The ability of broadacre grain cropping systems to meet future demands for substantially larger quantities of grain is increasingly being challenged by negative nutrient budgets, declining native fertility and rising fertiliser costs. At current prices replacing N, P, K and S removed in grain with fertiliser inputs would cost $50-$150/ha in additional fertiliser costs. This is an underestimate, as application costs will also rise – especially if periodic deep placement of immobile P and K is undertaken. These costs challenge profitability, and short of significantly higher food prices, alternate ways of managing nutrient budgets need to be considered.
The most effective strategy could be using legumes to fix atmospheric N, as at least half of the nutrient deficit is N. However this would require grain legumes that have a low N harvest index (i.e. leave lots of N in residues) and a fixation system that is insensitive to elevated soil mineral N. Peanut fulfils that criteria in coastal systems, but mungbean and chickpea do not meet those needs for inland areas. Forage/ley legumes need mixed cropping enterprises to be viable. It is also worth noting that grain legumes remove higher quantities of P, K and S per tonne of grain than cereals, so while N is free other costs may rise.
Availability of organic wastes, particularly from intensive livestock industries and in some localities urban wastewater treatment plants, is increasing. These products can be significant (and economic) sources of nutrients, but need to be considered as supplements rather than a fertiliser replacement. Nutrient ratios in these products do not match crop removal and they are relatively dilute sources of nutrient, so freight costs can be high. A promising future strategy will be reclaiming nutrients from waste streams and concentrating them in forms that more closely resemble bag fertilisers.