Productivity and water balance of soybean-based systems on a Vertic Inceptisol watershed

Background

Vertic Inceptisols, which occur in association with Vertisols in a toposequence, occupy about 60 Mha area in India. These soils have similar physical and chemical properties as the Vertisols, except that these are shallower and somewhat lighter in texture and occur on slopes not exceeding 5%. Because of their location in toposequence, Vertic Inceptisols are prone to severe land degradation. Major constraints for crop production on these soils are a high runoff of rainwater and associated soil erosion, depletion of nutrients and beneficial organisms leading to decline in crop productivity. Various land surface management practices, e.g., tillage, ridges- and-furrows, broadbed-and-furrows (BBF), etc., for Vertisols have been investigated in India to control the flow of excess rain water, thereby minimizing soil erosion and increasing infiltration. While improved landform systems have been reported to decrease runoff and soil erosion, concomitant yield improvements of crops have not been achieved in various field studies. To provide answers to some of these questions, we studied the productivity and resource use of soybean-based cropping systems on a Vertic Inceptisol watershed at the ICRISAT research center, Patancheru.


The Watershed Experiment

On the basis of a topographical survey a small watershed of 4.7 ha was designed and developed. The general slope of the land was less than 2%. Because of the natural variability in soil depth (the depth of the black soil material), the whole watershed area was divided into shallow (<50 cm soil depth) and medium-deep (>50 cm soil depth) blocks. Each block was further divided into two parts to which two landform treatments, i.e., sowing on broadbed-and-furrow (BBF) and flat systems were assigned. The whole watershed thus consisted of four hydrological units arising from the factorial combination of two soil depths and two landforms, which were: (1) flat shallow, (2) BBF shallow, (3) flat medium-deep, and (4) BBF medium-deep. Two cropping systems, namely soybean-chickpea sequential system and soybean/pigeonpea intercrop system, were sown in each of these hydrological units (Fig. 1). Glyricidia ws planted on the bunds of the plots laid to BBF system. Glyricidia loppings and soybean crop residues were added to BBF plots during the rainy season to provide additional nitrogen. Sowing of crops in the BBF system was done on a 0.8% grade, while in the flat system it was done along the contour lines. Observations were taken on crop growth, light interception, surface runoff and soil water changes.

Figure 1. Soybean-chickpea sequential system and soybean/pigeonpea intercrop systems sown in BW7 at ICRISAT Center, Patancheru, India.

Climatic data were recorded daily from the class ‘A' agrometeorological observatory situated adjacent to the watershed. Interception of photosynthetically active radiation (PAR) in various treatments was determined with a line quantum sensor (LI-COR instruments, USA). Plant samples were taken at 7-10 days intervals for growth analysis. To monitor changes in soil water content neutron probe readings were taken in each subplot at every 7-10 days intervals. Runoff from each hydrological unit was measured with automatic water stage recorders. The yields of rainy and postrainy season crops at final harvest were determined based on large area samples. Because of simultaneous occurrence of runoff and deep drainage during the rainy season the daily water balance components of crops (evapotranspiration and deep drainage) for each hydrological unit were estimated using the water balance model of Ritchie (1998). During the postrainy season there was no surface runoff and deep drainage. Water use (evapotranspiration) by the postrainy season crops was equal to profile water depletion from sowing to harvest, plus any rainfall during the crop growth period.


Results

Crop growth, leaf area index, and light interception: Crops grown during the rainy season on flat landform on medium-deep soil had a higher leaf area index and more light interception compared to those grown on the BBF landform, except on the shallow soil where the differences were inconsistent. Crop growth during the postrainy season was primarily determined by the soil depth (available water) than by landform treatments.

Water balance and soil erosion: Because of greater time of concentration, total runoff and peak runoff rates were lower on the BBF landform compared to those on the flat landform. Soil erosion was lower in the BBF compared to that in the flat landform (Table 1). BBF landform decreased surface runoff and increased deep drainage on both the soils compared to the flat landform treatment. Water use by the 2-crops ranged from 50 to 100% of seasonal rainfall over the years (Table 2). Surface runoff and deep drainage water was captured in surface tanks and dug wells to provide irrigation to horticultural crops. Overall rainfall use efficiency on watershed basis was greater than 50% in most years.

Table 1. Measured runoff, peak runoff rate, and soil loss in two landform treatments on a Vertic Inceptisol watershed, ICRISAT Center.
	Rainfall (mm)	Runoff (mm) BBF          Flat	Peak runoff rate (m3 s-1 ha-1) BBF          Flat	Soil loss* (t ha-1) BBF          Flat
Medium depth (1995-99 season)
Mean Range	805 546-1043	150        188 1-232      3-290	.082              .096 .003-.135    .003-.145	1.5        2.2 --           --
Shallow depth (1996-99 season)
Mean Range	854 546-1043	128        160 2-251      2-283	.081              .136 .003-.130    .004-.235	1.2        2.1 --           --
*Only 1998 season data.

 

Table 2. Simulated soil water balance of the soybean-chickpea sequential system in a Vertic Inceptisol watershed, 1995-1999 seasons, ICRISAT Center.
	Rainfall (mm)	Water balance components (% of rainfall) Surface runoff                   Deep drainage          Crop water use BBF               Flat              BBF                Flat             BBF              Flat
Medium depth
Mean Range	758 532-973	14        18 0-24      1-30	15              13 0-25          0-21	74        74 57-100  58-100
Shallow depth
Mean Range	758 532-973	13        17 0-26      1-31	25              21 7-36           4-33	70        71 51-100  52-100

Nitrogen balance: Integrated nutrient management (INM) followed in improved system (sowing on BBF + Glyricidia on bunds) resulted in balanced N budget for soybean + chickpea sequential and soybean/pigeonpea intercrop system (Table 3). Glyricidia loppings provided 25 kg N ha-1 yr-1 without adversely affecting crop yields of the nearby rows. Pigeonpea derived up to 89%, soybean up to 75%, and chickpea up to 42% of their N requirement through BNF. In the conventional system (sowing on flat) relying only on biological source resulted in depletion of soil N (about 50 kg ha-1 yr-1) during first four years.

Table 3. Average nitrogen balance of soybean-based cropping systems in a Vertic Inceptisol watershed, 1995-98 seasons, ICRISAT Center.
	Cropping systems Soybean-chickpea                              Soybean/pigeonpea BBF                    Flat                          BBF                   Flat
Total 'N' uptake (kg ha-1) Total 'N' loss (kg ha-1)(runoff + deep drainage) 'N' additions (kg ha-1)(rainfall, fallen leaves, roots & BNF) 'N' additions (kg ha-1)(compost, Glyricidia loppings) 'N' balance (kg ha-1)	197 13.4 165 45 0.0	198 17 167.7 0 -47.3	220 14.1 200 44 +10	214 17.4 182.7 0 -48.7

Crop yields: Total productivity of the soybean-chickpea sequential system ranged from 2.5 to 2.8 t ha-1, and that of soybean/pigeonpea intercrop system ranged from 2.0 to 2.2 t ha-1 (Table 4). These yields are much higher than the soybean yields (<1.0 t ha-1) reported for the target region of Madhya Pradesh, India. Landform treatments did not influence the yields on soil types.

Table 4. Grain yield (t ha-1) of soybean-chickpea sequential and soybean/pigeonpea intercrop systems in a Vertic Inceptisol watershed, 1995-99 seasons, ICRISAT Center.
	Cropping systems Soybean-chickpea                              Soybean/pigeonpea BBF                    Flat                          BBF                   Flat
Medium depth
Mean Range	2.8 2.2-3.6	2.8 2.0-3.7	2.0 1.2-2.4	2.2 1.3-2.9
Shallow depth
Mean Range	2.5 2.0-3.3	2.5 1.9-3.3	2.1 1.5-2.4	2.0 1.3-2.7

Conclusions

These results indicate that while the BBF system is useful in decreasing runoff and increasing infiltration of rainfall on Vertic Inceptisols, there is a need to increase light use by crops on BBF during the rainy season to increase their productivity. The productivity of postrainy season crops can be increased by developing practices that will increase soil water extraction by crops. Integrated use of nutrients from the organic sources resulted in balanced N budget of soil. A watershed-based farming system needs to be adopted to capture significant amount of rainwater lost as runoff and deep drainage. The stored water in the surface ponds and dug-wells can be used for supplemental irrigation to increase productivity of soybean-based systems leading to overall increases in resource-use efficiency, crop productivity, and sustainability.

For more information please contact:

Dr Piara Singh
Senior Scientist
(Agroclimatology and Systems Modeling Unit),
ICRISAT-Patancheru 502 324.