NAGAA PLANT POT TRIAL

 

Land Use Planning Note AAH/2/88

 

CONTENTS

1 INTRODUCTION

2 PURPOSE OF THE TRIAL

3 SOIL

4 GYPSUM

5 OBSERVATIONS / DETERMINATIONS

5.1 Emerson Test

5.2 Soil Texture and Soil pH

6 FINDINGS

7 CONCLUSIONS

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 NAGAA PLANT POT TRIAL LUP Note AAH/02/88

1 Introduction

Land Use Planning Notes AAH/7/86,AAH/10/86,AAH/12/86,AAH/13/86 and AAH/10/87 all give background data and discuss the need for gypsum addition to the Nagaa soils. Observations on structural stability made in AAH/10/87,and field observations, show that gypsum appears to aid the structural stability of the soils but field observations show that the method of hand broadcasting the gypsum does not give a very even distribution of the material. Trials to date have generally used gypsum at the rate of 10 tonnes per hectare and some observations on lesser applications are indicated.

2 Purpose of the Plant Pot Trial

An attempt was made to crush the coarse soil clods of Nagaa and produce units of about 1 cm diameter or less and to apply different rates of gypsum using a method whereby the gypsum comes into close contact with the soil being treated. The hope being that better distribution of the gypsum might indicate that lesser amounts could be used and that there could be the possibility of applying the gypsum whilst loosening the soil surface with a ROTAVATOR type machine.

3 Soil

The soil used consisted of a sample of the top-soil from the location of the tillage trial at Jumeiza DC described in LUP Note AAH/10/87. The air-dried soil was lightly crushed until most units were less than 1 cm across. The crushed soil fragments were then thoroughly mixed. Five samples of 1.5 Kg each were weighed into large polythene bags, this weight of soil being just enough to fill a small plant pot to a depth of about 15 cm.

4 Gypsum

Gypsum was weighed out as shown in Table 1 below and the powder added to the soil in the polythene bags and thoroughly shaken to ensure even distribution. The treated soil was then placed into the plant pots and the pots placed out-doors in a place where they received rainfall with as even a distribution as possible.

Table 1 Gypsum Treatments

Pot Number

Weight of Soil

Kg

Weight of Gypsum

g

Rate of Gypsum

t/ha

1

1.5

0

0

2

1.5

2

1

3

1.5

4

2

4

1.5

10

5

5

1.5

20

10

 

5 Observations / Determinations

Some 4 months later, after the wet season, the air-dried soils were placed in polythene storage bags until such time as a few simple laboratory tests and observations could be carried out.

 5.1 Emmerson Test

Replicate samples of each soil sample were subjected to the Emmerson Test to establish the structural stability class number. The results are shown in Table 2 below.

Table 2 Emmerson Test Observations

Sample

Dry Soil: Slaking

Dry Soil: Dispersion

Plastic Limit: Dispersion

Emmerson Class

0 t/ha

Total

None

Total

3

1 t/ha

Total

None

Some

3+

2 t/ha

Almost total

None

Some

3+

5 t/ha

Almost total

None

None

4

10 t/ha

Obvious peds

None

None

4

Even after several days the 5 and 10 t/ha treated soil had not dispersed and appeared relatively stable. The 1 and 2 t/ha treated soil showed less dispersion than the untreated soil and lay between Emmerson Classes 3 and 4.

5.2 Soil Texture and Soil pH

Hand (field texturing) and pH assessed texture by pH meter on a 1:5 soil:water suspension. Results are shown in Table 3.The supernatant used for pH determination was also studied to note the cloudiness or clarity - an indication of dispersion or stability of the soil.

Table 3 Soil Texture and pH

Sample

Texture

pH 1:5 water

Mean pH

Supernatant

0 t/ha

SiCl

8.32/8.46

8.39

Cloudy

1 t/ha

SiCl

8.09/8.08

8.09

Cloudy

2 t/ha

SiCl

8.02/7.93

7.99

Cloudy

5 t/ha

SiCl

7.37/7.37

7.37

+/- Clear

10 t/ha

SiCl

7.25/7.32

7.29

Clear

 

 

 

 

 

Note; SiCl = Silty clay loam

Methodology for pH was; 10g of air dried soil ground then mixed with 50ml of distilled water and thoroughly stirred. After standing for 60 minutes the supernatant was poured off and the pH measured on an electronic pH meter after the electrode had been immersed for 2 minutes. Duplicate samples were measured.

6 Findings

a) All levels of gypsum gave an improvement in structural stability but addition of up to 2 t/ha did not give any marked improvement.

b) Five and 10 t/ha of gypsum gave a marked improvement in structural stability and after 7 days immersion in water the soil did not disperse.

c) Dispersion of the soil during pH determination, as noted by clarity of the supernatant, correlated closely with results of the Emmerson Test.

d) All samples of the dry soil did slake when immersed in water but the 10 t/ha treated sample did remain as definable peds.

e) The pH of the soil increasingly fell with increasing amounts of gypsum and the soil changed from moderately or strongly alkaline to almost neutral with the addition of about 5 t/ha of gypsum.

 7 Conclusions

No significant change in pH or structural stability was noted with up to 2 t/ha of gypsum, and the changes brought about by addition of 5 and 10 t/ha were very similar. LUP Note AAH/12/86 showed that a soil pH of 7.5 correlated with an ESP of 5 (Exchangeable Sodium Percentage) and this was considered to be a desirable condition. The present study shows that a pH of 7.5 could be achieved with the addition of about 4 t/ha of gypsum and that soil structural stability would be improved by this input of gypsum.

Applications of 10 t/ha of gypsum in field trials to date have not shown any great improvement in soil structural stability and it could be that the method of application to date has not led to the best results. If a ROTAVATOR could be found the effect of applying smaller amounts of gypsum whilst rotavating could be tested.

 

A.A.Hutcheon

January 1988