Liebig and the discovery of mineral fertilizers

Today we almost take for granted the facts that sunlight delivers the energy needed for plant growth and that plants derive their nutrition - minerals and water - from the soil. Yet it took mankind almost 10,000 years to make this basic, but very important discovery.

Man’s very existence depends primarily on an adequate supply of food. Our early ancestors sustained themselves by gathering food and by hunting, moving when necessary to find fresh sources. Then, about 10,000 years ago, more organized agricultural systems – based on soil cultivation – were developed in order to grow food crops. This change gradually took place as the very first settlements, the Sumerian and Mayan civilisations of Asia and Central America, evolved. These societies expanded as they learned how to overcome the problems of inherently infertile soils by transferring soil humus from forest to field. The resulting increase in soil fertility enabled the production of more food.

Humus theory

From Roman times onwards, techniques of cereal-fallow rotation were developed. The mineral nutrients contained in cereal residues were released during the fallow period to provide soil fertility and create the basis for acceptable cereal yield in the following season. Subsequent fallows were replaced by leguminous crops in the rotation cycle. The introduction of legumes also improved the nutrient supply for the next cereal crop, especially because of their ability to fix additional nitrogen from the air. In addition, legumes provided more feed for animals. More intensive husbandry increased the amount of farmyard manure returned to the soil, which also improved crop yields per unit of area.

As a consequence, farmers observed for centuries that crop yields were higher following the application of organic matter to the soil. Until the early 1800s researchers therefore believed that organic matter, and not minerals, were the essential plant nutrients (Fig. 1). This idea became well known as the ‘humus theory’. In a way similar to human and animal nutrition, organic substance was regarded as the primary energy source for plants. However, in the early part of the 19th century, the idea that plants might need only minerals from the soil gained wider acceptance. The first researcher to doubt the validity of the ‘humus theory’ was Philipp Carl Sprengel. In 1838 he stated: “ The conviction should have been reached long ago that humus is not such an important substance as we have been led to believe, and that the current doctrine of humus is exceedingly full of contradictions.”

Liebig’s “Law of the Minimum”

But Sprengel’s ideas remained of largely theoretical importance and had only limited influence on agricultural practice. However, the way had now been firmly paved for a systematic study of the problem of plant nutrition. Shortly afterwards, in 1840, the renowned researcher Justus von Liebig rejected the humus theory and constructed a scientific case demonstrating that crops require mineral elements from the soil (Fig. 2), hydrogen and oxygen from water, and carbon from CO₂ in the air.

At first, Liebig thought that plants also got most of their nitrogen from the air. He concluded that the supply of mineral elements in the soil is limited because the soil does not contain an indefinite amount of minerals. In his famous “Law of the Minimum” published in 1843, Liebig stated that crop yields are proportional to the amount of the most limited nutrient, whichever nutrient that may be. He then concluded that crop yields increase if the limited nutrient is applied. However, yield is increased only to the point at which another nutrient is needed when the “Law of the Minimum” applies in turn to that nutrient. Liebig’s model is often illustrated by a barrel with varying stave lengths (Fig. 3), where the capacity of the barrel to hold water is limited by the length of the shortest stave.

As a result of Liebig’s theory and the experimental work pursued at Rothamsted in the UK, the first mineral fertilizers were produced in the mid-1800s. The introduction of mineral fertilizers was a milestone in the history of agriculture, as they enabled agriculture to supply sufficient food for the rapidly growing world population (Fig. 4).