Assessing soil fertility and decomposition patterns of G. sepium green manure at three slope categories of Sri Lanka’s hill country

The hilly region of Sri Lanka is characterized by terrain of different slopes, cultivated by smallholder farmers. Due to its latitudinal position, Sri Lanka is under the influence of two monsoons, which define the minor and the major crop seasons. The intensity of cultivation varies on the basis of slopes and rainfall patterns. However, during the major crop season (October to March), farmers cultivate all types of terrain with crops such as maize for consumption and sale of surplus.

Soils of the humid tropics are characterized by low amounts of organic matter and thus, nitrogen often becomes the most limiting nutrient in these systems. Thus, farmers try to maintain soil quality through the incorporation of green manures. The tropical legume tree called Gliricidia sepium is widely used as green manure in Asian tropics and especially in Sri Lanka. This tree produces large quantities of good quality biomass within a short period of time, as well as fuel wood for sale.

The farmer fields present on the test site of Meegahakiula were classified into three slope categories (flat: < 10 %, moderate: 10-30 %, steep: > 30 %). These slope categories were considered to have an influence on the present soil profile and on the chemical and physical parameters of soil fertility in the fields. The different inclinations were also thought to have an impact on the decomposition pattern of the green manure in the farmer fields. Due to depth specific macroclimates, variation in soil decomposer populations and in chemical and physical properties of the soil, the depth of G. sepium placement was suggested to have an impact on the leaf decomposition. These assumptions were studied within a master thesis conducted over the major crop season 2007-2008.

 

Soil fertility assessment

The three slope categories had an impact on the state of soil fertility in the fields. However, clear differences in the soil properties were observed only within the upper 20 cm to 30 cm of the soil profiles. The knowledge gained from the soil analyses of the upper soil layers seems to provide enough information to explain the relationship between the crop yield and the state of soil fertility in the three slope categories.

Soil degradation, as measured by soil organic matter, varied with the slope categories. However, the carbon content found in the upper soil layers of the steep slope category was higher than in the flat slope category. These observations pointed out that anthropogenic accelerated erosion may sometimes overweight the effect of natural erosion, which was expected to be highest in steep lands. Therefore, the hypothesis stating that the organic matter content is lowest in the steep slopes had to be rejected.

The soil inclination had an impact on the thickness of the Ap horizon. In some focal points of the steep slope category, the Ap horizon was non existent and the A0 horizon was very thin, thus suggesting the action of superficial erosion. Missing Ap horizons as well as very thin A0 horizons were the most relevant proof of active erosion forces on the test site. The soil depths observed in the three slope categories were in the same range and did not indicate any prominent influence of erosion on deeper soil layers.
Decomposition patterns of G. sepium at the three slope categories

The decomposition rate of G. sepium, as measured by the biomass decline of organic material in the soil, was marginally higher in the moderate when compared to the flat slope category. However, if this was true for the biomass decline, there was no impact of the inclination on the rates of carbon and nitrogen release. This probably allows the consideration that the three slope categories do not have any relevant impact on the availability of crop nutrients from organic inputs.

As observed within the progression of this study, the state of soil fertility depended on a big range of factors and could not be explained through the impact of the topography as a unique factor. The cropping history, the agricultural practices as well as the income and even the interest of the farmers probably had a considerable impact on the soil fertility level at each focal point. These factors influenced in turn the decomposition of the green manure applied to the fields

 

Decomposition patterns of G. sepium at the three placement depths

The decomposition of G. sepium green manure was not as rapid as hypothesized and less than half of the leaf material decomposed within the five week duration of the experimental period. The precipitation pattern being unforeseeable, the farmer is confronted to a considerable uncertainty regarding the synchrony between nutrient release and crop needs. Even with a good knowledge of the crop nutrient demand and of the green manure decomposition pattern, asynchrony may happen as the result of a lack of precipitation before the period of high crop nutrient demand.

Depending on the way of green manure application, as a surface mulch or incorporated at 15 cm and 30 cm depth by tillage, the decomposition pattern of the leaf material was different. Therefore, the hypothesis of an impact of the placement depth on the decomposition pattern was confirmed.

Considering the high rate of nitrogen release when green manure is applied as soil mulch and the rate of biomass decline which did not differ from the incorporation at 15 cm and 30 cm, green manure application at the surface is recommended. For the farmers, this type of application is also the most comfortable. The pattern of nutrient demand of the crop is given for each climatic sub-zone of the Tropics. Therefore, synchrony between the time of nitrogen release from the decomposing organic matter and the time of nitrogen needs of the crop can be achieved. Knowing that G. sepium leaf material used as surface mulch results in a nitrogen release peak in the second week after application, the time point of green manure application to the field can be adjusted to the time of sowing.

 

Figure 1 : Excavation of a soil profile in the flat slope category
(in the background: G. sepium legume trees; © E. Berset)

 

Swiss participant                                                            Sri Lankan institution

ETHZ                                                                                University of Peradeniya
Estelle Berset                                                                     Prof. Dr. U.R. Sangakkara
Master student in Environmental Sciences                        Department of Crop Science
Chemin de la Prairie 12                                                     Faculty of Agriculture
1720 Corminboeuf                                                            Peradeniya, 20400
Switzerland                                                                       Sri Lanka
                                                                                         
estelle.berset@gmail.com                                                  ravisangakkara@sltnet.lk