Land degradation caused by deforestation seriously affects the soil environmental capacity (SEC), with potential risks to soil health. However, conventional SEC theory is unable to quantitatively describe the saturation capacity of different soil fractions and environmental pollution levels of a heterogeneous system. Thus, a new concept, the soil pool capacity (SPC), was introduced to fill this gap. We undertook space‐for‐time substitution to investigate the responses of SPCand soil safety capacity (SSC) to changes in the soil physiochemical properties, chemical species, and molecular characteristics of soil lead (Pb) in the conversion of tropical secondary rainforest (TSR) to rubber (Hevea brasiliensis) monoculture plantations with 15‐ and 60‐year histories (RP15/60). Conversion of TSR to RP15/60 caused adverse effects on soil properties (i.e., lower soil organic matter [SOM] and higher bulk density [BD]). The saturated SPC (SPCsat) for Pb in bulk soils under RP15 (2,203.17 g m−2) and RP60 (1,634.09 g m−2) decreased compared with that in TSR (2,227.10 g m−2). The contribution rates of SPCsat in the exchangeable (P1) and carbonate (P2) pools to the total capacity stabilized at approximately 94%. The contribution rate of potential SPCsat in P4 (OM‐bound Pb) significantly decreased with increasing agricultural intensity. The SSC of labile fractions (P1 + P2) accounted for 81–85% (a critical level) of the total SSC when the soil loading capacity for Pb reached the SSC in bulk soil regardless of land‐use type. Fourier transform infrared and X‐ray diffraction spectra revealed the Pb‐loaded species of SPCsat in TSR and RP15/60 (i.e., TSR: Pb2SiO4 in P2, (CH3COO)2Pb in P4, RP15/60: Pb5Si8O21 and Pb2(P4O12) in P2, and (CHOO)2Pb in P4). Correlation and factor analyzes showed that SOM and BD were key factors that had an opposite impact on SPCsat and SSC. These results are helpful to choose appropriate agronomic measurements to reduce land degradation and crop safety risks caused by deforestation.