Disentangling the interactions of soil nutrient cycles for sustainable agriculture

Balanced soil nutrient budgets are important for sustainable agriculture. Soil nutrient pools are interconnected within and between elements and balanced by natural processes. In intensive agriculture the excess of nutrients disrupts the balance, leading to increased leaching, volatilisation or other losses to the environment. Stabilisation of ions and molecules stores nutrients in the soil, which can make them unavailable, while mobilisation moves nutrients into available state, potentially causing leaching. This spectrum of availability is driven by different factors for each nutrient is connected to to other cycles in the soil system.

We propose a concept of "soil safe operating space" for agriculture - a state of soil  where mobilisation and stabilisation are balanced, making just enough nutrients available for plant uptake to grow optimally, so leaching is reduced. Finding that state and developing practices to reach it would improve the sustainability of agriculture greatly. To aid in this, first the interactions of nutrient dynamics and its drivers need to be clear. Therefore we synthesised connections between cycles of macronutrients C, N, P, K, Ca, Mg, S and micronutrients Cu and Zn based on a literature review of 175 articles in the field of soil chemistry. These connections also established the driving factors of the mechanisms and corresponding feedbacks. We analysed the results in the context of the classical management practice of target pH 6.5 as per the old nutrient availability diagrams. They showed that not all nutrients benefit from it, such as sulphate being leached, ammonium oxidated and phosphate adsorbed. This suggests that, depending on the agricultural targets, pH may be adjusted to achieve certain goals. The cross-cycle interactions also played a great role, with the concentrations and forms of one nutrient heavily affecting others via precipitation, fixation or adsorption. Phosphorus has the most mechanisms of interactions and factors reducing its availability, soil organic matter affects storage of most other nutrients and cycles of other macronutrients have nuanced reactions to soil factors like mineralogy, surface charge and solution concentrations. Facilitating some processes over others via controlling pH and moisture, organic matter addition or other amelioration practices would have more defined effects than setting a target pH while ignoring other factors.

This study provides a unique framework that helps modelling of soil chemistry because of its broad scope, connecting the various individual studies into an integrated system of cycles. Examples of such developments are presented in the model FarmSAFE of the project: 101060455 — NutriBudget.