Factors affecting the nitrogen requirement of sugarcane .1 Economics

Fertilizer nitrogen requirements of sugarcane have traditionally been based on an economic yield optimum derived from rates of nitrogen experiments. A number of studies have been reported uring the early years, with trial results reported from South Africa (du Toit 1957), Hawaii (Stanford and Ayres 1964), Brazil (Malavolta 1982), Louisiana (Golden and Ricaud 1965), India (Singh 1963), Peru (Husz 1972) and Puerto Rico (Samuels and Alers-Alers(1964). These early trial studies were usually conducted in different agro-climatic regions, on a range of soil types with little or no consideration for the large differences in N mineralizing potential that existed between soils.

Invariably the conclusions led to broad distinctions being made between nitrogen rates for the production potential of environments. In terms of commercial practice, the recommended nitrogen rates were based on expected cane yield, with rates of N varying from 1.5 to 2.0 kg N/t cane. The emphasis was mainly on fertilizing the crop rather than on managing the soil, which led to a period of excessive and inefficient N usage.

5.2.5.2 Climate

Climatic factors such as moisture, radiation and temperature have a huge impact on crop yield, which in turn will have a major impact on the rate and quantity of nitrogen removed by the crop. A climatic benchmark of yield is essential if the contribution of management and especially nitrogen management is to be fully assessed. In practice climatic potential has been determined by a well known formula derived by Thompson (1976):

Y = Eo/100*9.8 (5.1)

where Y = yield (tc/ha/y), and Eo = Class A pan evaporation per annum in mm fully replenished by rainfall and irrigation.

These yields are possible where moisture is not limiting under experimental conditions. Effective rainfall (70 % of total precipitation) can be used in place of Eo but this is a much less reliable measure.

Based on the author’s experience, under African conditions climatic yield potentials can vary anywhere from 120 to 170 tc/ha/y subject to no moisture limitations . However, under commercial practice, and under the highly variable soil conditions that occur in Africa, the achievable estate yields under irrigation are likely to be between 20 and 35 % lower than potential yield. These days a whole range of crop simulation models are available that combine crop, soil and weather databases which can be used to simulate multi-year outcomes of crop management strategies for sugarcane at any location in the world. The use of models is more fully discussed in Chapter 14.

5.2.5.3 Crop stage (plant or ratoon) and cycle

Research findings from trials conducted worldwide have confirmed that plant cane is far less

responsive to applied N than ratoon cane. In general, results suggest that the N requirement of plant cane is on average about 40 kg/ha N lower than that of ratoon cane. The main reason for this

difference is the extra release of mineral N from the soil following a fallow period.

Age of cane and season are important factors responsible for most of the variation in N uptake. The variable impact of season on crop removal of nutrients was illustrated earlier in this chapter by nutrient uptake data of cultivar N14 grown in a lysimeter at Pongola in South Africa (Thompson 1988). Favorable seasonal cycle crops can accumulate N rapidly, to the extent that over 80 % of the N requirement can be taken up after four months, whereas in an unfavorable cycle just over 10 % of the N may be taken up by the crop. This differential N uptake has important implications for the timing of fertilizer placement and especially when designing a fertigation program. Research findings in South Africa have shown that fertigation is more effective than conventional surface applied N for cane grown on an

unfavorable cycle (Autumn to Autumn), whereas with a favorable growth cycle there was little or no difference in efficacy between conventional and fertigation N applications (Weigel et al. 2008).

5.2.5.4 Age

In South Africa, an 18 month old crop growing in the elevated areas of the Midlands, starting in May (Autumn) and ending in November (Summer), will invariably be out-yielded by an 18 month old crop starting in November and ending in May, because the first crop effectively has one summer

compared with the second crop which has two summers. More mineral soil N becomes available during two summer cycles compared with the single summer cycle, implying that less N may be needed in a favorable cycle.

5.2.5.5 Cultivars

In some countries such as Taiwan, India, Argentina and more recently in South Africa, differences in N fertilizer use efficiency amongst cultivars is becoming an increasingly important factor. Trials conducted in Pongola and Mpumalanga in South Africa have shown that using the ratio of sucrose yield to N accumulation, cultivars may be classified into one of three categories: (i) efficient N use responders, (ii) inefficient non-responders and (iii) inefficient responders (Schumann 2000).

High N use efficiencies for N12 and N19 were 65 % and 63 % above the ‘reference’ cultivar NCo376.

In contrast, N14 was 19 % less efficient than NCo376 (Fig. 5.8). These data suggested that substantial benefits may be derived from cultivar-specific N fertilizer recommendations for sugarcane, rather than a single recommendation based on NCo376. Current fertilizer recommendations for N14 have been increased by 30 kg N/ha, based on field trial data, and a reduction of 20 kg N/ha will be recommended for N19. Further confirmation of genotypic differences to N treatment was obtained in a large scale pot trial in which 30 clones from the well known AA40 population, comprising 153 clones from a single cross, were subjected to a low and high rate of N nutrition.

Figure 5.8. Comparative N nutrient use for a range of South African cultivars.

(after Meyer et al. 2007)

5.2.5.6 Soil N mineralization potential

More recent studies in determining the N requirement of sugarcane have focused on factors such as climate, soil type, cane cultivar, crop class, legume usage during the fallow, rainfall, length of time green cane trash blanketing has been practiced, and irrigation. For example, early studies conducted in South Africa have shown that mineralization of organic N to ammonium and subsequent nitrification of the ammonium to nitrate, is a continuous process but is largely driven by the organic matter status of soils, drying and wetting cycles, the duration of drying prior to wetting up, temperature changes, soil pH, biological activity and soil disturbance through tillage operations (Wood 1964).

South African studies

Follow-up investigations based mainly on N rate trials conducted in South Africa, Swaziland and Zimbabwe have shown that the relative response obtained to applied N was inversely related to the amount of N released as measured by soil incubation and the organic matter content of soils:

 Soils with low (< 25 mg N/kg), moderate (20-40 mg N/kg), high (40-60 mg N/kg) and very high (> 60 mg N/kg) N release, were associated with average responses to applied N of approximately 50, 30, 20 and 10 % respectively (Moberly et al. 1982).

 The results implied that the probability of a response to applied N decreased markedly with increasing soil organic matter content and greater N release (Meyer et al. 1983).

 The results also showed that the response to applied N was broadly related to the nature of the diagnostic topsoil horizon when classified according to the South African Binomial system (MacVicar et al. 1977).

 Average response declined in the order grey orthic > melanic/vertic > red to brown orthic >

humic A horizons.

These early studies formed the basis of an improved system of N recommendations for the sugar industries in South Africa and the SADEC countries (SASRI Information sheets 7.1 and 7.13), that take into account regional differences in climatic potential, whether the crop is rainfed or irrigated, stage of crop (plant or ratoon), soil mineralization potential index and more recently cultivars (Meyer et al.

1986; Meyer et al. 2007). The current N recommendations for plant and ratoon cane are

summarized in Table 5.4. For ratoon cane the lower value in the range applies to rainfed cane with

an achievable yield potential of 100 tc/ha over 15 months, and the upper part of the range refers to irrigated cane with a yield potential of 125 tc/ha/an. Soils with the lowest organic matter content (< 2 %) receive the highest N recommendation of 140 kg/ha N for dryland cane and 100 kg/ha N for soils with a high organic matter content (> 4 %) due to the higher N mineralization potential. These rates approximate baseline rates of 1.6, 1.4, 1.2 and 1 kg N/t cane used respectively for the given soil N mineralization categories of low, moderate, high and very high N mineralization potential (Meyer et al 1983). Economic assessments that have been conducted periodically have shown that these rates can all be economically justified (Thompson 1980, Prins et al. 1997 and Schumann 2000).A more detailed explanation of the economic benefits is given in section 5.4 of this chapter.

Australian studies

In Australia, Chapman (1994) and Calcino (1994) underlined the importance of climatic regions when assessing the N requirement of sugarcane. For example, the Burdekin region with its fertile soils, higher temperatures and access to water, has a higher yield potential than most other districts, and higher N rates were needed for cane growing on the same soils in other regions. For some time the N requirement of sugarcane was based on a general rule of 1.4 kg N/t expected sugarcane yield plus an additional 1 kg N/t cane yield above 100 t/ha (Keating et al. 1997).

Subsequent investigations highlighted the importance of soil organic matter and the nitrogen mineralization potential and, as is the case in South Africa, a soil nitrogen mineralization index, based on soil organic matter content was proposed for refining nitrogen inputs for two regions of the Australian sugar industry (Schroeder and Wood 2001). Further advances in rationalizing the N requirement of sugarcane was made with the introduction of revised N fertilizer recommendation guidelines based on a combination of district yield potential and a six point soil mineralization index (Wood et al. 2003; Schroeder et al.2005; Schroeder et al. 2007). For dryland ratoon cane the N recommendations vary from 100 kg/ha N for cane growing on soils with more than 2.4 % organic matter to 160 kg/ha N for soils with less than 0.4 % organic matter (Table 5.4). The N rates for plant cane show a similar trend but are 20 kg/ha lower than the ratoon rates where there has been no green fallow. Plant cane N rates are reduced further by as much as 80 kg/ha N following a

soybean/cowpea fallow. As in South Africa, the improved system of making N recommendations has led to more efficient N use and an overall reduction in N application, with economic and

environmental benefits in terms of reduced costs and reduced N losses.

Other countries

The important effect of organic matter and soil mineralization potential on the N requirement of sugarcane has also been recognized in Brazil (Penatti et al. 1997) and in Florida with its large areas of peat, as well as sandy soils where four categories of soil mineralization potential are recognized.

In India, N rates range from 0-50 kg/ha N in Bihar, to 250-300 kg/ha N in Karnataka and

Maharashtra, to over 350 kg/ha N in the south-east coastal area of Tamil Nadu. In general, the rate matches the intensity of irrigation which is higher in the tropics than in the subtropics (except the Punjab). As a simple rule, 1 kg N/t cane expected is given for plant cane and 1.25-1.50 kg N/t cane expected for ratoon crops. The optimum for ratoons is at least 25 % greater than for plant cane.

The rate is adjusted to the extent of 10 % of the recommended rate to allow for leguminous green manure of compost and farm yard manure which has been used, but no correction is permissible for any residual N from one sugarcane crop to the next.

Table 5.4. General recommendations for application of nitrogen fertilizer for sugar industries in South Africa, Australia, Florida, Brazil, Hawaii and India.

(adapted from data given by Kingston 2000 )

Country

Organic matter status (%)

N mineralizing

capacity

Crop stage

and kg N/ha Comments

Plant Ratoon South Africa

(Meyer et al.

1986)

< 2 Low 120-140 160-200

Actual rate depends on bioclimatic region, rainfed or irrigated 2-4* Moderate 100-120 140-160

2-4** High 80 120

> 4 Very high 60 100

Australia

(Schroeder et al.

2005)

< 0.7 Very Low 140 160

Plant N rates must be reduced where legume fallows have been used

0.7-1.4 Low 130 150

1.4-2.1 Moderately

low 120 140

2.1-2.8 Moderate 110 130

2.8-3.5 Moderately

high 100 120

3.5-4.2 High 90 110

> 4.2 Very high 80 100

Florida

(Anderson 1990)

Sandy Low 200 200? Sandy soils, split

applications OM < 35 % Moderate 120 120 Sandy muck soils

OM 35-85

% Very high 34 34 Mucky sand soils

OM > 85 % Very high 0 0 Muck soils

Brazil

(Penatti et al.

1997)

General −

50 + fixation;

legumes , wastes

100 + fixation;

legumes, wastes

Applies mainly to deep red latosol soils

India General − 50-100 150-200

Rates tend to be lower for smallholder grown

cane Hawaii

(Stanford and Ayres 1962)

General − 224 224

Mostly as split applications in drip

irrigation

*Refers to mainly non-red soils **Refers to mainly deep red clays (Nitosols, Rhodic Luvisols and Ferralsols)

5.2.6 How to adopt soil specific nitrogen recommendations