Moisture Sensor Reading

Hi all,

As I think a lot of you have tried, I am building a project that consists of measuring the soil moisture, if it drops below a certain value, trigger an alarm of sorts.

So I bought a Capacitive Moisture Sensor (It looks like this one)

I made a simple sketch (on a nodeMCU) that reads the value of the sensor and sends it to a webpage. Then goes to deepsleep and wakes up in about an hour. And send the reading again.
I did a baseline test (Dipped the sensor in water -> Raw Reading was around 400 and Hold it in the air -> Raw Reading was about 850).

Next up I wanted to see what it does when I put it in actual soil. (In a pot of a House plant.)
The reading I get are strange:

2018-11-29 23:11:28	plant	764
2018-11-30 00:08:43	plant	762
2018-11-30 01:06:18	plant	761
2018-11-30 02:03:20	plant	760
2018-11-30 03:00:42	plant	759
2018-11-30 03:58:02	plant	758
2018-11-30 04:54:53	plant	758
2018-11-30 05:52:12	plant	757
2018-11-30 06:49:42	plant	756
2018-11-30 07:46:41	plant	754
2018-11-30 08:44:07	plant	754
2018-11-30 09:41:56	plant	754
2018-11-30 10:39:14	plant	752
2018-11-30 11:36:58	plant	749
2018-11-30 12:34:12	plant	749
2018-11-30 13:31:43	plant	749
2018-11-30 14:29:29	plant	747
2018-11-30 15:26:47	plant	746
2018-11-30 16:24:30	plant	745
2018-11-30 17:22:14	plant	745
2018-11-30 18:19:39	plant	744
2018-11-30 19:17:00	plant	740
2018-11-30 20:14:09	plant	739
2018-11-30 21:11:31	plant	738
2018-11-30 22:09:17	plant	739
2018-11-30 23:07:12	plant	738
2018-12-01 00:03:54	plant	738
2018-12-01 01:01:26	plant	737
2018-12-01 01:58:15	plant	735
2018-12-01 02:55:32	plant	736
2018-12-01 03:53:01	plant	735
2018-12-01 04:50:17	plant	735
2018-12-01 05:47:39	plant	733
2018-12-01 06:44:45	plant	732
2018-12-01 07:42:09	plant	732
2018-12-01 08:39:21	plant	729
2018-12-01 09:36:25	plant	728
2018-12-01 10:33:51	plant	726
2018-12-01 11:31:24	plant	724
2018-12-01 12:28:31	plant	725
2018-12-01 13:26:01	plant	725
2018-12-01 14:23:21	plant	724
2018-12-01 15:20:19	plant	723
2018-12-01 16:17:51	plant	724
2018-12-01 17:15:05	plant	723
2018-12-01 18:12:13	plant	723
2018-12-01 19:09:59	plant	721
2018-12-01 20:07:32	plant	720
2018-12-01 21:04:31	plant	719
2018-12-01 22:02:02	plant	714
2018-12-01 22:59:36	plant	714
2018-12-01 23:56:47	plant	715
2018-12-02 00:54:14	plant	715
2018-12-02 01:51:30	plant	713
2018-12-02 02:48:36	plant	712
2018-12-02 03:46:08	plant	714
2018-12-02 04:43:36	plant	710
2018-12-02 05:41:18	plant	708
2018-12-02 06:38:22	plant	710
2018-12-02 07:35:35	plant	710
2018-12-02 08:32:52	plant	711
2018-12-02 09:30:38	plant	710
2018-12-02 10:27:52	plant	710

As you can see here the value drops. (According to the baseline) it means that the soil is getting wetter.
Do you guys have any idea what I am doing wrong? (Or what other conditions are that pollutes the readings?)

Thanks in advance

Post a link to the schematics of that sensor or at least to a manual for it. Without this information and the sketch you used to get these values we would have to guess and that’s just a waste of time.