How fertilization sustained agricultural dominance in northern Europe

O rye (Secale cereale), a drought- and cold-resistant cereal, did not become one of the pillars of European agriculture because it was an undemanding plant. The real reason for its success is now emerging with scientific precision. A study reveals that, since Late Antiquity, farmers in northern Europe cultivated rye using intensive fertilization techniques, challenging established ideas about the origin and expansion of this grain.

The research employed stable isotope analysis on archaeological and modern rye grains, revealing that the practice of fertilizing fields with animal manure was common as early as the 4th century AD.

The discovery breaks with the idea that rye thrived solely on poor soils due to its hardiness. In fact, the isotopic data show that its cultivation was integrated into sophisticated agricultural systems, which included the strategic use of animal waste, peat and even marine debris.

The team analyzed grains from 11 archaeological sites in Germany and the Caucasus, dating from antiquity to the late Middle Ages. The results indicate that up to 15 tons of manure were applied per hectare per year, similar to modern agricultural experiments. This suggests that even in the earliest stages, rye was grown in fields enriched by intensive fertilization.

Comparison with agricultural experiments conducted at the Thyrow and Halle centers in Germany reinforces this conclusion. There, plots cultivated with and without fertilizer produced grains with contrasting levels of nitrogen isotopes.

Fertilization increased the rates by up to 7%, revealing the direct impact of fertilization on the grain. Interestingly, the type of soil — sandy or loam — modulated this effect: more fertile soils amplified nitrogen absorption, even under moderate fertilization regimes.

From the 10th century onwards, the practice of cultivating rye in a system known as Ewiger Roggenbau (perpetual rye cultivation) became established. This technique involved the deposition of a mixture of soil and stable manure over delimited fields. The constant application of organic matter raised the level of the land in relation to the surrounding area and allowed for permanent crops without crop rotation — something rare in pre-modern agriculture.

In addition to nitrogen, the study analyzed sulfur levels, identifying lesser-known agricultural practices.

In Neermoor, for example, low sulfur isotope values ​​indicated the use of bog peat as fertilizer, a practice linked to salinization and anaerobic processes. This type of analysis makes it possible to distinguish the origin of the fertilizer — whether manure, plant compost or marine sediments.

Carbon isotope values, in turn, made it possible to estimate the relative yields of the fields. The best results were obtained in coastal areas, especially on artificial elevations known as Wurten (or terps), where proximity to the sea and organic fertilization ensured ideal conditions. There, cultivation probably took place in the summer, after the winter tides had receded. The productivity of these fields rivaled that of the best continental soils.

Contrary to expectations, no clear trend of increasing fertilization intensity was observed over time.

Ancient sites such as Göritz and Loxstedt already show signs of intense fertilization, comparable to those of medieval castles and villages such as Neermoor or Diepensee.

Fertilization varied more with local context—soil type, proximity to stables, availability of manure—than with historical period.

Domestication of rye

The domestication of rye followed a peculiar path. Originally a weed that invaded wheat and barley fields, rye gained ground with the adoption of the moldboard plow, which turned the soil deeply.

This technological advance reduced the competitiveness of wild forms and favored varieties with non-brittle ears, better adapted to human harvesting and sowing.

Over time, rye's productivity surpassed its image as a rustic cereal.

In the Middle Ages, it became the staple food of castles, churches and urban centers. Archaeology reveals significant stocks at sites such as Groß Lübbenau, Bremen and Starigard.

Abundance not only allowed for feeding growing populations but also for generating surpluses, which were fundamental for the consolidation of political-religious powers.

The data also disprove the theory that the rye found in Wurten was imported from Geest areas (high sandy areas). Isotopic analysis clearly differentiates the grains grown in each region. Those from the coast exhibit distinct signatures, indicating local cultivation, even in frequently flooded and salinized lands.

More information at doi.org/10.1098/rstb.2024.0195