The Nutrient Cycle

Where the cycle breaks in ordinary land use

The usual way of farming has turned this loop into a one-way street at almost every station.

The harvest leaves. Every load of grain, fruit or hay carries nitrogen, phosphorus and potassium off the field. In a forest these would return as litter. On the farm they travel to cities and rarely come back. Crop residues are often burned or removed, and with them the organic matter that would have fed the soil.

Excreta go to the sewer. Most of what the harvest carried away passes through people and ends up in wastewater. In the Swedish reference values of a guideline by the Stockholm Environment Institute, urine alone holds about 88 percent of the nitrogen and two thirds of the phosphorus that a person excretes. In the sewer these nutrients become a pollution problem instead of a fertiliser.

Rain washes them out. Bare soil in winter is the classic leak. Nitrate dissolves in water and is carried below the roots into groundwater and streams. A global meta-analysis by Nouri and colleagues found that cover crops reduced nitrate leaching by 69 percent compared with bare fallow.

Erosion carries them off. The topsoil that runs off a bare slope is the richest layer, full of humus and nutrients. Italy has the highest mean rate of soil loss by water among EU member states, according to the European Commission's harmonised model.

The gap is filled from outside. Synthetic nitrogen from the Haber-Bosch process has made modern harvests possible; Erisman and colleagues estimated in 2008 that nearly half of the world's population was fed thanks to it. But the efficiency is low: by an analysis of fifty years of data from 124 countries, more than half of the nitrogen added to cropland never reaches the harvest, and most of the rest is lost to the environment. Phosphorus comes from rock deposits that are finite and concentrated in a few countries. The updated planetary boundaries of 2023 count the flows of nitrogen and phosphorus among the limits humanity has already overstepped.

Techniques that close the nutrient cycle

All of them follow one rule: give back what you take, and keep the soil alive and covered so that it can hold what it gets. The graphic above shows where each technique works.

Air and rock. Nitrogen-fixing plants bring nitrogen from the air into the system, through the rhizobia in their root nodules. When they are cut or shed leaves, the nitrogen reaches their neighbours. A dynamic accumulator draws minerals up from the subsoil with deep roots and lays them on the surface with its leaves.

Roots and fungi. Mycorrhiza extends the reach of roots and is especially important for phosphorus, which moves very little in soil. Polyculture and the plant guild combine plants with different root depths and needs, so that more of the soil is used and less is lost.

Growing plants. Biomass plants are grown to be pruned again and again; their cuttings are fertiliser grown on the spot. Green manure does the same in a season. Syntropic agriculture, the food forest and agroforestry build whole systems in which pruning and leaf fall feed the soil continuously.

Harvest and litter. Mulch and living mulch return organic matter to the surface, where soil life can work it in. Sheet mulching builds soil from layers of organic material, and hügelkultur turns rotting wood into a slow source of nutrients.

Animals and people. In silvopasture animals graze among trees and leave their dung where the nutrients came from. The zaï pit concentrates manure or compost exactly where a plant will grow. Composting kitchen waste and using urine as fertiliser close the loop at the house.

Soil life. The soil food web is the engine of the cycle: without it nothing is returned. A cover crop feeds it all year and catches nitrate in winter before rain can carry it away.

Humus store. Humus holds nutrients and releases them slowly. Biochar adds a stable, porous carbon that can hold nutrients and give soil life a home. Terra preta, the dark soil of the Amazon, shows what can grow when settlements return their waste, ash and charcoal to the land over centuries. Good soil structure keeps all of it in place.

How it looks at Sa Medada

Sa Medada is a slope in Jerzu, in Ogliastra on the east side of Sardinia, with an old olive grove and macchia on the steeper slopes. A laboratory analysis of the soil does not exist yet. We therefore cannot say which nutrients are scarce or plentiful there, and we make no claims about it. That would be the first step before any targeted fertilising, read together with simple field tests and indicator plants.

What we can say is how the cycle is meant to be started. Project Eden begins with pioneer plants and ground cover, so that the soil is never bare and what falls stays on the land. Several nitrogen fixers are already in the ground, all without inoculation with root-nodule bacteria: Tephrosia, coral tree, Inga edulis, Pacay, Bambara groundnut, fava bean and Leucaena. Whether each species finds its bacteria without help is one of the open questions. Leucaena is regarded as invasive in many tropical countries. For Terra Humana, invasive species are nature's own solution strategy and hold the greatest potential for rapid renaturation; species on the EU list of invasive alien species of Union concern are never used.

The nitrogen fixers have work to do. In the open ground grow, among others, bananas, mango, avocado, guava, cherimoya, papaya, passion fruit, lychee, moringa, persimmon and citrus. Many of them are hungry plants. Whether their demand can be met from within the land, through nitrogen fixers, ground cover and the return of everything that grows there, is the question the nutrient cycle at Sa Medada has to answer.