Freyer D N

Sangakkara

Weerasekera

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16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
Green manuring for tropical organic cropping – A comparative 
analysis 
Sangakkara, U R1, Weerasekera1, D N & Freyer, B2 
Key words: green manures, application, soils, yields, seasons 
Abstract  
Green manuring is an essential component of tropical organic farming. Field studies 
evaluated the use of three legumes as in situ or ex situ green manures, along with a 
nonlegume green manure and a control to ascertain their impacts on soil properties 
and yields of maize and mung bean grown in major (wet) and minor (dry) seasons.  In 
situ green manuring, especially with legumes, had the most beneficial impact on soil 
properties, while with ex situ methods, the use of leaves alone improved soil 
properties. Yields were increased to a greater extent by green manuring in the minor 
season, and the in situ system proved to be more beneficial. In ex situ green 
manuring, greater benefits were obtained by the application of leaves alone.  The 
impact of different green manures and their application methods is presented. 
Introduction  
Green manures are an ideal method of sustaining soil fertility in the tropics 
(Joergensen, 2002, Fageria, 2007), and in organic farming, for both soil fertility and 
microbial activity (Palm et al., 2001). Many studies in Asia (e.g. Katyal et al., 2001) 
highlight the value of green manures, although no studies report the benefits of ex situ 
or in situ application of the same material in comparison to that of only ex situ green 
manures on tropical upland crops. Field studies were carried out to compare how 
green manures grown in situ and ex situ affected selected soil properties and yields of 
maize (Zea mays) and mungbean (Vigna radiata) cultivated in the major and minor 
seasons of tropical Asia, in contrast to application of only ex situ manures. 
Materials and methods  
The study was carried out at the Experimental Station (418 m above sea level, 8°N, 
81°E) of the University of Peradeniya, Sri Lanka, located in the mid-country 
intermediate zone, over the period October 2004 to August 2005, to encompass the 
major (WET) and minor (DRY) season corresponding to the Northeast and Southwest 
monsoons. The soil was an Ultisol (Rhodoult) with a sandy clay loam texture. The site 
received 722 mm and 236 mm of rainfall in the major and minor seasons and the me 
an temperature and humidity were 29°C + 2.3°C and  69.5 + 2.33% 
  
The experiment had 9 treatments per species (maize and mungbean), namely ex situ 
application of gliricidia (Gliricidia sepium) or tithonia ( Tithonia diversifolia ) leaves, or 
twigs and leaves, in situ or ex situ application of crotolaria (Crotalaria juncea) or 
sesbania (Sesbania rostrata), and a control with no green manures, replicated three 
times in a randomized block design. 
 
 
1. Faculty of Agriculture, University of Peradeniya, Sri Lanka (ravisangakkara[a]sltnet.lk) 
2  Institute of Ecological Agriculture, University of Natural Resources and Applied Life Sciences, 
Vienna, Austria 
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
 
The land was well prepared manually in August 2004 and March 2005 on adjacent 
blocks, with 18 plots of 2x2 m demarcated per replicate. Seeds of crotalaria or 
sesbania were broadcast on the selected plots for the two crops, and populations of 
these two species were maintained in an adjacent block for ex situ green manuring. In 
early October 2004 or May 2005, with the rains of the major and minor seasons, the 
biomass of the two planted green manures were estimated and the selected green 
manures were applied in situ or ex situ  to the plots at a rate equivalent to 4 t/ ha of dry 
matter. The ex situ green manures – gliricidia and tithonia leaves or leaves and twigs, 
and sesbania or crotalaria ex situ were obtained nearby. C:N ratios of samples were 
determined. At 15 days after incorporation, soils of the plots were sampled to a depth 
of 30 cm using cores and analyzed for pH, bulk density, water holding capacity, and 
soil N by standard techniques as described by Anderson and Ingram (1993). 
Thereafter, seeds of maize (Var Ruwan OPV) or mungbean (Var MI5) were planted 
and maintained as per local recommendations without any chemical inputs. Seed 
yields were determined at crop maturity. The data was subjected to analysis of 
variance using a general linear model. 
Results and Discussion 
Sesbania and crotalaria had the highest N contents (Table 1) and the lowest C:N 
ratios, which are important in green manures. The inclusion of twigs, which is done by 
most tropical farmers in ex situ mulching, reduced N content and increased the C:N 
ratios of gliricidia and tithonia. Furthermore, tithonia had the lowest N contents and 
hence the highest C: N ratios thus indicating its inability to provide N to plants in 
organic systems, although it is known to provide P to crops (Cong and Merkcx, 2005). 
Tab. 1. Nitrogen and C: N ratios of the selected green manures 
Green manure N % C:N ratio 
Gliricidia leaves 3.25 18.4 
Gliricidia leaves and twigs 2.91 20.9 
Crotalaria juncea plants 4.08 14.8 
Sesbania rostrata plants 4.56 14.1 
Tithonia leaves 0.58 25.7 
Tithonia leaves and twigs 0.45 29.4 
Probability (n=24) 0.018 0.009 
 
Green manures had no significant impact on soil pH, although bulk density declined 
significantly when compared to the control (Table 2). In situ green manures reduced 
bulk density to the greatest extent. In ex situ mulching, leaves and twigs reduced bulk 
density more than when leaves alone were added, because of the higher lignin 
content of the former. The WHC followed the same trend as bulk density, and there 
was a significant positive correlation (r= 0.76*) between these two variables. Soil N 
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
was increased significantly by the legume green manures when compared to the 
control and tithonia. Again, in situ green manuring had a greater significant impact and 
also the use of leaves alone of gliricidia. This illustrated the benefits of in situ green 
manuring for enhancing soil properties and N. If ex situ manuring is adopted, the use 
of leaves alone would develop a better soil for organic farming 
Tab. 2 Selected soil properties at planting as affected by green manures in wet 
(S1) and dry (S2) seasons, Es = addition ex situ, IS = addition in situ 
Green 
manure 
Ad. pH 
(1:2.5 H2O) 
Bulk density 
Mg.m-3 
WHC % Soil N  
(% Dry wt) 
  S1 S2 S1 S2 S1 S2 S1 S2 
Gliricidia L* Es 6.14 6.23 1.24 1.25 18.4 18.7 2.15 2.02 
Gliricidia L & 
T 
Es 6.25 6.39 1.22 1.21 19.9 19.2 2.04 1.98 
Crotalaria Is 6.46 6.52 1.18 1.21 20.2 19.6 2.24 2.15 
 Es 6.38 6.27 1.21 1.24 18.5 19.3 2.14 2.05 
Sesbania Is 6.34 6.18 1.19 1.23 19.8 18.7 2.21 2.11 
 Es 6.15 6.22 1.23 1.25 18.6 19.1 2.11 1.99 
Tithonia L Es 6.50 6.58 1.29 1.31 19.5 19.4 1.76 1.77 
Tithonia  
L & T 
Es 6.24 6.43 1.24 1.25 18.2 19.8 1.62 1.65 
Control  6.15 6.13 1.35 1.36 15.6 14.8 1.58 1.54 
Probability 
(p=0.05) 
 0.049 0.057 0.038 0.024 0.046 0.018 0.021 0.017 
*L & T refer to leaves and twigs respectively 
Green manures enhanced yields of both crops, especially the lower yields in the minor 
dry season, which could be attributed to the enhancement of soil water holding 
capacity. The legume material when grown in situ had a greater beneficial impact. In 
ex situ green manuring, the use of leaves alone had the greatest beneficial impact. 
Tithonia, although it provides P to plants, could not have the same impact on yields as 
N is the most limiting nutrient in most tropical soils (de Costa and Sangakkara (2006). 
 
 
 
 
16th IFOAM Organic World Congress, Modena, Italy, June 16-20, 2008 
Archived at http://orgprints.org/view/projects/conference.html 
 
 
Tab. 3. Yields of maize and mungbean (kg  ha-1) in major and minor seasons as     
affected by green manures and method of application 
Green manure Addition Maize Mungbean 
  Major Minor Major Minor 
Gliricidia L Ex situ 3251 2471 956 674 
Gliricidia L & T  2433 1958 825 615 
Crotalaria In situ 3998 2941 999 701 
 Ex situ 3704 2665 921 756 
Sesbania In situ 3790 2781 1001 795 
 Ex situ 3410 2485 954 741 
Tithonia L Ex situ 3041 2104 825 642 
Tithonia L & T Ex situ 2534 1917 758 542 
Control  1844 1452 458 329 
Probability (p=0.05)  0.038 0.027 0.005 0.019 
 
Thus, the study highlighted the importance of green manuring for tropical organic 
cropping. Legumes, especially as in situ green manures, had a greater beneficial 
impact, especially in the minor dry seasons. If ex situ green manuring is adopted, the 
use of leaves is the best option rather than the common practice of adding the entire 
shoot.  
References  
Anderson, J M ., Ingram, J.S. (1993). Tropical soil biology and fertility. A handbook of methods. 2nd  
Ed. CAB International. Wallingford, U.K. 221p 
Cong, P T., Merckx, R (2005) Improving P availability in two upland soils of Vietnam using Tithonia 
diversifolia H. Plant & soil 269: 11 – 23. 
De Costa, W.A.J.M, Sangakkara, U.R. (2006). Agronomic regeneration of soil fertility in tropical 
Asian smallholder uplands for sustainable food production. J Agric. Sci (Camb) 144: 111 – 
133 
Fageria, N K. (2007). Green manuring in crop production. J Plant Nutrition 30: 691 – 719. 
Joergensen, R.G. (2002). Challenges to organi farming and sustainable land use in the tropics and 
subtropics. J. Agric. Tropics and Subtropics 103: 105 – 105 
Katyal, J.C., Rao, N.H., Reddy, M.N. (2001) Critical aspects of organic matter in the tropics: The 
example of India. Nut. Cyc. Agroecosys. 61: 77 – 88 
Palm, C.A, Gachengo, C.N., Delve, R.J., Cadish, G, Giller, K.E. (2001) Organic inputs for soil 
fertility management in tropical agroecosystems: application of an organic resource base. 
Agric. Ecosys. Env. 83: 27 – 42. 


Green manuring for tropical organic cropping -A comparative analysis

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Green manuring for tropical organic cropping -A comparative analysis

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