The long-term mineral fertilizer use is sustainable

Based on the scientific work of Justus von Liebig, the introduction of mineral fertilizers in the middle of the 19th century was a milestone in the history of agriculture. Mineral fertilizers as a new and additional source of plant nutrients enabled farmers to increase their crop yields significantly. For many people the astonishing effect that mineral fertilizer application had on crop growth raised concern whether this effect of a higher crop production would be sustainable in the long-term. The only solution to address this concern was to start long-term field trials in order to monitor the effect of mineral fertilizers.

Long term field trials
The methodology of a field trial is to divide a large field into different smaller plots, which receive different fertilizer applications (e.g. no fertilization, a specific rate of mineral fertilizers or a manure application) (Fig. 1). The impact of those different treatments on crop yield and soil quality can then be studied at a location with the same soil and the same climatic conditions.

Figure 1.

The world's oldest long-term agricultural field experiments started between 1843 and 1856 by Lawes and Gilbert at Rothamsted in England. Similar experiments started all over Europe, for example in France at Grignon in 1875, in Germany at Bad Lauchstädt in 1902, in Denmark at Askov 1894, in Russia at Moscow 1912, in Norway at Møystad 1922, and in Poland at Skierniewice 1923. In the US there are four classical long-term experiments. These are Morrow Plots at the Univerity of Illinois (1875), Sanborn Field at University of Missouri (1888), Magruder Plots at Oklahoma State University (1892) and Old Rotation at Auburn University (1896). Many of these experiments still continue today and provide valuable data on the impact of different fertilizing strategies on soil properties and crop yield.

Before the introduction of mineral fertilizers, organic matter had been the only source of nutrients and it was the target of any grower to increase soil organic matter in order to have more nutrients for the crop. Typical application rates were about 10-30 tonnes of organic matter per ha. Much smaller rates in the form of mineral fertilizers (about 1 t/ha) were recommended to be sufficient for maintaining or even increasing soil quality and crop yields. One can imagine that there were some doubts upon the sustainability of the application of such small amounts of fertilizers specifically on the soil organic matter content. Soil organic matter content is an important factor of soil quality because it maintains a good soil structure, it reduces the risk of soil erosion and improves the water retention capacity of soils.

Impact on soil organic matter
Changes in soil organic matter can only be studied in long-term field trials because properties such as organic matter content change usually only slowly in response to different management practices like mineral fertilizer applications. Soil properties vary with soil type, climatic conditions and management system, therefore the long-term field trials provide data that are primarily characteristic for the given site. Even so, the overall quality and quantity of the data allow for generally applicable answers to the two main questions that triggered the trials: firstly, what is the long-term impact of mineral fertilizer application on soil organic matter, and secondly, what is the long-term impact on crop yield. Most of the long-term experiments have shown that mineral fertilizers do not reduce soil organic matter content but keep it stable or increase it slightly (Fig. 2). No input of any fertilizers reduces organic matter in the long-term.

Fig. 2

• Long-term trial started in 1902, Bad Lauchstädt, Germany
• Organic matter shown as % organic carbon in the top soil layer
• Mineral fertilizer NPK (160 kg mineral N/ha)
  
  

The long-term trials also confirm that without any fertilizer application, crop yields do not increase but stay at relatively low levels. This is shown in Fig. 3 for the long-term trial in Rothamsted as an example. With increasing inputs of mineral fertilizers crop yield increases up to a level where other factors like water become limiting and hence restrict further growth. Application of only manure results in lower yields compared to the mineral fertilizer application, because the amount of nutrients applied with the manure is lower than with mineral fertilizers. In principle a further increase in the rate of manure applied in the trials would also increase crop yields further. However, as manure is not available in indefinite amounts, this is of no practical relevance.

Fig. 3

• Long-term trial started in 1843, Rothamsted – Broadbalk, UK
• Winter wheat, average grain yield 1996 – 2000, mineral fertilizer = 100%
• Manure: 35 t/ha (175 kg total N/ha)
• Mineral fertilizer - NPK (190 kg mineral N/ha)
  

Reviewing all long-term experiments it can be concluded, that with mineral fertilizers the soils get more productive and thereby produce more yields and hence more crop residues that in turn maintain or increase the soil organic matter content. Furthermore, all long-term experiments confirm Liebig’s theory that plants need minerals to grow. In addition they confirm that mineral fertilizer applications are neccesary to replenish nutrients removed from the soil by the harvested crop, in order to sustain sufficiently high yield levels in the long-term.

During the last century farmers more and more appreciated the significance of the results of the long-term experiments and, over the years, there was a steep increase in the use of fertilizers. Today plant nutrition has become a business that delivers about 440 Mio. tonnes of fertilizers to the farmers each year.