ifc
international fertilizer correspondent
No 8



News from other sources

Editorial

Feed the soil to feed the people, the IPI jubilee
- Session 1: Policy issues related to food supply and the environment
- Session 2a: Economic and social issues
- Session 2b: Economic constraints
- Session 3a: Plant nutrients for sustainable agriculture
- Session 3b: Imbalance in nutrient supply
- Session 4: Potash in agriculture
- Acquiring and putting knowledge into practice - the role of IPI

News
- The law of the minimum
- Soil fertility in Czech Republic in decline

IPI Publications

Publications from other sources

Other editions of IFC

The law of the minimum

This well-known maxim is instantly associated with the famous agricultural chemist Justus von Liebig (1803- 1873). He formulated in 1855 that "By the deficiency or absence of one necessary constituent, all others being present, the soil is rendered barren for all crops to the life of which that one constituent is indispensable. With equal supplies of the atmospheric conditions for the growth of plants, the yields are directly proportional to the mineral nutrients supplied in the manure. In a soil rich in mineral nutrients, the yield of a field cannot be increased by adding more of the same substances"
(Ploeg et al., 1999).

Optimum Nutrient Supply - The Law of Minimum

So far so good, but wait a minute.

There was another German agronomist and chemist, Carl Sprengel, 1787-1859 (the article "The early years" in ifc No. 7 referred to him).

Carl Sprengel was a student of Albrecht Daniel Thaer. Thaer was one of the most renowned agronomists in Germany and an avid supporter of the humus theory for plant nutrition. After working for some time as an agronomic adviser on large estates, Sprengel registered as a student of natural science at the University of Göttingen, where he became a lecturer in 1826. He also conducted pioneering research in agricultural chemistry, dealing mainly with the analysis of plants and the soils on which they grew.

His first publications (1826) dealt with the humus theory. Based on his analytical work, Sprengel concluded that the soluble salts in the humus extract were the real plant nutrients, and with that conclusion he disproved the humus theory.

In a publication about the use of salt (NaCl) as a fertilizer, Sprengel stated (1828) that "Because, as mentioned before, rainwater often contains salt, it follows that this is always present in the soil; however this does not mean that the soil always contains it in sufficiently large amounts to allow luxurious plant growth; this is especially true for cultivated plants, and, for this reason, fertilization with salt has recently gained much recognition; and if the same beneficial results were not always seen, this was either because the soil already had enough salt in it, or one of the other substances necessary for crop growth was missing; because it is indisputable that, when a plant needs 12 substances to develop, it will not grow if any one of these is missing, and it will always grow poorly, when one of these is not available in a sufficiently large amount as required by the nature of the plant".

More details on the work of Carl Sprengel is in the paper "On the origin of the theory of mineral nutrition of plants and the law of the minimum" by R.R. van der Ploeg, W. Böhm, and M.B. Kirkham, Soil Sci. Soc. Am. J. 63:1055-1062 (1999) from which the above article is extracted.

In recognition of the work of Carl Sprengel, van der Ploeg and his associates feel it would be more appropriate in future to call the law of the minimum, the Sprengel-Liebig Law of the Minimum.

Soil fertility in Czech Republic in decline

After the economic reform of 1989, agriculture in the Czech Republic underwent substantial changes. J. Vostal, from the Agricultural Faculty in Prague, recently reported on the considerable decline in the number of livestock, reduction in the use of inputs and even that the labour force has decreased. Consequently, agricultural production has also decreased. This, together with insufficient income after liberalization of the agrarian market, caused a strong reduction in investments in agriculture.

To stabilize the situation it is necessary to revive production by restoring soil fertility with the corresponding inputs in order to ensure at least a minimum level of profitability for farmers.

The following graphs depict the situation with respect to fertilizer use and soil fertility. Taking the K balance as an example, it shows that the use of potash fertilizers declined drastically from some 50 kg/ha K before the reform to currently 5 kg/ha K.

K balance of arable soils in Czech Republic
Graph showing K balance of arable soils in Czech Republic
(Vostal, 2001)

Also the K input with manure halved from 50 to 25 kg/ha. At the same time the K removal by crops was reduced from 95 kg/ha in the prereform period to currently 60 kg/ha, reflecting the decline in agricultural production. In consequence, the resulting K balance changed from a surplus of +9 kg/ha K to a deficit of -27 kg/ha.

Evolution of the nutrient balance of arable soils in Czech Republic
Graph showing Evolution of the nutrient balance of arable soils in Czech Republic
(Vostal, 2001)

The decline in the use of other nutrients is as serious as for K although the N balance still shows a surplus of 5 kg/ha N. Use of N fertilizers went down from 95 to less than 60 kg/ha N, the use of P from almost 30 to less than 5 kg/ha P, the use of Mg from 21 to 1.5 kg/ha Mg and the use of lime from 232 kg/ha to 25 kg/ha Ca. The corresponding nutrient balances are negative with the exception of N as mentioned earlier.

With the change in the application rate the nutrient ratio in fertilizer use changed as well and became more unbalanced. The pre-reform ratio of 95-29-53 kg/ha N-P-K (1:0.31:0.56) deteriorated to 56-5-6 kg/ha N-P-K (1:0.09:0.11) during the period 1996-2000.

Share of N in fertilizer use in relation to application rate in Czech Republic
Graph showing Share of N in fertilizer use in relation to application rate in Czech Republic
(Vostal, 2001)

It is also of interest to see that the obvious imbalance in nutrient supply becomes larger the lower the fertilizer rate. The share of N in fertilizer use increases from 64% at an application rate of more than 100 kg/ha NPK to 84% when the farmer uses less than 50 kg/ha NPK. This confirms what is also observed in developing countries that in times of economic constraints, fertilizer use becomes more unbalanced in favour of N because of its immediate response. However, the long-term consequences of soil nutrient mining as seen in the Czech Republic should give decision-makers serious cause for alarm.

References:

Vostal, J. (2001): Intensity of fertilization in relation to selected production characteristics and gross plant production (IPI internal report).
Vostal, J. (2001): Economic balance of nutrient mining in Czech agriculture, 1996-2000.