Odp: An error in fitting a non linear regression

Hi

I have a data set with two variables "q" and "depth" as follows:

q<-c(tapply(weight[Soil=="Jy"], Depth[Soil=="Jy"], mean)). This commns
returns 7 "q" values:
0.687900000 0.845550000 0.405416667 0.152766667 0.033100000 0.031400000
0.005183333

The "depth" values are produced using this command whish "depth" shows 

soil depth: 
depth <--c(5,15,25,35,45,55,65). This return these values for "depth":
[1]  -5 -15 -25 -35 -45 -55 -65

I have uploaded the plot file in below:



http://www.nabble.com/file/p22118160/plot.doc plot.doc 

Now I want to fit the following decaying exponential model to our data
presented in the plot file such that depth is X and q is Y.
nreg<-nls(q~a*exp(-b*depth), start=list(a=1, b=0.5)). This command 

the following error:
Error in nls(q ~ a * exp(-b * depth), start = list(a = 1, b = 0.5)) : 
  singular gradient

However, if I use positive values of depth i.e. 
[1]  5 15 25 35 45 55 65

then I recieve the following error:
Error in numericDeriv(form[[3]], names(ind), env) : 
  Missing value or an infinity produced when evaluating the model

Notice that though in the plot depth is drawn on Y axis but in the
regression model "depth" in independent. Plot shows the real world of 

happne to the crop root growth.

Thank you.
Saeed Ahmadi

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