GREEN MANURING

EXTENSION BULLETIN No.

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ACKNOWLEDGEMENT

The authors are grateful to Prof. V. 0 . Chude of Soil Science De­

partment, Ahmadu Bello University, Zaria for his useful suggestions

and for providing some of the materials used for writing this bulle­

tin.

J.M. Jibrin, C.K. Daudu, J.O. Adeosun, A. Ibrahim and Y.A. Sani

(Authors)

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INTRODUCTION

Increasing concern for sustainable soil productivity and ecological

stability in relation to the use of chemical fertilizers have led to re­

newed interest in organic fertilizers such as compost, farmyard ma­

nure and green manure. Composts and farmyard manure have lo~

and variable nutrient contents, the more readily available green ma­

nures therefore constitute an important potential source of soil pro­

ductivity improvement. Trees, shrubs, cover crops, grain legumes,

grasses, weeds, ferns and algae could all be used as green manure to

provide inexpensive source of organic fertilizer for building up and

maintaining soil organic matter and fertility.

This bulletin will be useful to extension agents in teaching farmen

the preparation and use of green manures.

2.0 Advantages of Green Manuring

The fast population growth in Nigeria has resulted in considerable

pressure on land resources of the country. Increasing demand fm

food and energy has led to continuous intensive cropping with high

yielding crop varieties, resulting in increased demand for nutrient

supply on an already over-exploited land, and the application of in­

organic fertilizers to maintain high crop yields. Recently, high cost~

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of fertilizers and the increasing concern for ecological stability and

sustainable soil productivity have led to renewed interest in green

manuring. Advantages of green manuring include:

a) Nitrogen fixation. Some green manures crops such

as Sesbania, and Velvet bean (Mucuna pruriens)

could accumulate up to 1 OOkg N ha·1 or more in 7 to

8 weeks. . . .-: --

b) Increasing the availability of several plant nutrients

through its favorable impact on the physical, chemi-

cal and biological properties of the soil.

c) Minimizing losses of soil organic matter and reduc-- . ..r· ing soil compaction and erosion.

d) Green manuring is employed in the reclamation of _ ...

salt-affected soils.---:: ...

e) Green manures enhance the efficiency of chemical

fertilizers in increasing crop yields when used as

supplement to chemical fertilizers.

Ancient Chinese farmers had used the practice of green manuring in I •

soil productivity improvement more than 2000 years ago. Green

manuring is simply the incorporation of undecomposed green plant

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materials into the soil in order to improve soil productivity. The

material may either be grown in situ using quick-growing plant spe­

cies or may be harvested elsewhere and brought in. The most com­

monly used green manure species belong to the legume family. Plate5

1 to 10 show some commonly used green manure crops available in

Nigeria.

Green manuring is different from short- or long-duration cover crop­

ping in which a crop (usually a legume) is sown primarily to protect

the soil from erosion, suppress weeds, conserve moisture as well a5

improve soil N status and fertility. It is also different from rotational

ploughing-in of green crop residues such as mungbean and cowpea,

which also contributes to soil fertility improvement through deca)

of the root system with its associated nitrogen-fixing nodules. In

practice, however, it is difficult to make a clear distinction between

the three practices.

3.0 Cultivation Practices

Normally, a green manure is grown in monoculture

for various periods and incorporated into the same field. A

green manure crop may also be grown as an intercrop or in

mixed cropping. Alternatively, short duration varieties o1

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crops such as mungbean and cowpea are grown for the dual

purpose of grains and green manuring, the residues after pick­

ing the pods being used for green manuring the subsequent

crop. In the alley - or hedge or avenue - cropping system,

food crops are grown in alleys formed by hedgerows of trees

or shrubs. The hedgerows are periodically cut back at plant­

ing and used as green manure.

The cultural practices commonly employed in green manur­

ing under tropical conditions are described below.

3.1 Method of sowing

Green manures are usually broadcast and covered by

one of the following basic methods:

a) Broadcast over ploughed field followed by planking (lev­

eling) to cover the seed with soil.

b) Broadcast on a leveled field followed by incorporating

the seed with harrow or cultivator and then planking.

c) Broadcast on an uncultivated field after rains and mixed-

in with a cultivator followed by planking.

Seeds can also be sown by manual drilling in rows

about 20 - 30cm apart.

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3.2 Time of Sowing:

Green manure crop is normally grown for a period oJ

6 - 8 weeks before planting/transplanting the main crop. It i~

important that the green manure crop is not allowed to gro\\o

woody.

3.3 Seed Rate:

In order to delay development of woodiness, a high

seed rate is recommended for green manuring. In reclama-

tion of salt affected soils higher seed rates are recommended

because germination is likely to be poor on such soils. The

seed rates for some green manure crops are given in Table 1.

3.4 Inoculation:

Most legumes can produce root nodules and fix ni­

trogen without prior inoculation with Rhizobium. However,

where such a crop is introduced for the first time, it may be

necessary to inoculate. This will enhance the onset and num­

ber of effective nodules and increase the amount of nitrogen

fixed by the legume.

3.5 Fertilizer application Generally, leguminous green manures do not require

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supplemental N. However, a starter dose of 15-25kg N ha-1

is beneficial in N-deficient soils, and in soils with low or­ganic carbon content.

Most leguminous green manures utilize soil P quite well. However, on P-deficient soils, green manures respond to P application in terms of N-accumulation and shoot dry weight.

3.6 Age of incorporation: Green manure crop should be turned into the soil not

later than the time of flowering. It is important that the green manure crop is incorporated into the soil when the plant is still tender and succulent before the on-set of woodiness. Usually the best time of incorporation for most crops is 6 to 8 weeks after sowing, at which time there is sufficient biom­ass and the plants are not yet woody.

3. 7 Method of incorporation For rapid decomposition, it is important to incorpo­

rate green manure into soil containing sufficient moisture. Complete incorporation of green manure into the soil is im­portant for full effectiveness, however this may be difficult to accomplish in practice. Standard tractor-drawn disc har­row can be used for incorporating green manures. Soil-in­verting plough can also be used after the crop has been cut and laid flat in the field. Where these implements are not available or affordable, as is the case with most resource -poor Nigerian farmers, simple hand hoes could be used.

3.8 Interval between incorporation and crop establishment. A period of between 2 to 8 weeks is necessary be­

tween time of green manure incorporation and planting/trans­planting of the main crop. Where there is adequate moisture and high temperature decomposition takes place faster, there­fore the period between incorporation and planting/transplant-

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ing is short. 4.0 EFFECT OF GREEN MANURING ON SOIL PROPER­

TIES 4.1 Soil physical properties:

By improving the organic matter content of the soil, green manuring improves a number of soil physical proper­ties such as aggregation, bulk density, water retention and transmission and reduce soil erodibility, thereby encourag­ing greater penetration and proliferation of crop roots.

4.2 Soil chemical properties: Apart from improving theN fertility of the soil, green

manuring also improves the availability of other nutrients. Carbon dioxide and organic acids released during decompo­sition of green manures act on insoluble soil minerals thereb) releasing nutrients into the soil solution. Green manuring ~ known to increase the levels ofP, K, Ca, Mg and Sin the soil solution.

4.3 Soil biological properties: Green manuring markedly increases soil microbial

activity. By providing energy and nutrients, green manure promotes the growth and activity of desirable micro-organ­isms that play key roles in transforming and liberating nutri­ents in the soil. The use of certain green manures is known to increase the sol populations of vesicular arbuscular mycorrhizal fungi, which improve phosphate uptake by crops. Some green ma­nures could reduce soil populations of nematodes. However, the use of certain green manures may also encourage popu­lation build-up of some pathogenic organisms.

4.4 Reclamation of salt affected soils

Green manuring has long been known to be a useful practice for reclaiming saline and sodic soils. Incorporating a salt-tolerant green manure crop brings about favorable

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changes in physico-chemical properties of such soils. Sesbania is known to do well on salt-affected soils; it with­stands drought and water-logging and is particularly well­suited to sodic soils. Incorporating a salt-tolerant green ma­nure crop results in favorable changes in the physico-chemi­cal properties of such soils. The reclamation of sodic soils essentially involves the re­placement of Na ions on the exchange complex with more desirable Ca ions. An increase in carbon dioxide production in the soil following he incorporation of readily-decompos­able plant material through green manuring enhances the solubility of Ca in the soil. This in turn helps replace ex­changeable Na resulting in an improved soil structure.

3.0 Conclusion Tremendous benefits are obtained from green manuring. 1bese

include N-accumulation in the soil through N fixation, absorption of nutrients from lower soil depths by the roots of the legumes which translocate them to the leaves and eventually the leaves on the soil surface also lower temperature, improve soil structure, wa­

ter-holding capacity, aeration, chemical and biological properties of the soil There are, however, some draw backs that could militate against furmers adoption of green manuring practices. Some of these drawbacks include:

a) Green manuring may not give immediate (cash or kind) income.

b) Theeffects of green manuring in tropical soils are of.. ten short-lived.

c) It is difficult to dig-in and incorporate green manures into the soil annually with hand tools.

d) Green manuring does not :fit the farmer's traditional mixed-cropping systems and is therefore not easily ac­cepted.

e) It requires what the fimner considers unnecessary la-

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bor. To solve some of these drawbacks it is suggested that

a) Grain legumes such as early cowpea (Vigna unguiculata) could be grown as sole crop. This will serve as food crop while also perfonning a manurial function on the subsequent crop.

b) Sole-cropping perennial legumes that can be harvested as food crops and their leaves used as green manures eg. Pigeon pea ( Cajanus cajan ).

c) Intercropping shade-tolerant legumes with different food crops, since this fits more into prevailing funning systems.

References: 1. Agboola AA (1974). Problems of improving soil fertility

by the use of green manuring in tropical funning systems. FAO Soils Bulletin 27: 147 - 164.

2. Agboola A A (1982). Organic manuring and green ma-nuring in tropical agricultural production systerm. Non­symbiotic N fixation and organic matter in the tropics. 12th Int. Soil Cong. Symposium,lbadan, Nigeria Pp198-222

3. Jibrin J.M. (1999). Increasing the phosphate use efficiency of crops on Nigerian acid soils through cultural practi~ . P~ 6. ;"~ ... l s - A-h.~ck t'b ..(.lio v("'"«.n;!J, ~~. "' ·~ ·

4. Meelu, O.P., Yadvinder- Singh and Bijay-Singh (1994). Green Manuring for soil productivity Improve­mentFAOWorldSoilsResourcesReportsNo.76

5. Reijn~es,c., B. Haverkort and A Waters-Baye (1992).

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250p.

Farming for the future. An introduction to Low-External-Input and sustainable Agricul ture. Macmillan Education Ltd., London

6. Sanginga N., Mulongoy K and Ayanaba A (1988). Response of leucaena/Rhizobiurn Symbio sis to Mineral Nutrients in South Western Nigeria. Plant soilll2:121-127.

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>late 1: Maize intercropped with Chamaecrista rotundifolia

'late 2: Cajanus cajan

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Plate 3: Centrosema pubescens

Plate 4: Crotalaria ochroleuca

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Plate 9: Clitoria ternatea

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TOOle 1: SeWing rae cf sare green mm~.re

crql>.

Oup Seed rate (Kg lD-1)

1. O:Mp;:a Cfligp:u1guiaictf:9 30-35

2 I.rolrb (I.diciJ ]Jllpll'(!l6) 30-40

3. Seslxnia cr:lieae 50-ffi

4. Pig;m rea ( CcjCI'TU5 a§ en) 30-40

5. Scyrem(~ rrm) 50-iU

6. MmgJxm 30-40

7. Seslxniarastraa 30- 40

8. Mampuiens 30-40

9. Qitcria terrrtea 30- 40

10. a~ 50-ffi

11. Plr:tsrolus vlig;ris 30-40

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Plate 7: Mucuna pruriens

Plate 8: Stylosanthes hamata

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Plate 5: Glycine max

Plate 6: Lablab purpureus

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