Remember put an reproducible example of data, maybe with dput() for find better help.
You could use dput(data[1:20,]) , for select the first 20 rows and all colums.
Check the data because in somes cases the date_measurement is low that fertil_applied. 
data <- data.frame(
fertil_applied=c("03/09/2014","23/02/2015","20/03/2015","20/04/2015"),
date_measurement=c("17/10/2014","16/11/2014","15/12/2014","15/12/2014 ")
)
data$date_diff <- as.Date(as.character(data$date_measurement), format="%d/%m/%Y")-
as.Date(as.character(data$fertil_applied), format="%d/%m/%Y")
data