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Removal of nutrients in harvested grain

G.M. Proudfoot

CSBP & Farmers Ltd, 40 The Esplanade Perth WA 6000

When crop production is more profitable than animal production pasture leys are shorter and the soil is cultivated more often. The result is a decrease in organic matter, a breakdown of soil structure and an overall decrease in soil fertility. A major cause of the depletion of soil fertility is the removal of nutrients in harvested grain (1). The aim of this study is to determine the average composition of wheat from CSBP trials and the annual total removal of nutrients.


Data was collected from 1988 CSBP wheat trials which were located throughout the agricultural areas of Western Australia. The crops were harvested and the grain analysed by standard Methods.

Results and discussion

The harvest yield from Western Australia in 1988 was 5.1 million tonnes and, based on the grain composition shown in Table 1, this represents the removal of 95,880 tonnes of N; 11,730 tonnes of P; 18,355 tonnes of K; 2,040 tonnes of Ca; 4,590 tonnes of Mg and lesser amounts of trace elements.

Table 1: Composition of wheat grain (CSBP trial results)

The average wheat yield in WA for 1988 was 1.36 t/ha (2). This represents the removal of 25 kg/ha N, 3.1 kg/ha P and 4.9 kg/ha K. In practice, the amount of fertiliser required to avoid depletion of soil reserves would be considerably more (3). WA farmers applied on average, 15.7 kg/ha N and 9.8 kg/ha P.

Nitrogen is renewable from the atmosphere, however fixation by legumes can be variable and not easily quantified. The application of phosphorus exceeds removal, with the excess having some benefit to following legume pastures. Calcium and sulphur are provided by superphosphate, but not by some higher analysis fertilisers such as diammonium phosphate (4).

Potassium is the major constituent of grain which is derived almost entirely from soil reserves and the possibility of future deficiencies is likely.

We need to be aware of the less visible aspects of soil degradation, such as the decline in soil fertility. Through soil and plant analysis the status of nutrients car he monitored and deficiencies corrected. Nutrients of high soil supply should be used (at a recognised cost) and the status monitored and maintenance rates adopted when the status falls to marginal levels.

1. Lipset, J. and Dann, P.R. (1983). J. Aust. Inst. Agric. Sci. 49, 81-9

2. Australian Bureau of Statistics (1989).

3. Helvar, K.R. and Godden, D. (1977). J. Aust. Inst. Agric. Sci. 43, 22-30

4. W.A. Dept of Agriculture (1986). List of Registered Fertilisers 505

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