diff -ru a/R/soc.R b/R/soc.R
--- a/R/soc.R	2026-09-29 10:31:04
+++ b/R/soc.R	2026-09-29 10:31:04
@@ -33,6 +33,12 @@
 #' @export
 
 soil_organic_carbon <- function(para, stock_change_para, land_required, biomass) {
+
+  # Previous land use / management key; defaults to the current one (no change).
+  soc_prev_key <- function(para, key) {
+    k <- para[[paste0(key, "_previous")]]
+    if (is.null(k) || length(k) == 0 || is.na(k) || !nzchar(k)) para[[key]] else k
+  }
   
   co2_conversion_factor <- 44 / 12
   
@@ -122,12 +128,27 @@
         para[["cropland_orgmatter"]],
         "cropland_orgmatter"
       ),
+      prev_factor_land_use = lookup_soc_factor(
+        crop_landuse_tbl,
+        soc_prev_key(para, "cropland_system"),
+        "cropland_system"
+      ),
+      prev_factor_management = lookup_soc_factor(
+        crop_tillage_tbl,
+        soc_prev_key(para, "cropland_tillage"),
+        "cropland_tillage"
+      ),
+      prev_factor_input = lookup_soc_factor(
+        crop_input_tbl,
+        soc_prev_key(para, "cropland_orgmatter"),
+        "cropland_orgmatter"
+      ),
+      # IPCC 2006 Eq. 2.25: dC = (SOC0 - SOC(0-T)) / D. The measured stock is SOC0,
+      # so SOC(0-T) = SOC0 * F(0-T) / F0. No previous state given means no change.
       annual_change_carbon_stocks_mineral_soils =
-        ((carbon_stock_last_year_inventory_period *
-            stock_change_factor_land_use *
-            stock_change_factor_management *
-            stock_change_factor_input) -
-           carbon_stock_last_year_inventory_period) /
+        (carbon_stock_last_year_inventory_period *
+           (1 - (prev_factor_land_use * prev_factor_management * prev_factor_input) /
+              (stock_change_factor_land_use * stock_change_factor_management * stock_change_factor_input))) /
         time_dependence_stock_change *
         area_last_year_inventory_period
     )
@@ -156,12 +177,27 @@
         para[["grassland_implevel"]],
         "grassland_implevel"
       ),
+      prev_factor_land_use = lookup_soc_factor(
+        grass_landuse_tbl,
+        "All",
+        "grassland landuse"
+      ),
+      prev_factor_management = lookup_soc_factor(
+        grass_mgmt_tbl,
+        soc_prev_key(para, "grassland_management"),
+        "grassland_management"
+      ),
+      prev_factor_input = lookup_soc_factor(
+        grass_input_tbl,
+        soc_prev_key(para, "grassland_implevel"),
+        "grassland_implevel"
+      ),
+      # IPCC 2006 Eq. 2.25: dC = (SOC0 - SOC(0-T)) / D. The measured stock is SOC0,
+      # so SOC(0-T) = SOC0 * F(0-T) / F0. No previous state given means no change.
       annual_change_carbon_stocks_mineral_soils =
-        ((carbon_stock_last_year_inventory_period *
-            stock_change_factor_land_use *
-            stock_change_factor_management *
-            stock_change_factor_input) -
-           carbon_stock_last_year_inventory_period) /
+        (carbon_stock_last_year_inventory_period *
+           (1 - (prev_factor_land_use * prev_factor_management * prev_factor_input) /
+              (stock_change_factor_land_use * stock_change_factor_management * stock_change_factor_input))) /
         time_dependence_stock_change *
         area_last_year_inventory_period
     )
@@ -190,12 +226,27 @@
         para[["grassland_implevel"]],
         "grassland_implevel"
       ),
+      prev_factor_land_use = lookup_soc_factor(
+        grass_landuse_tbl,
+        "All",
+        "grassland landuse"
+      ),
+      prev_factor_management = lookup_soc_factor(
+        grass_mgmt_tbl,
+        soc_prev_key(para, "grassland_management"),
+        "grassland_management"
+      ),
+      prev_factor_input = lookup_soc_factor(
+        grass_input_tbl,
+        soc_prev_key(para, "grassland_implevel"),
+        "grassland_implevel"
+      ),
+      # IPCC 2006 Eq. 2.25: dC = (SOC0 - SOC(0-T)) / D. The measured stock is SOC0,
+      # so SOC(0-T) = SOC0 * F(0-T) / F0. No previous state given means no change.
       annual_change_carbon_stocks_mineral_soils =
-        ((carbon_stock_last_year_inventory_period *
-            stock_change_factor_land_use *
-            stock_change_factor_management *
-            stock_change_factor_input) -
-           carbon_stock_last_year_inventory_period) /
+        (carbon_stock_last_year_inventory_period *
+           (1 - (prev_factor_land_use * prev_factor_management * prev_factor_input) /
+              (stock_change_factor_land_use * stock_change_factor_management * stock_change_factor_input))) /
         time_dependence_stock_change *
         area_last_year_inventory_period
     )
@@ -248,4 +299,4 @@
   )
   
   return(results)
-}
\ No newline at end of file
+}