Walkers and Syers addressed this question in a seminal paper published in 1976, which the so-called Walkers and Syers model is based on (Figure 1). They used soil chronosequences, which are series of soils developed over a period of time under similar conditions of vegetation, topography, and climate, in New Zealand in their investigations.
They determined the dynamics of nutrient concentrations in the soils of different ages and, in the case of phosphorus (P), distributions among different fractions or pools. They showed that young soils have low concentrations of nitrogen (N) because very little N exists in most parent materials. Yet, soil N content increases rapidly during the early stages of ecosystem development as biological nitrogen fixation kicks in.
In contrast, P is derived almost exclusively from the parent material. P concentrations are generally highest in young soils but decrease gradually as soil ages. This is the result of greater P losses through runoff and leaching than replenishing by bedrock weathering. When P losses are balanced by P inputs from atmospheric deposition, it reaches a “terminal steady state”. Importantly, P availability also decreases as the primary P forms (e.g. apatite) are transformed into occluded P (e.g. P associated with iron oxides) and organic P (Figure 1).
Other nutrients such as Ca, Mg and K behave similarly to P. The model, that Walker and Syers (1976) proposed, implies that plant production is generally limited by N in young soils and by P in old soils; N and P supply should more or less equilibrate in intermediate-aged soils. During the process of soil formation, the so-called pedogenesis, soil fertility first increases and then decreases. Obviously, the rates of changes of P and other nutrients in the soil also depend on factors such as climate, relief, parent material, plants as well as other living organisms in the soil.
In the years after its publication, results from soil chronosequence studies in other parts of the world generally supported the Walker and Syers model. As such, this model provides a robust framework for understanding the biogeochemistry of soil nutrients during pedogenesis.
Read more discussion on the topic by Zhao F.J. 2023. The Walker & Syers model and the age of soil. Soil Ecology Letters. 5(3): 220170.
