 |
This is on a site in North
Lincolnshire recently. Watering with a can appeared to wet
the soil well, but monitoring with the Diviner
in the field quickly showed that a very large amount of water
was needed - subsequent use of the spade soon showed that
what the Diviner was saying was true - the water had not gone
below the top 10cm, even though over 30mm of water was applied.
The likely deficit from cultivation, de-clodding and planting
was in the region of 50-70mm!!
There are a number of questions
the grower needed to answer:
How dry is my soil?
How much water do I need
to apply to wet it fully for scab control?
How do I get it wet enough
for good 'scab' control?
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|
|
With
most water monitoring systems it is important to know when
a soil is 'full'. We see many fields that have poor soil structure
due to the de-clodding and planting activities. This, we believe
helps cause 'scab' in soils that in some years have no problem,
as well as poor growth early in the season. The 'normal' practice
is for growers to go over their crop with 15mms of irrigation
to overcome these difficulties. Since we have been involved
with the EnviroSCAN we have
been able to 'see' how much water it takes to refill a soil,
but many growers who do not have the luxury of such equipment
do not know how much water is needed. (In 1996, after a dry
winter many soils needed 70mms to refill the soil to 50cms.
Not many growers were able to achieve this and hence had disappointing
crops. If you 'guesstimated a 30mm deficit, you would have
been 40mms out for the season!).
Recently
we conducted an experiment in filling a heavy soil after planting
potatoes. Access tubes were installed in a field and using
our new Portable 'EnviroSCAN' probe, called the Diviner,
we were able to take readings, then water the tube with a
watering can and take more readings. The agronomist who did
the work was 'amazed' at how long it took and how much water
was needed. You can see below the figures that show our conclusions.
Our conclusions are that it took 1.5 hours to refill the soil
and it needed 42mm of water! No wonder the 'average' grower
does not refill his soil and hence has poor crops. (Probably
much more water was needed to achieve this.)
You
do need good equipment that is agronomically correct to be
able to see this. The EnviroSCAN has always done this, but
the new Portable Diviner, used sensibly, does it admirably.
|
Time
|
10cm |
20cm |
30cm |
40cm |
50cm |
60cm |
0
-30cm
|
| 12.48 |
4.52 |
16.24 |
28.16 |
36.75 |
27.67 |
28.89 |
48.92
|
| 13.43 |
15.99 |
39.73 |
32.67 |
37.71 |
34.17 |
35.63 |
88.39
|
| 13.58 |
23.61 |
40.81 |
31.41 |
36.09 |
30.70 |
35.88 |
95.83
|
| 14.28 |
29.35 |
38.97 |
31.87 |
36.95 |
31.61 |
35.59 |
100.19
|
another example in
light soil
Soil
Moisture readings in a light sandy soil, growing a
crop of Turnips
This
is an experiment you can do at any time on your farm
when you need to know the real deficit - try it -
you may not like what it tells you, but it will explain
why your irrigation is not as effective as you would
wish.
|
| Tube
installed when the soil was very dry - the customer was
going to irrigate 12mm - not quite enough! |
|
|
10cms
|
20cms
|
30cms
|
40cms
|
50cms
|
Total
0-40cms
|
|
18-Jul
11.14
|
4.28
|
4.22
|
5.26
|
5.57
|
8.67
|
19.33mm
|
|
18-Jul
11.19
|
10.54
|
4.27
|
5.27
|
5.55
|
8.58
|
25.63mm
|
|
18-Jul
11.21
|
17.58
|
4.28
|
5.31
|
5.57
|
8.52
|
32.74mm
|
|
18-Jul
11.23
|
23.20
|
4.41
|
5.4
|
5.65
|
8.77
|
38.66mm
|
|
18-Jul
11.25
|
36.07
|
4.57
|
5.42
|
5.6
|
8.26
|
51.66mm
|
|
18-Jul
11.51
|
37.83
|
13.93
|
5.37
|
5.56
|
8.55
|
62.69mm
|
|
19-Jul
12.44
|
32.15
|
25.35
|
26.77
|
24.80
|
11.39
|
109.07mm
|
| The
soil has now wetted down to 50cm (but it is not full yet)
so we conclude that the top 40cm is now full |
| Note
that the 10cm layer wets up fully and only when over full
does water reach the 20cm layer |
| Note
the repeatability of the Diviner at the lower depths -
it is very accurate! |
|
