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Advances in sugarcane soil fertility research inSouthern AfricaJ.H. Meyer a & R. van Antwerpen ba 16 Delaware Ave, Durbanb SASRI, Private Bag X02, Mount Edgecombe , 4300Published online: 07 Mar 2013.

To cite this article: J.H. Meyer & R. van Antwerpen (2010) Advances in sugarcane soil fertility research in SouthernAfrica, South African Journal of Plant and Soil, 27:1, 19-31, DOI: 10.1080/02571862.2010.10639967

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S. Afr. J. Plant & Soil, 27(1): 25th Anniversary Edition 1983-2008 19

Advances in sugarcane soil fertility research in Southern Africa

J.H. Meyer1 and R. van Antwerpen2*116 Delaware Ave, Durban, 2SASRI, Private Bag X02, Mount Edgecombe 4300.

The South African sugar industry is a modern, well organised industry and highly cost-competitive in world terms, producing some 2.3 million tons of sugar annually from 20 million tons of cane. The bulk of the cane is produced by some 1626 large scale growers (80%) and the rest is split between 36 500 small scale growers (11%) and milling companies with sugar estates (9%). As part of the 25th anniversary of SA Journal of Plant and Soil, a three-part review is given of recent soil and crop science based research initiatives conducted by the South African Sugar Industry in studying the impact of: (a) Soil fertility in determining the main nutritional require-ments of sugarcane, (b) Soil factors limiting climatic potential including water intake due to surface crusting, soil loss through erosion, low available moisture capacity, soil organic matter loss, acidification and water logging during wet seasons, and (c) Better soil management strategies based on crop residue retention, nutrient recy-cling, minimum tillage, ridge tillage, cover crops and intercropping, to conserve soil and water more effectively, increase soil organic matter, improve fertiliser use efficiency and reduce physical damage to soils during har-vesting. It is imperative that the continued success of the industry be underpinned by high standards in environ-mental management.

Keywords: Fertiliser, nutritional requirements, Saccharum hybrid sp., soil management, systems agronomy

*To whom correspondence should be addressed: (E-mail: rianto.van.antwerpen@sugar.org.za)

IntroductionThe South African sugar industry is one of the world’s most modern well organised industries served by the grower and miller sectors with an estimated annual production of 2.3 mil-lion tons of sugar per season. Approximately 38 200 regis-tered sugarcane growers produce on average 20 million tons of sugarcane annually from 14 mill supply areas, extending from Northern Pondoland in the Eastern Cape to the Mpuma-langa Lowveld. The South Sugar Industry is unique in many respects, as it is the most southerly cane growing area in the world, lying between latitudes 25° 21' and 31° 00' S and lon-gitudes 29° 54' and 32° 20' E. Of a total of 413 566 ha under cane, about 80% of the area is dry land, with an average rain-fall of less than 1000 mm per annum making it one of the dri-est rain fed cane areas in the world, with average yields of just over 70 t cane ha-1 per crop or about 50 t cane ha-1 yr-1. Under irrigation, yields are considerably higher, with an aver-age of more than 100 t cane ha-1 yr-1 per crop in Mpuma-langa, while further north in Zambia cane yields at the estate level average about 120 t cane ha-1 yr-1 under irrigation.

Adding to the uniqueness of this industry is its own inter-nationally acclaimed Sugarcane Research Institute at Mt. Edgecombe (SASRI, formerly known as SASEX) that forms part of the South African Sugar Association. This research facility was founded in 1925 to provide new varieties and agricultural practices that would enhance the profitability of the industry and ensure its long-term survival. History has shown that many of the agricultural advances in productivity in the industry have been due to the release of new disease resistant varieties, more efficient weed control and fertiliser practices, integrated pest management procedures, the use of chemical ripeners and various soil best management practices based on outcomes from research conducted by SASRI staff, estate agronomists and researchers based at the University of KZN or employed by private companies. The published pro-ceedings of the local Sugarcane Technologist Congresses

over the years provide the best testimony to the advances that have been made, with over 2600 papers that have been pre-sented at the annual meetings of the association, covering most aspects of technology in cane sugar production, both from an agricultural and a milling perspective. In this review paper some of the more notable milestones of research achievements in soil fertility and crop nutrition are high-lighted that have impacted on the agricultural sector of the industry during the past quarter century.

Crop nutrition researchEver since SASRI was established in 1925, there has been an awareness of soil fertility problems in the sugar industry and the economic importance of correctly fertilising sugarcane. Over this period, N and K usage per ha increased three to four fold and this increase was largely influenced by recommenda-tions made by the Fertiliser Advisory Service (FAS), based on the results of the numerous N, P and K factorial trials that were conducted over this period (Figure 1). Since 1979/80 yields have levelled off due to a host of factors, especially harvesting cane younger to reduce the risk of Eldana stalk borer damage in older cane. The drought between 1992 and 1994 also took its toll with markedly reduced yields. More efficient fertiliser practices have played a significant part in maintaining cane productivity and soil health and reducing fertiliser costs. Some of the more important research out-comes include: (a) Threshold values for interpreting soil and leaf analysis (Du Toit, 1960), (b) Use of radio isotopes in nitrogen use efficiency and P nutrition studies (Wood & Wood, 1967), (c) Modified P recommendations that take soil P fixation into account (Meyer, 1978), (d) Amelioration of soil acidity and the development of an exchangeable Al Satu-ration Index to determine lime requirement (Schroeder et al.,1995), (e) Soil specific nitrogen fertiliser advice based on near infra-red measured N mineralisation potential that has resulted in large savings of N use for many growers (Meyer et

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20 S. Afr. J. Plant & Soil, 27(1): 25th Anniversary Edition 1983-2008

al., 1986), (f) Assessing organic amendments used by sugar-cane growers for improving soil physical, chemical and bio-logical properties (Van Antwerpen et al., 2003), (g) N carrier advice based on a test to measure volatilisation of ammonia from N carriers applied to soil (Schumann, 2000), (h) Varia-ble threshold values for K based on clay and the Ca+Mg/K ratio (Donaldson et al., 1990) and (i) Improved N advice based on classifying varieties into low, medium and high N use efficiency categories (Schumann et al., 1998).

Nitrogen managementDuring the early 1970s, N fertiliser recommendations were developed at a time when maximisation of production and short term economic gain were the key factors driving the industry forward. N recommendations for ratoon cane were based on expected cane yield, but the actual N usage of 1.90 kg N t-1 cane far exceeded the recommended FAS rate of 1.25 kg N t-1. Little account was taken of the large differences in N mineralising potential that existed between soils, and that res-ident soil N may in a number of soils play a more important role for sugarcane nutrition than fertiliser N. The emphasis was mainly on fertilising the crop rather than on managing the soil.

First soil specific nitrogen fertiliser recommendationsBecause this method of establishing N requirement led to over-application of N fertiliser, an alternative system of N recommendation for plant and ratoon cane was developed for growers in 1984, using the results of extensive laboratory studies and fertiliser trials (Wood, 1968; Moberly, 1982; Meyer et al., 1983) which have shown that the amount of nitrogen available to the crop differs markedly between soils and is probably influenced by factors such as climate, aera-tion, moisture availability, organic matter status and the depth of soil. Examination of trial data showed that the relative yield response to applied N fertiliser was inversely related to the organic matter content of the soil and measured N release by the incubation method. Soils with low (<25 mg kg-1), moderate (20-40 mg kg-1), high (40-60 mg kg-1) and very high N (>60 mg kg-1), were associated with average responses to applied N of about 50%, 30%, 20% and 10%

respectively (Figure 2). For advisory purposes, soils are clas-sified into four categories according to their potential to min-eralise N from organic matter (low, medium, high and very high; Meyer et al., 1986). In practice, the N mineralisation potential is estimated using the soil form. When this informa-tion is not provided, N mineralisation potential ratings are determined by near infra-red reflectance spectrometry (Meyer, 1989).

New soil test for predicting ammonia volatilization losses from surface-applied ureaIn 1999 a new laboratory soil test method was developed to predict the potential ammonia volatilization losses from sur-face-applied urea fertiliser used in the sugarcane industry Schumann, 2000). The Fertiliser Advisory Services (FAS) has implemented both methods as an added value service for all routine soil samples, and the soil-specific advice given will be devised to minimise nitrogen losses and maximise yield response to fertilisers. The revised N recommendations have led to a reduction in N use particularly for cane growing on high to very high N mineralising category soils. Overall, there has been about a 20% decline in N usage between 1975 and 2005 throughout the industry and N use efficiency now aver-ages about 1.5 kg N t cane-1, compared to a level of over 2 kg N t cane-1 in the Australian sugar industry.

Improved N fertiliser use efficiency through fertigationFertigation trials are currently being conducted in Mpuma-langa and Swaziland to measure the benefits of fertigation in terms of nitrogen use efficiency by comparing sugarcane yield response to a range of nitrogen rates applied both in dry form and by fertigation (Butler et al., 2002). Experiments have been established in both early and late season cycles to account for the effect of climatic conditions on crop nitrogen demand and supply. Four rates of N (0, 45, 90 and 135 kg ha-1) were applied as urea N both in dry form and by fertigation. The fertigation treatments were applied in four equal splits at monthly intervals after harvesting. For the early season crop there was a marked improvement in N use effi-

05000

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2000025000

30000

3500040000

45000

1951 1963 1975 1987 1997Year

N, P

& K

(ton

s yr

-1)

NPK

Figure 1 Average amounts of N, P and K (t yr-1) used by the South African sugar industry between 1951 and 1997.

Figure 2 Average sugar yield response to applied N in relation to diagnostic topsoil horizon (after Meyer et al., 1986).

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S. Afr. J. Plant & Soil, 27(1): 25th Anniversary Edition 1983-2008 21

ciency where the N was applied through the fertigation sys-tem (10% overall) but the treatment differences were not statistically significant. The biggest improvement in N use from fertigation occurred at the 90 kg ha-1 rate where the fer-tigation N treatment gave an additional 2.6 t cane ha-1

response. The indications at this stage are that for an early season cycle, a saving of between 40 to 50 kg ha-1 N is possi-ble where the urea is applied by fertigation (Weigel et al., 2008).

Quantifying genotypic differences in N-use efficiency between varietiesRecent nitrogen research has focused on important genotypic differences in N-use efficiency between varieties as demon-strated in trials conducted in Pongola and Mpumalanga. Using the ratio of sucrose yield to N accumulation, varieties may be classified into one of three categories: efficient N-use responders, inefficient non-responders or inefficient respond-ers (Schumann et al., 1998). While urea is still the favoured nitrogen (N) source for the sugarcane industry, due to a lower price and higher N content (46%) than the competing prod-ucts LAN (28%) and ammonium sulphate (21%), there is still concern about how growers can minimise losses of ammonia N from urea when soil and climatic conditions are not favour-able.

PotassiumFor many years, the threshold value used by FAS was 112 mg kg-1 for all soils, but this was modified in 1982 to allow for differences in soil texture following results from glasshouse trials (Wood & Burrows, 1980), and a re-assess-ment of fertiliser trials (Meyer & Wood, 1985). K threshold levels of 150 and 225 mg kg-1 were introduced for soils with clay contents of 30 to 40%, and >40% respectively (Wood & Meyer, 1986). The availability of potassium to sugarcane can be affected adversely in soils with potassium selective clay minerals such as smectite that dominate vertisols, allowing such soils to fix potassium within the clay. Higher applica-tions of potassium fertiliser are required for potassium fixing soils for generation of levels of exchangeable and soil solu-tion potassium similar to those encountered in soil with less capacity to fix potassium. Availability and uptake of potas-sium is inhibited by high levels of calcium and/or magnesium which favour uptake of these elements in mass flow in the transpiration stream thus interfering with potassium uptake by diffusion across an electrolyte gradient between soil and roots (Wood & Meyer, 1986). Donaldson et al. (1990) and Henry et al. (1992a) suggested that potassium nutrition needs special attention when base saturation measured by the ratio (Ca+Mg)/K is greater than 20 (when units of measurement are mg kg-1 for cations). Release of potassium from clay min-erals is also inhibited by low soil temperature in winter and spring and by high moisture levels in soils dominated by 2:1 clay minerals (Donaldson et al., 1990). The latter authors rec-ommend a higher threshold value for soil potassium for base saturated soils with more than 40% clay and where irrigation generally does not allow drying and cracking of the soil. The higher threshold recognises that potassium is released from the exchange complex when soil is dried for analysis and that the "conventional" threshold will not allow sufficient potas-

sium in soil solution for optimal growth of sugarcane in cool and wet soils. For best results, potassium should be applied soon after harvest for cane grown on 2:1 clay soils, particu-larly during the latter conditions (Donaldson et al., 1990).

PhosphorusThe difference between the measured amount of P in the soil and the amount needed for optimum cane growth is used to determine the P fertiliser requirement. Prior to 1980, P rec-ommendations were based on the modified Truog procedure in which the P is extracted with 0.02 N sulphuric acid for 30 minutes (Du Toit, 1962). The amount of P recommended is the difference between the Truog value and 90 kg P ha-1 for plant cane or 30 kg P ha-1 for ratoon cane. While the Truog soil test method was reliable in predicting a likely response to applied P it did not account for the fate of applied P in the soil, which is a major factor in determining P fertiliser use efficiency. This became important when the industry expanded to the high P fixing soils in the Natal Midlands. During the late seventies laboratory and glasshouse studies with these soils indicated that soils with similar acid extracta-ble P levels, but different P sorption characteristics, were likely to have different P requirements (Meyer, 1974). Subse-quent field trials confirmed that economic returns were possi-ble from broadcasting P fertiliser in excess of the highest rate recommended by FAS (Meyer & Dicks, 1979). A rapid phos-phorus desorption index (PDI) soil test was introduced to sup-plement the standard soil extraction procedure for midlands soils (Reeve & Sumner, 1970). Depending on whether the soil is strongly, moderately or weakly P fixing, the furrow appli-cation is increased to 120, 100 or 90 kg P ha-1. For medium and high P fixing soils with Truog P levels below 13 mg kg-1, supplementary broadcast P applications are also now rec-ommended in conjunction with the normal furrow P applica-tions.

In recent years, growers in Pongola and Mpumalanga have made increasing use of laboratories where the soil tests have not been calibrated for sugarcane. This has led to diverse fertiliser recommendations and to considerable confusion among growers, as the recommendations call for excess application of phosphorus (P) and potassium (K) fertilisers in particular. An investigation was carried to show that standard-ising soil extraction on the Truog method will lead to greater consistency in results, better interpretation of fertiliser advice and generally less P being applied, thereby reducing costs (Botha & Meyer, 2004) The potential saving at 2004 prices was approximately R600 ha-1 for ratoon cane, which would generate a saving of R14 million per annum, based on the assumption that 60% of the region does not require P applica-tion to ratoon cane. Based on 2008 prices of P fertiliser the savings will have more than doubled.

Lime requirementInvestigations to determine the reasons for the stimulating effects of wattle brushwood ash on cane growth (Meyer, 1970) led to the identification of Al toxicity as a growth limit-ing factor. The beneficial effects of the ash were simulated by the application of lime even where the need for this was not indicated by the FAS criteria based on pH, and exchangeable Ca and Mg levels. A modified exchangeable Al index (EAI)

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22 S. Afr. J. Plant & Soil, 27(1): 25th Anniversary Edition 1983-2008

method proved effective in establishing the degree of Al tox-icity in the soil. The relationship between EAI and clay con-tent was used on a routine basis for predicting the toxic effects of Al, and the lime requirements of acid soils. Soils with EAI/clay ratios in excess of 3.5 responded to liming (Moberly & Meyer, 1975). For humic soils the EAI/clay crite-ria was supplemented with an Al:S ratio (Schroeder et al., 1993). The sulphate anion is considered to have "self liming" properties and substantial amounts of mineral S may occur in humic soils. For diagnostic purposes soils with an Al:S ratio of 2 or higher are expected to respond to liming.

Further investigations revealed that the use of an alumin-ium saturation index (ASI) improved the prediction of lime requirement even further. Lime requirement was modified to include an Aluminium Saturation Index (ASI), calculated as 100 x Al / 100 g clay / (Ca + Mg + K + Al / 100 g clay), with all elements are expressed as cmolc kg-1 (Schroeder et al., 1995). This improvement was considered more robust than using Al / 100 g clay or the Exchangeable Aluminium Index (EAI) of Moberly and Meyer (1975) alone. Varietal differ-ences in terms of tolerance to Al toxicity have also been iden-tified. A soil ASI threshold value of 40% is used to determine the lime requirement of N12 compared with 20% for all other varieties (Schroeder et al., 1995). Other studies have involved the combined use of lime and phosphogypsum for the amelio-ration of steeply sloping lands under minimum tillage as well as subsoil acidity (Turner et al., 1992; Nixon et al., 2003).

SiliconFor many years, silicon deficiency in crops was relatively unknown and this element was widely regarded as non-essen-tial for plant growth, despite the fact that silicon is often present in the highest concentration amongst inorganic con-stituents.

In the South African Sugar Industry, early investigations have emphasized the abiotic role of Si in alleviating Al toxic-ity problems in highly weathered oxisols that occur in the Kwa-Zulu/Natal midlands. Initially six field trials were estab-lished, covering a total of 14 crops, to compare the relative efficacy of Si carriers such as Slagsil [SiO2 35%], Hulsar lime [SiO2 <2%], Amcor slag [SiO2 37%] and Hawaiian calcium metasilicate [SiO2 49%]. The results showed that the silicon carriers were superior to lime treatments, with nine signifi-cant responses to Si and six for the lime treatment (Moberly & Meyer, 1975). Residual treatment effects were also pro-nounced and in one of the trials at Seven Oaks, the Slagsil treatment increased yield significantly in the plant crop and significant residual treatment effects were also obtained in the following three ratoon crops (Table 1). The efficacy of the sil-icon carriers in increasing plant available silicon levels in the soil declined in the order Slagsil>Hawaiian slag>>Amcor blast furnace slag at both the low and high rates.

Table 1 Residual effectiveness of a silicon treatment versus limestone incorporated to a depth of 0.60 m (t cane ha-1)

Crop stage Control Extra P Lime SlagsilPlant

1 st Ratoon

2 nd Ratoon

3 rd Ratoon

4 th Ratoon

116

97

47

97

46

128

113

70**

113

59*

123

113

69**

115*

60*

133**

128**

72**

114*

59*Average 81 97 96 101(after Moberly & Meyer, 1975) ** Statistically significant, P < 0,01 • Statistically significant, P < 0,05

Crop and sucrose loss from Eldana saccharina stalk borer damage still ranks as the most important factor limiting pro-ductivity in the South African sugar industry. Recent studies at the South African Sugarcane Research Institute have emphasized the important role of applied silicon in improving the resistance of sugar cane to Eldana infestation even in the more tolerant varieties such as N21 and N33 (Keeping & Meyer, 2000; 2003; Meyer & Keeping, 2005). Greenhouse and field trials have been conducted to compare the efficacy of four silicon sources. In the greenhouse, sugarcane varieties were artificially inoculated with E. saccharina and treated with three dosage rates (0, 2.5 and 5 t ha-1) of calcium sili-cate. At 5 t ha-1 calcium silicate, there was a reduction of 30% in borer damage and 20% in borer mass. The most susceptible varieties showed the highest silicon uptake and the greatest response. Of the four carriers tested, stalk borer incidence declined as follows: local Namibian calcium sili-cate>imported USA calcium silicate>local Slagment>flyash.

In the field experiment, similar results were recorded.Of the four field trials with silicon treatments that have

been harvested, three have shown positive results in reducing damage from the Eldana borer. The plant crop results of a recently established silicon x Eldana field trial with two sus-ceptible and one resistant cane variety showed maximum reductions in percent stalk length damaged (depending on Si carrier and treatment level) of 24%, 30% and 35% in varie-ties N27, N29 and N35 respectively. On average (Meyer & Keeping, 2005), the most susceptible variety, N35, also gave the greatest yield response to silicon treatment (+23%), fol-lowed by N27 (+9%) and N29 (+4%).

There is ample evidence from the literature that nutrients such as nitrogen and silicon play important roles in the sus-ceptibility and resistance of a range of crops to stalk borer damage. In our own industry, studies conducted in 1989 showed that high N usage under conditions of moisture stress greatly increased survival of larvae and infestation of sugar-

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S. Afr. J. Plant & Soil, 27(1): 25th Anniversary Edition 1983-2008 23

cane by E. saccharina (Atkinson & Nuss, 1989). The results of a recent glasshouse trial investigation confirmed that the effect of nitrogen and silicon treatment on varietal resistance to E. saccharina was significant but that the effects were opposite, with nitrogen increasing susceptibility to the borer and applied Si reducing susceptibility to the borer (Meyer et al., 2005). Maximum reductions in percent stalks damaged from silicon treatment ranged from a 70% average at the low-est N level to 39% at the intermediate N level, down to 35% at the highest N treatment. The beneficial effect of Si in reducing borer damage at the high N level was also evident in the reduced stalk length bored, lower number of internodes damaged, lower borer numbers and borer mass. In general, nitrogen has a greater impact on E. saccharina susceptibility than silicon (Figure 3).

Another valuable outcome from this work was that the N/Si ratio in sugarcane leaves was better correlated with poten-tial E. saccharina damage than Si alone and that sugarcane with values in excess of 2:1 was associated with increasing risk from borer damage. The silicon status of the stalk was also closely related to borer damage and silicon levels below 0.2% are associated with an increasing risk of borer damage.

Using organic nutrient carriersWhile FAS has mainly offered advice in terms of both straights and fertiliser mixtures, there has been an increasing demand for advice based on the use of biofertilisers such as chicken litter, filtercake, kraal manure and pig slurries. Poul-try litter is potentially the most valuable, as it usually contains less than 35% moisture, and at least 60% of the total N and 50% of the total P is considered to be immediately available to the crop (Moberly & Stevenson, 1971). Five t ha-1 applied in the furrow can provide sufficient N, P and K for a plant cane crop growing on a humic soil without using additional fertiliser. Similar savings may be obtained for fertilising ratoon cane with poultry litter.

Filtercake is also frequently applied in the furrow at plant-ing but it has a considerably higher moisture content than

poultry litter, as well as lower total N and K contents, which makes it less attractive to use as a fertiliser source. It is prima-rily regarded as a substitute for inorganic P fertiliser (Moberly & Meyer, 1978). A furrow application of 20 t filtercake ha-1

(at 60% moisture content) will provide approximately 25 kg N, 80 kg P and 16 kg K. On average 3 t of fresh filtercake is equivalent in value to 1 t of fresh chicken litter.

Given the huge escalation in fertiliser prices over the past five years, there has been renewed interest in the use of CMS, a waste product from the distillation of ethanol from molasses (Turner et al., 2002). Today at least 30% of nutrients applied to sugarcane are applied as CMS fortified with additional N and P. Another waste product commanding attention is sewerage sludge, which was evaluated as a nutrient source over 20 years ago (Turner et al., 1991).

Economics of fertiliser usageAlthough the principle of maximum economic yield ha-1 was emphasised by Du Toit (1960), it was only in the late 1970s that the concept of maximum return on fertiliser investment using marginal return curves was applied to fertiliser recom-mendations for sugarcane (Thompson, 1980). The assessment showed that between 1970 and 1980 there was very little change in the ratio between the value of a t of cane and the cost of a t of fertiliser, and essentially the two approaches pro-duced similar recommendations for N, P and K. However, since 1980 the ratio has become less favourable, requiring 11 instead of 8 t of A-pool cane to cover the cost of one t of fertiliser. With an increasing squeeze on profit margins and the likelihood that output value/input price ratios will be more volatile, it may be expected that economic optima for N, P and K will be more vulnerable to such changes. The Kyna-Cane computer programme was recently developed using FAS soil norms to quantify the consequences of economic changes on optimal nutrient recommendations more accu-rately (Prins et al., 1997). SASRI is also developing the Cane-man model which will enable growers to evaluate the economic consequences of different fertiliser strategies under various risk scenarios.

Improved fertiliser management through leaf analy-sisFoliar diagnosis has been used to great advantage in deter-mining the nutrient status of sugarcane and is widely accepted as a means of improving the effectiveness of fertiliser use. With the introduction of X-ray fluorescence and NIR (Wood et al., 1985), leaf analysis has become more accessible as a diagnostic tool. It compares favourably with soil analysis in correlating with fertiliser responses and provides a useful check on the uptake of fertilisers already applied. A disadvan-tage is that it is often carried out too late and the results can only be used for adjusting the fertiliser programme of the next crop. However, investigations with the Diagnosis and Recom-mendation Integrated System (DRIS) (Meyer, 1981), in which nutrient indices derived from ratios between nutrients, rather than nutrient percentages in the leaf, have indicated that this system can help to expedite corrective fertiliser treat-ment of the crop that has been sampled. Since nutrient ratios vary less than nutrient percentages as the crop ages, diagnosis on irrigated cane can be made at two months, compared to

0

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N1 N2 N3 N1+Si N2+Si N3+Si

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NCo376N31N35N36N37

Figure 3 Effect of nitrogen treatment in the absence and pres-ence of silicon on susceptibility to E. saccharina in five sugar-cane varieties, where N1=60 kg N ha-1, N2=120 kg N ha-1 and N3=180 kg N ha-1. E. saccharina susceptibility is calculated as a weighted % of the trial mean of borer performance and damage.

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four months with the conventional approach. In fertiliser management, foliar diagnosis has the important advantage over soil analysis that it is the only means of assessing the efficacy of fertiliser applications, or detecting the effects of N losses due to N volatilization, leaching or denitrification.

Leaf analysis has been useful in identifying the reasons for yield decline in past research projects. These include the Upper Tongaat Syndrome (Thompson, 1985) in which Zn and P deficiencies were identified as limiting factors, the weak sands project in which sulphur deficiency was prominent (Thompson, 1983), the low leaf/high soil K problem in the Lowveld (Donaldson et al., 1991), acid chlorosis as a result of Fe deficiency induced by Mn toxicity and a suspected boron deficiency in Malawi. More recently, a suspected compaction problem on a large estate in the Lowveld was linked to a high incidence of leaf P deficiency as identified by foliar diagno-sis.

Fertiliser Advisory ServiceOutcomes from most of the research conducted by staff over the years have been incorporated into FAS both in the form of new diagnostic methods and new recommendations on lime and fertiliser advice as well as reclaiming saline/sodic soils.

During the first 20 years of FAS, considerable historical data from soil and leaf analyses, fertiliser recommendations and crop performance had accumulated, and as a result FAS was able to introduce whole cycle fertiliser advice in 1975. This provided growers with fertiliser recommendations for cane covering a plant crop and four succeeding ratoons, referred to as a "whole crop cycle". Each recommendation was based on the chemical analysis of a representative pre-plant soil sample taken after the previous crop had been ploughed out. The computer at SASRI was programmed to evaluate the analytical data based on available soil threshold values as well as a set of tables from which the crop nutrient requirement could be determined.

With the introduction of whole cycle advice, the need for taking soil samples after each harvest largely fell away. Ini-tially this led to a decline in the numbers of samples sent to FAS, from 16 000 in 1974 to 12 000 samples in 1975. None-theless, the number of grower samples analysed gradually increased and by 1983 had reached a peak of 25 000. With the introduction of the "User Pay Entity” (UPE) in the following year, sample throughput again declined and has since been on a downward trend. The drought between 1993 and 1995 also contributed to the decline in numbers of soil and leaf samples analysed, but since 1997 there has been a slow recovery although the 16 812 soil and leaf samples analysed in 1998/99 is still well below the 25 000 samples that were processed in 1983.

During the past 25 years a quiet “revolution” in analytical methodology due to the introduction of sophisticated scien-tific instruments has occurred Analytical operations have moved away from dispensing liquid samples from a macro to semi and micro scale, making it possible to dispense and ana-lyse very small quantities of liquid very accurately. Another milestone in this “revolution” was the commissioning of a new X-ray fluorescence spectrometer in 1996 which enabled grower leaf samples to be analysed directly without using time consuming acid digestion procedures. This streamlining

meant that a grower would receive his recommendations within 12 days of receipt of his samples at the laboratory. Other important developments have included: (a) New NIR based soil tests to rapidly assess soil organic matter and N mineralisation potential, (b) Introduction of Al saturation index soil tests for lime requirement, (c) New soil tests to pre-dict phosphorus fixation [PDI] in midland soils, (d) New soil tests for zinc and sulphur, (e) Fast X-ray analysis for major and minor nutrients in leaf samples not only from sugar cane but also other fruit crops such as coffee, tea, citrus, macada-mias and various species of commercial forest trees, (f) Com-puterised advice to ameliorate soil salinity/sodicity problems, (g) Water quality assessment for irrigation, and (h) Advice on the use of organic sources of N, P and K such as filtercake, chicken litter and vinasse.

Thus in the last 25 years, although growers are now charged for soil and leaf samples that are analysed by FAS, the range and scope of tests offered to growers has markedly increased, as well as the extent of practical advice that is needed in determining fertiliser policy at the farm level while at the industry level considerable savings have been made in the fertiliser bill.

The impact of new challenges from precision agriculture and the rapidly growing soil sustainability school on soil health evaluation and soil management will also need to be considered in the future development of FAS.

Monitoring long term fertility trendsThe FAS databank of soil and leaf analysis is now large enough to determine fertility trends in grower fields, and a programme referred to as NIRS (Nutrient Information Retrieval System) was specifically developed to capture and store soil and leaf analysis data and to carry out surveys in which the frequency distribution of important soil and plant nutrients are categorised into various stages of sufficiency for various extension areas. The initial assessment based on the capture of 130 000 soil and 50 000 leaf samples showed that there was a relatively high proportion of leaf samples defi-cient in potassium (28%), somewhat fewer deficiencies in N (13%) and P (12%), and only minor occurrences of defi-ciencies in Ca (4%), Mg (1%) and Zn (8%) for the whole industry (Meyer et al., 1989).

Since the industry wide nutrient survey that was con-ducted in 1970, little use had been made of analytical data until the computerisation of FAS recommendations in 1980. In 1988, a programme referred to as NIRS (Nutrient Informa-tion Retrieval System) was developed to make meaningful comparisons of nutrient trends between the various extension areas. Currently, analyses from more than 200 000 soil and 75 000 leaf samples are stored in the data bank. Since 1980/82 there has been a steady decline in the average leaf N and P levels, which is partly attributed to the increased use of varie-ties N12 and N14, both known to be less efficient in N and P uptake than NCo376. Other factors include reductions in N and P usage due to E. saccharina and the recent drought. The highest N deficiency occurred in Zululand South (36%), P deficiency in the Midlands North (32%), K deficiency in Mpumalanga (24%), and Zn deficiency amongst the North Coast small scale growers (18%).

An important observation from the soil data bank assess-

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ment is the rapid acidification that has occurred in the light textured soils since 1980/82. Results of a survey of paired cultivated and virgin sites also revealed accelerated acidifica-tion of soils under cultivation.

Advances in soil management practicesSoil health is currently under the spotlight in the industry as there have been concerns that monocropping of sugarcane for many decades has in many parts of the industry resulted in nutrient mining, declining levels of soil organic matter and an increase in soil acidity. These factors, together with a concern over the possibility that a ‘yield plateau’ has been reached, have led growers and researchers to look critically at the way soil is presently being managed. There is evidence for a decline in the productivity of sugarcane lands as measured by sucrose yield per harvested ha. In the South African context the loss in productive potential is estimated to be equivalent to about 20% of current output.

Practices that conserve soil organic matter such as green manuring, minimum tillage, the use of organic nutrient carri-ers such as filtercake, chicken and kraal manure, and trashing of cane at harvest have greatly increased in the industry as there is a need to sustain the all important functions of the soil food web by maintaining soil humus (Van Antwerpen et al., 2003).

During the past three decades several studies have been carried out in South Africa on the effects of some of the main soil degradation processes affecting soil health and cane pro-ductivity. These include: (a) Minimum tillage practices (Moberly & Iggo, 1976; Turner, 1980), (b) Soil erodibility research (Platford, 1982), (c) Soil crusting and runoff (Dewey & Meyer, 1989), (d) Vertical mulching under rainfed and irri-gated cane conditions (Henry et al., 1992b), (e) Reclamation guidelines for saline sodic soils (Johnston, 1980), (f) Electro-magnetic induction as a technique for detecting and mapping salinity (Johnston et al., 1994), (g) Impact of irrigation water quality on soils (Culverwell & Swinford, 1986), (h) Drainage (Meyer et al., 1991), (i) Surveys paired old and new (virgin) sites throughout the industry (Van Antwerpen & Meyer, 1996), (j) Assessing soil compaction in the South African Sugar Industry (Van Antwerpen et al., 2008), (k) Organic amendments for improving soil properties and water use effi-ciency using long term burning and trashing trials (BT1) (Van Antwerpen et al., 2003), and (l) Impact of green manuring on improved soil properties (Schumann et al., 2000).

Minimum tillageA lot of research was carried out in the 1970s to establish whether deep ploughing and ripping were economically justi-fied. After several years of extensive trial work it was found that the benefits of deep ploughing were short-lived and did not justify the costs (Moberley, 1972). Subsequent research has shown that the minimum tillage system (strip tilllage), in which glyphosate is used to kill the old crop, results in mini-mal soil erosion and improved cane yield when compared with the conventional methods of land preparation (Iggo & Moberly, 1976). Other measured benefits included increased soil organic matter and reduced soil bulk density (Moberly & Iggo, 1976; Turner, 1980). A comparison of soil and water losses from conventional and minimum tillage replanting

methods on a range of soil forms using the rainfall simulator technique showed that soil and water loss under a minimum tillage system could be reduced by 60% and 30% respec-tively, provided the crop had grown to the 6th leaf stage at the time of spraying (Haywood & Mitchell, 1987). Over 40% of the coastal areas with slopes in excess of 10% use a strip cropping/minimum tillage system, and the impact on soil ero-sion control has been dramatic.

Soil crusting and erosionMany soils in the sugar industry are subject to various degrees of crusting under both rainfed conditions and irriga-tion before crop canopy. Physical disaggregation of soil parti-cles occurs in response to the impact of raindrops, causing compaction of the surface layer which limits water penetra-tion into the soil. This physical breakdown is accelerated where the soil surface solution has an electrolyte concentra-tion too low to maintain physical structure during raindrop impact. Soil crusting is the precursor to soil loss through ero-sion. Erodibility ratings of some South African sugar industry soils have been determined by Platford (1982) using run-off plot measurements.

The results of both laboratory and field experiments with rainfall simulators conducted on Entisol soils have shown that strong crusts do not form under a surface mulch such as trash. Average results from five trials conducted with a rainfall sim-ulator over a five year period showed that trash saved 89% of the soil and 58% of the water lost from plots where burnt tops had been spread (Platford, 1982). More recently, the results of both laboratory and field rainfall simulator work have shown that preventing surface crusting under raindrop impact is one of the main reasons for reducing soil loss and improving water intake rates (Meyer, et al., 1988). Where no cover existed, measured soil loss was found to be seven times higher than on a surface protected by a trash blanket. Amelio-rants such as phosphogysum, molasses meal, polyvinyl alco-hol and various polymers were less effective and far more costly than a trash blanket in reducing runoff and increasing rainfall use efficiency.

Chemical dispersion of clays leading to surface crusting is also greatly influenced by the level of exchangeable sodium in the soil and the salt concentration of the percolating solu-tion (Dewey & Meyer, 1989). In general, the intake rate was found to be inversely proportional to soil Exchangeable Sodium Percentage (ESP).

Soil conservationThe need for improved soil conservation standards in sugar-cane production on steep slopes led to further research into soil erosion and conservation systems, such as the La Mercy catchment project, to measure runoff and soil loss from sugar-cane catchments with varying management practices (Plat-ford, 1979). This pioneering work together with associated plot and rainfall simulator experiments proved very valuable in guiding soil and water conservation standards in terms of minimum tillage, trash management and the design of water control structures. This information in turn enabled produc-tion and extraction systems to be developed which best suited the condition of the farm and the grower's preferences and circumstances.

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Further research into developing standards for resource protection and conservation of the environment continued (Maher, 2000) and in 2002 led to a revised Manual of Stand-ards and Guidelines for Conservation and Environmental Management in the South African Sugar Industry that cov-ered the basics of farm planning, how a Land Use Plan should be implemented, management practices such as land prepara-tion, use of agrochemicals, burning and trashing and Codes of Burning Practice (Anon SASA, 2002).

Green manuringPast studies have shown that the incorporation of ecological practices into sugarcane production and management has the potential to arrest and ameliorate the negative effects of monocropping on soil degradation and yield decline. Com-parison of commercial yields in Swaziland indicated a response to fallowing and green manuring of about 50% in the plant crop and 25% in the first and second ratoons, with no response thereafter (Nixon, 1992). A further investigation was conducted to assess the effects of fallowing and green manuring practices over a seven-month period on sugarcane yields and the physical properties of a poorly draining clay soil (Nixon & Simmonds, 2004). There were yield increases of 10 and 8% in the subsequent plant and first ratoon crops respectively, after fallowing and green manuring, but no sig-nificant yield responses in the second ratoon. Topsoil air-filled porosity increased from 11% under continuous sugar-cane to 16-19% after fallowing, and steady state ponded infil-tration rates were increased from 0.6 to 1.3 mm h-1.

In another interesting study, seven green manure crops (sunn hemp, marigold, oats, dolichos beans, velvet beans, for-age peanuts and cowpeas), as well as tomato and sugarcane variety N12, were evaluated in a glasshouse trial to assess their susceptibility to the root knot nematode Meloidogyne javanica, as well as their influence on other nematode popula-tions. The results indicated resistance of forage peanuts, mar-igolds and sunn hemp to M. javanica. Cowpeas, dolichos beans and tomato were particularly good hosts. The host sta-tus of velvet beans and sugarcane variety N12 differed in inoculated and naturally infested soils. Forage peanuts and sunn hemp increased the free-living nematode populations more so than did the other crops (Berry & Wiseman, 2003). The findings of these studies are consistent with those of other studies conducted in Australia, Louisiana, and Thailand, which showed that the use of green manures improved soil properties and reduced the incidence of disease and certain pests with accompanying increases in sugarcane yields.

Trash managementThere is a clear and unfortunate trend in the industry towards more burning at harvest compared with trashing. There are logical reasons for this trend; it is not only retrogressive in regard to conservation of soil and moisture, but conflicts with growing public pressures regarding the pollution from burn-ing in South Africa. Thompson (1966) reported average yield responses of 10 t cane ha-1 to trash retention in trials con-ducted under rainfed conditions on a cross section of soils. He also noted significant increases in soil organic matter and cat-ion exchange capacity, particularly in the top few centimetres of soil. Under irrigation the response to trash retention was

much lower (Thompson, 1966). Gosnell (1970) came to a similar conclusion with low levels of irrigation in Zimbabwe, but showed a substantial yield depression with trash retention under full irrigation. Trash conservation is a very effective means of reducing soil and water losses from sugarcane fields. This is particularly important in KwaZulu-Natal, where slopes are often steep and many of the soil types are highly erodible. It was shown that on a grey Longlands form soil with an 11% slope, a trash blanket prevented 90% of the rainfall loss and more than 60% of the soil loss in ratoon cane during the pre-canopy stage (Thompson, 1966).

Analysis of data from the long-term trash management trial at Mount Edgecombe (BT1) has shown that after 71 years of green cane harvesting, with trash retention, there was a significant increase in soil organic matter content in the sur-face 10 cm of soil compared to burning (Graham et al., 1999; Van Antwerpen et al., 2001). The size of the microbial bio-mass and its respiratory rate, dehydrogenase activity and arginine ammonification rate were also increased by trash retention. Fertilised treatments tended to have a higher organic matter and microbial biomass C content than unferti-lised ones, reflecting the higher yields and greater organic matter returns under fertilisation. Soil microbial activity was however inhibited by fertiliser applications. This inhibitory effect was attributed to fertiliser – N induced soil acidification (Graham et al., 2001).

In a recent review paper Van Antwerpen et al. (2006) indi-cated that the average trashed over burnt yield response for eight BT trials under rainfed conditions was 9 t ha-1 yr-1 and this figure is commonly used in the sugar industry to repre-sent the response that can be expected from a trashing system. However, responses have varied from 25 to -23 t cane ha-1 yr-1, tending to be better in below average rainfall years and poorer in very wet years.

A model was recently developed to simulate the physical state of trash on a field, and its effects on crop growth condi-tions, with particular reference to the water balance. The model was applied to 22 consecutive crops of the BT1 burn-ing vs. trashing trial at Mt. Edgecombe. On average, per crop, the modelled trash intercepted 44 mm of rain water, reduced runoff/stormflow by 39 mm and soil evaporation by 152 mm, resulting in increased drainage by 53 mm, and increased tran-spiration by 92 mm. High correlation coefficients between observed yields and simulated transpiration for burnt (R2=0.60) and trashed (R2=0.73) conditions were obtained and the model will be used to augment the Canesim model to improve SASRI crop forecasting of cane yield from both burnt and trashed cane fields (Van den Berg et al., 2006).

CompactionHarvesting and cane extraction during wet conditions is an unavoidable practice in many cane growing areas and uncon-trolled infield traffic will cause most damage in terms of soil compaction, sealing/capping and physical damage to cane stools. The effect tends to be exacerbated in irrigated areas where there has been insufficient drying off before harvesting or where soils are not adequately drained. Soil compaction is also difficult to quantify. Changes in soil physical properties such as bulk density and resistance to penetration caused by infield traffic have been the subject of extensive research.

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In South Africa, Maud (1960) showed that for most sugar belt soils, the tendency for compaction is greatest when their moisture content is near field capacity. At Pongola, where a deep Hutton form soil was severely compacted, bulk density increased and the macro pore space of the soil was reduced between 0 and 80 mm deep, but there was no adverse effect on the yield of ratoon cane (Johnston & Wood, 1971). A sub-sequent investigation of a poor yielding field on a similar soil showed that infield loading during wet conditions caused severe soil compaction and damage to stools.

Swinford and Boevey (1984) as well as Swinford and Meyer (1985) found that moderate and severe compaction on a grey structureless sandy loam caused an increase in bulk density and soil strength and decreased air filled porosity. Compaction over the row had a greater effect on yield than compaction of the inter row. Amelioration through ripping was only slightly beneficial. Tines seem to have a detrimental effect due to root pruning, which affects growth of the subse-quent crop. It was concluded that yield decline from infield traffic is as much due to physical damage to stools as to a breakdown in structure and sealing/capping from soil com-paction, particularly under critical soil moisture conditions.

In a review of previous compaction studies conducted in the sugar industry, Van Antwerpen and co-authors showed that harvesting and cane extraction during wet conditions are una-voidable at certain times of the year, and past field studies have shown that compactive force due to uncontrolled infield traffic will cause most of the damage to cane stools and soils (Van Antwerpen et al., 2000).

In a compaction trial conducted on a shallow Shortlands form at the Komati research farm, yields from the 1.5 m row spacing, where the wheels of the truck were close to the stools, were on average 18% lower compared to where no vehicles were allowed into the field. Results have confirmed that tramline row spacing, designed to eliminate driving on cane stools, is effective in nullifying cane yield loss due to stool damage and keeping soil bulk density on the cane row low. Good progress has also been made in the joint venture with the University of Kwa-Zulu/Natal (UKZN) and the Commercial Forestry Research Institute (FSRI) to develop a model that will predict the effect of infield traffic on soil com-paction and yield response. Yield and soil measurements from the Komati trial are being used to calibrate the model.

Salinity/sodicityThe effects of soil salinity and sodicity in the low rainfall regions of the Lowveld have been extensively studied (Von der Meden, 1966; Johnston, 1977). A primary cause of soil salinization in these regions is the development of high water tables, which allow capillary rise of saline ground water into the rooting depth of the crop. Poor quality irrigation water may be another source of salts. Major outcomes from these studies were the development of guidelines based on satu-rated paste analyses.

A serious decline in yield on an estate in northern Zulu-land was linked to soil degradation due to a build up of salts in the soil (Culverwell & Swinford, 1986). A more recent study of a cane yield decline on duplex soils in Swaziland showed that, under a system of monocropping, there was deterioration in both physical and chemical properties of soils

when compared with adjacent virgin land (Henry & Rheber-gen, 1994).

An interesting glasshouse and field study was conducted to assess the impact of increasing levels of soil salinity during the growing season on the physiology of sugarcane (Naidoo et al., 2004). It was evident throughout the trial that high lev-els of salt adversely affected growth for all three varieties; total above-ground biomass decreased significantly with increasing salinity. Although sucrose yields (t ha-1) of all three varieties showed a trend of decline with increasing salinity, only NCo376 showed significant treatment effects. Leaf water potentials at both pre-dawn and midday were found to be lower as EC increased, implying a mild water stress. N17 and NCo376 showed moderate sensitivity to salt because the response to salinity was never as severe as the response of N22, which displayed a more pronounced reac-tion at higher levels of salinity.

Yield decline and soil healthThe failure of industrial sugarcane yields to break through the “productivity plateau” during the past 15 years was the main reason for initiating a survey in which the differences in soil properties between virgin and adjoining cultivated soils were examined in northern KZN. At each site, soil samples were taken in triplicate at depths of 0-150, 150-300 and 300-450 mm for chemical examination. Duplicate undis-turbed soil core samples were taken at depths of 0-20 and 200-220 mm for physical analysis. Mean differences in tex-ture between paired sites were less than one percent and not significant. Mean soil water contents at saturation, 10 kPa and 1500 kPa were all slightly higher and bulk density slightly lower in the virgin areas compared to the cultivated areas. In terms of chemical soil reaction, cultivated areas on average showed higher levels of Al, titratable acidity and acid satura-tion at all depths with CEC lower and increasing with depth. Differences between areas in terms of P were small and not significant. In terms of threshold values the quantities of P were sufficient on average for ratoon cane and marginal for plant cane. S was slightly better and increased in the virgin areas with depth. Total N, K, Ca and Mg were all reduced in the cultivated areas and the (Ca+Mg)/K ratio indicated a greater imbalance between these nutrients in the cultivated soils. Although the ratio increased with depth, the difference between the mean values per depth interval showed that soil degradation is diminishing with increasing depth.

The cultivated areas showed significantly less organic matter in the top soil layer but the difference decreased with increasing depth. The same trend, though not significant, was found for total N. In the irrigated areas EC, SAR and Na val-ues were respectively 30, 30 and 53% higher in the topsoil of cultivated areas, suggesting an increased salinity/sodicity condition compared to virgin soils. Although, the values for Na were below the threshold value of 2 cmolc kg-1, the sub-soil (450 mm depth) values in the cultivated areas approached this threshold value (Van Antwerpen & Meyer, 1996). Similar studies have been carried out in other parts of the sugar indus-try confirming these initial findings (Dominy et al., 2002).

More recently, some valuable research has been con-ducted by post-graduate students from the University of KZN under the guidance of Prof. Haynes and with the collaboration

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of Dr. van Antwerpen from SASRI. The study of the impactof land management including sugarcane, horticultural crops (citrus, avocado and banana), forestry (gum, wattle and pine), kikuyu pasture, native forest and native grassland on the size and composition of the earthworm community on sugarcane estates in Northern KwaZulu-Natal was particularly notewor-thy. Earthworm numbers followed the order: burnt sugarcane < trashed sugarcane = grassveld = gum forest = pine forest = wattle forest = avocado orchard < citrus orchard < banana plantation < native forest < kikuyu pasture. The pattern of change in organic C, and particularly microbial biomass C, with land-use showed broadly similar trends to those of earth-worm numbers. This demonstrated that the C input to the soil and the amount of labile, metabolizable C present were the major determining factors to the size of both the soil micro-bial and earthworm communities (Dlamini et al., 2001). A number of other studies dealt with the benefits of various organic amendments in restoring soil health by means of mill wastes such as flyash and filtercake (Dee et al., 2002) and poultry manure as a source of nutrients and carbon as a food source for the soil microbial community (Judge & Haynes, 2003).

A most useful post-graduate study dealt with phospholipid fatty acid (PLFA) analysis and how it could be used to charac-terise the microbial communities in the soil under long-term pre-harvest burning of sugarcane and several other land uses, and on a long-term experiment comparing sugarcane burnt before harvest and sugarcane harvested green with retention of a trash blanket. Trends in total PLFAs (total microbial bio-mass) followed the order: kikuyu pasture > native grassland > ryegrass pasture > maize > burnt sugarcane, and were higher under trashed than burnt sugarcane. It was concluded that sugarcane production under pre-harvest burning is particu-larly detrimental to the structural diversity of soil microbial communities and that the conversion to green cane harvesting with trash retention increases soil microbial diversity (Haynes & Graham, 2004).

In recent years the focus of soil research at SASRI has shifted towards gaining a better understanding of the role of the soil microbiology community in soil and crop health. Soil microorganisms, including bacteria, fungi, nematodes and algae, have the potential to be important indicators of soil health. Micro-organisms are responsible for the decomposi-tion and transformation of organic matter in soils, and are also responsible for a significant number of mineral transforma-tions. These processes affect nutrient availability, and hence soil quality and crop yield. A review of available methods that could be used to measure the quantity and quality of soil microflora as indicators of soil health was recently conducted by Van Antwerpen et al. (2005). Of all the methods it was concluded that the molecular biological methods based on genetic fingerprinting showed the greatest potential for iden-tifying the presence of various micro-organisms through tar-geting the rRNA gene. It was pointed out that none of the methods was perfect and that they all have advantages and disadvantages. Only using different molecular biological techniques, microbial methods and methods to determine environmental parameters together will lead to an unbiased understanding of the role of microorganisms in their environ-ment.

Improvements in water use efficiencyResearch into water use by sugarcane grown in lysimeters led to the elucidation by Thompson of the relationship between cane yield and evaporation (Thompson & Boyce, 1968). This work paralleled that of world leaders like Penman and Mon-teith and was ahead of any other sugarcane country at the time. The major impact was improved irrigation scheduling leading to increased water use efficiency. Irrigation advice based on computer models began as early as 1985 following development of a useful water balance model on the main-frame computer at the Experiment Station (Thompson, 1988). This type of work continued using PC based models with the most prominent being the CANEGRO growth simulation model that was developed at Mount Edgecombe, comprising balances for water and energy and for carbon (McGlinchey & Inman-Bamber, 1995). Irrigated sugar estates outside South Africa that have largely been driven by South African tech-nology, management and finance include Ubombo Ranches and Mhlume in Swaziland, Triangle (Zimbabwe), Kilombero (Tanzania), Hippo Valley (Zimbabawe), Sucoma and Dwangwa (Malawi), and Nakambala (Zambia). These initia-tives changed the face of the sugar industry in Southern Afri-can from one dominated by rainfed agriculture in Kwa-Zulu/Natal to large irrigated estates outside (Gosnell, Personal communication, 2007).

The advent of soil specific cane management rec-ommendationsPrior to 1980, very little attention was paid to the possible impact of soil differences in cane husbandry practices, with the result that generally the same varieties were grown across all soil types; fertiliser recommendations were totally based on laboratory soil extraction results without recourse to the nature of the soil, which we know today largely determines the fate of applied fertilisers and therefore their nutrient use efficiency. The early pioneering studies by Beater (1944), to determine the nature and distribution of soils in the sugar industry, and mapping over 500 000 ha of cane land accord-ing to soil parent material on a 1:6000 scale are without peer. More recently the Binomial System of Classification (MacVicar et al., 1977) were important steps that laid the foundation of a comprehensive soils bulletin for the sugar industry. This is already in its third edition with over 49 dif-ferent soil forms described (Anon SASA, 1999). The over riding theme is that the same management practices cannot be applied across all 49 different soil forms (Moberly & Meyer, 1984). Best management practices that for example apply to the Arcadia soil form will not necessarily apply to sugarcane growing for example on the Hutton soil form.

Systems agronomyThe 1990s saw the development of crop growth simulation models and a number of decision support programs to assist with irrigation management, fertiliser application rates, plough-out decisions and trash management. The Cane-Sim model has been successfully used to forecast yields in the sugar industry (Bezuidenhoudt & Singels, 2003). Another innovation has been the development of a more user friendly crop modelling computer program called ‘My Cane’ that pro-vides simple, real-time and field-specific irrigation advice via

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an SMS on the grower’s cell phone. A pilot study involving 17 small scale growers in Pongola proved very successful with an overall saving of 20% in water use and an overall improvement in yield productivity (Singels, 2005). These and many other programs are available to all extension officers and will eventually be placed on the SASA website for grow-ers. A spin-off from this work has been the establishment of closer co-operative links with research workers in Australia, the USA, Zimbabwe and Swaziland.

ConclusionsWhile there can be little doubt that the contribution of soil science has had a marked impact on sugarcane productivity and the production of sugar in this country, it goes without saying that complex problems like the yield plateau cannot be solved by one discipline alone, but require solution through a multidisciplinary team approach. For example the E. saccha-rina stalk borer problem which has been nagging this industry for nearly 40 years has largely been investigated by entomol-ogists but now with the linkage to silicon deficiency and high N usage we are getting closer to reducing the risk of infesta-tion through an integrated approach. The rapid escalation in fertiliser prices will provide enormous opportunities for fur-ther improvements to be made by scientists in improving nutrient use efficiency and reduced fertiliser usage through soil and plant analysis, better choice of fertiliser carriers, tim-ing and placement in relation to crop cycle and soil type. For example the 25% improvement in N fertiliser use efficiency has come about by recognizing the importance of the nitrogen cycle in the soil from the potential to mineralize N from organic N sources to the fate of N through leaching, volatili-zation and denitrification. N recommendations for sugarcane now include the potential N release from a soil which can make up to 50 % of the N requirement of sugarcane. Where lime is applied to soils with moderate to high levels of organic matter, N release from organic N sources is further enhanced and up to 70% of the N requirement of plant cane may be met from a high organic matter containing soil.

There will be a greater focus on beneficiating organic wastes through composting and using waste by products such as filter cake, flyash and vinasse as potential nutrient sources. The global hunger for renewable energy, for power genera-tion and non-electrical technologies such as solar water heat-ing and bio-fuel, will also present soil scientists with opportunities in the future, in terms of value addition and diversification and to reduce its exposure to the world sugar market.

In conclusion, in the words of a former Director of SASRI, we need to be reminded “that research cannot guaran-tee success but no research guarantees failure, that research findings cannot be implemented without technology transfer and that development of people and conservation of the envi-ronment are equally important ingredients of the success equation”. There can be little doubt that technology will con-tinue to contribute towards the sustainability and economic progress of the South African Sugar industry.

ReferencesANON SASA, 1999. Identification and management of soils in the

South African Sugar Industry. South African Sugar Association

Experiment Station, Mount Edgecombe, South Africa.ANON SASA, 2002. Standards and guidelines for conservation and

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