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close this bookSustainable Management of Soil Resources in the Humid Tropics (UNU, 1995, 146 pages)
close this folderVIII. Nutrient management
close this folderD. Nutrient cycling
View the document(introductory text...)
View the document1. Crop residue mulch
View the document2. Agroforestry systems

(introductory text...)

Nutrient cycling and re-use is an important strategy for sustainable crop production in the humid tropics. It involves returning nutrients removed by crops and animals to the soil for future use. In addition to crops and animals, soil fauna (e.g., earthworms, termites) also play an important role in cycling of several elements, including C, N. P. S. B. Cu. Zn, and Mo. Growing deep-rooted plants is important in order to cycle nutrients from the sub-soil by returning them through crop residue to the surface where the following shallow-rooted crops can use them. Used effectively, recycling can substantially reduce chemical fertilizer requirements. Some important recycling strategies outlined in Fig. 17 include crop residue mulch, deep-rooted perennials, and animal wastes.


Fig. 17 Strategies for recycling nutrients

1. Crop residue mulch

Crop residues contain substantial quantities of plant nutrients. The data in Table 31 show the nutrient composition of the crop residues of some crops grown in the humid tropics. The concentration in oven-dry tissue ranges from 0.58% to 4.0% for N. 0.1% to 1.1% for P. and 0.2% to 3.4% for K. Nitrogen and phosphorus concentrations are generally higher in legumes than in cereals. On a weight basis, the major plant nutrients contained in 1 Mg of crop residue may range from 15 to 60 kg of N. P. and K (Table 32).

The beneficial effects of returning crop residue as mulch on crop yield are well known. These benefits are due not only to the recycling of plant nutrients but also to improvements in soil moisture and temperature regimes, enhancement of soil structure, and erosion control. However, the use of crop residues as fertilizers is especially important to resource-poor farmers. Some examples of the beneficial effects of crop residue mulch on crop yields are shown by the data in Tables 33 through 37. The data in Table 33 show that compared with an unmatched control, crop yields were improved with any mulch material. Rice husks increased maize yield by 0.7 Mg/ha and cassava yield by 12 Mg/ha. The data in Table 34 on yam production on an acid soil in eastern Nigeria show that mulching significantly increased the yam tuber yield. Mulching increased tuber yield by 20% on both ridge till and flat seedbed.

Table 31 Nutrient composition of crop residues of some crops grown in the humid tropics

(kg/ha/yr)

Crop/species

N

P

K

C/N ratio

Cowpea stem

1.07

1.14

2.54


Cowpea leaves

1.99

0.19

2.20


Rice

0.58

0.10

1.38

105.0

Maize

0.59

0.31

1.31

55.0

Oil palm (processed fibre)

1.24

0.10

0.36


Sesbania leaves

4.0

0.19

2.0


Crotolaria spp.

2.89

0.29

0.72


Tephrosia spp.

3.73

0.28

1.78


Water hyacinth

2.04

0.37

3.40

18.0

Azolla spp.

3.68

0.20

0. 15


Typha spp.

1.37

0.21

2.38


(Modified from FAO, 1990)

Returning crop residue as mulch may also have synergistic effects with fertilizer use. The data from the eastern Amazon by Schoningh and Alkamper (1984) showed that crop residue mulches with low C:N ratios had more beneficial effects than those with high C:N ratios (Table 35). On an Ultisol in eastern Nigeria (Tables 36 and 37), the yield of plantain and bananas was drastically improved by residue mulch.

Table 32 Plant nutrients contained in 1 Mg of dry straw

(kg/Mg)

Crop/species

N

P

K

Total

Cowpea stem

10.7

11.4

25.4

47.5

Cowpea leaves

19.9

1.9

22.0

43.8

Rice

5.8

1.0

1 3.8

20.6

Maize

5.9

3.1

13.1

22.1

Oil palm (fibre)

12.4

1.0

3.6

17.0

Sesbania leaves

40.0

1.9

20.0

61.9

Crotolaria spp.

28.9

2.9

7.2

39.0

Tephrosia spp.

37.3

2.8

17.8

57.9

Water hyacinth

20.4

3.7

34.0

58.1

Azolla spp.

36.8

2.0

1.5

40.3

Typha.spp

13.7

2.1

23.8

39 6

(Recalculated from the data in Table 31)

Plantain yield was five times more with mulch than with chemical fertilizers alone. The data in Fig. 18 show that returning crop residue mulch enhanced the beneficial effects of fertilizer application in maize yield (Kang, 1993). Without fertilizer application, residue retention had little effect on maize grain yield.

Table 33 Crop yield response to 22 different mulch materials applied on Alfisols in Nigeria

(Mg/ha)

Mulch

Cassava (fresh roots)

Maize

Cowpea

Soybean

Bare soil (control)

16.4 def

3.0 e

0.6 a

0.6 de

Maize stover

16.4 def

3.3 cd

1.1 a

1.5 abc

Maize cobs

17.8; cdef

3.3 cd

1.1 a

1.4 abed

Oil palm leaves

17.1 def

3.2 cd

1.2 a

0.9 bcde

Rice straw

17.9 cdef

3.5 bed

1.0 a

1.5 abc

Rice husks

28.3 a

3.7 abc

1.1 a

0.8 de

Kikuyu grass straw

14.2 ef

3.3 cd

1.2 a

1.4 abed

Elephant/napier grass (Pennisetum)

16.6 def

3.3 ed

0.9 a

1.3 bed

Guinea grass

15.5 f

3.6 bed

2.1 b

1.5 ab

Andropogon straw

18.5 cdef

3.5 bed

1.0 a

1.2 bcde

Cattail straw (Typha)

16.7 def

3.1 cd

1.0 a

1.1 bcde

Cassava stem (chipped)

20.9 cd

3.8 abc

0.9 a

1.4 abcd

Pigeon pea tops

22.9 be

3.7 abc

1.1 a

0.9 cde

Pigeon pea stem (chipped)

19.9 café

3.5 bed

1.0 a

1.3 bcd

Legume husks

26.4 ab

4.4 a

1.0 a

1.5 abc

Soybean tops

22.9 be

4.2 ab

1.0 a

1.2 bcde

Hemp (Eupatroium)

18.8 cdef

3.6 abc

1.0 a

1.2 bcde

Mixed twigs (chipped)

18.5 cdef

3.4 bed

1.0 a

1.2 bcde

Sawdust

20.5 cde

3.7 abc

0.9 a

1.9 a

Black plastic

30.5 ab

3.0 cd

0.9a

1.1 bcde

Transluscent plastic

27.7 ab

2.7 d

1.0 a

1.1 bcde

Fine gravel

22.9 be

3.1 cd

1.0 a

1.0 bcde

Figures followed by similar Ietters are stastically similar within vertical roust (Okigbo and Lal. 1980)

Table 34 Effects of tillage methods and mulching on yield and yield components of yam tubers in eastern Nigeria

Treatment

Diameter (cm)

Length (cm)

Number

Mean tuber yield (Mg/ha)

Ridge, mulch

16.7 a

23.5 b

9936 b

15.4 a

Flat, mulch

17.1 a

21.7a

12916a

16.1 a

Ridge, no mulch

13.3 b

20.7 a

10385 b

12.8 b

Flat, no mulch

13.8 b

20.6 a

10128 b

13.4 b

Figures followed by similar letters are statistically similar within vertical rows.

(Maduakor et al., 1984}

Table 35 Yield response of maize and cowpea to different mulch materials and mineral fertilizer in an eastern Amazon Oxisol, Capitao Poco (CPATU) Para, Brazil, 1983

(kg/Mg)

Mulches used (10 Mg/ha of dm)

First crop (maize)*

Second crop (cowpea)**


NPK (kg/ha) 20-80-60

NPK 0-0-0

NPK 30-80-60

NPK 0-0-0

Elephant grass

4646

2144

1227

80

Pueraria

5697

3342

1187

114

Weeds

4911

2215

1394

105

Sec. Forest (2 3 years)

4462

1560

1191

35

Sec. Forest (4 5 years)

4479

1807

1397

95

Rice husks

4398

1146

1487

163

Maize cobs + husks

4863

2101

1302

41

Bare soil

3539

78

1169

7

LSD. 5%

987

281

LSD. 1%

1321

376

LSD. 0.1%

1729

493

* Grain moisture content 14. 5%
** Grain moisture content 13, %

(Schoningh and Alkamper 1984)

2. Agroforestry systems

Agroforestry systems involve growing woody herbaceous species and perennials in association with food crops and livestock on the same piece of land. Agroforestry systems have been described extensively in several reports (i.e., Kang et al., 1981, 1989, 1990; Harwood, 1987;
Nair. 1989; Szott et al., 1991). They are known to increase ecological diversity within a landscape unit and optimize the use of limited resources through the integration of complementary components. There are three principal types of agroforestry systems (Fig. 19).

Table 36 Effect of mulching and fertilizer on yield of plantain and banana on an acid soil in eastern Nigeria

(Mg/ha)

Treatment

Plantain

Banana


Giant

Medium

No mulch, no fertilizer*



No mulch, fertilizer

18.0 a

16.7 a 7.5 a

Mulch, no fertilizer

17.2 a

15.8 a 9.5 a

Mulch and fertilizer

31.3 b

19.8 a 13.3 b

* Most plants broke.

Numbers in the same column followed by the same letter are not significantly different at 5%.

(IITA. 1981)

Table 37 Comparative effects of fertilizer and mulch on plantain yield on an acid soil in eastern Nigeria

Parameter

Mulch

Fertilizer

Yield (Mg/ha)

22.8

4.8

Bunch weight (kg)

11.8

8.1

Plants harvested (% of planted)

116.0

36.0

Harvest duration (months)

10.0

6.0

(IITA. 1981)

(i) Agrisilvicultural: This system involves simultaneously growing crops and trees on the same piece of land. Some commonly used agrisilviculture systems include alley cropping (Plate 38) and hedgerow cropping.


Fig. 18 Synergistic effects of using fertilizers in combination with returning crop residue

* N0P0K0, = control with no fertilizer. (a) years 1 4: N1 = 80 kg N/ha, N2 = 160 kg N/ha; (b) years 5 8: Ni = 100 kg N/ha, N1 = 200 kg N/ha; (c) years 9 10: Ni = 75 kg N/ha, N2 = 150 kg N/ha. P1 = 30 kg P/ha. P2 = 60 kg P/ha, K1 = 40 kg K/ha. K2 = 80 kg K/ha.

(Kang, 1993)*


Fig. 19 Types of agroforestry systems

(ii) Silvopastoral: This system involves raising livestock on improved pastures grown in association with trees. Some commonly used systems are alley farming and live fences (Plate 39).

Table 38 Commonly recommended species for agroforestry systems in the humid tropics

Species

Growth characteristics

Uses

Acioa bateri

Fast-growing shrub

Alley cropping, nitrogen fixation

Albizia falcate

Tree grows to 30 m

Erosion control, nitrogen fixation

Albizia lebbeck

Tree grows to 25 m

Erosion control, nitrogen fixation

Anthonotha

Fast-growing shrub

Alley cropping, nitrogen fixation macrophylla

Calliandra calothyrsus

Fast-growing shrub to 8 m, on acid soils

Alley cropping, nitrogen fixation

Cassia siamea

Shrub grows to 8 m. vigorous coppicing

Fuelwood, nitrogen fixation, lumber

Erythrina spp.

Tree grows to 20 m, often thorny, coppices shell

Live fences, nitrogen fixation, fuelwood, fodder

Flemingia macrophylla

Shrub grows to 3 m

Alley cropping, nitrogen fixation

Gliridia sepium

Fast-growing tree to 20 m. vigorous coppicing

Alley cropping, nitrogen fixation. forage, fodder. staking material

Inga spp.

Nitrogen-fixing shrub, acid- tolerant

Alley cropping. nitrogen fixation

Leucaena leucocephala

Tree grows to 20 m, fast-growing on non-acid soils. vigorous coppicinig

Fodder. fuelwood. erosion control, nitrogen fixation, alley cropping. staking material

Panagomia pinneta

Small tree. grows to 8m

Erosion control live hedges

Sesbania spp.

Fast-growing loss: tree

Erosion control, nitrogen fixations

(NRC, 1993a)

Table 39 Merits and limitations of agroforestry systems

Merits

Limitations

Reduction in fallow period and high cropping intensity over longer time period

High labor input

Erosion control and runoff management

Highly skilled management

Strengthening of nutrient cycling mechanisms leading to savings in fertilizer use

Low yields due to allelopathic effect and competition among trees and food crops for light, water, and nutrients

Alternate products (e.g., fodder, fuel, staking water mulch and food crops)

Limited application on soils of moderate to low soil fertility

Saving, land and decrease in need for clearing new land

Potentially high risks of pests and diseases

Improved traditional system, therefore, ecologically compatible

Difficulties in adoption due to traditional land tenure system

(iii) Agrisilvopastoral: This system involves a three-way mixture based on a combination of crops, trees. and animals. Such a system requires skillful management, and can be sustainable even in harsh environments and fragile soils.

A wide selection of tree species and woody shrubs can be used for agroforestry systems (Table 38). Some of these trees are suited for acid soil conditions and others for erosion control, some are more appropriate as forage trees, and still others are useful for pruning to be used as mulch. The choice of appropriate species is critical to the success of agroforestry systems. In addition to the intended use, the choice of tree and associated crop species also depends on cultural and ethnic factors of social importance.

The merits and limitations of agroforestry systems are shown in Table 39. A principal advantage of these systems is the reduction in the length of the fallow period and the potential for continuous and intensive cropping. Agroforestry may facilitate intensive land use for multiple uses on relatively fertile soils. It may also enable relatively more intensive use on steep lands and marginal soils, which cannot be used otherwise. A major advantage of agroforestry systems on sloping lands is erosion control. Closely spaced contour hedgerows of suitable woody perennials and shrubs can drastically reduce the risks of runoff and accelerated soil erosion. The data in Table 40 indicate large reductions in runoff and soil erosion with hedgerows of Gliricidia and Leucaena established at 2 and 4 m intervals.

Table 40 Alley-cropping effects on runoff and soil erosion from maize-cowpea rotation measured in 1984

Treatment

Runoff

Erosion (Mg/ha/yr)


Total (mm)

Fraction of rainfall (%)


Plow-till

232

17.1

14.9

No-till

6

0.4

0.03

Leucaena 4 m

10

0.7

0.9

Leucaena 2 m

13

1.0

0.1

Gliricidia, 4 m

50

1.5

1.7

Gliricidia. 2 m

38

2.8

3.3

(Lal, 1989a)

Table 41 Net primary production of biomass for commonly recommended multi-purpose tree species in the humid tropics

Species

Net primary production of biomass (ka/ha/yr)

Acacia auriculiformis

3000-4000

Acacia mangium

2500 3500

Albizia falcata

4000-5000

Alchornea cordifolia

2000-3000

Calliandra calothyrsus

2500-3500

Cordia alliodora

2500-3500

Dalbergia latifolia

4000-5000

Erythrina poeppigiana

4000-6000

Gmelina arborea

1500-5000

Leucaena leucocephala

3000-5000

(NRC, 1993a)

Table 42 Nutrient composition of foliage of some trees and woody perennials grown with agroforestry systems in the humid tropics

(%)

Species

N

P

K

Cassia siamea (leaves)

1.91

0.18

1.03

Tephrosia sp.

3.73

0.28

1.78

gliricidia sp.

4 15

0.27

300

Leucaena leucocephala

3.85

0.17

1.46

Erythrina sp.

400

0.29

3.05

(FAO. 1190)

Another benefit of growing woody perennials and trees in association with crops is the large quantity of biomass produced. The net primary production of biomass for perennials ranges from 1.5 to 6 Mg/ha/yr (Table 41). This biomass is a valuable resource for small land holders of the humid tropics. In addition to its use as forage, fuel, and staking material, the biomass can also be returned to the soil as mulch for soil protection and nutrient recycling. The nutrient content of the foliage of some of these species ranges from 2% to 4% for N. 0.2% to 0.3% for P. and 1% to 3% for K (Table 42). Consequently, a substantial amount of plant-available nutrients can be added to the soil by returning 3 to 7 Mg/ha of biomass. The data in Table 43 show that nutrients contributed by the biomass returned to the soil from Leucaenu can be 275 to 440 kg/ha of N. 15 to 49 kg/ha of P. 133 to 264 kg/ha of K, 74 to 195 kg/ha of Ca, and 17 to 52 kg/ha of Mg. Nutrients contributed vary widely among different species (Table 44). Rather than prunings, even the litter fall from some trees can add substantial amounts of nutrients (Table 45). Not all the nutrients recycled in the biomass, however, are available to crops. The nutrient use efficiency may be only 20% to 30% or less.

Table 43 Amount of nutrients in Leucaena leucocephala prunings

(kg/ha)

Year

Nutrients


N

P

K

Ca

Mg

Total

1985

440

49

264

195

52

1489

1988

408

37

244

155

29

873

1989

275

15

133

74

17

514

1990

281

16

188

106

26

617

(Hauser and Kang, 1993)

Table 44 Estimated nutrient addition through prunings of four woody shrubs grown at 4 m intervals on an Alfisol in western Nigeria

(%)

Species

Biomass yield (Mg/ha/yr)

Nutrient yield



N

P

K

Ca

Mg

Total

Acioa barteri

30

41

4

20

15

5

85

Alchornea cordifolia

4.0

85

6

48

42

8

189

Gliricidia sepium

5.5

169

11

149

104

18

451

Leucaena leucocephala

74

247

70

184

98

16

565

(Kang and Wilson. 1987)

Table 45 Biomass and nutrient addition by litter fall of a three-year-old plantation of Cassia siamea

(kg/ha)

Month

Leaf fall (Mg/ha)

Nutrient addition



N

Ca

Mg

K

January

5.9

110.3

88.3

13.0

39.1

February

5.7

106.4

85.2

12.6

37.7

March

2.4

43.7

35.0

5.2

15.5

April

1.5

28.1

22.5

3.3

10.0

May

1.9

35.1

28.1

4.1

12.5

June

1.4

25.8

20.7

3.0

9.2

July

2.1

40.0

32.0

4.7

14.2

August

3.4

62.5

50.1

7.3

22.2

September

9.0

167.8

134.4

19.8

59.5

October

9.6

179.3

143.6

21.2

63.6

November

12.7

236.5

189.7

28.0

84.0

December

16.4

305.0

244.3

36.1

108.2

Total

172.0

1340.8

1073.9

158.4

475.7

(Ghuman and Lal. 1990)

Table 46 Grain yield of maize alley cropped with Leucaena leucocephala hedgerows and in plots with no hedgerows (control) on an Alfisol in southwestern Nigeria, and absolute and relative yield advantages of alley cropping

(kg ha)

Grain yield


1985

1986

1988

1989

1990

Mean

Alley cropping

4890 a

3470 a

5657 a

4141

5376

4707

Control

3990 b

2240 a

5084 b

3353

4032

3740

Difference

900

1230

573

788

1344

967

Relative %

2.6

54.9

11.3

23.5

33.3

25.8

(Kang., 1993)

Table 47 Effects of agroforestry systems on relative grain yields of cowpea a tropical Alfisol

Treatment

Relative grain yield for different years *


1982

1983

1984

1985

1986

1987

Mean

Plow-till

128

78

79

77

176

65

100

No-till

270

147

212

139

177

38

164

Leucuena, 4 m

177

91

103

73

51

39

89

Leucaena, 2 m

129

57

89

28

26

42

62

Gliricidia 4 m

168

106

119

105

80

37

103

Gliricidia, 2 m

124

95

120

72

41

40

82

* Relative yield is calculated as a ratio of actual yield to average yield of all seasons and all treatments. expressed in percent. No fertilizer was applied.

(Lal 1989a)


Fig. 20 Grain yield as affected by position of the crop row from the perennial hedgerow

Table 48 Grain yield of rice from an experiment conducted at Yurimaguas, Peru, with three species of shrubs

Species

Row from the shrub

Grain yield (kg/ha)

Inga

1

723


2

1942


3

2162


4

1986


5

1975


6

2052


8

1923


10

1875

Erythrina

1

1582


2

2014


3

1888


4

2059


5

2056


6

2044


8

2107


10

1997

Leucaena

1

1718


2

2084


3

1948


4

2128


5

2253


6

2321


8

2106


10

2419

(TROPSOIL.1987)

Because of added nutrients and other favorable soil physical factors, total production can be more with agroforestry than with simple crop-based, tree-based, or animal-based systems. However, the yield of individual components may be decreased. The data in Table 46 show that the yield of maize was the same or better with alley cropping than without. However, the data in Table 47 on cowpea yield show a significant yield reduction with alley cropping, probably due to allelopathic effects. The reduction in average cowpea yield was as much as 11% by Leucaena at 4 m intervals and 38% by Leucaena at 2 m intervals. Yield reduction may also happen due to competition for nutrients between perennials and annuals. An example of the competition is shown by the data in Table 48 on an acid soil at Yurimaquas, Peru. Rice crop yields increased with wider spacing between hedgerows, and with increase in distance from the hedgerow (Fig. 20). The grain yield of the row next to the hedge of Inga or Erythrina was about 1300 kg/ha. The yield was about 1500 kg/ha or less for the fourth spacing The grain yield increased to more than 2000 kg/ha for the eighth row from the hedgerow. The grain yield of the row next to the hedgerow decreased by about 40% regardless of the hedgerow species. In addition, the high labor requirement is another limitation of agroforestry systems. The system is labor-intensive and complete mechanization is often difficult and not feasible to achieve.