I have approximately made the dataset according to the table (NO NEED TO GO TO DETAILS, MAY NOT BE ACCURATE)
data<-data.frame("fertilization"=c("phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","phosphorus","no phosphorus","no phosphorus","no phosphorus","no phosphorus","no phosphorus",
"no phosphorus","no phosphorus","no phosphorus"),
"root.exclusion"=c("no p","no p","p","p","p","p","p","p","no p","no p","p","p","no p","no p","p","p","no p","no p","p","p"),
"species"=c("CALE","ACMI","CALE","FERO","CALE","ACMI","CALE","FERO","CALE","FERO","CALE","ACMI","CALE","FERO","CALE","FERO","CALE","ACMI","CALE","ACMI"),
"P"=c(4980,5348,7078,4589,6344,4097,5578,2967,3524,4396,5337,4343,3075,3621,4086,2772,2175,4240,4825, 2615))
THE FUNCTION I AM USING IS
selected_P<-data$P[data$fertilization=="phosphorus"&data$species=="FERO"&data$root.exclusion=="p"]
selected_P
O/P
[1] 4589 2967
Is this the solution.
Apologies if I didn't understand the problem correctly.
As I am a newbie myself. 