© CAB International 2003. Nutrients for Sugar Beet Production: Soil–Plant Relationships
(A.P. Draycott and D.R. Christenson) 180
High-quality roots are critical in the extrac- tion of sugar and directly affect the econom- ics of both growing and processing the crop.
To make pure crystalline (or liquid) sugar, some processors have introduced payment systems to encourage growers to produce roots with the required quality characteris- tics. There are many references in previous chapters showing that nutrition during crop growth plays a large part in final root qual- ity. This chapter defines quality and identi- fies optimum nutrition to achieve it.
With most crops it is difficult to define quality because subjective criteria, such as texture, taste, shape and colour, are involved.
Fortunately, with sugar beet, most of the important aspects of quality can be defined and measured. Physical characteristics important in harvesting and processing, such as root shape and ease of slicing (Rasmusson and Wiklund, 1960), are more difficult to define but are affected little by nutrition.
Root constituents are: (i) water; (ii) dry matter; (iii) total soluble solids; (iv) total insoluble solids; (v) sugar; (vi) non-sugar, soluble solids; (vii) soluble nitrogenous organic compounds; (viii) soluble, non- nitrogenous organic compounds; and (ix) soluble mineral matter (ash). Approximate relationships among these constituents and their relative concentration in the root are given in Fig. 13.1.
Thus only 20% of the weight of beet as harvested makes up the material in solution processed in the factory. Water and insoluble solids are easily removed but this soluble portion containing the sugar must be puri- fied during processing, removing the non- sugar soluble solids. The quality of sugar beet is based on the compounds in this group of constituents. Since nutrition of the growing beet affects the quantity and nature of non-sugar soluble solids, it becomes a vital link in producing good-quality beets.
Sugar concentration and juice purity are the two most important components of beet as harvested (Carruthers and Oldfield, 1961).
Sugar is expressed as a percentage of the fresh weight of the beet, commonly called the
‘sugar percentage’ or ‘sugar content’. It is an important quality, affecting the amount of roots handled and transported and the fac- tory throughput. Purity is the ratio of sugar to total soluble solids, expressed as a percent- age. After soluble components are removed from beet, sugar is purified by a series of chemical and physical steps. Impurities decrease the extraction of sugar because some sugar is removed from solution with the impurities. The amount of sugar lost to molasses ranges from 1.5 to 1.8 parts for each part of impurity (non-sugar components).
Harvey and Dutton (1993) have reviewed other aspects of root quality and processing.
Determination of Sugar Percentage
The amount of sugar in roots is normally determined polarimetrically on an extract of fresh macerated root (brei) by the method used by Sachs and described by Le Docte (1927). Sugar percentage so mea- sured is usually in the range 15–20%.
Nutrients added as fertilizers may have considerable effects on the sugar percent- age: some decrease it and some are benefi- cial. Where a nutrient decreases sugar percentage but increases root yield, it is important to know the ‘break-even’ point where the increase in root yield equals the decrease in sugar percentage, resulting in maximum sugar production.
Over the past 50 years, considerable strides have been made in increasing sugar percentage. Many of these improvements result from plant breeding efforts world- wide, mostly by decreasing water content (increasing dry matter). The sugar percent- age of fresh roots is usually closely related to the amount of water in the roots.
Consequently, climatic conditions before harvest may have a marked effect. Some nutrients also affect the water content of roots and thereby the sugar percentage.
The concentration of sugar in roots expressed as a percentage of root dry mat- ter removes the effects of changes in water content. In most of the literature this is not reported. A few reports have shown that some nutrients also slightly affect the amount of sugar in dry matter.
Determination of Juice Purity
Juice purity can only be determined directly in the laboratory by a time-consuming com- bination of refractometric and polarimetric measurements on juice expressed from fresh roots (Carruthers and Oldfield, 1961). Many laboratories have therefore adopted an alter- native, indirect and quicker method of esti- mating juice purity (Carruthers et al., 1962).
Sodium, potassium and alpha-amino nitro- gen concentrations in the extract (prepared for the determination of sugar percentage) are determined and the values used in a regression, e.g. juice purity = 97.0 − 0.0008(2.5K + 3.5Na + 10 alpha-amino N);
potassium, sodium and nitrogen being expressed as mg 100 g1of sugar.
Carruthers et al.(1962) found a close rela- tionship between the two methods of assess- ing juice purity (r2 = 0.74). Harvey and Dutton (1993) provided a more detailed assessment of non-sugars in roots and meth- ods of measurement.
Effect of Nitrogen Fertilizer UK
The outstanding contributions of Carruthers and his team in the 1950s and 1960s greatly helped in understanding the criteria for sugar beet quality. His formulae have been tested, adapted and adopted in many coun- tries. Often nitrogen fertilizer was mentioned
Water – 75% Dry matter – 25%
Total soluble solids – 20% Total insoluble solids – 5%
Non-sugar, soluble solids – 4%
Sugar – 16%
Soluble, nitrogenous, organic compounds – 1.8%
Soluble, nitrogen-free, organic compounds – 1.4%
Soluble, mineral matter – 0.8%
Fig. 13.1. Approximate composition of sugar beet roots by weight (from Alexander, 1971).
by him because it decreased quality. This led to field trials to test the effects of fertilizers and how to optimize their use, not only for yield (as in the past) but for quality too (Adams, 1962; Draycott and Cooke, 1966;
Collier, 1967; Boyd et al., 1970). Tables 13.1 and 13.2 summarize this early work.
Generally a small amount of nitrogen fer- tilizer was found to have little, if any, delete- rious effect on sugar percentage and in a few trials it was beneficial. Larger amounts (100 kg N ha1or more) always decreased sugar percentage (see Chapter 2). The mode of action was mainly through decreasing the dry-matter percentage of the roots.
Results with juice purity were different because even the smallest amount of nitro- gen fertilizer (or organic manure) decreased it. Later work has shown that this was because all the nitrogen sources increased the concentration of nitrogen-containing components of roots at harvest.
Increasing nitrate concentration in the sugar beet plant would be expected to depress sugar percentage and juice purity.
Last and Tinker (1968) examined this effect
in detail. They varied nitrate concentration in sugar beet leaves and petioles using dif- ferent quantities of nitrogen fertilizer. Sugar percentage and juice purity were measured in beet roots at harvest. Increases in nitrate nitrogen in petioles up to 700 p.p.m. (sam- pled in June) had little effect on sugar per- centage. Above 700 p.p.m. an increase of 180 p.p.m. in petioles corresponded to a decrease of 1% in sugar percentage. Increasing plant nitrate concentration in June also had serious consequences for juice purity. The authors stressed that sugar percentage and juice purity depend on many factors and, although nitrate concentrations were related to sugar percentage and juice purity in single experiments, they found no dependable rela- tionships between them for all experiments.
Nitrogen has a surprisingly similar effect on widely differing soils (Fig. 13.2). Roots of sugar beet grown on peat soils had an aver- age sugar percentage of only 14.7 and a juice purity of 90.8%, whereas averages on sandy soil were 17.0 and 96.0%, respectively. A wide range of amounts of nitrogen fertilizer had parallel effects on all soil types for sugar percentage and juice purity.
Armstrong and Milford (1985) reported on a study of nitrogen uptake, sugar yield and amino N concentration. Their work sup- ported the view (Last et al., 1983; Burcky, 1991) that about 200 kg N ha1total uptake and no more is needed, even for very high- yielding crops, e.g. 15 t sugar ha1. Optimum amino N concentration in roots at harvest was more difficult to define.
Armstrong and Milford pointed out that, in an analysis of a large number of crops on a wide range of soils, amino N rose rapidly when total uptake exceeded 220 kg N ha1. For disease- and drought-free crops, an uptake of 200 kg N ha1coincided with an amino N concentration of less than 150 mg 100 g1sugar. In droughty years the equiva- lent amino N value was 200 mg 100 g1 sugar, and with virus yellows infection 300 mg 100 g1sugar. On peaty soils, even in the absence of drought or disease, total uptakes were excessive (300 plus kg N ha1) and so were amino N concentrations (up to 450 mg 100 g1sugar). They thought that this effect on peaty soils was an inevitable result of the uptake of much nitrogen late into the Table 13.1. Effect of nitrogen fertilizer and
farmyard manure on sugar percentage and juice purity.
kg N ha1
75 150 225
Without farmyard manure
Sugar percentage 16.6 0.4 0.8
Juice purity 88.8 0.5 1.3
With farmyard manure
Sugar percentage 16.4 0.4 0.8
Juice purity 88.2 0.7 1.4
Table 13.2. Effect of nitrogen fertilizer on sugar percentage and juice purity (from Boyd et al., 1970).
kg N ha1 No. fields 0 75 150 225 Sugar percentage 110 17.3 17.3 16.8 15.8 Juice purity 73 94.7 94.5 93.9 93.2
autumn, which accumulated in roots, greatly exceeding that needed for growth.
In a later investigation by the same group (Pocock et al., 1990; Allison et al., 1996a), nitrogen (0–180 kg N ha1) increased amino N exponentially from 57 to 130 mg 100 g1 sugar at the extremes of application. The increase was accompanied by a small increase in the other two impurities, sodium and potassium. In the report by Pocock et al.
(1990), results from Belgium were also exam- ined to extend the range of values of nitro- gen uptake and amino N. Respectively, they ranged from 65 to 383 kg N ha1and 44 to 410 mg amino N 100 g1sugar. It appeared that, in both countries, optimum uptake was about 200 kg N ha1to produce best yield and quality. Unfortunately, on some very fer- tile soils, uptakes were 200–400 kg N ha1 without any nitrogen fertilizer.
Europe
Results of 28 experiments in Ireland (McDonnell et al., 1966) showed that 45 kg N
ha1 had no effect on sugar percentage.
Further additions of fertilizer decreased sugar percentage by 0.1% for each 23 kg N ha1. Juice purity was reduced in a near-lin- ear fashion from 96.4 to 95.3 over the range of 0 to 125 kg N ha1. Carolan (1960), also work- ing in Ireland, found that nitrogen fertilizers caused large increases in the glutamine con- centration of sugar beet and this partly accounted for the decrease in juice purity.
Climatic conditions in Ireland are such that sugar beet is often still growing at harvest.
In France, Dubourg et al.(1957) examined the effect of a wide range of amounts (0–250 kg N ha1) on sugar percentage and concen- tration of amino acids in the soluble fraction of sugar beet roots. All amounts of nitrogen used decreased sugar percentage and increased amino acid concentration.
Von Lüdecke and Nitzsche (1967) investi- gated the effects of ‘normal’ and excessive amounts of fertilizer nitrogen on sugar per- centage and juice purity in Germany. Normal fertilizing (125, 125 and 175 kg N, P2O5and K2O ha1, respectively) had little effect on quality, but overfertilizing (400, 400 and 570 14
15 16 17 18
0 75 150 225
Sugar percentage
Sands 1959–1963 Mineral soils 1956–1969 Mean of all soils 1956–1969 Organic–mineral soils 1966–1969 Peats 1963–1965
90 92 94 96 98
0 75 150 225
Nitrogen (kg ha–1)
Juice purity (%)
Fig. 13.2. Effect of nitrogen fertilizer and soil type on sugar percentage and juice purity (from Draycott et al., 1971b).