diff --git a/R/apus.R b/R/apus.R
index 1dc4562..3a23644 100644
--- a/R/apus.R
+++ b/R/apus.R
@@ -67,7 +67,7 @@ Apus <- R6::R6Class(
     #' @description
     #' Add a field to the apus object
     #'
-    #' @param b_id_field (character) ID or (unique) name of the field
+    #' @param b_id_field (integer) ID of the field
     #' @param b_area (number) The area of the field (ha)
     #' @param b_lu (character) The cultivation code for this field
     #' @param d_n_req (number) The required amount of Nitrogen for this field (kg N / ha / year)
@@ -83,8 +83,8 @@ Apus <- R6::R6Class(
     addField = function(b_id_field, b_area, b_lu, d_n_req = NA, d_p_req = NA, d_k_req = NA, d_n_norm = NA, d_n_norm_man = NA, d_p_norm = NA, b_lu_yield = NA, b_lu_price = NA) {
 
       # Check arguments ---------------------------------------------------------
-      checkmate::assert_character(b_id_field,len = 1)
-      checkmate::assert_numeric(b_area,lower = 0, upper = 100)
+      checkmate::assert_integerish(b_id_field,len = 1)
+      checkmate::assert_numeric(b_area,lower = 0, upper = 50000)
       checkmate::assert_subset(b_lu,choices = apus::cultivations$b_lu)
       checkmate::assert_numeric(d_n_req,lower = 0, upper = 400)
       checkmate::assert_numeric(d_p_req,lower = 0, upper = 150)
diff --git a/README.md b/README.md
index 7c8951c..fc39c73 100644
--- a/README.md
+++ b/README.md
@@ -43,7 +43,7 @@ apus$addField(
 
 # Add a second field
 apus$addField(
-  b_id_field = 1L,
+  b_id_field = 2L,
   b_area = 5000,
   b_lu = 'nl_262',
   d_n_req = 270,
diff --git a/man/Apus.Rd b/man/Apus.Rd
index bf3e9c7..b49b802 100644
--- a/man/Apus.Rd
+++ b/man/Apus.Rd
@@ -81,7 +81,7 @@ Add a field to the apus object
 \subsection{Arguments}{
 \if{html}{\out{<div class="arguments">}}
 \describe{
-\item{\code{b_id_field}}{(character) ID or (unique) name of the field}
+\item{\code{b_id_field}}{(integer) ID of the field}
 
 \item{\code{b_area}}{(number) The area of the field (ha)}
 
diff --git a/tests/testthat/test-003-apus.R b/tests/testthat/test-003-apus.R
index d95e7be..cb59da2 100644
--- a/tests/testthat/test-003-apus.R
+++ b/tests/testthat/test-003-apus.R
@@ -96,12 +96,12 @@ test_that("Second field is added", {
 field3 <- data.frame(
   b_id_field = 3L,
   b_area = 5000,
-  b_lu = 'nl_366',
+  b_lu = 'nl_372',
   d_n_req = 370,
   d_p_req = 130,
   d_k_req = 50,
   d_n_norm = 330,
-  d_n_norm_man = 300,
+  d_n_norm_man = 200,
   d_p_norm = 80
 )