Managing N fertilizer .1 Choice of nitrogen carrier

Despite the fact that urea is subject to NH3-N losses by volatilization and LAN, AS and urea suffer field losses of NO3-N by leaching and denitrification, for many years the quoted average yield response to applied N from different carriers tested across a wide range of soils, have largely shown that different forms of N tended to give the same yield response (du Toit 1957; Wood 1968,

Prammanee 1989; Wood et al. 1990; Denmead et al. 1993; Meyer 1995). In practice, the choice of N carrier is still largely driven by the price of a unit of N ($/kg N). The higher N content forms are usually the most cost effective, but increasing attention is being paid to improving fertilizer use through better placement and timing of operations.

5.2.8.2 Placement of urea

In Australia subsurface placement of urea fertilizer is preferred to surface application, as this operation avoids volatilization NH3-N loss and has out-yielded surface applied urea in field

experiments (Calcino and Burgess 1995). The environmental impact is also a lot lower as the risk of nitrogen loss in surface runoff is minimized. In practice with ratoon cane, the urea is placed below the surface on either side of the cane row, or knifed into the centre of the stool, behind a coulter disk. Equipment has been developed to bury the urea through a trash blanket but this increases the cost of N application. Urea should under no circumstances be placed on top of the trash as the urease enzyme contained in trash will generate a huge loss of N through volatilization NH3-N loss.

Delaying application of urea until the canopy is approximately 50 cm in height (Freney et al. 1994) also reduces volatilization losses, but was not as yield effective as was the subsurface placement (Calcino and Burgess 1995).

Recent research conducted in the South African sugar industry has underlined the importance of basing the choice of N carriers on soil-specific conditions (Schumann 2000). Field tests showed that volatilization loss of N from urea was greatest with band placement compared with broadcast placement and that light textured soils especially sands and loamy sands were associated with the highest risk of N loss. A new soil test permits soil-specific recommendations for good management practices (GMP) of urea, and advises the use of more stable solid alternatives such as LAN, ASN or ammonium sulfate using the % volatilization loss threshold values shown in Box 5.10 where applicable.

Box 5.10 Thresholds for rating potential ammonia volatilization loss(after Schumann 2000)

> 15 % Recommend a change from urea to a different N source such as LAN. Ammonium sulfate can be recommended where S is needed or where mild soil acidification is desirable, but definitely not on acid sandy soils.

5-15 % Recommend improving urea N efficiency by broadcasting rather than banding urea.

< 5 % No special advice is given for the use of urea, which is the preferred source, provided its price remains competitive.

5.2.8.3 Timing of N fertilizer applications

With plant cane, timing of N fertilizer is not as critical as ratoon cane because the growth pattern is generally slower and there is an abundance of released mineral N following a crop fallow. With ratoon cane, timing is more important if fertilizer N use efficiency is to be maximized. Ideally, timing should be synchronized with the pattern of N uptake by the crop. In southern Africa the following guidelines are generally recommended for timing N fertilizer application to ratoon cane.

Summer harvest: top-dress all N within two weeks of harvesting previous crop.

Winter harvest: In the warm irrigated areas no significant benefits have been obtained from splitting N, except during very cold winters when regrowth is very slow and N uptake is inefficient. Under these conditions splitting is recommended for all N carriers as follows:

 March-May harvest: 1/3 N within two weeks of harvest, + 2/3 in August/September.

 June-July harvest: 1/2 N within two weeks of harvest, + 1/2 in August/September.

 August onwards harvest: Apply all N within two week of harvest. In the cooler region of the Midlands, N application can be delayed until September.

Under certain conditions, splitting may be beneficial on soils with restricted drainage to prevent loss of N through denitrification and on sandy soils (Meyer and Wood 1994). Use a six to eight week split according to the harvesting season:

• May-June harvest: apply 1/3 initial, + 2/3 top-dressed.

• July-August harvest: apply 1/2 initial, + 1/2 top-dressed.

• September onwards: apply all initial.

5.2.8.4 Fertigation

Drip irrigation is considered efficient as it facilitates more accurate and flexible application of soluble fertilizers through the irrigation system (fertigation), leading to greater N fertilizer efficiency

compared with conventional N application. It has been demonstrated in the Australian and

Mauritian sugar industries that N applied as split treatments through the dripper can reduce N usage by between 30 and 50 % (Thorburn et al. 1998; Ng Kee Kwong et al. 1999). However, a major

disadvantage is that fertigation schedules will rely heavily on seasonal factors and the phenology of the crop. Nutrients are continuously taken up by sugarcane in accordance with the different growth stages. There is initially a great emphasis on N and P during the germination and canopy phases, increasing for a summer crop in an exponential manner, followed by a linear uptake of N, P and K during the grand growth phase (see Chapter 1), with a marked tailing off of N but a continued uptake of K and P during the crop maturation phase. The variation in N uptake for different cropping cycles is illustrated by the N growth curves shown in Fig. 5.9.

Figure 5.9. Nitrogen accumulation in cultivar N14 for three cropping cycles (Thompson 1991).

In the irrigated areas of Swaziland, cumulative nitrogen uptake curves were tested for a late season summer cycle plant crop and for an early season winter cycle first ratoon crop. Results indicated that the winter N uptake curve correctly predicted the proportional monthly N demand of a winter ratoon crop, but appeared to underestimate the N demand of a summer plant crop between January and April (Butler et al. 2002). Follow-up trials conducted in neighboring Mpumalanga province of South Africa showed that splitting the fertilizer applications and applying N via the irrigation system was of special advantage in early season cycles. This is attributed to the lengthy demand for N in early season crops as reflected by the N demand curve being better matched by the split doses. In the late season summer cycle trial there was no yield advantage to splitting N compared to the single incorporated urea placement treatment (Weigel et al. 2008).

Box 5.11 Advantages and disadvantages of fertigation Advantages include:

 Better control of nutrient supply according to growth stages.

 Flexibility in adapting nutrient supply according to different crop cycles as determined by season.

 Control of leaching losses of nutrients especially in sandy soils which has environmental implications.

 Where urea is used as the N carrier, N losses through volatilization as ammonia can be greatly reduced.

 Less labor is needed.

Disadvantages include:

 Crop removal of nutrients is highly dependent on season, locality, soils and cultivar. This implies that multiple growth curves must be drawn up and levels and ratios of nutrients determined for the conditions in question.

 Management factor must be very high and an understanding of the physiology of the plant is needed.

 Invariably the inherent soil fertility status in this approach is ignored and there is likely to be a build-up of nutrients over time, which will lead to luxury uptake of nutrients and a negative impact on quality.

 High purity nutrient sources are required, which increases the cost of crop production.

 Fertigation with vinasse or CMS may result in blockages, depending on the type of dripper used.

5.2.8.5 Side effects of nitrogenous fertilizers

An important side effect of nitrogenous fertilizers is the acidifying effect of ammonium and amide fertilizers during the nitrification process. The proton release H⁺, associated with the nitric acid that is formed, is a powerful acidification source. Since this has to be counteracted by liming, it is convenient to express it in terms of the equivalent loss of CaO (Table 5.6).

Table 5.6 Soil acidification by nitrogen fertilizers (after IFA World Fertilizer Use Manual 1999)

Fertilizer Amount of lime (kg)needed to neutralise acidification induced by 1 kg of N

Calcium ammonium nitrate (27 % N) 1

Ammonia, urea, ammonium nitrate 2

Diammonium phosphate,

ammonium sulfate nitrate 3.5

Ammonium sulfate 5

The transformation of ammonium to nitrate by bacteria, proceeds quickly when temperatures increase. At temperatures of 20-25 °C an application supplying 50-100 kg/ha N has the potential to nitrify in about one to two weeks. Special nitrification inhibitors added to fertilizers can delay nitrification for several weeks.

5.2.8.6 Organic nitrogen carriers

Inorganic fertilizers are generally more popular than organic by-products such as chicken litter and filter press mud, mainly because organic products are bulky to transport and are sometimes difficult to apply. If not packaged and certified they need to be analyzed before use, as their nutrient balance is often not suitable for sugarcane and might need to be supplemented with inorganic fertilizers.

However, given the recent volatility in the price of fertilizers, as well as environmental and health concerns, there has been an increasing demand for advice based on the use of bio-fertilizers such as chicken litter, filter cake, kraal manure and pig slurries. Poultry 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 et al 1971). Five t/ha in the furrow can provide sufficient N, P and K for a plant cane crop growing on a humic soil without using additional fertilizer. This provides a saving of between 10 to 25 % to the grower, subject to the landed cost of the poultry litter. Similar savings may be obtained for fertilizing ratoon cane with poultry litter. Where co- products or agricultural wastes are available at reasonable prices, they are certainly worth

considering (Table 5.7).

Some farm wastes are used because recycling is the only effective and beneficial means of disposing of them. An example is vinasse, also known as stillage or distillery slops, which is produced at about 13 liters for each liter of ethanol from molasses, or around 8 liters from cane juice. The product is widely used as a potassium source in Brazil as well as in other countries producing ethanol from sugarcane. In South Africa, Condensed Molasses Solubles (CMS) which contain 45-50 % solids, has recently been developed from the evaporation of vinasse to a Brix value of between 50 and 60 %.

The main driver has been to reduce the cost of transport of raw vinasse from the distillery to distant farms (Lyle 2006). Impressive yield and soil fertility benefits have also been obtained from using vinasse in Brazil (Korndörfer and Anderson 1997), Taiwan (Wang et al. 1996) and Zimbabwe (Matibiri 1996), and with CMS in Swaziland (Turner et al. 2002). More detailed information on the use of both products is given in Chapter 13.5.

Table 5.7. Range in nitrogen content for selected organic N carriers.

(after Barry et al. 1998)

Organic carrier N % range

Mill Waste materials (Filter mud

Filter mud/ash mixtures Mill ash

Vinasse/dunder

0.8 - 2.2 0.4 - 0.8 0.1 - 0.5 0.05 - 0.8 General wastes

Sewage sludge

Animal manures (farmyard manure, liquid manure, slurry) Compost (mixture of decomposed plant residues, etc.)

Green manures (leguminous or other crops incorporated into the soil)

0.5 - 6.6 0.5 - 5.0 0.5 - 3.5

1 - 3.5

Green manures as a natural source of N, and as a means of breaking the monoculture to promote soil health, are dealt with in Chapter 2.9.3.

There are health and environmental risks associated with the use of organic manures, resulting from high rates of application of slurry (from cattle or from pigs) in certain areas, and from town compost and sewage sludge which may contain an excess of certain nutrients or be contaminated with heavy metals or toxic organic substances which attract flies. Organic materials with a low C/N ratio (below 15:1) also have a high potential for nitrification and the generation of soil acidity.