temperature of soils which areintrinsically wet (owing to texture, structureor drainage), tends to lag behind airtemperature and the onset of rapid growthcan differ by more than a week betweenneighbouring but contrasting soils. In thecase of grassland, this can be important forthe turning out of livestock in the spring.Raised beach soils in the Lothians weretraditionally used to grow early vegetablesbecause their sandy texture allowed themto warm up quickly in spring, but they werealso vulnerable to frost. Although cerealand grassland crops are resistant to thelevels of freezing stress experienced in theEast of Scotland, frosts as late as 20 May(Figure 2.1) can delay the canopydevelopment of potato and sensitivevegetable crops, with consequent yieldlosses.WaterLong term average precipitation is fairlyevenly distributed throughout the year,Figure 2.1 Date of last spring frost (grass minimum temperature) at threemeteorological stations in the East of Scotland, 1961 – 1999.19
Crop Production in the East of ScotlandFigure 2.2 Mean August and September rainfall at three meteorological stations inthe East of Scotland, 1961 – 1999. The curve is fitted to five-year runningmeans.although there is considerable variabilityfrom year to year (Table 2.1). There is atendency towards drier weather in Februaryto April, and the harvest months of Augustand September can be very wet (Figure2.2). The driest areas are those on theSouth-East and Moray Firth coasts wherethe average annual rainfall is less than600 mm. Annual precipitation increases tothe west because of an increase in altitudeand proximity to the wetter west coast. As arule of thumb, arable agriculture is notviable when annual rainfall exceeds1000 mm.In Scotland, rainfall per se is not usually thelimitation to agriculture but rather soilwater status, which depends on the balancebetween precipitation on the one hand andlosses from evapotranspiration anddrainage on the other. Temperature sets thepotential length of the growing season forall crops, as outlined above; however, withmechanised agriculture, the actual growingseason is effectively restricted to the periodwhen there is a soil water deficit (Table 2.1),so that the soil is sufficiently strong tosupport the weight of cultivating, plantingand harvesting machinery without sufferingdamage. There is very considerable interannualvariation in soil water content,superimposed on the annual cycle. Forexample, Francis (1981) estimated that, overthe years 1957-75, the first soil moisturedeficit in spring occurred outside the period21 February to 20 March in half of thegrowing seasons; the corresponding rangefor autumn return to field capacity was 20October to 30 November (Hay, 1993). Thedifference between the actual and potentialgrowing season can, of course, be reducedby the use of winter-sown or perennialcrops, thus reducing the need for traffic inspring.Over the twenty year period up to 1999, thecumulative rainfall from April to July (themain period of crop growth) ranged from50 to 150% of the mean, such that cropestablishment could be hindered as muchby too little as by too much soil water.August and September rainfall (Figure 2.2),which is important for the harvest ofcereals, bulking of main crop potatoes and(October) drilling of winter cereals, is alsorather variable. Winter rain, whichinfluences the leaching of nutrients overthe winter period and the degree of20
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Crop Production in the East of Scotland - SASA

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