cstapels on Extraer datos de thingspeak sin utilizar libreria

The soil monitor example shows how to send data without the library. 

You can see the correct syntax for the GET to retrieve data on Get a Channel Feed. 

It looks like you missed the text of the request in the code above.

" GET to 

Here is a snippet that might help:

if (client.connect(thingSpeakAddress, 80))
{

    client.println("GET /channels/"+tsChannel+tsFolder+tsField+"/last.txt");
    client.println();
}
else
{
  failedCounter++;

    lastConnectionTime = millis();
}
delay(500);
String response;
char charIn;

do {
charIn = client.read(); // read a char from the buffer
response += charIn; // append that char to the string response
}
while (client.available() > 0);
client.stop();