SummaryThe growth and water use of sugar beet affected by early (ED) and late (LD) drought was compared with that of irrigated (I) and unirrigated (NI) controls. Mobile shelters were used to exclude rain from ED plots during June and July, and LD plots during August and September, respectively, whereas outside these periods the ED and LD plots were irrigated as necessary.The ED treatment affected the fibrous roots severely. Many of the roots in the top 60 cm of soil died and development of the root system below this depth was slow. Expansion of the leaf canopy slowed, radiation interception was reduced and the rate of water use fell from about 1·2 times to 0·6 times Penman potential transpiration rate. The LD treatment, which was imposed when the fibrous root system was already extensive, had little effect on the fibrous roots except in the top soil. The accessible soil water was quickly depleted and the resulting stress was accompanied by earlier senescence of leaves. The rate of converting intercepted light to crop dry matter was reduced in both treatments. However, the ED treatment was the most detrimental because the amount of light intercepted in the months of highest radiation was greatly reduced owing to the restricted leaf cover. The relative effects on growth are reflected in the final sugar yields which were 8·7, 10·5, 9·9 and 12·0 (±0·30) t/ha in the ED, LD, NI and I treatments respectively.More of the deep soil water was used in the drought-affected plots (particularly LD) than in the irrigated controls. Maximum depths of water extraction were 140–150 cm in ED and I plots and > 170 cm in LD plots. The highest uptake rates per unit length of root (20–40 μl/cm per day) were measured in the deepest part of the root system. At all depths, uptake rates declined as the soil dried. After correcting for overestimated water use where necessary, the ratios of final dry matter and sugar yields respectively to season-long water use (June–October) were close to 1·4 and 0·8 t/ha per 25 mm for all four treatments.

During 1973-8 six field experiments examined the effect of 0, 41, 82, 124, 166 and 207 kg N/ha with and without irrigation on the growth, yield and quality of sugar beet. The culture of the crops was planned to produce a large yield in order to determine the optimal nitrogen application for the above-average crops which many growers are now seeking to produce. Ammonium nitrate was used as the nitrogen source, broadcast in one dose before sowing as was recommended practice in the early 1970s. The growth of the crop was monitored from the seedling stage to harvest in December, as was nitrogen uptake by the crop, and water removal from the soil using a neutron probe.In 3 years when the weather was dry after drilling, the fertilizer significantly depressed the number of plants which established but plant weights showed that some nitrogen fertilizer was needed early for rapid seedling growth. Changes in the method of applying fertilizer for sugar beet are therefore suggested and are being tested. Soil analyses in the plough layer during establishment (May-June) indicated an optimum concentration of mineral nitrogen of about 40 mg N/kg soil at this stage.Nitrogen fertilizer was very important for a high yield; throughout the growth of the crop it greatly increased total dry-matter yield and at final harvest this was reflected in sugar yield. Considering the six years together, sugar yield was linearly related to both dry-matter yield and total nitrogen uptake. However, within a year, increasing nitrogen uptake above 200 kg N/ha with nitrogen fertilizer did not increase sugar yield; maximum yields of sugar each year were normally obtained with 125 kg N/ha fertilizer or less, and irrigation had little effect on the optimum amount. Explanations for the lack of responsiveness of sugar beet to greater applications of nitrogen fertilizer are being sought in further more detailed analyses of the crop and its environment. INTRODUCTIONfrom the seedling stage until harvest with and without irrigation and nitrogen fertilizer. ThroughDespite experiments at Broom's Barn, Woburn out the experiments, special effort was made to and elsewhere (Draycott & Webb, 1971; Penman, ensure a high yield so that the data obtained were 1952,1962Harris, 1972; Wolley& Bennett, 1962), applicable to the above-average crops at which there is still widespread belief amongst sugar-beet growers were aiming. growers aiming to produce consistently high yields, At the seedling stage, particular attention was that when the crop is irrigated more nitrogen paid to the effect of nitrogen fertilizer on number of fertilizer is needed to achieve potential growth and plants and growth. The former became more releyield than is usually recommended, e.g. by Sugar vant with the adoption of precision sowing to a Beet Research and Education Committee (1982). final plant stand because Jaggard (1979) has shown Previous experiments measured final yield only and the necessity of a full, regular plant stand for showed that there was little interaction between maximum yield. I...

The influence of season, and certain agronomic treatments (irrigation, nitrogen fertiliser, density of planting and sowing date) on leaf number were analysed in a series of sugar-beet crops grown during the five seasons 1978-82. Leaf appearance was a linear function of thermal time (accumulated temperature above 1 "C) and could be described by four variables: a) the thermal duration of the seedling establishment phase, d's; b) the thermal time interval between appearance of each of the early leaves, Be; c) the thermal duration of the early phase of leaf appearance, d'a, and d) the thermal time interval between the appearance of each of the later leaves, 81. The progression of leaf death could also be described by a thermal time interval, Bd.There were only small differences in the number of leaves produced by the eleven crops grown during the five seasons. Such differences as appeared, were largely attributable to changes in d'a and 81, which were interpreted as responses to increasing competition for mineral nutrients and assimilate at the shoot apex. Be was similar in all crops; 30"Cdays were needed between the appearance of each of the early leaves. Only the early leaves died. Each one was retained by the plant longer than its predecessor. Increasing soil moisture deficit under an unirrigated crop shortened Bd and depriving crops of nitrogen lengthened it. It is concluded that small differences in the rates of leaf appearance did not greatly influence the rates at which leaf canopies expanded early in the season, but that the rates of leaf death influenced both the time at which the canopies reached their maximum sizes and the rates at which leaf areas subsequently declined.

Sugar yield increases from irrigation in 19 experiments on a sandy-loam soil at Broom's Barn over an 11-year period were examined in relation to rainfall and to both potential and measured soil moisture deficit. Irrigation increased average yield from 7-6 to 8-3 t sugar/ha and in six of the years significantly increased yield by more than 11 sugar/ha (15%). The experiments also tested plant density, nitrogen, harvest date and time and amount of irrigation. Without irrigation, maximum sugar yield was from a density of 74000 plants/ha but larger densities gave slightly more yield when irrigated. Irrigation affected the magnitude of response to nitrogen but 100 kg N/ha gave the most profitable yield increase, both with and without irrigation. Yield increases of about 11 sugar/ha (15%) between early and late harvesting were also independent of irrigation. Early irrigation of 25 mm and 50 mm in June and July respectively increased yield in 4 of the 5 years but in all years applications in late summer did not increase sugar yield. The main factors controlling the yield response to irrigation were period and size of deficit. The soil type and summer rainfall at Broom's Barn were compared with those in 36 other experiments at five localities between 1947 and 1973; yield increases at Broom's Barn were smaller, probably because the others were on lighter soils.

SUMMARYGrowth of the roots of sugar beet, potato and barley in the field was observed through glass panels and related to changes in soil moisture measured by a neutron probe during 1969–71. The depth of observed root growth was generally related to, but 10–15 cm deeper than, the maximum depth of soil‐moisture extraction. On average of three years, sugar beet, potato and barley used water from the top 23, 33 and 45 cm soil respectively by the beginning of June, and from the top 70, 68 and > 100 cm soil by the end of June. Maximum soil drying in each horizon gave an in situ measure of available water capacity, and showed that sugar beet and barley eventually extracted similar amounts of water from each horizon, but potatoes extracted less, especially from below 60 cm. Between 30 and 100 cm deep, the in situ available water capacity (per 10 cm soil) progressively decreased from 16 to 10, 15 to 5 and 16 to 8 mm under sugar beet, potato and barley respectively. The calculated soil‐moisture deficit (potential evapotranspiration minus rainfall) and measured soil moisture deficit were not related early in the growing period before the crops established much leaf cover.