16.3.3  Deficiencies and responses

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Figure 16.5 An idealised relationship between dry matter yield (% of maximum) and nutrient concentration in plant parts. (Based on Smith and Loneragan 1997)

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Table 16.3

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Figure 16.6 Wheat response to nutrient additions on a sand from Lancelin, Western Australia. Plants responded poorly, of not at all, to nutrient additions if any of the key elements including P, N, Zn or Cu were omitted. 'All' = full nutrients (Based on J.F. Loneragan, unpublished data; but see Loneragan 1996 for more information on diagnosis of nutrient disorders)

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Table 16.4

The early German soil chemist Liebig postulated that plants growing on a nutrient-deficient soil responded in direct proportion to the addition of the deficient nutrient. A later German worker, Mitscherlich, showed that responses more frequently followed the economists’ Law of Diminishing Returns with successive additions of a deficient nutrient giving progressively smaller increments of growth approaching a plateau of maximum yield. His ‘Mitscherlich Equation’ for representing response curves provides a good fit to the ascending asymptote of many experimental data, but does not describe a supraoptimal depression in plant growth which follows the yield plateau with excess nutrient supply (Figure 16.5).

Early German workers (Sprengel and later Liebig) recognised that plants require an adequate supply of all essential nutrients for maximal growth, so that a deficiency of any one nutrient would restrict growth regardless of all others (Figure 16.6). Liebig also proposed that growth was determined by the nutrient in most limiting supply and that when that nutrient was supplied, further growth was controlled by the next most limiting nutrient. This ‘Law of the Minimum’ often holds when a deficiency of one nutrient is appreciably more severe than others as in wheat grown on a soil more severely deficient in N than in Zn and Cu (Table 16.3). Notably, wheat plants did not respond to either Zn or Cu fertilisers in the absence of N fertiliser, but responded strongly to both when N was present.

In other situations, plants deficient in two nutrients respond to additions of either nutrient when added alone, but when added together each modifies the other’s response according to the Law of Limiting Factors as stated by Blackman in 1905: ‘the effect of a factor is least when another factor limits growth and greatest when all other factors are in optimal supply’. Plants growing in soils severely deficient in both N and P frequently respond in this way to addition of N and P fertilisers (Table 16.4). A positive interaction between N and P on shoot dry mass is obvious in this case, and discussed further in Section 16.3.5 with N, P and K interactions on growth of oats.

Soils with multiple nutrient deficiencies are common in Australia, so that it is important to recognise this principle of key limiting factors by supplying all other limiting nutrients when testing for a specific nutrient deficiency (Figure 16.6; Section 16.3.5).

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