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:

  1. 0 - 25cm
  2. 25 - 50cm
  3. 50 - 100cm

 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