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Group Title: TREC-H research report - Tropical Research and Education Center-Homestead ; SB-85-3
Title: Feasibility of measuring field soil moisture content of marl soil using a surface moisture neutron probe
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Permanent Link: http://ufdc.ufl.edu/UF00067850/00001
 Material Information
Title: Feasibility of measuring field soil moisture content of marl soil using a surface moisture neutron probe
Series Title: Homestead TREC research report
Physical Description: 12 leaves : ; 28 cm.
Language: English
Creator: Orth, Paul G
TREC (Agency)
Publisher: University of Florida, Agricultural Research and Education Center
Place of Publication: Homestead Fla
Publication Date: 1985
 Subjects
Subject: Soil moisture -- Measurement -- Florida   ( lcsh )
Neutron counters   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
non-fiction   ( marcgt )
 Notes
Statement of Responsibility: Paul G. Orth.
General Note: "December 30, 1985."
 Record Information
Bibliographic ID: UF00067850
Volume ID: VID00001
Source Institution: University of Florida
Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: oclc - 72816842

Table of Contents
    Historic note
        Historic note
    Main
        Page 1
        Page 2
        Page 3
        Page 4
        Page 5
        Page 6
        Page 7
        Page 8
        Page 9
        Page 10
        Page 11
        Page 12
Full Text





HISTORIC NOTE


The publications in this collection do
not reflect current scientific knowledge
or recommendations. These texts
represent the historic publishing
record of the Institute for Food and
Agricultural Sciences and should be
used only to trace the historic work of
the Institute and its staff. Current IFAS
research may be found on the
Electronic Data Information Source
(EDIS)

site maintained by the Florida
Cooperative Extension Service.






Copyright 2005, Board of Trustees, University
of Florida










Homestead TREC ReseaFchRe ort SB85-3 December 30, 1985

S\ \ Paul G. Orth
Sr '-'versity of Florida, IFAS
?' 5 )p opical research and Education Center
\ ~ ,C0 Lo tHomestead, FL 33031


Fe of Measuring Field Soil Moisture Content of
\.H'XF4Soil Using a Surface Moisture Neutron Probe


ABSTRACT

The purpose of this research was to evaluate the use of a soil moisture probe as
a tool for the measurement of soil moisture. This in turn could be used to
calibrate aerial photographs showing soil moisture differences. The goal was to
save time and cost of calibrating the aerial photographs. The soil moisture
probe works on the principle of neutron thermalization. To be time efficient a
surface probe was used rather than one inserted into the soil for which special
preparations usually must be made. Fields used in this trial were selected by
the South Dade Soil and Water Conservation District.

Data were collected from four fields with two being sampled twice about a week
apart to give a total of 6 sets of data. Each set of data consisted of 2-3
sites with 3-4 replications at a site. There were a total of 15 sites and 46
replications (samples). A total of 3 sites out of the 6 sets of data did not
fit data from the remaining sites. In addition 4 sites fit poorly, but their
departure was not great enough to exclude the data from the initial calibration.
One of the latter fit well if percent soil moisture (%SM) from both depths were
averaged and the corresponding neutron probe ratio (NPR) was used.

A significant linear correlation (adjusted R2 of 85%) was obtained using the
data collected at 12 sites. A second approximation of the curve, 9 sites gave
better correlation (adjusted R2 of 97%). The regression equation was:

NPR = 0.573 + 0.00592 X % SOIL MOISTURE.

This means that a completely dry soil should read 0.573, and the reading will
increase by slightly less than 0.006 for every 1% increase in soil moisture.
Factors contributing to inaccuracies between probe readings and actual soil
moisture are unsmooth fields, recent rain or irrigation, and too fluffy a
surface condition.

There is considerable potential for the use of the neutron soil moisture probe
for quickly obtaining moisture data to be correlated with aerial photography in
efforts to determine inherent field variability in soil moisture management
needs. Also, its use in soil water management could probably be developed
through the determination of calibration curves applicable to cultivated and
irrigated fields. The results of this experiment have provided information to
facilitate the designing of experiments to refine the procedures for obtaining
and processing field and laboratory data relating soil moisture to neutron probe
readings.










INTRODUCTION

Aerial photographs with false-color infra-red film register differences in soil
moisture. This is because water reflects less infra-red radiation than the
soil. Aerial photography may be a useful tool to determine variability in water
retention and/or potential water supply within a field. Such knowledge would
facilitate field cultural practices. It would be necessary to calibrate the
photos in terms of soil moisture percentage to apply such a tool in the field.

Such calibration would be more efficient (cost effective) if field soil moisture
could be measured much more quickly than the standard method of sampling the
soil and gravimetrically measuring its water content. One possible method is to
use a neutron probe designed to measure soil moisture. A neutron probe emits
fast neutrons, some of which are slowed when encountering hydrogen atoms and
detected by a special sensor. A surface neutron probe can be placed on the
surface of the soil and will emit neutrons into the soil beneath. The neutrons
penetrate into a volume of soil hemispheric in shape with the distance
penetrated dependent upon the composition and water content of the material.
The greater the density of hydrogen atoms, the less the distance penetrated.
The primary difficulty encountered in using this technology is that the greater
the distance of the neutron source and sensor from the hydrogen atoms, the less
sensitivity in detection. Two implications of this fact are: (1) if two soils
have the same moisture content, but the surface of one is rough and the other is
smooth, the latter will give a larger reading on the neutron probe and (2) if
the change in moisture content with depth is not the same in two soils, then the
soil containing relatively more moisture closer to the probe than the other will
produce the greater reading. Thus a field must be in some type of "normal"
state for calibration and field evaluation for a neutron probe to be a viable
tool. For example, if a small amount of rain had recently fallen on the field,
the photograph likely would indicate a greater moisture content than the neutron
probe would indicate because the latter might be measuring into drier zones
below the wet surface. On the other hand, if a thin layer of dry soil, possibly
from recent cultivation, lies over moist soil, then the photograph will indicate
drier conditions than will the neutron probe.

The purpose of this research was to evaluate the use of a soil moisture probe as
described above as a tool for the measurement of soil moisture which in turn
could be used to calibrate aerial photographs showing soil moisture differences.

MATERIALS AND METHODS

Fields used in this trial were selected by the South Dade Soil and Water
Conservation District. All selected fields previously had been photographed
from the air on one or more occasions. Indications were that soil moisture
differences within and/or between fields showed up in the photographs. The
primary differences of interest were: (1) a steady-state soil condition in
which the surface, although drier than soil at depth, indicated the soil
moisture status of the field and (2) differences within a field where greater
moisture at the soil surface was associated with a deeper profile and
correspondingly better supply of soil moisture for crops.










Most fields were planted with corn, potatoes, or beans, but enough soil was
exposed to predict a successful attempt at aerial photography. Readings were
taken and the soil sampled from 6 fields on four days between December 4 and 13,
1984. Sampling location was selected for flatness of the area and
representativeness of the site. Where beds were planted and stand good, the
sampling site was between rows, but not a wheel track area. In some fields
there were plant skips on the bed and thus the bed was sampled, especially when
the furrows lacked adequate flatness. In one field the soil was especially soft
as it had been cultivated recently and settling had not occurred.

Neutron probe readings were taken both after slight physical compaction with a
board and after moderate compaction, about 70 gm/cm2, of pressure with the same
board. In one field, the south facing slope of the bed was sampled since it was
the smoothest surface available. One site had very dry soil thus providing an
opportunity to test a situation where a layer of dry soil might mask, from
photography, the true soil moisture situation in a field. This soil required
compaction also which was done by applying foot pressure to a board lying on the
slope. This produced a very fine surface for taking readings with the neutron
probe resting against the indentation near the bottom of the slope. In this
latter case soil samples were 0-1, 1-6 and 6-10 cm. All other samplings were
0-2 and 2-10 cm. Soil sampling was done with routine care. That is, no
extraordinary efforts were made to monitor the depth sampled or prevent small
amounts of soil from one layer falling from the sides of the sampling hole and
being collected with soil from the next deeper level. Thus the calibration
curves obtained would be representative of an average sampling crew. Samples
were placed in plastic bags and protected from direct heating by the sun to
reduce the chance of moisture loss before weighing. Samples were reweighed when
dry, bag weight was subtracted to get net soil weight, and percent moisture was
calculated. Some samples contained rocks which were included in calculating
moisture percent. The rocks were later separated and weighed, in case any of
that information might be useful in interpreting the results.

The neutron probe used was the surface model made by Troxler The rechargeable
batteries in the scaler were weak, requiring frequent recharge and thus limiting
the amount of sampling possible in one day. Three half minute counts were
generally used for field readings and calibration. A calibration was done
before and after a series of readings at each site in a field. The calibration
readings were taken with the neutron probe placed on a polyethylene block
designed for that purpose. The average calibration reading was used in
calculating ratios from the field readings by the standard method of dividing
the field reading by the calibration reading. This compensates for any changes
in instrument operation from day to day. Generally 3 nearby sites were used at
one location in the field to obtain replication. Soil samples consisted of
several cores to give 0.5-1.5 kg per sample. Soil samples were taken with a
coring device or with a trowel if the soil was too dry or rocky for core
sampling.

Neutron probe readings and soil samples were taken by different teams on
some occasions, and this may have introduced some variability into the results.
One aim of this experiment was to determine a good method both for sampling and


Commercial products are mentioned for information only and not as a
recommendation.










taking readings to serve as a guide in a more refined investigation. The fine
textured particles in all fields were marl. No sampling was done in Rockdale
soils. Data were statistically analyzed using the Minitab software on the IFAS
VAX computer in Gainesville. Simple regression analyses were run as well as
data plotting. Statistics were run on many sub-groups of data as well as on the
averages described above. Three sample averages did not fit the general curve
and were not included in the final analyses. These are discussed separately.
Aerial photographs were taken of all fields on one of the sampling days, but
their evaluation is not included in this report. The sampling sites were not
clearly identifiable in the photographs. Also, soil moisture patterns were not
clear. Thus, in light of the limitations of both the field data collected and
the aerial photography, further study of the photographs did not seem
appropriate. This report can serve as a guide to good technique in soil
sampling and neutron probe data collection for future research on marl and other
soils in Dade County.

RESULTS AND DISCUSSION

The fields that were sampled are listed in table 1; some comments about the
fields are included. Fields designated 1 and 5 are the same field sampled on
different days and in a different manner as listed in table 1. Similarly,
fields 2 and 6 are the same field. However, there was one more site sampled for
field 6 than for field 2. First, the generalized results will be discussed, and
then specific references to detailed data will be made.


Table 1. Information on fields sampled.
Field date site description* location/depth comments
No. sampled (cm)

1 12/4 Williams M P NC/C furrows freshly
field 0-2, 2-10 cultivated
2 12/4 Alger RM B/F NC/C between rows soil firm
field 0-2, 2-10 in beans
3 12/11 Alger M P bed tops
field 0-1, 1-6, 6-10
4 12/11 Alger M C bed tops
field 0-2, 2-10
5 12/12 Williams M P bed side
field 0-1, 1-6
6 12/13 Alger RM B/F between rows
field 0-2, 2-10

* M = marl, RM = rocky marl, NC/C = both non-campacted and compacted readings,
P = potatoes, C = corn, B/F = beans in part of area, fallow in part


Averaged data are summarized in Table 2 with the center column containing the
data giving the better correlation. The selection of data used was based on
detailed study of the data from each field. These data were used in the
regression analysis and are plotted in Figure 1. The regression equation is:

RATIO = 0.600 + 0.00480 % SOIL MOISTURE.
Adjusted R2 = 85.2% and s = 0.02454.









Table 2. Summary of neutron probe and soil moisture data from
12 of the 15 areas sampled.

Soil Moisture (%)
Field Neutron Probe Sampling Depth (cm)
No. Ratio 0-2 2-10 2-10

1 0.769 29.0 32.0 32.0
1 0.810 39.0 39.0 39.0
2 0.644 5.5 13.0 13.0
2 0.638 7.4 11.5 11.5

0-1 0-1 1-6

3 0.787 44.1 44.1 53.0
3 0.764 34.6 34.6 42.0
3 0.780 44.9 44.9 46.0
3 0.738 5.2 5.2 30.0

0-2 2-10 2-10

5 0.797 37.0 41.0 41.0
5 0.823 38.0 40.0 40.0
6 0.668 13.0 16.0 16.0
6 0.737 19.0 20.0 20.0


The regression equation indicates a ratio of .600 would correspond to completely
dry soil. Also, the ratio changes .0048 for every 1% change in soil moisture.
The correlation is significant with an R2 of 85%. However the graph gives a
striking example of how much variation might be encountered. Each sample point
on the graph is indicated by an asterisk. The number on the same line is the
field number corresponding to that data point. A location in field 5 and one in
field 6 gave nearly identical neutron probe ratios (NPR) but differed about 10%
in moisture. (See discussion of table 8 for comments and discussion of this
observation.) Also two locations in field 3 may not be a good fit on this curve
for the reasons discussed for table 5. Primarily this points to the need for
additional refinement in technique. Thus the discussion that follows refers to
the data in more detail in order to focus on the needed refinements. It also
helps verify the usefulness of the data collected and to assist in the planning
of any future experiments. Statistics were run on many sub-groups of data in
addition to those needed to obtain the averages described above. These
statistics are the source of the comments to follow.

Table 3 is the data from the two fields sampled on December 4, 1984. These data
show a low to non-significant correlation between SM and NPR within a field.
The range in soil moisture was too narrow and the variability between readings
was too wide to give good correlation. Also, a greater number of sites should
be used if variability within a field is to be examined. However, the aim of
this experiment was to use average data from each site within a field, compile
data from a number of fields differing significantly in moisture, and obtain a
first approximation calibration curve. Of some interest is the result that in
field 1 the best correlation, R2 = .659, was between the NPR on compacted soil










Figure 1. Graphic representation of 12 of the 15 sampled areas.


5
1 *


* 33


0.86+

0.84+

0.817

0.79+

0.76+

0.74+

0.71+

0.69+

0.66+

0.64+


6 *


5 *


* 6


2
2 *
---------------------------------- ---------


9.00


18.00 27.00 36.00
SOIL MOISTURE (PERCENT)


45.00


54.00


and the 0-2 cm SM. R2 between NPR on compacted and non-compacted soil was .226
and between 0-2 and 2-10 SM was .364. Combining data from these two fields gave
the largest R2, .952, between the NPR on compacted soil and the 0-2 SM. This is
summarized in table 4. The differences between correlation coefficients are not
significant, but the comparison giving the highest value was used in making the
general summation previously shown in figure 1.

The detailed data obtained from sampling field 3 are given in table 5. Note
that the moisture content was more uniform between depths, and that relatively
the surface was not as dry as in fields 1, 2, 5, and 6. Only at site 2 where,
on the average, the surface soil was somewhat drier than at the two lower depths
do the SM and NPR data seem to fit the data from fields 1 and 2. Sites 1 and 3
seem to have too low NPR values for the SM measured. The reason is not obvious.
The probe readings were taken on top of the beds while those for fields 1 and 2
were taken between the rows. Perhaps the air space on either side of the bed
contributed to the low readings.

Similar data for field 4 are given in table 6. In field 4 the 0-2 cm samples
averaged the same or more moisture than 2-10 cm, and this may be the reason they
do not fit the data in figure 1. A possible reason for this moisture
distribution may be irrigation. These readings were taken on top of beds as in
field 3. These are two of the three averages not included in the general
summary.


1 *












Table 3. Data from two fields including values for each replicate.


Site Sample


Neutron Probe
Ratio
NC *
NC C


.7127
.7676
.7820
.7329
.7896
.7651
.7542
.6126
.6274
.6197


.7699
.7755
.7794
.7524
.8171
.8010
.8118
.6507
.6426
.6393
.6516
.6331
.6299


Percent Moisture
0-2cm 2-10cm


34.23
34.98
27.44
22.07
39.66
39.35
38.55
6.27
4.52
5.58
7.74
8.35
6.07


34.08
**
31.55
30.04
42.49
42.52
31.04
11.84
13.04
14.29
12.54
10.68
11.33


* NC = not compacted; C = compacted
** This sampling site was very shallow, and
be obtained.


a good 2-10 sample could not


Table 4. Statistical comparison between NPR and SM on compacted and
non-compacted soil.

Soil status Soil Sample r2
not compacted 0-2 cm .862
not compacted 2-10 .874
compacted 0-2 .952
compacted 2-10 .886


Table 5. Detailed data from field 3.

Field Site Sample Neutron Probe Percent Moisture
No. Ratio
0-1 1-6 6-10

3 1 1 .776 44.2 58.4 49.1
3 1 2 .792 44.3 48.6 45.2
3 1 3 .793 43.7 51.1 49.4
3 2 1 .764 44.0 36.6 45.4
3 2 2 .769 35.0 44.7 45.8
3 2 3 .759 24.8 44.3 45.9
3 3 1 .781 43.6 47.2 48.6
3 3 2 .775 44.6 49.7 49.5
3 3 3 .784 46.6 41.0 49.6


Field
No.










Table 6. Detailed data from field 4.

Field Site Sample Neutron Probe Percent Moisture
No. Ratio
0-2 cm 2-10 cm
4 1 1 .729 67.0 63.0
4 1 2 .728 63.5 51.9
4 1 3 .770 53.5 54.8
4 2 1 .731 53.1 61.7
4 2 2 .758 59.9 51.1
4 2 3 .771 50.4 50.5


Data from field 5 are given in table 7. (This is the same field as field 1.)
Site 1 was extremely dry on the surface, and drier than the other two sites at
the 2 to 10 cm depth. These data fit figure 1 well. However, data from sites 2
and 3 are additional examples of variability in NPR encountered within a field.
The data from field 5 shows how a very dry 0-2 cm depth requires use of the 2-10
cm soil moisture data for calibration purposes.



Table 7. Detailed data from field 5.

Field Site Sample Neutron Probe Percent Moisture
No. Ratio
0-2 cm 2-10 cm
5 1 1 .729 3.2 28.7
5 1 2 .728 5.0 27.6
5 1 3 .759 7.3 33.1
5 2 1 .798 36.3 40.6
5 2 2 .795 36.6 40.8
5 2 3 .797 38.0 41.9
5 3 1 .813 37.9 40.8
5 3 2 .825 38.2 39.6
5 3 3 .831 38.7 40.2


Table 8 gives the data from field 6. Field 2 and 6 were the same general area,
and the designation field in this case was not a single field but represents an
area. In site 2 under field 6 it can be seen the soil was much wetter than at
the other 2 sites. Site 2 was the third location whose data did not fit the
curve in figure 1. In this case the cause seems to be roughness of the soil
surface. This was a fallow area that was very rough and thus there was a
greater than usual air gap between the neutron source and the soil moisture.
This reduced the sensitivity of the instrument to the water present.

Additional data in tables 7 and 8 serve to illustrate further the relationship
between soil moisture content and distance from the probe. Although the NPR for'
site 1 in field 5 and site 3 in field 6 are very similar, soil moisture is quite
different. (These data are the source of two data points previously referred to
in figure 1.) There was a very dry zone at the soil surface at site 1 in field
5 with 28-33% moisture in the 2-10 cm depth. At site 3 in field 6, both depths









Table 8. Detailed data from field 6.

Field Site Sample Neutron Probe Percent Moisture
No. Ratio
0-2 cm 2-10 cm
6 1 1 .683 13.8 16.2
6 1 2 .671 13.8 16.2
6 1 3 .650 10.0 15.3
6 2 1 .816 52.4 55.0
6 2 2 .781 51.0 54.9
6 2 3 .773 56.2 59.8
6 3 1 .726 18.8 19.8
6 3 2 .743 18.4 19.9
6 3 3 .741 20.9 18.8


had moisture in the 19-20% range. A calculation of average soil moisture for
0-10 cm gives 24.9% in field 5 and 19.5% in field 6. The NPR and SM data do not
fit the same calibration curve. However, the average SM for 0-10 cm in field 5
and the corresponding NPR does fit the curve well as indicated by the slightly
better correlation associated with the following regression equation for the
eleven data points:

RATIO = 0.592 + 0.00501 % SOIL MOISTURE.

Adjusted R2 = 88.7 % and s = 0.02246

Elimination of two more data points from field 3 which do not fit well gives
figure 2. The regression equation for this revised curve is:

RATIO = 0.573 + 0.00592 % SOIL MOISTURE.

Adjusted R2 = 96.8 % and s = 0.01282

These data show that to develop and use a calibration curve, or curves, certain
standards regarding the field situation must be developed. In this case there
must be a predictable change in SM with depth. Not only is this important in
generating calibration curves, but it also has a use when the technique is
applied in the field. For example, one of the potential uses of the soil
moisture probe is to monitor soil moisture for scheduling of irrigation. To
accomplish this, it is necessary to estimate the total available water in the
soil profile using the neutron probe reading. This could be done if the change
in SM with depth is predictable and the depth of the soil is known.

The data indicated, in general, a correlation between soil moisture at different
depths in an individual field. This means that when there is good correlation
between neutron probe ratios and soil moisture at one depth, the correlation
with another depth would be reasonably good also. However, this does not mean
data from different fields can be combined as has already been shown. On the
contrary, in order to obtain the best R2 from all the data here described it was
necessary to select the depth of soil with corresponding soil moisture to be
used for each field. Thus, although there was a relationship between SM at
different depths in a field, this relationship was not the same in all fields
studied. Further research is needed to determine the number of typical soil





-10-


Figure 2. Adjusted graphic representation of 9 of the 15 sampled areas.

0.86+


0.84+
5


0.81+


0.797


0.76+


0.74+-


0.71+


0.69+


0.66+


0.64+


8.00


1 *


* 5


1 *


* 3


* 5


2 *


6


* 2


16.00


24.00


32.00


40.UU


48.00


SOIL MOISTURE (PERCENT)


moisture profiles (variation of SM with depth) that should be included in a
series of curves relating NPR to SM. The prime consideration is to be able to
make the soil moisture measurement at the soil surface; the soil profile factors
should be characterized previously. Two factors important in characterizing the
profile are depth of soil and organic and inorganic make-up of the soil.

The time required for a set of readings, 3 one minute readings at each of 3
locations was about 15 minutes. This was no more than twice the length of time
to collect the necessary soil samples, 3 at each of two depths. Time to process
the samples would bring the total time to more than the time required when using
the probe. Also, the results from the probe readings are available sooner.

SUMMARY AND CONCLUSIONS

Data were collected from four fields with two being sampled twice about a week
apart to give a total of 6 sets of data. Each set of data consisted of 2-3
sites and 3-4 replications at a site. There were a total of 15 sites and 46
replications (samples). A total of 3 sites out of the 6 sets of data did not
fit data from the remaining sites. In addition 4 sites fit poorly, but their
departure was not great enough to exclude the data from the initial calibration.
One of these fit well if SM from both depths were averaged and the corresponding
NPR was used.


S- -- -- ------+- -- ------- -





-11-


Using the data collected at 12 sites it was possible to obtain a calibration
curve relating the NPR, the ratio between the field reading and the calibration
reading of the surface neutron probe, to the soil moisture percentage. The
correlation was significant, adjusted R2 of 85%. A second approximation of the
curve including 9 sites gave better correlation, adjusted R2 of 97%. The
precision with which data from a single field could be used to establish a
calibration curve, or the usefulness of a calibration curve for monitoring a
single field, was not explored in depth. However, variation within a field was
one of a number of aspects examined in order to indicate refinements in
technique to be adopted.

The suggested standard technique includes starting with a soil surface as smooth
as possible and as compact as practical. This places the neutron source and
sensor in a reproducible relationship with the soil moisture. Second, the rate
of change in soil moisture with depth must be a characteristic which can be
specified as a characteristic of the calibration curve. This involves knowing
if a recent rain or irrigation has enhanced the quantity of water at the
surface, or if cultivation has reduced water at the surface by reducing
capillary flow from below. This aspect is one topic that will require more
research if neutron probe data are to be used in applied situations. Third,
characteristics of the profile also may affect the calibration curve.

Not enough data were taken in individual fields to determine whether the
differences in soil moisture previously detected in photographs are great enough
to allow calibration of the photos using the neutron probe. Related to this is
whether the neutron probe would be a useful tool in mapping field variability
which then can be related to crop management requirements. Success in finding
an affirmative answer to these questions seems likely with continued research.
All these questions could be investigated at the same time research is done on
calibration curves and how well sub-surface features related to soil moisture
supply correlate with the combined aerial photography neutron probe
information.

This experiment suggests the following outline for an experiment with major
emphasis of the aerial photography aspect. First, the field must be tilled and
left with a smooth surface. Second, the field must lie idle for a period of
time, but not enough time to allow the development of sufficient vegetation to
mask the soil surface from the camera. At the present time an estimate of 1 to
2 weeks of idle time is suggested. Third, if there is significant rainfall,
probably more than 1/2 inch, an additional waiting period must be allowed or a
different calibration curve used. (Probably it is beneficial for some rain to
occur between cultivation and photography to firm the soil.) Fourth, a
photograph must be available to assist in the selection of areas in the field to
be sampled. Fifth, the sampling must take place in areas that are marked so
they can be detected in another photograph taken at approximately the same time
as the sampling.

Some of the limitations for development of calibration curves and using the
neutron probe, coupled with aerial photographs, as a tool for making soil
moisture maps have been discussed above. Results of the research suggest that a
neutron probe of a slightly different design might prove more efficient in
making and interpreting soil moisture readings. Such an instrument would have a
probe that could be lowered a short distance, 1-4 cm, into the soil, perhaps
into a small access tube which could be inserted just prior to taking the




, a


-12-



reading. This should avoid the problems of unusually dry surface soil and
surface roughness.

Thus there is considerable potential for the use of the neutron soil moisture
probe in obtaining moisture data quickly to be correlated with aerial
photography in efforts to determine inherent field variability in soil moisture
management needs. Also its use in soil water management could probably be
developed through the determination of calibration curves applicable to
cultivated and irrigated fields. The results of this experiment have provided
information to facilitate the designing of experiments needed to refine the
procedures for obtaining and processing field and laboratory data.

REFERENCE

Gardner, W. H. Water Content, chapter 7 of Methods of Soil Analysis, American
Society of Agronomy, Inc, pub. 1965 pp 104-114.




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