The establishment of the foundations of agricultural chemistry and plant physiology during the Renaissance

Modern technologies of mineral nutrition and the calculation of fertilizer application rates are based on the fundamental laws of chemistry and plant physiology. These principles did not emerge spontaneously—their foundation was laid during the Renaissance, when science replaced church dogmas with experiment and practice. Understanding how a plant interacts with the soil, water, and air allowed for a shift from blindly copying ancient techniques to the managed formation of a harvest.

From experience to the first laws of plant physiology

In the XIV–XV centuries, the development of manufactories and commodity production in Europe forced agriculture to seek ways to increase productivity. The invention of the printing press in the mid-XV century provided an impetus for the development of agricultural science. Between 1466 and 1515, works by ancient writers, philosophers, and agronomists were printed in Europe. This returned forgotten knowledge about soils to practice and launched the study of water plant nutrition.

During this period, botany emerged as an independent biological discipline for the first time. An outstanding Italian scientist and painter (1452–1519) made experimentation the primary tool of cognition. He described leaf arrangement, heliotropism, and geotropism in detail, and also identified root pressure and the movement of plant sap. It was he who first spoke about the circulation of substances and began to study the influence of air, water, and the mineral part of the soil on plant life.

Following him, another Italian naturalist (1519–1603) attempted to provide an accurate morphological and physiological characterization of plants. He suggested that nutrients are drawn by rootlets from the soil much like iron is attracted to a magnet. In his opinion, saps move up the stem under the influence of "vital warmth" and heating by the sun, while the root itself absorbs moisture like a lantern wick dipped in kerosene.

• The start of printing scientific works — 1466
• Discovery of root pressure — XV–XVI centuries
• Theory of mineral nutrition of plants — 1563

Mineral nutrition theory and the law of return of elements

Scrupulously accumulated knowledge of plant physiology became closely intertwined with practical agrochemistry. For a long time, farmers applied fertilizers intuitively, without understanding the reasons for their effectiveness. In most agriculture books of the XVI century, no step forward was taken on this issue compared to ancient Roman authors.

The situation changed with a French naturalist (1510–1589) who, in 1563, published "A Scientific Treatise on Various Soils (Salts) and Agriculture." He was the first to formulate a view of the soil as a source of mineral substances necessary for crop nutrition. He proved that plants extract salts from the earth, gradually depleting it, which causes yields to fall.

To preserve soil fertility, the scientist recommended regularly returning nutrients to the soil. One of the techniques was burning straw and returning the resulting ash, which contains the salts absorbed from the soil. It was also recommended to add soluble salts and marl to manure and other organic fertilizers. Without such compensation or without a rest period (fallowing), the soil loses the mineral substances brought by rains and dews.

The understanding of exactly how plants are nourished was formed from contradictory hypotheses. In 1629, a classic experiment was established that long solidified the erroneous water theory of nutrition. A willow cutting was planted in a tub with carefully weighed soil and was regularly watered with rainwater. The results of this five-year experiment showed a colossal increase in the plant's mass with minimal change in the weight of the earth.

• Mass of soil at the beginning of the experiment — 91 kg
• Mass of the willow cutting — 2.25 kg
• Duration of the experiment — 5 years
• Increase in willow mass — 74.4 kg
• Decrease in soil mass — only 56.7 g

Based on these data, researchers concluded that for plant life, water alone is sufficient, which allegedly densifies and turns into organic matter. The error lay in the fact that, at the time, science did not yet know about the aerial nutrition of plants and the assimilation of carbon dioxide. There were also no methods for accounting for ash elements, which the nursery plant absorbed from the soil in extremely small but vital quantities.

The failure of the water hypothesis was proven during more precise tests conducted between 1665 and 1728. Scientists doubted that water could completely replace soil nutrition and compared the development of mint seedlings in various media. The results of weighing the green mass clearly demonstrated the dependence of plant development on the composition of the water and the presence of organic impurities in it.

This experiment proved that the main building material for a plant organism is "earthy matter," not pure water. Most of the moisture entering through the roots is not retained in the tissues but evaporates into the atmosphere through the pores of the leaves. Thus, scientists for the first time separated the physiological role of water as a transport medium and soil as a source of nutrition.

The discovery of "fertility salt" and the first concepts of the nitrogen cycle

In parallel with the study of water and soil balance, a search was conducted for specific active ingredients responsible for yield. In 1650, the hypothesis about the existence of a special "fertility salt" was first proposed. In a 1656 treatise, researchers directly identified saltpeter as the basis for the growth of all plants. The "soul" of saltpeter (referred to as nitrum in historical sources) was effectively understood as the nitrate anion, and the description of its properties anticipated the discovery of the nitrogen cycle in nature.

In those years, it was assumed that the active principle of saltpeter rises from the depths of the earth into the air and then returns with dew and precipitation. Indeed, modern analyses confirm the presence of traces of nitric acid in atmospheric moisture. And although the authors of the hypothesis operated with terms like "soul of saltpeter," they correctly identified the mechanism of the nitrogen cycle.