Introduction
2.6 Fertility Management
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results with up to half the nitrogen being lost. Higher cost (per unit of N). Ammonium sulphate is far less subject to volatilisation with average losses of 10%. When applied underground, both materials were not lost to volatilisation to any significant degree (ibid), however they would still be subject to leaching.
Recent studies at four coastal Queensland sites show losses to volatilisation of up to 66% over the month following application (Freney et al, 1992). The urea moves to a volatile ammonia form with moisture (dew, condensed soil evaporation, rain) and escapes into the atmosphere contributing to greenhouse gases. Thirteen millimetres of rainfall or irrigation appeared sufficient to water in the urea but losses of 2 0 % to volatilisation were still seen. With the addition of other losses such as denitrification, leaching, and run-off, Freney estimates that typically less than 2 0 % of nitrogen applied this way actually gets into the plants. Surface banding urea fertiliser along the trash layered rows instead of broadcasting has been shown to double the early rate of atmospheric loss and result in higher overall losses compared to broadcasting (Freney, 1991).
Volatilisation of urea may be significantly reduced through application immediately prior to rains or spray irrigation, thus ensuring the urea is watered into the soil. Predictive models of rainfall to assist application timing are under development (Anich and Wegener, 1992). Plant establishment appears to be the best application time for growers still broadcasting urea. Later, canopy formation protects gaseous removal of ammonia, reduces dew formation leading to trash surface losses, and reduces surface temperatures.
Chapman (1990) suggests that the minimum 20% loss also occurs with urea buried in the inter-row due to denitrification in moist conditions during the months it takes for cane roots to reach the area.
The best available practice seems to be underground side stool application as late as possible.
Alternative fertilisers like nitram which are stable and resist volatilisation are a possibility for broadcasting. While more expensive, they have higher marginal returns.
Chapman (1991) argues that when broadcasting fertiliser is the only option, it is economically better to use urea and accept the losses than to use alternative sources of nitrogen. Growers would however also have to accept the nitrogen contributing to atmospheric problems and justify this to the rest of society.
Surface application of urea is not a sustainable practice and should be avoided: it wastes energy, farmers' money, and moreover, contributes nitrogenous greenhouse gases. Improving nitrogen fertiliser regimes will reduce waste, lower fertiliser costs, decrease greenhouse gas emissions from nitrous oxides and reduce nitrogen in runoff (reducing the risk of eutrophication in streams and waterways).
2.6.2 Biological Nitrogen Fixation
Soil and root samples from virgin and cultivated cane lands at Tully, Queensland were examined for bacterial populations (Murphy & Macrae, 1985). Of the 150 strains found, more than half were nitrogen fixing (mostly buterobacteriaceae). Species were similar in soil under cane for over 50 years and in virgin soils. This finding holds promise for salvaging natural biological nitrogen fixation in old cane soils.
Murphy and Macrae (1985) noted different microbes with different cane varieties. They speculate that each cane cultivar may have its own particular mix of adapted N-fixing bacteria and that
Keeping It Sweet - ACF certain cultivars may prove to be more suitable hosts. When comparing nitrogen fixing organisms in Queensland with those reported in Brazil, the researchers found that Q 9 0 (a cane variety) grown in Tully, Queensland had m o r e potential nitrogen fixing associates. Further research is suggested to optimise bacteria and cane variety hosts and to increase on-site biological nitrogen fixation.
Nitrogen fixing bacteria associated with sugar cane have been k n o w n since the 1960s (Doebereiner, 1961). Biological alternatives to chemical fertilisation may be feasible despite E v a n s ' (1977) view that biological nitrogen fixation is not sufficient to meet the requirements of intensive crops, at current yields.
The following are important indicators of the importance and potential for biological nitrogen fixation
• Sugar cane has been grown in monoculture for over 100 years at Sao Paulo, Brazil without any addition of nitrogen fertiliser. In this situation, only half the fields even responded to nitrogen fertiliser if P and K w e r e also supplied. (Lin, 1986).
• Ruschel (1978) reported that sugarcane took up 2 5 - 3 0 % of its total nitrogen through biolog cal nitrogen fixation in addition to that supplied from chemical fertiliser and soil nitrogen.
• In Taiwan, a Beijerinckia-like bacteria was isolated from cane fields and inoculated onto cane in water and sand culture lab experiments (Lin, 1986). T h e results indicated that this bacteria in association with the cane roots produced sufficient nitrogen from the air to provide about 5 0 % of the dry weight of crops with typical artificial fertilisation.
Furthermore, natural soil systems will have a far m o r e complex soil ecology than the single species isolated in Taiwan, and the combined effects of a number of complimentary species could explain the total biological nitrogen fixation at Sao Paulo. T h e Beijerinckia bacteria occur mainly in tropical regions but are extremely cold hardy (Becking, 1961).
Chapman et al (1992) identified free living nitrogen fixing microbes associated with sugar cane trash at Mackay. Significant levels of nitrogen fixation can occur within green trash layers as they decompose, especially under moist conditions. T h e highest levels expected w e r e 36-49 kg/ha/yr of N2. Urea fertiliser added to trash at typical broadcast application rates, restricted this biological nitrogen fixation.
There is both great potential and a pressing need for further investigation of biological nitrogen fixation associated with cane plants. Unfortunately, Chapman reports (pers comra) that several research proposals in this field have not been funded. Current research by B S E S looking at total soil sterilisation by fumigants via trickle irrigation may eliminate any chance of help from beneficial soil organisms.
2.6.3 A l t e r n a t i v e F e r t i l i t y M a n a g e m e n t
Some recent developments in fertilisation should be investigated for their influence on sugar cane growth. These include:
Green manures — many growers use legumes for green manure plantings during replant
luteola), centra (centrosema pascuvorum) and phasey bean (macropilium phaseolus) were found well suited to late wet conditions. They contributed 40-50 kg n/ha if well grown and produced clear improvements in soil structure, organic matter and weed control compared to bare cultivation. At Tully, vigna, mung and soy bean proved especially useful under wet conditions.
• Biostimulants — biostimulants, non-fertiliser compounds which increase plant growth and vigour, have shown impressive results for some grass species. Preliminary trials using biostimulants have also indicated that the amount of fertiliser can be diminished without affecting biomass growth.
• Rock dusts — the use of basalt dust and reactive phosphate rock have also shown positive results for plant growth.
• Micro nutrients — in Bundaberg cane farmers were told that sugarcane did not need micro nutrient amendments of zinc and boron. Cane farmers who had been adding these materials as part of vegetable small cropping rotating through their farm had seen a clear response in cane following vegetables. They have taken the lead in changing to this practice and are gaining healthy productive increases in yield (Don Hall, Crop Tech Labs, pers comm).
Investigation of these materials and processes should include broad environmental implications as well as cane yield trials.
Recent research (Russo, 1990) into a new approach to fertilisation points to some potentially valuable materials called bio-stimulants. These compounds increase plant growth and vigour through increased efficiency of nutrient and water uptake. They are non-fertiliser products which have a beneficial effect on plant growth and most are naturally sourced. The typical product consists of a mix of humic acids, marine algae extracts, a non-hormonal reductant plant metabolite and vitamins. These bio-stimulants increase root and top growth of plants while decreasing fertiliser requirements up to 50% in a number of species (coffee, grasses, pines, alder, Douglas fir).
They also increase the plant's resistance to stress from drought or residual herbicides.
Humic acids comprise 65-70% of the organic matter in soil. Humic acids promote growth by increasing cell membrane permeability, increasing oxygen uptake, respiration, and photosynthesis, increasing phosphorus uptake, increasing root and cell elongation, increasing ion transport and acting as growth hormones. Algae extracts are commonly used organic supplements for increasing plant growth and stress resistance. The active ingredient, cytokinin growth hormones, act to encourage wound healing, delay cell death and chlorosis, increase chloroplast development, promote cell division, organ formation, and stimulation of cell enlargement. Cytokinins have also been shown to increase root elongation and root hair development.
In research at Yale University, greenhouse and field studies on one bio-stimulator produced impressive results. With rye grass, chlorophyll content increased 207% and regrowth after mowing was twice as high compared to the control. Lawn sod treated with the bio-stimulator produced 6 5 % higher root weight and 3 5 % deeper root growth with a similar result for bent grass. Forest nursery trees showed 10-100% increases in root mass. Improved root and shoot growth, better root growth potential, and better stress resistance seem to be the most common result of using this material.
Furthermore, bio-stimulants may cut down application of synthetic fertiliser without affecting growth. Russo (unpublished) has done preliminary research showing that in the presence of the bio-
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stimulant, coffee seedlings treated with half the amount of fertiliser yielded the same shoot biomass and higher root biomass than those fully fertilised.
Evans (1948) reported a marked increase in root proliferation w h e n basalt dust was added to the leached volcanic soils of Mauritius. Andrew W o o d (pers c o m m ) is currently trialing basalt dust on cane in the Ingham area and commercial basaltic and granitic rock dust amendments are n o w available in S.E. Queensland.
Anon (Aug 1991) reports that reactive phosphate rock (RPR) will finally be generally available in Queensland. As a ground rock containing the equivalent of half a b a g of lime per bag of RPR, it differs from super phosphate in having a tendency to moderate rather than increase soil acidity. Per tonne of actual phosphorous, it is 3 0 % cheaper than single super. W h i l e most quickly available in soils below pH of 5.5, the R P R acts as a long term source of phosphorous in the soils. Since phosphorous is one of the few nutrients increasing in Queensland cane soils and at more than adequate levels on most farms, the changeover should be easy. T h e rock is sourced from North Africa and does not have worrisome levels of cadmium contamination like widely used marine island sources of rock phosphate used in superphosphate manufacture.
2 . 6 . 4 T r a s h I n c o r p o r a t i o n
In North Queensland, trash incorporators are used that mix a 10-15cm thick trash layer to a depth of 20cm in two passes at a rate far faster than rotary hoeing, leaving a surface much less likely to erode. Practicing growers state they have noted a spectacular increase in earthworm count and a marked improvement in soil structure. (Baxter, 1983)
2 . 6 . 5 I n n o v a t i v e E q u i p m e n t for F e r t i l i s i n g U n d e r T r a s h B l a n k e t s
An innovative Mulgrave farmer has developed a machine without outside funding which cultivates and fertilises under green trash blankets. Named the Rossi Hurricane, after its designer, M a r k Rossi, who built the machine to combat problems associated with fertilising under trash blankets.
The Rossi Hurricane can combine the benefits of cultivation and subsurface fertiliser application with the moisture retention, weed control and cost savings of trash blanketing in one operation. The inter-row trash blanket is lifted at the front of the Hurricane where it is mulched, sucked through the machine by an extractor, and discharged back onto the inter -row space after a series of grubber tines thoroughly cultivates the trash free soil and applies fertiliser in a subsurface band beside the cane stool. Finally, a combination of tines and levelling bars reform the inter r o w space leaving a profile suitable for t o d a y ' s harvesters. Trash cover on the cane stool remains undisturbed through this process. (Chapman, 1992).
2 . 6 . 6 M i n / Z e r o Till P l a n t i n g
While there has been significant progress toward adoption of m i n i m u m till ratoon management and green cane trash blanketing, most growers still use largely traditional practices at planting. Planting is a costly exercise, McMahon and Teske (1989) report unpublished data at Proserpine indicating a cost of conventional planting at $536/ha variable costs plus 35 hours labour per hectare. The period immediately following planting has the most severe erosion risk.
Several on-farm trials with minimum tillage planting produced a yield loss equivalent to $325/ha.
maintenance. There are several current research projects looking further at minimum till planting techniques and results.
While minimum tillage does reduce erosion, nutrient losses per unit of soil erosion is higher (Hunter et al, 1988). Greater nutrient losses from crop residues and surface applied fertilisers occur in the absence of tillage or protective trash blankets. Surface applied fertilisers are unavailable to plants in dry times and this can lead to higher losses off-site and higher fertiliser use in minimum tillage systems.