The Boserupian theory holds that population density growth can be accompanied by sustainable agricultural intensification (Boserup, 1965). However, it is not certain whether the positive link between population density and environmental/economic benefits associated with agricultural intensification are indefinite. The current study utilizes cross sectional data from a random sample of farm households drawn from two densely populated Counties in Kenya to assess whether Boserupian agricultural intensification is sustainable at high population densities. The study utilizes a robust approach that incorporates soil quality parameters into economic analysis to assess the effect of population density on soil quality and crop productivity. It employs non- parametric regression, OLS regression and asymmetric trans-log production function estimation methods. Results indicate that at low a population density, endogenous sustainable agricultural intensification occurs, which is associated with improvements in soil quality and crop yields. However, as population densities exceed 600 persons/Km2, soil quality attributes such as soil texture, soil pH levels and fertility indicators such as soil organic matter (SOM) and electrical conductivity (EC) start to deteriorate. The end result of deteriorating soil quality is binding of critical nutrients and thus reduction in the crop yield response to fertilizer application. This reduces crop productivity and consequently returns to agriculture. These findings have imperative policy bearing on livelihoods and smallholder agriculture considering that a large proportion of sub-Saharan Africa’s population is dependent on rain-fed agriculture and population densities continue grow.