NSW Agriculture, Agricultural Research Institute, Wagga Wagga. NSW 2650
Phosphatic (P) fertilisers are used extensively in conventional farming in the southern and central NSW wheatbelt. However, there is limited use of nitrogenous (N) fertilisers.
The extensive farming system is based on rotational farming practice using subterranean clover, lupins and peas to fix N and using that N to grow cereal and oilseed crops. A typical rotation would be three years of subterranean clover-based pastures followed by a single crop of canola or oats, then a wheat crop, a lupin or pea crop, followed by a wheat and perhaps a barley crop.
The spacing of the legumes in the cropping sequence is designed to minimise the use of fertiliser N. Phosphatic fertiliser (some superphosphate) is used throughout the cropping portion of the rotation. When required, low rates of application of N are applied as ammoniate phosphates (monoammonium phosphate (MAP), diammonium phosphate (DAP)) at sowing. Higher rates of N are applied as ammonium nitrate or urea. Table 1 shows that P is the main fertiliser input while most N is applied at sowing as ammoniated phosphate. As the ammoniated phosphates are mainly MAP, these fertilisers supply twice as much P as N. The phosphates are mainly superphosphate and are used on long-term pastures to the east of Wagga Wagga.
There is some limited use of lime (calcium carbonate) and trace elements, mainly molybdenum as sodium molybdate or molybdenum trioxides.
In its broad thrust towards a rotational farming system the conventional system shares commonality with the organic systems of farming. The use of lime and trace elements is generally permitted in the organic systems. However, the differences are in the use of "chemical" or "synthetic" P and N fertilisers.
Table 1. Estimates of fertiliser type (% of total tonnage) distributed by one major supplier in the Wagga district.
High analysis phosphates 8%
Ammoniated phosphates 42%
N and P blends and other compounds 7%
N fertilisers 4%
The organic systems rely mainly on reactive phosphate rock (RPR) for P fertiliser. This is not used in the conventional system as it is not as cost-effective in our environment as the water soluble P fertilisers.
Rock phosphates are apatites, either hydroxyapatite (Ca5(PO4)3OH), fluorapatite (Ca5(PO4)3F) or carbonated apatite (Ca5(PO4)3CO3). These materials differ in their behaviour and uses. Rock phosphates contain mixtures of these forms but the amount of each varies and the higher the degree of fluoridation the less reactive the rock is and the higher the carbonation the more reactive is the phosphate rock. The phosphate in rock phosphates which are predominantly fluorapatite is not soluble in water with low solubility in ammonium citrate. These rock phosphates are used predominantly in the manufacture of superphosphates. Table 2 gives a typical analyses of Christmas Island phosphate rock which is used in this way.
The reactive phosphate rock (RPR) (e.g. North Carolina, Table 2) has more P soluble in ammonium citrate.
RPR can be an effective P fertiliser provided:
• it is finely ground
• the soil is acid (< pH 5.5. in CaCl2 and preferably < pH 4.9)
• the crop grown is perennial rather than annual
• the rainfall is high (> 500 mm and preferably > 800 mm)
Table 2. Typical composition of some different phosphatic fertilisers, phosphate rock products and phosphate rock.
Product Total Water Ammonium Ammonium
P soluble citrate citrate
(%) P (%) soluble P (%) insoluble P (%)
Phosphate Greenleaf single 8.8 7.2 1.4 0.2
super 17.5 14 2.2 1.3
___________________________________________________________________________phosphate Christmas Island P 16.4 - 0.8 15.6
rock (used to
Reactive North Carolina P 13.0 - 2.9 10.1
phosphate rock (Reactive)
(From Burgess, J. (1991). Reactive phosphate rock. NSW Agriculture, Agdex 541, No.6.
A recent review of the effectiveness of RPR in Australian agriculture (Bolland et al. 1988) concluded that their effectiveness was about 30% that of superphosphate although it varied widely, the outcome of which is that RPR is generally not cost-effective compared to other P fertilisers in the wheatbelt. In the higher rain slopes and tablelands on acid soils with perennial pastures, RPR may be a realistic option. In the end the decision for the conventional farmers is one of cost-effectiveness.
Calcined phosphate rock is permitted in some organic farming systems (Biological Farmers of Australia) but not by others (National Association for Sustainable Agriculture). This product is a kilned (heated) RPR in which carbon dioxide is driven off to form a more reactive product.
Nitrogen fertilisers are generally not permitted in the organic system. Nitrogen is derived from legumes or from the import of animal (e.g. blood and bone) or vegetable materials.
Urea use is permitted by the National Association for Sustainable Agriculture in its "sustainable" category (level C) but not in the "organic" category (level A).
The level of the heavy metal cadmium (Cd) is found as a contaminant in some sources of phosphate rock. It varies widely. Typically, superphosphate has 45-50 ppm Cd which comes mostly from the phosphate rock. MAP and DAP have less than 10 ppm Cd. The source of phosphate rock and the rate of application are the main controllers of Cd input into soils.
Adverse off-site environmental effects from "chemical" fertilisers are cited as one reason for avoiding their use. The presence of P and N in waterways can be a problem.
Generally P in the soil is immobile. It will only enter waterways if the soil is sandy or the fertiliser P use is excessive. This combination has caused problems in vegetable production (high fertiliser use) or on sandy soils (Pee Inlet, WA). In the central and southern wheatbelt the rate of P applied is biologically suboptimal due to economic constraints. The soils are clays or loams and hold P effectively.
Nitrogen (as NO3-) is more mobile in soils. Again our use of N fertilisers is suboptimal. However, most N in the farming system is produced by legumes and this N is mainly used by plants as NO3-. The use of the legume in the system is likely to be the main source of NO3- in ground water and waterways.
The organic farming movements suggest that with an understanding of the dynamics of the cropping or livestock system, dependency on even permitted substances will be reduced (Anon, 1990a).The assumption appears to be that the soil and biological activity in the soil are able to provide full nutrition for plants and imports of nutrient are unnecessary.
This may be true for nitrogen as legumes can provide an input of N. However the situation with P in the northern wheatbelt of NSW is dramatically different. Phosphorus applied to the soil is fixed chemically by the soil, as calcium, iron or aluminium phosphates. Some P is removed from the property in produce. Unless more P is imported the system is exploitive and unsustainable. A similar situation exists with sulphur.
The organic farming ethic of aiming to reduce inputs from outside the farm and operate a "closed" system is a serious impediment to sustainability.
Soil acidification is driven by agricultural production and the leaching of nitrate below the rooting depth of crops and pastures. As a result productive organic farming will also lead to acidification. Only one practice, the use of RPR, will work against this process. However, the application of RPR at reasonable P application rates would reduce the rate of acidification only marginally; it is likely that an organic system would be about 10% less acidifying than a conventional system.
In this area the conventional and organic systems share the main approach of the use of legumes. The organic systems prefer the use of naturally-occurring ground rock as fertiliser (e.g. phosphate rock, lime) and prohibit N fertilisers. The use of N fertilisers is minor compared to the legume inputs but is used to supplement N in deficient situations.
1. Anon (1990a). The National Association for Sustainable Agriculture Australia Ltd; Standards. National Association for Sustainable Agriculture Ltd, PO Box A366, Sydney South, 2000.
2. Anon (1990b). Biological Farmers of Australia Co-op Ltd; Production Standards, Secretary BFA Standards Council, "Demeter", Breeza, NSW 2381.
3. Bolland, M.D.A., Gilkes, R.J. and D'Antuono, M.F. (1988). The effectiveness of rock phosphate fertilisers in Australian agriculture: a review. Australian Journal of Experimental Agriculture. 28:655-68.
4. Burgess, J. (1991). Reactive phosphate rock. NSW Agriculture, Agdex 541, No.6.