DIMSU
SURVEY 1987
Land
Use Planning Note AAH/6/88
CONTENTS
1 INTRODUCTION
1.1 Background
1.2 Previous
Studies
1.3 Objectives
2 SOIL
SURVEY and SOIL SAMPLING
2.1 Soil
Survey Procedure
2.2 Soil
Sampling
2.3 Soil
Analysis
2.3.1 UK Analyses
2.3.2 Local Analyses
2.4 Findings
3 SOIL
FERTILITY
3.1
Soil pH and Exchangeable Cations
3.1.1 Introduction
3.1.2 Weighted Mean Values
3.1.3 Interpretation of Data
3.2 Macro-nutrients
and Fertility Potential
3.2.1 Introduction
3.2.2 Data
3.2.3 Interpretation of Data
3.
Micro-nutrients
3.3.1 General
3.3.2 Data
4 SOIL
ACIDITY and LIMING
4.1 Soil
pH and Aluminium Saturation Percentage (ASP)
4.1.1 Introduction
4.1.2 Crop Tolerances
4.1.3 Soil pH Data
4.2 Soil
pH, Rooting Depths and Relative Millet Yield
4.2.1 Introduction
4.2.2 Soil pH and Millet Roots
4.2.3 Observations and Interpretation of Data
4.2.4 Soil pH, Exchangeable Aluminium, ASP and
Rooting Depth
4.2.5 Interpretation of Data and Discussion on
Rooting
4.3 Lime
Requirements
4.3.1 Introduction
4.3.2 Liming Formula
4.3.3 Lime Requirements
4.3.4 Conclusions and Discussions
5 FINDINGS
and CONCLUSIONS
Appendix
1 Soil
Profile Descriptions and Laboratory Data
2 Nyala
Determined Soil pH
______________________________________________________________________________________________________
1 INTRODUCTION
1.1 Background
There has been a farm on Dimsu Development
Centre (Dimsu DC) for over 10 years where various agronomic trials have been
conducted. The total farm size is 100 hectares but to date less than one third
of this area has been utilised though about half of the area has been cleared
of the virgin tree savannah.
Over time it has proved to be impossible to
obtain consistent yields and expected responses with the various trials and, in
fact, crops grown within the D.C. have appeared to be poorer than those grown
on farmers fields locally. During the Donor's review mission in 1987 it was
suggested that a full soil survey and soil sampling exercise, for detailed
laboratory analysis, be undertaken to try and establish if there were major
problems and variations within the D.C.
1.2 Previous
Studies
During the original surveys carried out by
Hunting Technical Services in the early 1970s there were some soil samples
taken and analyses done and those data were reviewed on the arrival of the Land
Use Planner for Phase 2 of the WSP. Many of the sites were, of course, not
within the Qoz of the Dimsu area but nevertheless gave some indication of the
soil parameters.
During 1976/77 there was a limited programme
of soil sampling and analysis and the data were presented in the Agronomy
Section Annual report for that year, these data have been reviewed and utilised
in this study. During 1986 the Land Use Planner carried out a review of
existing data that could be found in the WSDC library and an analysis of those
data were presented in the LUP Inception Report 1986.
There was a limited soil sampling exercise
carried out during 1985/86 and the data resulting from that study were
presented in An Indicative Land Use Plan for the Western Savannah Project,
September 1986. Unfortunately some critical factors contained in those data
were not noted at the time and were not brought into focus until the LUP
Inception Report mentioned above.
During the 1986/87 season several soil
profile pits were excavated, described and sampled (at Dimsu DC) to try and
find correlations between millet yields, rooting depths and soil reaction (pH).
A plant pot trial was also carried out testing various forms of liming
materials on samples of Qoz soil. Due to the lack of laboratory equipment soil
pH could not be determined until the 1987/88 season. Land Use Planning Notes on
the studies done and observations made are as follows:
- AAH/01/86: Qoz Soils - 1985 Laboratory Data
- AAH/05/86: Aluminium Saturation and Liming
- AAH/06/86: Analysis of Available Laboratory
Data -Qoz
- AAH/09/86: Observations on Qoz Trials
- AAH/01/87: Crop Yield versus Soil pH at
Dimsu DC
- AAH/05/87: Calcium Deficiency in Qoz Soils
- AAH/01/88: Liming of Qoz Soils
1.3 Objectives
The main objectives of the recent survey of
Dimsu DC were to establish the uniformity, or otherwise, of the soil
distribution and identify, if possible, the main soil constraints to crop
production. Specific attention being paid to soil pH, levels of exchangeable
aluminium and aluminium saturation and the suspected deficiency of calcium. If,
in fact, these three factors proved to be unfavourable it would be of little
importance if other macro and micro-nutrients were deficient and additions of
fertilisers would not give reliable responses until the major constraints of
pH, aluminium and calcium deficiency were rectified.
2 SOIL
SURVEY AND SOIL SAMPLING
2.1 Soil
Survey Procedure
A detailed survey was carried out and, as
there were no suitable aerial photographs of the area, a grid survey was done.
Some 100 auger bores were excavated to 100cm depth and all were described to
FAO standards. Just over 60 samples were collected from auger sites; composite
samples being taken from 0 - 25, 25 - 50 and 50 - 100 cm depths.
After study of the auger descriptions
representative sites were selected and 10 soil profile pits were dug, described
and sampled, by described horizon, to a depth of 150 cm. Soil samples from
these pits were sent to the laboratory along with the samples collected from a
further 7 profile pits dug and described in 1986 (refer AAH/01/87). The location
of all sites can be seen in Figure number 1 and in more detail on map number
ASD/66.
2.2 Soil
Sampling
Five percent of the auger bores were bulk
sampled over the depth ranges stated in 2.1 above to allow soil pH to be
measured. All the 1987 profile pits were sampled to full depth, by natural
horizons, and samples were subjected to detailed laboratory analysis. The
profile pits dug in 1986 were also sampled to full depth, by horizon, but the
samples were subjected to only limited analyses.
2.3
Soil Analyses
2.3.1 U.K.
Analyses
All samples from the profile pits, 88 samples
from 17 pits, were sent to the Tropical Soils Analysis Unit (TSAU) Reading, UK
for detailed analyses - this was done to allow direct comparison with the data
from the studies done in 1985/86. The range of analyses was as detailed below
and results can be seen on the laboratory record sheets on file LUP/5/6.
The soil analyses carried out were as
follows:
Exchangeable Cations
- Sodium (Na)
- Potassium (K)
- Magnesium (Mg)
- Calcium (Ca)
Exchangeable Aluminium
Cation Exchange Capacity
Total Nitrogen
Organic Carbon
Available Phosphorus
- Bray
- Olsen
Micro-nutrients
- Copper
- Manganese
- Zinc
- Boron
- Molybdenum
- Iron
Particle Size Analysis
2.3.2 Local Analyses
The soil pH (1:5 Soil:Water) was determined
on all auger bore samples and results are on file LUP/5/6. The exact
methodology was as detailed in LUP Note AAH/01/88. The soil pH was also
measured on duplicate samples from the profile pits to allow comparison of
local results with the overseas results (this comparison is presented in
Section 3.4). Contact was made with WSARP and a request submitted that they
carry out some analyses on (duplicate) samples to allow further comparisons but
there was never any response to the request.
2.4 Findings
From the survey there appears to be
uniformity in the soils pattern within the DC apart from the western edge of
the area which is not typical Qoz. In fact this area is not within the present
utilised part of the farm and nor is it likely that it will be. Labourers have
several small personal cultivated plots in that area. This area has been mapped
out and can be seen in Figure 1.
Textures are generally sand, with not quite
enough fine sand fraction to give textures of fine-sand, and often with depth
there is loamy sand. Topsoil colours are dark yellowish brown (10YR4/4) to
brown (7.5YR4/4) and sub-soils are typically strong brown (7.5YR4/6) to
yellowish red (5YR5/6). Besides colour and texture there are very few features
of note in these soils - rooting distribution is discussed in Section 3.5.
The soils of the atypical part of the DC are
not discussed at any length as the production of a soil map, with full soil
descriptions, was not one of the main aims of this study. It would appear
sufficient to have separated out the area that is not typical. In fact the area
in question surrounds a small depression (rahad) where there are cracking clays
(buta soil) and less well-drained soils are found within some distance from
this depression. Profile pit number PD9 was dug in the atypical area and the
analyses show textures to be loamy sand throughout (with associated slightly
higher cation exchange capacities), reaction is about pH 5.6 throughout the
profile but aluminium levels are high (refer Section 3.4). Soil colours tend to
be paler than in the typical Qoz and there can be gleyic (pale coloured) and
ferric (reddish) mottles at depth within the profile suggesting imperfectly
drained conditions and a transient water table.
3 SOIL
FERTILITY
It has long been known that on clearing
virgin Qoz farmers can expect to get acceptable crop yields for several years.
However, with time yields always fall and this is not to be unexpected when
farming sands, such as Qoz, since the total reserves of nutrients, and soil
minerals, are low. In fact what cropping does is to mine the available
nutrients from these soils.
Dimsu DC was established to carry out
agronomic trials to try and supply the data, on several aspects of agronomy, to
produce extension packages and messages. The aim being to assist the farming
population extend the length of time the Qoz could be farmed, if not actually
improve yields and achieve sustainable arable agriculture. Up to the end of
Phase 1 and early in Phase 2 the answers were not forthcoming and the aspects
of fertility which have led to this lack of success had not been identified.
Much of this study has been devoted to
interpretation of the laboratory data and how the parameters that exist might
effect the crops that are normally grown on Qoz, and on the natural vegetation.
The dominant species of natural vegetation surrounding each sample point (auger
sites and profile pits) was noted but the natural vegetation has not been
discussed here. The full field descriptions can be referred to if data are
required on the species noted.
Sections are presented covering the following
aspects:
- pH and Exchangeable Cations
- Macro-nutrients and Fertility Potential
- Micro-nutrients
- Soil pH and Aluminium Saturation (Section
4)
- Soil pH, Rooting Depths and Millet Yields
(Section 4)
Weighted mean values for all available data
are presented and original data can be found on file LUP/5/6, there being too
much data to present in this note.
3.1 Soil
pH and Exchangeable Cations
3.1.1 Introduction
One of the main aims of the present study was
determine if soil pH and the level of calcium were having adverse effects on
crop production. To allow for easier comparisons of all data weighted mean
values for various depth ranges are presented.
2. Weighted Mean Values
Weighted mean values for depths 0-25, 25-50
and 50-100cm for all existing data are shown in Tables 1 & 2.
Table 1 Soil
pH and Exchangeable Cations
|
Exchangeable Cations me/100g |
|
||||||||
Depth cm |
Study |
pH |
Ca |
Mg |
K |
Na |
Al |
CEC |
TEB |
Samples |
0 - 25 |
WSDC 76/77 |
5.8 |
0.86 |
0.43 |
0.11 |
0.06 |
ND |
3.54 |
1.46 |
11 |
|
TSAU 86/86 |
5.5 |
0.72 |
0.49 |
0.09 |
0.0 |
0.30 |
2.12 |
1.30 |
5 |
|
TSAU 87/88 |
5.3 |
0.75 |
0.47 |
0.13 |
0.0 |
0.26 |
1.53 |
1.47 |
9 |
|
Mean |
5.6 |
0.75 |
0.46 |
0.11 |
0.03 |
0.27 |
2.53 |
1.43 |
25 |
|
Rating |
Mod |
L |
H |
M |
VL |
VL |
VL |
VL |
|
25 - 50 |
WSDC 76/77 |
5.2 |
0.38 |
0.47 |
0.15 |
0.07 |
ND |
3.35 |
1.06 |
2 |
|
TSAU 87/88 |
4.8 |
0.29 |
0.46 |
0.09 |
0.0 |
1.02 |
2.17 |
0.85 |
9 |
|
Mean |
4.9 |
0.31 |
0.46 |
0.10 |
0.01 |
1.02 |
2.38 |
0.89 |
11 |
|
Rating |
Very |
VL |
H |
M |
VL |
M |
VL |
VL |
|
50 - 100 |
WSDC 76/77 |
4.6 |
0.35 |
0.37 |
0.11 |
0.07 |
ND |
4.20 |
0.90 |
2 |
|
TSAU 87/88 |
4.8 |
0.26 |
0.40 |
0.10 |
0.0 |
1.64 |
2.69 |
0.77 |
9 |
|
Mean |
4.8 |
0.28 |
0.39 |
0.10 |
0.01 |
1.64 |
3.07 |
0.79 |
11 |
|
Rating |
Very |
VL |
H |
M |
VL |
M |
L |
VL |
|
Notes: CEC
- Cation Exchange Capacity TEB -
Total Exchangeable Bases
Ratings: Ratings for Ca, Mg and K taken from
FAO 1980, Soil and Plant Testing, Soils Bulletin 38/2.
H High |
S Severe (deficiency) |
M Moderate / Medium |
Mod Moderately acid |
L Low |
Very Very strongly acid |
VL Very Low |
|
Table 2 Saturation
Percentages and Ratios
|
Saturation Percentages |
Ratio |
|||||
Depth cm |
Study |
Aluminium |
Base |
Calcium |
Ca:Mg |
||
0 - 25 |
WSDC 76/77 |
ND |
41.2 |
24.3 |
2.0 |
||
|
TSAU 86/86 |
14.2 |
61.3 |
34.0 |
1.5 |
||
|
TSAU 87/88 |
17.0 |
96.1 |
49.0 |
1.6 |
||
|
Mean |
16.0 |
65.0 |
35.1 |
1.8 |
||
|
Rating |
L |
P* |
P/S* |
L |
||
25 - 50 |
WSDC 76/77 |
ND |
31.6 |
11.3 |
0.8 |
||
|
TSAU 87/88 |
47.0 |
39.2 |
13.4 |
0.6 |
||
|
Mean |
47.0 |
37.4 |
13.0 |
0.7 |
||
|
Rating |
M |
S* |
S/SV* |
VL |
||
50 - 100 |
WSDC 76/77 |
ND |
21.4 |
8.3 |
0.9 |
||
|
TSAU 87/88 |
61.0 |
28.6 |
9.7 |
0.7 |
||
|
Mean |
61.0 |
25.7 |
9.1 |
0.7 |
||
|
Rating |
H |
S* |
S/VS* |
VL |
||
L - Low, not problematic |
* Deficiency designation (FAO 1980) |
||||||
M - Moderate, problems expected |
P - Poor to moderate deficiency |
||||||
H - High, problems almost certain |
P/S - Poor to moderate deficiency |
||||||
|
S - Severe deficiency |
||||||
|
S/VS - Severe to very severe deficiency |
||||||
3.1.3 Interpretation
of Data
For a sand the topsoil (0 -25 cm) could be
considered as a reasonable rooting zone with Base Saturation (BS) of about 65%
and magnesium and potassium rated high and medium respectively. But the calcium
level is low, the Ca:Mg ratio is low and the calcium saturation percentage of
35 % all indicate a moderate to severe deficiency of exchangeable calcium.
Reaction in the topsoil, pH of 5.6, is
acceptable and the level of exchangeable aluminium, ASP of 16%, would not be
considered a constraint to cropping.
With depth the situation gets worse and BS
falls to about 25% in the deeper subsoil (50 - 100 cm), soil acidity increases
(pH of 4.8 - 4.9) and the aluminium saturation percentage (ASP) increases from
about 47% in the upper sub-soil (25 - 50cm) to over 60% in the lower sub-soil.
Throughout the sub-soil the Ca:Mg ratio is less than unity (0.7) and the
calcium saturation percentage is extremely low (13 - 9%) both parameters
indicating a severe calcium deficiency.
At pH values less than 5.5 phosphate can
combine with iron and aluminium, both of which are present in considerable
amounts in these soils, to form compounds which are not readily available to
plants. Hence any added phosphate fertiliser could be tied-up and nil or poor
responses result. Also, below about pH 5.5 bacterial activity is normally
reduced and nitrification of organic matter is significantly retarded and so
any resources that the soil does have may well not become available for plant
growth. Not that these soils have much organic matter anyway, refer Section 3.2
3.2 Macro-nutrients
and Fertility Potential
3.2.1 Introduction
To date the main nutrient element that has
been recognised as being deficient in Qoz has been phosphorus (P) and
application of P, as Triple Super Phosphate (TSP) has figured largely in
previous trials and is the basis of the extension message supplied to farmers.
It has also been well recognised that nitrogen (N) and organic matter (OM) are
deficient and as a matter of course these elements have been determined in most
surveys and studies.
Fertility potential is an estimate of how
efficiently the soil will retain any added nutrients and is usually estimated
by the cation exchange capacity (CEC) of the soil. Data on these topics are
presented in this section.
3.2.2 Data
Weighted means for all available data are
presented in Table 3 below and the full data can be found in file LUP/5/6 and
previous reports.
Table 3 Weighted Mean Values for Macronutrients and CEC
Depth |
Study |
Avail P (ppm) Bray |
Avail P (ppm) Olsen |
Avail P (ppm) Truog |
Total N% |
Org C % |
CEC Me/100g |
No. of Samples |
0 - 25 cm |
WSDC 76/77 |
/ |
/ |
0.84 |
0.022 |
0.22 |
3.54 |
11 |
|
TSAU 86/86 |
6.6 |
2.08 |
/ |
0.010 |
/ |
2.12 |
5 |
|
TSAU 87/88 |
4.5 |
0.42 |
/ |
0.020 |
0.14 |
1.53 |
9 |
|
Mean |
5.3 |
1.01 |
0.84 |
0.018 |
0.18 |
2.53 |
* |
|
Rating |
/ |
L |
/ |
EL |
EL |
VL |
|
25 - 50 cm |
WSDC 76/77 |
/ |
/ |
1.15 |
0.01 |
0.16 |
3.35 |
2 |
|
TSAU 87/88 |
2.3 |
0.0 |
/ |
ND |
ND |
2.17 |
9 |
|
Mean |
2.3 |
0.0 |
1.15 |
0.01 |
0.16 |
2.38 |
11 |
|
Rating |
/ |
TD |
/ |
EL |
EL |
VL |
* |
50 - 100cm |
WSDC 76/77 |
/ |
/ |
1.09 |
0.01 |
0.13 |
4.20 |
2 |
|
TSAU 87/88 |
2.5 |
0.0 |
/ |
ND |
ND |
2.69 |
9 |
|
Mean |
2.5 |
0.0 |
1.09 |
0.01 |
0.13 |
3.07 |
* |
|
Rating |
/ |
TD |
/ |
EL |
EL |
L |
|
Notes / ND
Not determined
* No value given as various sample
numbers in mean
Ratings:
L |
VL |
EL |
TD |
ND |
Low |
Very Low |
Extremely Low |
Totally Deficient |
Not Determined |
3.2.3 Interpretation
of Data
The macronutrients P, N and organic matter
appear to be very deficient with nitrogen and organic matter being extremely
low throughout the soil depth. Available-P (based on the results of the Olsen
method) is low in the topsoil and totally deficient in the subsoil. Data from
the Bray and Truog methods have not been used as reliable rating classes were
not to hand.
Evidence that addition of phosphate
fertiliser has increased the soil reserves of available-P was not convincing
since there were very few samples from plots with a known history of
application of P. The only data that exist are as follows.
Profile Pit(s) |
Available-P in Surface Soil ( ppm Bray Method) |
Phosphate Applied |
11 |
10.0 |
20 units |
12 - 16 |
4.6 |
10 units |
17 |
4.0 |
0 units |
These results would suggest that, in general,
applied phosphate has been used up by the crop grown on the plot. Only where 20
units of P were applied does there seem to be any increase, but as there is
only one sample, no significance can be given to this observation.
The fertility potential of these soils, based
on CEC, is very low, as would be expected for sands, and the soil would have
very limited ability to retain any added nutrients and hence fertilisers would
easily be leached out. Conversely, any amendments, such as liming materials,
added to the surface soil should reach the lower horizons easily.
3.3 Micro-nutrients
3.3.1 General
It has been recognised in the past that some
of the micronutrients essential for crop growth are deficient in the Qoz sands
and the detailed analyses included further determination of micronutrient
contents. In the past several trials by the Agronomy Section have been carried
out and the data gathered together here may assist with the design of further
trials.
3.3.2 Data
As for the other parameters weighted means of
all available data are presented and estimates of the status, ratings, given.
It should be noted that several of the determinations made during the 1976/77
studies were of total contents of elements as compared to extractable contents
measured by the TSAU. For the sake of completeness these total contents are
presented but no interpretation made of them. The data are presented in Table
4.
Table 4 Micronutrient Contents
|
ppm |
% |
No. |
|||||||
Depth |
Study |
Cu |
Zn |
Mn |
Mo |
B |
Fe |
Total-S |
Samples |
|
0 - 25 cm |
WSDC 76/77 |
3.6* |
7.50* |
68.9* |
0.30* |
0.09 |
0.45 |
0.004 |
11 |
|
|
TSAU 85/86 |
0.2 |
0.10 |
16.3 |
0.07 |
ND |
9.6 |
ND |
4 |
|
|
TSAU 87/88 |
0.2 |
0.17 |
11.5 |
0.04 |
0.04 |
9.3 |
<0.005 |
9 |
|
|
Mean |
0.2 |
0.15 |
13.0 |
0.05 |
0.07 |
9.4 |
0.004 |
|
|
|
Rating |
+/-D** |
D |
D |
+/-D** |
D |
S |
D |
|
|
25 - 50 cm |
WSDC 76/77 |
4.5* |
11.0* |
58.0* |
0.40* |
0.06 |
0.56 |
ND |
2 |
|
|
TSAU 85/86 |
ND |
ND |
ND |
ND |
ND |
ND |
<0.005 |
9 |
|
|
TSAU 87/88 |
ND |
ND |
ND |
ND |
0.06 |
ND |
<0.005 |
/ |
|
|
Mean |
ND |
ND |
ND |
ND |
0.06 |
ND |
<0.005 |
|
|
|
Rating |
ND |
ND |
ND |
ND |
D |
ND |
D |
|
|
50 - 100cm |
WSDC 76/77 |
4.5* |
12.5* |
48.0* |
0.03 |
0.17 |
0.60 |
ND |
2 |
|
|
TSAU 85/86 |
ND |
ND |
ND |
ND |
ND |
ND |
ND |
0 |
|
|
TSAU 87/88 |
ND |
ND |
ND |
ND |
ND |
ND |
<0.005 |
9 |
|
|
Mean |
ND |
ND |
ND |
ND |
0.17 |
ND |
<0.005 |
|
|
|
Rating |
ND |
ND |
ND |
ND |
D |
ND |
D |
|
|
Notes * Total contents and data not used for
means
ppm |
ND |
D |
+/-D** |
S |
Parts per million |
Not determined / No data |
Deficient |
Borderline for sufficient |
Sufficient |
Ratings taken from Tropical Soil Manual,
Booker Agricultural International Ltd., 1984
Based on the ratings employed virtually all
the micro-nutrients are apparently deficient apart from iron (Fe), of which
there is sufficient, and copper (Cu) and molybdenum (Mo), both of which are
borderline between being deficient and sufficient for crop growth.
Deficiencies of micronutrients are not
normally expected in acidic soils. But, if the soils are very to extremely
acidic, which the Dimsu Qoz soils are, then many of the micronutrients can be
so mobile that they could easily be leached out of the soil (this is assuming
that the soil ever contained any of the elements in the first place). This
would appear to be rational explanation for the status of the Dimsu soils as
almost all the micronutrients are deficient, or almost so, in that the elements
have been lost through the leaching process in the acidic conditions that
prevail. The exception to the above explanation would be molybdenum (Mo) which
becomes less mobile (and available) at low soil pH values.
4 SOIL
ACIDITY and LIMING
4.1 Soil
pH and Aluminium Saturation Percentage (ASP)
4.1.1 Introduction
Land Use Planning Note AAH/01/86 detailed the
relationship between soil pH and ASP that appeared to exist based on the very
limited data to hand at that time. The important point was that there was a
very strong correlation between pH and ASP and once soil pH fell below 5.5 very
high ASPs were found.
All samples taken during the recent Dimsu
survey had the soil pH measured and most of the profile pit samples had the
exchangeable aluminium level determined. The data obtained has allowed
correlations to be made between pH as determined locally and by TSAU - this
correlation can be seen in Figure 2. The correlation between soil pH and ASP
can be seen in Figure 3.
4.1.2 Crop
Tolerances
There is not a great deal of published data
on the levels of ASP that are tolerated by crops grown in arid and semi-arid
regions. A literature search undertaken in the HTS Head Office, UK came up with
the following information:
·
Millet and sorghum are
both very sensitive to low soil pH and high ASP values, and a level of 20% ASP
is regarded as the critical level for these crops. This means that when ASP
exceeds 20% reduced crop performance and yield can be expected.
·
Groundnuts are more
tolerant to acidity and ASP, the critical level is estimated to be 30 - 40%
ASP.
·
No information could be
found for sesame but it is considered (by the researcher who undertook the
study) that sesame is probably sensitive.
4.1.3 Soil
pH Data
All the data collected on the pH of the soils
sampled during the recent survey are presented in Table 5. The values presented
for the auger samples have been converted to what is estimated to be the
equivalent pH as determined by the TSAU. The estimated overall ASP presented as
the last line of Table 5 was estimated using the following figure which shows
the correlation between pH and ASP.
Table 5 Soil
pH (1:5 soil/water) - Dimsu 1987 - Weighted
Mean pH Values*
Observation |
Land Use Category |
Number |
|
Depth Range |
|
Type |
|
of Samples |
0 - 25cm |
25-50cm |
50-100cm |
Auger Bores |
Virgin tree savannah |
3 |
5.63 |
4.81 |
4.76 |
|
Virgin tree savannah + grass |
5 |
5.63 |
5.15 |
5.45 |
|
Virgin tree savannah + other |
5 |
5.55 |
4.90 |
4.97 |
|
Weighted Mean |
(13) |
5.60 |
4.98 |
5.11 |
|
Cleared |
3 |
5.15 |
4.71 |
4.71 |
|
Cleared + grass |
4 |
5.02 |
4.46 |
4.60 |
|
Weighted Mean |
(7) |
5.08 |
4.57 |
4.67 |
|
Cultivated |
2 |
5.25 |
4.77 |
4.77 |
|
Weighted Mean |
(2) |
5.25 |
4.77 |
4.77 |
|
Weighted Mean for all bores |
(22) |
5.40 |
4.83 |
4.94 |
|
|
|
|
|
|
Profile Pits |
Virgin tree savannah |
2** |
5.40 |
4.85 |
4.80 |
|
Regrowth |
1 |
5.30 |
4.90 |
4.70 |
|
Cleared not cultivated |
2 |
6.00 |
4.85 |
4.70 |
|
Cultivated |
10 |
5.06 |
4.77 |
4.63 |
|
Long term rotation |
1 |
4.90 |
4.60 |
4.70 |
|
Weighted Mean for all pits |
(16) |
5.23 |
4.81 |
4.83 |
|
Overall weighted mean |
(38) |
5.33 |
4.81 |
4.83 |
|
Estimated overall ASP*** |
|
9.5 |
52.5 |
50.6 |
Notes : * - For auger bores estimated value
from Figure 2
: ** - One profile, PD9, excluded as not
typical Qoz
: *** - ASP estimated from Figure 3
4.1.4
Interpretation of Data
As can be seen from the data in Table 5
topsoil pH values are in excess of 5 whilst the subsoil values are all less
than 5 and, overall, show a mean value somewhere about 4.8. When the ASP values
are estimated, using Figure 3, it can be seen that ASP in the topsoil is not a
problem as the overall mean would appear to be less than the critical values
stated in Section 4.1.2 above and are about 10%. This would indicate that the
topsoil is a reasonable rooting zone and the pH and ASP levels should not be a
constraint to seed germination and initial plant growth.
However, the subsoils present quite a
different picture and show pH values of less than 5 with associated ASP values
in excess of 50%. From the critical levels being assumed to be appropriate for
the crops normally grown on Qoz it is apparent that a hostile root zone exists
from about 25 cm depth. This depth is the mean depth and there are minor
variations throughout the surveyed area of the DC farm.
From the above observations one would expect
that crop roots would not penetrate the subsoils due to the adverse conditions
caused by the pH and ASP and this topic is discussed in Section 4.2.
4.2 Soil
pH, Rooting Depths and Relative Millet Yields
4.2.1 Introduction
Several of the previous sections have highlighted
soil parameters that, alone and in combination, would limit the depth to which
roots would penetrate. The possibility of such a situation was recognised early
in year 1 of WSP-2 and several profile pits were dug down through the rooting
system of millet plants on some trial plots on Dimsu DC. These pits were
numbers PD11 to PD17 and the samples taken were subjected to limited laboratory
analyses, mainly soil pH.
During the soil survey undertaken at the end
of 1987 all profile pits were fully described and that description included
estimates of roots in each soil horizon, this being done in cultivated and
uncultivated (virgin) areas.
This section deals with the observations made
and the correlations found between soil pH and root distribution.
4.2.2 Soil pH and Millet Roots
The data assembled from analysis of the 7
profile pits dug specifically to study the rooting pattern of millet roots are
presented in Tables 6 and 7.
Table 6 Weighted
Mean pH Values
Depth (cm) |
|
|
Profile |
Pit |
Number |
|
|
|
|
11 |
12 |
13 |
14 |
15 |
16 |
17 |
Mean |
0 - 25 |
4.86 |
5.20 |
4.97 |
5.01 |
5.47 |
5.12 |
4.88 |
5.07 |
25 - 50 |
4.62 |
4.78 |
4.65 |
4.78 |
5.06 |
4.58 |
4.52 |
4.71 |
50 - 100 |
4.58 |
4.94 |
4.40 |
4.48 |
4.68 |
4.42 |
4.45 |
4.56 |
Mean |
4.66 |
4.97 |
4.61 |
4.69 |
4.97 |
4.64 |
4.58 |
4.73 |
Source : TSAU 1987/88
Table 7 Relative
Millet Yield and Mean Soil pH
Relative Yield * |
Profile Pit |
Mean |
pH at |
Depth |
Root |
Depths |
Treatment |
Millet |
Numbers |
0 - 25 |
25 - 50 |
50 - 100 |
Main |
Max |
|
Very High |
16 |
5.12 |
4.58 |
4.42 |
60 |
80 |
N, P + Lime |
High |
11 & 15 |
5.17 |
4.84 |
4.63 |
20-30 |
60 |
P (20 units) |
Average |
12 & 14 |
5.11 |
4.78 |
4.71 |
40 |
60 |
P (10 units) |
Low |
13 & 17 |
4.93 |
4.59 |
4.43 |
<40** |
<40** |
Nil and P |
* Relative
yields as supplied by Agronomy Section
** Very
few and all <40cm depth
4.2.3 Observations and Interpretation of Data
From Tables 6 and 7 it can be seen that as
soil pH falls, apart from in profile number PD16, the relative yield falls.
This observation being made on the soil to a depth of 50cm as it seems
irrelevant to include the deeper subsoil (50-100) as the roots did not
penetrate this deep.
On the assumption that the optimum, or
expected, rooting depth of millet is 70 cm (from literature search, various
sources), it is obvious that the main roots of the crop do not penetrate to
this depth and generally seem to be in the 30 to 40 cm depth range. This would
certainly correlate with the critical soil pH for millet being about 5.2
(literature search). In fact even the surface soil (0-25cm) pH values are all
rather close to, and generally less than, this critical value and reduced plant
performance and yield would be expected.
On the assumption that the expected rooting
depth of millet is 70 cm and the critical pH value is 5.2 it is apparent that
the roots of the millet on the plots investigated do not penetrate to the expected
depth and hence can not forage for nutrients and moisture. There was ample
evidence that there was soil moisture to well below the depth to which the
roots did penetrate and even below the expected rooting depths (refer Section
4.2.5).
There would appear to be an anomaly in that
the relative yield at site PD16 was very high yet the soil pH values,
especially in the subsoil, were no less acidic despite lime having been
applied. It can only be assumed that the crop utilised the added lime
(100Kg/ha) before the lime had any effect on changing the soil pH. The
additional calcium from the lime, along with the added N and P, would have
boosted crop yield on this plot.
4.2.4 Soil
pH, Exchangeable Aluminium, ASP and Rooting Depths
In this section weighted mean values for the
soil pH, exchangeable aluminium and aluminium saturation percentage (ASP) have
been presented for the various different land use categories within the Dimsu
DC farm, and these have been related to the depth (dominant) of root
penetration noted during the study. The full data can be found in file LUP/5/6
and only weighted means are presented in Table 8.
Table 8 Land Use, Rooting Depth and Soil Acidity
Land Use Mean Rooting Depth |
Mean* Values Within Main Root Zone |
|
Values in Horizon Below Root Zone |
|
||||||
|
(cm) |
pH |
Al |
ASP |
No. |
pH |
Al |
ASP |
No. |
|
Virgin |
50 |
5.33 |
0/75 |
25 |
3 |
5.00 |
2.2 |
54 |
3 |
|
Cleared |
42 |
5.20 |
0.33 |
21 |
6 |
4.65 |
1.5 |
71 |
6 |
|
Cultivated |
45 |
4.91 |
0.55 |
37 |
8 |
4.50 |
1.5 |
69 |
3 |
|
Notes: Al - Exchangeable Aluminium as me/100g
soil
ASP - Aluminium Saturation Percentage
No - Number of Samples in Mean*
* - Weighted mean
4.2.5 Interpretation
of Data and Discussion on Rooting
Under virgin conditions roots go slightly
deeper, 50 cm as compared to about 45cm in cultivated areas, but this is still
to a very shallow depth. If the virgin vegetation is a climax community then
either the community is not very tolerant to the prevailing soil conditions or
it is a shallow rooting community that has developed to compensate for the
prevailing conditions.
From the soil survey (1987) and other
observations (1986) it was noted that soil moisture extends to well below the
depths to which the main roots penetrate. Soil moisture was noted down to
depths of 165 cm and lamellae (evidence of a wetting front) were noted at
depths between 60 and 140 cm. Lack of soil moisture should hence not be the reason
for roots (virgin vegetation or crops) penetrating to any great depth.
Within the main rooting zones weighted mean
soil pHs range from 4.91 to 5.33, with an overall mean of 5.09, whilst
immediately below the main rooting zone the pH ranges from 5.00 to 4.50 with an
overall mean of 4.64. There is a progressive reduction in soil pH from virgin
(5.33) through cleared (5.20) to cultivated (4.91) - presumably due to the
removal of calcium from the nutrient cycle. This removal would have come about
by the removal of the trees, for firewood and building materials, as the land
was cleared and by the harvesting of crops. Both actions leading to a relative
concentration of hydrogen and aluminium ions (acidifying agents).
Levels of exchangeable aluminium increase dramatically
from within the main root zones (overall weighted mean value of 0.49 me/100g)
as compared to the soil horizon directly below the root zone (overall mean
value of 1.68 me/100g). These values can be seen on reference to the original
laboratory data sheets. Values such as are found in the subsoils are generally
considered to be highly unfavourable, if not toxic, to most crops.
Aluminium saturation percentages (ASP) range
from about 21 - 37% in the main root zones, with an overall mean of 26.2%, and would
be considered as acceptable for tolerant varieties of crop. In the subsoils ASP
values range from 54 to 70.5%, with an overall weighted mean value of 66%, a
value that is tolerated by few crops and, even then, major yield depressions
would be expected. As quoted earlier (Section 4.1.3) the critical level for
millet and sorghum is estimated to be 20% and for groundnuts about 30%, hence
it is apparent that there is a major constraint to crop growth in the soils of
Dimsu DC.
The correlation between depth of root
penetration and soil pH / ASP seems quite clear. The evidence suggests that an
unfavourable, or hostile, condition prevails in the subsoils and that roots, of
not only arable crops but also native vegetation, do not penetrate much below
40 - 50 cm depth to forage for whatever nutrients and moisture that exist in
the subsoils.
Data for ASP values for 13 profile pits are
shown in Figure 4 where normal rooting depths of millet, sorghum and groundnuts
are shown along with the critical levels of ASP tolerated by these crops. As
can be seen from this figure millet is able to utilise about 50% of the
normally accepted rooting depth, sorghum about 35% and groundnuts between 25
and 50%.
4.3 Lime
Requirements
4.3.1 Introduction
To overcome the effects of soil acidity and
exchangeable aluminium, it is necessary to add a liming material, source of
calcium, to the soil. Normally a soluble source of calcium is best as this will
mobilise and be leached down the profile and replace the aluminium ions, on the
exchange complex, and thereby reduce soil acidity and increase the level of
nutrient calcium in the soil.
In Landuse Planning Note AAH/1/86 lime
requirements were calculated using the formula of Salinas, Cochrane et al
(1980) as, from experience in other areas this formula appears to give the best
estimate of how much lime to add.
The formula is designed to achieve a target
level of ASP within the topsoil and for the Dimsu soils a target level of zero
ASP has been set. This may appear unrealistic but the hope is that there would
be enough liming material applied to move down into the subsoil and help
ameliorate the conditions that exist there. It would be possible to calculate
liming rates for the subsoil but there is no practical way of applying the
liming material at depth with the equipment presently available on the project.
4.3.2 Liming
Rate Formula*
The formula utilised is as follows;
LR = 1.8{Al-X x (Al+TEB+H)/100} Tonnes / ha
Where LR is the Lime Requirement in
tonnes of calcium carbonate per hectare and the elements, Al, H and TEB are
expressed in me/100g soil, as determined by the laboratory, and X is the target
level of ASP expressed as a percentage.
* An Equation for Liming Acid Mineral Soils
to Compensate Crop Aluminium Tolerance. Cochrane T.T., Salinas J.G. and Sanchez
P.A. Tropical Agriculture (Trinidad), Volume 57, No 2, April 1980.
4.3.3 Lime
Requirements
The pertinent data for the 10 profile pits
for which full laboratory analyses were done are presented in Table 9 along
with the calculated Lime Requirements to achieve zero and 10% ASP.
Table 9 Lime
Requirements for Topsoils
Profile No. |
Al |
TEB |
Mean ASP |
(t/ha) Lime (CaCO3) |
Requirement |
|
me/100g |
me/100g |
% |
10% ASP |
0% ASP |
PD1 |
0.45 |
0.80 |
41 |
0.59 |
0.81 |
PD2 |
0.32 |
1.72 |
13 |
0.21 |
0.58 |
PD3 |
0.02 |
1.19 |
1 |
0.00 |
0.04 |
PD4 |
0.21 |
1.29 |
10 |
0.11 |
0.38 |
PD5 |
0.14 |
2.33 |
16 |
0.00 |
0.25 |
PD6 |
0.26 |
0.96 |
20 |
0.25 |
0.47 |
PD7 |
0.28 |
1.74 |
12 |
0.14 |
0.50 |
PD8 |
0.28 |
1.60 |
11 |
0.17 |
0.50 |
PD9 |
0.18 |
2.94 |
4 |
0.00 |
0.32 |
PD10 |
0.40 |
0.61 |
30 |
0.54 |
0.72 |
|
|
|
Weighted Mean |
0.20 |
0.46* |
* The figure of 0.46 tonnes per hectare
(t/ha) of calcium carbonate compares very closely with the figure calculated
for all Qoz soils, for which there was data, in AAH/1/86.
4.3.4 Conclusions
and Discussion
It is obvious that a field trial to check the
calculated rates of lime required is required and the intention is that the
Agronomy Section will lay down such a trial during the 1988 wet season and
changes in soil pH will be determined after the rains. A part of the 1988/89
Soils/Land Use work programme is a suggested trial which should include a
cropped component and an un-cropped component, the liming material to be used
to be restricted to locally available materials.
It is also obvious that there is quite a wide
a variation in the amount of lime that is required and it is strongly
recommended that the average rate calculated in Table 9 be used and also that
double this rate should be applied as an extreme application.
The locally available materials are discussed
in Land Use Planning Note AAH/1/88. In that note it was shown that all of the
materials did bring about changes in soil pH - but only a depth of about 20 cm
was monitored in the plant pot experiment (AAH/1/88). It has to be seen if
application of sufficient liming materials to reduce the surface soil ASP to
zero, as calculated, will have any effect on the subsoils, and to what depth.
It may take some time before the applied materials can work down through the profile
and bring about changes in the soil pH and there is every chance that any crop
growing on the trials plots will intercept and use up all, or much, of the
applied material.
Application of the locally materials should
bring about reductions in the soil acidity, should reduce ASP levels and should
also increase the supply of calcium in the soil and hence reduce the noted
calcium deficiency. Of course, it should be borne in mind that any aluminium
displaced from the surface horizons, by the added calcium (lime), will move
down the profile and, to begin with, the acidity and ASP of the subsoils could
increase. Amelioration of the subsoil will only occur once the displaced
aluminium has been leached to sufficient depth to be below the rooting depth of
the crops being grown. This could take several years and, perhaps, further
applications of lime.
5 FINDINGS
and CONCLUSIONS
Each section of the report has some
discussion on the conclusions drawn on the topic of that section. All that is
done in this section is to highlight the various findings.
5.1 Most of the Dimsu DC is covered by what has always
been considered to be typical, deep Qoz sand. The survey showed that the farm
has yellowish brown to brown topsoils and strong brown to yellowish red
subsoils. There is a small area of imperfectly to poorly drained soils near the
western boundary. This area has been mapped out and is not currently part of
the cultivated farm and should be avoided for trials work in the future.
5.2 There is ample evidence of soil moisture resources to
considerable depth (150 cm) in these Qoz soils and lack of moisture resources
can not be blamed for poor crop and agronomic trials performance. However, the
availability of the moisture resources that exist is a totally different matter
and it has been concluded that plant roots do not forage throughout the depth
of the soil for moisture (refer 5.3 and 5.4 below) nor, by implication, for
nutrients.
5.3 The soils are moderately to very
strongly acid, their inherent fertility is low to very low and fertility
potential is very low. There is an identified calcium deficiency which is
considered to be severe in the topsoil and severe to very severe in the
subsoil.
Nitrogen, phosphorus and organic matter
contents are all extremely low and phosphorus is considered to be totally
deficient in most subsoils. With the very low fertility potential (CEC) these
soils have very limited ability to retain any added nutrients and fertilisers,
if of a readily soluble nature, would be easily leached out.
Micronutrient contents are all very low,
apart from iron, and are classified as deficient apart from copper and
molybdenum which are on the borderline of being deficient.
5.4 As stated in 5.3 above, the soils are
rather acidic and topsoil pH values are just over 5 whilst subsoil values are
less than 5. Aluminium saturation percentage (ASP) is not considered a problem
in the topsoil but it does form a considerable constraint in the subsoil where
values in excess of 60% are found.
A literature search indicated that the
acidity and ASP values found in the subsoils are beyond the critical,
acceptable levels for the main crops normally grown. There is hence considered
to be limited effective depth of soil in which the roots can forage for
moisture and nutrients.
A study of the rooting depths of millet, and
natural vegetation, showed that roots do not penetrate the lower horizons where
the low pH and high ASP values are found. This finding would appear to confirm
that there is a major constraint in the subsoils. One would expect that in an
environment such as at Dimsu, sandy soils with low and erratic rainfall, that
there would be exploitation of the maximum (apparently available) rooting depth
by plants. The fact that this does not happen means that plants could suffer from
drought far more quickly than they should.
5.5 Lime requirements have been calculated
and to achieve zero percent ASP in the topsoils about 0.5 tonnes per hectare of
calcium carbonate needs to be added. Since there is not an ASP problem in the
topsoils it may seem odd to lime the topsoil. Lime, in theory, should be
injected, or buried, at about 25cm depth but at present there is no suitable
equipment on the project to do this. Consequently, the soils will be surface
limed (using ground local limestone) and monitored to see to what extent and
depth changes in soil pH occur.
If the project does develop a Technology
Generation Unit (TGU) perhaps the agricultural engineer could either procure or
develop some suitable equipment for injecting lime at depth in the Qoz.
5.6 Until such time as the presently
recognised excessive acidity, with associated ASP problem, and calcium
deficiency is ameliorated it is considered that reliable, meaningful results
from agronomic trials will not be forthcoming.
A.A.Hutcheon
12 June 1988
APPENDIX 1
Soil Profile Descriptions and Laboratory Data
(Data not available - on file in Nyala)
APPENDIX 2
Nyala Determined
Soil pH
Table A2/1 Soil pH (1:5 water) Soil Auger Bores - Dimsu
Line No. |
Site No. |
Sample |
pH |
Line No. |
Site No. |
Sample |
pH |
0 |
2 |
A |
6.11 |
6 |
3 |
A |
6.10 |
|
|
B |
5.52 |
|
|
B |
5.73 |
|
|
C |
5.47 |
|
|
C |
5.70 |
0 |
8 |
A |
5.73 |
6 |
7 |
A |
5.94 |
|
|
B |
5.52 |
|
|
B |
5.27 |
|
|
C |
5.66 |
|
|
C |
5.48 |
1 |
6 |
A |
5.80 |
7 |
5 |
A |
5.83 |
|
|
B |
5.21 |
|
|
B |
5.63 |
|
|
C |
5.53 |
|
|
C |
5.63 |
2 |
3 |
A |
5.78 |
7 |
8 |
A |
6.03 |
|
|
B |
5.58 |
|
|
B |
5.60 |
|
|
C |
5.52 |
|
|
C |
5.85 |
2 |
10 |
A |
5.61 |
8 |
2 |
A |
5.34 |
|
|
B |
5.64 |
|
|
B |
5.26 |
|
|
C |
5.60 |
|
|
C |
5.47 |
3 |
3 |
A |
5.90 |
8 |
6 |
A |
6.58 |
|
|
B |
5.58 |
|
|
B |
5.97 |
|
|
C |
5.64 |
|
|
C |
5.76 |
3 |
8 |
A |
5.90 |
9 |
3 |
A |
5.70 |
|
|
B |
5.74 |
|
|
B |
5.96 |
|
|
C |
5.68 |
|
|
C |
6.13 |
4 |
5 |
A |
5.48 |
9 |
4 |
A |
6.41 |
|
|
B |
5.19 |
|
|
B |
6.02 |
|
|
C |
5.16 |
|
|
C |
6.52 |
4 |
9 |
A |
7.94 |
9 |
8 |
A |
5.88 |
|
|
B |
6.63 |
|
|
B |
5.58 |
|
|
C |
5.68 |
|
|
C |
5.56 |
5 |
6 |
A |
6.34 |
10 |
5 |
A |
6.12 |
|
|
B |
5.88 |
|
|
B |
5.70 |
|
|
C |
5.59 |
|
|
C |
5.95 |
|
|
|
|
10 |
9 |
A |
5.85 |
|
|
|
|
|
|
B |
5.57 |
|
|
|
|
|
|
C |
5.56 |
Notes pH is pH 1:5
water and samples are:
Table A2/2 Soil pH (1:5 water) Profile Pits -Dimsu
Profile No. |
Sample |
Depth (cm) |
pH |
Profile No. |
Sample |
Depth (cm) |
pH |
PD1 |
A |
0 - 9 |
6.09 |
PD6 |
A |
0 - 14 |
5.62 |
|
B |
9 - 35 |
5.59 |
|
B |
14 - 37 |
5.20 |
|
C |
35 - 70 |
5.58 |
|
C |
37 - 75 |
5.27 |
|
D |
70 - 100 |
5.52 |
|
D |
75 - 150 |
5.53 |
|
E |
100 - 161 |
5.53 |
PD7 |
A |
0 - 15 |
5.97 |
PD2 |
A |
0 - 21 |
5.61 |
|
B |
15 - 26 |
5.62 |
|
B |
21 - 37 |
5.51 |
|
C |
26 - 45 |
5.57 |
|
C |
37 - 83 |
5.33 |
|
D |
45 - 79 |
5.58 |
|
D |
83 - 150 |
5.77 |
|
E |
79 - 110 |
5.76 |
PD3 |
A |
0 - 23 |
5.86 |
|
F |
110 - 155 |
5.92 |
|
B |
23 - 41 |
5.69 |
PD8 |
A |
0 -18 |
6.07 |
|
C |
41 - 83 |
5.63 |
|
B |
18 - 44 |
5.73 |
|
D |
83 - 128 |
5.52 |
|
C |
44 - 72 |
5.67 |
|
E |
128 - 170 |
5.56 |
|
D |
72 - 101 |
5.73 |
PD4 |
A |
0 - 22 |
5.74 |
|
E |
101 - 156 |
5.84 |
|
B |
22 - 48 |
5.69 |
PD9 |
A |
0 - 11 |
6.19 |
|
C |
48 - 106 |
5.58 |
|
B |
11 - 23 |
6.00 |
|
D |
106 - 158 |
5.68 |
|
C |
23 - 38 |
6.19 |
PD5 |
A |
0 - 18 |
6.85 |
|
D |
38 - 62 |
6.10 |
|
B |
18 - 34 |
6.07 |
|
E |
62 - 95 |
6.12 |
|
C |
34 - 54 |
5.35 |
|
F |
95 - 145 |
6.00 |
|
D |
54 - 87 |
5.47 |
PD10 |
A |
0 - 9 |
5.87 |
|
E |
87 - 159 |
5.65 |
|
B |
9 - 22 |
5.70 |
|
|
|
|
|
C |
22 - 48 |
5.69 |
|
|
|
|
|
D |
48 - 96 |
5.76 |
|
|
|
|
|
E |
96 - 152 |
5.82 |
Note : Refer
Section 4.1.1 for relationship between pH Nyala and TSAU