African Crop Science Journal, Vol. 7. No. 4, pp. 365-373, 1999
COMBINED INPUTS OF CROP RESIDUES AND FERTILISER FOR SMALLHOLDER MAIZE PRODUCTION IN SOUTHERN MALAWI
I.L. MWATO, A.B.C. MKANDAWIRE and S.K. MUGHOGHO
Code Number: CS99027
Chronic shortage of food exists in Southern Malawi because of low maize (Zea mays L.) yields due, in large part, to soil fertility depletion. An approach to ameliorating soil fertility is strategic use of crop residues and mineral fertilisers. An on-farm experiment was conducted in two contrasting locations, Kasonga (9 farms) and Songani (8 farms) that examined the effects and interactions of applying fertilisers and crop residues to soils. Crop residues consisted of maize stover and soybean (Glycine max (L) Merr.) trash of four cvs. Bossier, Ocepara 4, Kaleya and Magoye that were obtained from a previous crop at each farm. Average organic N inputs ranged between 15 kg N ha-1 for maize stover and 74 kg N ha-1 for soybeans and fertiliser N inputs were 0, 20, 40 and 60 kg N ha-1 with small amounts of accompanying P (0-13 kg P ha-1). ANOVA of maize grain yield revealed significant effects of location (P<0.001), fertiliser addition (P<0.001) and crop residue type (P=0.002). Overall grain yields were much larger at Songani (2374 kg ha-1) than Kasonga (812 kg ha-1). Fertilisation resulted in maize yields ranging between 901 kg ha-1 with no fertiliser to 1955 kg ha-1 with addition of 60 kg N and 13 kg P ha-1. ANOVA of total N uptake by maize revealed highly significant effects of location and fertilisation (P<0.001) and a significant effect of residue type (P=0.02). Overall N uptake by maize was 19.6 kg ha-1 at Kasonga and 62.7 kg ha-1 at Songani. The highest N uptake resulted from maximum fertilisation and with residues of soybean cvs. Bossier and Magoye. Use of fertilisers and soybean residues proved an effective strategy to increase maize yields but further studies that examine the economics of these interventions are necessary.
Key Words: Crop rotation, Glycine max, maize stover, nutrient recycling, smallholder farming systems, soil fertility
La pénurie chronique alimentaire existe dans le sud du Malawi à cause des rendements faibles du maïs (Zea mays L.) due en grande partie à lépuisement de la fertilité du sol. Une approche à lamélioration de la fertilité du sol est une utilisation stratégique des résidus des cultures et dengrais mineraux. Un essai en milieu réed a été conduit dans deux localités défférentes, Kasonga (9 fermes) et Songani (8 fermes) pour examiner des effets et des interactions de lapplication dengrais et résidus de cultures sur les sols. Les résidus de cultures étaient des fanes du maïs et des ordures de soja (Glycine max (L) Merr.) de quatre cultivars, Bossier, Ocepara 4, Kaleya et Magoye qui étaient obtenus des cultures précédentes à chaque ferme. La moyenne des intrants dN organique variait entre 15 kg N ha-1 pour les fanes du maïs et 74 kg N ha-1 pour le soja et les intrants dengrais dN étaient 0, 20, 40 et 60 kg N ha-1 avec une petite quantité accompagnante de P (0-13 kg P ha-1). ANOVA du rendement en grains du maïs a indiqué des effets significatifs de la localité (P<0.001), addition dengrais (P<0.001) et du type de résidu de culture (P=0.002). Générallement les rendement en grains étaient les meilleurs à Songani (2374 kg ha-1)plus que Kasonga (812 kg ha-1). La fertilisation a conduit aux rendements en grains variant de 901 kg ha-1 avec sans engrais à 1955 kg ha-1 avec addition de 60 kg N et 13 kg P ha-1. ANOVA de la consommation totale d N pour le maïs a révélé des effets significatifs de localité et de fertilisation (P<0.001) et un effet significatif (P=0.02) du type de résidu. La consommation totale d N par le maïs était 19.6 kg ha-1 à Kasonga et 62.7 kg ha-1 à Songani. La consommation la plus supérieure dN est provenue de la fertilisation maximale et avec les résidus des cultivars de soja, Bossier et Magoye. Lutilisation dengrais et des résidus de soja ont prouvé un effet stratégique pour augmenter des rendements du maïs, cependant des études future examinant léconomie de ces interventions sont nécessaires.
Mots Clés: Rotation de culture, Glycine max, recyclage des éléments nutritifs, fanes du maïs, fertilité du sol, systèmes culturaux des petits agriculteurs
Decline in soil fertility is a widespread limitation to yield improvement in many maize-based cropping systems throughout East and Southern Africa (Buresh et al., 1997). Mineral fertilisers improve soil fertility and result in increased yields but are expensive and often beyond the reach of resource poor farmers resulting in the chronic food insecurity in Africa (World Bank, 1996). Solutions to smallholder farmers soil fertility problems may be found in the strategic combination of organic resources, particularly from nitrogen-fixing legumes, with small amounts of mineral fertiliser (Palm et al., 1997). Soybeans (Glycine max (L) Merr.) are able to derive much of their nitrogen (N) requirement through biological N-fixation (BNF) in association with Bradyrhizobium spp.(Giller and Wilson, 1991). Estimates of BNF by soybeans vary between 40 and 206 kg N ha-1 annually (Franco, 1978). The N in soybean residues is rapidly mineralised and is efficiently utilised by subsequent crops (Power et al., 1986) although others report that soybean residues have higher C:N ratios and lignin contents than other field legumes (Giller et al., 1997). Soybean is widely perceived as a legume host that requires a specific rhizobia, Bradyrhizobium japonicum, and this bacteria is generally absent from African soils (Woomer et al., 1997). However, promiscuously nodulating soybeans that are associated with the near-ubiquitous cowpea type rhizobia are known and now being popularised in Southern Africa (Kasasa et al., 1998)
Promiscuous soybeans may have higher nitrogen contents in their crop residues because of the reduced translocation to the reproductive sink as evidenced by the lack of leaf chlorosis immediately before maturity (Mpepereki et al., 1996). Significant contributions to soil fertility improvement through BNF by promiscuous soybean varieties with relatively lower N harvest indices have been reported by Giller and Wilson (1991). Furthermore, legumes that do not require rhizobial inoculation offer considerable advantage to smallholder farmers with low incomes and poor access to legume inoculants and information about their use (Woomer et al., 1997). The potential of promiscuous nodulating soybeans to produce economic grain yields of up to 2 t ha-1 without inoculation was reported in Zambia (Giller and Wilson, 1991).
The objectives of this study were to compare the benefits of incorporating soybean residues from different promiscuous and specific soybean cultivars to the more widely-available maize stover and to determine whether these residues offer useful alternatives to mineral fertilisers. The studies were conducted through on-farm experimentation at two contrasting locations in Southern Malawi. The locations are characterised by densely settled smallholdings, histories of drought and soil nutrient depletion and chronic food insecurity (Kanyama-Phiri et al., 1997).
MATERIALS AND METHODS
On-farm experimentation. On-farm research was initiated during the 1996/97 cropping season to introduce promiscuously nodulating soybeans into maize-based cropping systems. The research was conducted on 17 smallholder farmers fields at Songani and Kasonga in Zomba district, southern Malawi. Experiments were arranged as Randomised Complete Block Designs. During the 1996/97 growing season, soybean varieties Magoye and Kaleya (promiscuous varieties) and Bossier and Ocepara 4 (specific varieties) were planted in farmers fields. Continuous maize monocrops were also established as controls where subsequent crops received additions of maize stover from the previous season. Each farmer grew either Magoye, Bossier and maize or Kaleya, Ocepara 4 and maize. Each plot had 8 ridges spaced 90 cm apart. The maize was planted at a spacing of 90 cm and 3 plants per hole. Soybeans were planted with two rows on a ridge at a spacing of 5 cm between plants. Harvesting was done in March 1997 in all fields. The plant samples of the different soybean varieties and maize from all plots were weighted, ground and analysed for nitrogen using Kjeldahl procedures and data were expressed as kg N ha-1.
During the 1997/98 cropping season, maize and soybean residues were incorporated into their respective plots before the onset of rains. Fertiliser was applied at different levels of 0, 20, 40, 60 kg N ha-1 as 50 % CAN and 50% compound fertiliser 23:21:0 resulting in addition of 0, 4.2, 8.4 and 12.6 kg P ha-1. Compound fertiliser was applied during land preparation and CAN was topdressed 3 weeks following emergence. Maize hybrid MH 18 was planted at the density described above. Plots were weeded according to farmer practises beginning 2 weeks after maize emergence. Maize plants were recovered at harvest maturity from sample plots 3.6 m x 1.8 m and divided between shelled grain, cobs and husks, and stover. These samples were weighed and analysed for total N by digestion as described above.
Data analysis. Data were compiled onto a computer spreadsheet with variables and measurements representing different columns and cases (farms) entered as rows. The data were inspected for accuracy and imported into a computer statistical software. Summary statistics were collected by stepwise sorting of data by location, residue type, fertiliser application rate and different combinations of these variables (e.g. location x fertiliser rate). ANOVA was conducted by testing location effects with replicates nested in locations and all other effects and interactions with the Mean Square Error. Linear regression was performed with total N uptake as a dependant variable and N inputs as the independent variable after sorting by location, residue type and or N fertilisation rate. Y-intercepts were used to estimate maize uptake of soil N and N use efficiency was calculated by dividing adjusted N uptake by their respective sources (crop residue N or fertiliser N).
RESULTS AND DISCUSSION
Maize grain yield. ANOVA of maize grain yield revealed significant effects of location (P<0.001), fertiliser addition (P<0.001) and crop residue type (P=0.002). Significant interactions were observed between location and both fertiliser addition (P<0.001) and residue type (P=0.02). Overall grain yields were much larger at Songani (2374 kg ha-1) than Kasonga (812 kg ha-1). Fertilisation resulted in maize yields ranging between 901 kg ha-1 with no fertiliser to 1955 kg ha-1 with addition of 60 kg N and 13 kg P ha-1. The location x fertiliser interaction resulted from a much greater yield response to fertiliser addition at Songani (+1588 kg ha-1 with maximum fertilisation) than Kasonga (+580 kg ha-1). The location x residue type interaction was based upon difference in maize performance with soybean cvs. Bossier and Ocepara 4 at the two locations. Application of soybean residues from Bossier resulted in the largest yields at Songani and the poorest in Kasonga, and vice versa for Ocepara 4. Incorporation of maize residues resulted in the poorest maize yields at both locations, 699 and 1912 kg ha-1 at Kasonga and Songani, respectively.
Components of maize yield at Kasonga and Songani at different rates of fertiliser addition are presented in Figure 1. This data averages the different residue additions and the Standard Error indicators represent variation in total biomass. Note that maize productivity with maximum fertilisation at Kasonga is less than unfertilised maize at Songani and that fertiliser response at Kasonga is somewhat attenuated. Maize grain yield for all treatments is presented for Kasonga (Fig. 2A) and Songani (Fig. 2B) with different scaling of the y axis. Residues of maize consistently resulted in lowest yields at both locations but the effectiveness of different residues from soybean cultivars varied between locations. Residues of Ocepara 4 resulted in relatively strong performance of maize at Kasonga, but not Songani but was extremely variable across the farms in many location x fertiliser rate treatments.
N uptake and use efficiency. ANOVA of total N uptake by maize revealed highly significant effects of location and fertilisation (P<0.001) and significant effect of residue type (P=0.02). Overall N uptake by maize was 19.6 kg ha-1 at Kasonga and 62.7 kg ha-1 at Songani. The highest N uptake resulted from maximum fertilisation (Table 1) and with residues of soybean cvs. Bossier and Magoye (51 and 49 kg N ha-1, respectively). There was no difference between the maize performance resulting from incorporation of soybean residues from promiscuously nodulated varieties (Kaleya and Magoye) and more microsymbiont specific ones (Bossier and Ocepara 4).
Partial N budgets of the maize crop receiving either stover or soybean residues at different rates of mineral fertilisation are presented in Table 1. Significant differences were observed for total inputs, uptake and offtake for both residue types. Note that N inputs from the organic inputs is constant across fertilisation rate with an average 15.1 kg N and 74.4 kg N ha-1 provided by stover and residues of all soybean varieties across all farms, respectively. Total N uptake by maize stover and soybean residues were 20.6 and 32.5 kg N ha-1 in absence of fertilisation, respectively. Total N uptake across both residue types and fertiliser rates correlated with N supplied from mineral fertiliser (r=0.23, P<0.001), suggesting that fertilisers featured in the N nutrition with both types of organic inputs. N offtake, that is the N contained in shelled grain, was 49% of total N uptake by maize.
Table 1. Effects of mineral fertiliser application and crop residue
incorporation on total nitrogen inputs, maize uptake of nitrogen and nitrogen
removed as grain based on 17 on farm trials in Southern Malawi
Linear regression of total N inputs across all fertiliser additions and maize N uptake were significant for addition of maize stover (N uptake = 16.90 + 0.41N input, r=0.33, P=0.006) and soybean residues (N uptake = 26.40 + 0.15 N input, r=0.31, P<0.001). This relationship was not significant for maize stover at Kasonga location suggesting that N immobilisation may have occurred. The specific relationship for N inputs derived from soybean cv. Kaleya combined with mineral fertilisers and maize N uptake is presented in Figure 3. Note that the slope (0.26) suggest the rate at which N inputs were assimilated.
Direct separation of use efficiency between residues, soil and fertilisers N sources was not possible owing to the absence of a complete control (no residues or fertilisers applied) in this on-farm experiment. However, it is possible to derive this value from the y-intercept of linear regressions of N uptake (dependent variable) and N inputs (independent variable). When these regressions are performed separately for Kasonga and Songani, soil N uptake values of 10.5 and 28.5 kg ha-1 are obtained (data not presented). The N use efficiency of the organic inputs may then be calculated for the unfertilised treatments by dividing adjusted (for soil N) N uptake by N contained in residues. In this case, maize stover and soybean residues were utilised at efficiencies of 0.10 and 0.25 at Kasonga and 0.14 and 0.24 at Songani, respectively. It is then possible to calculate the N use efficiency of fertilisers by adjusting total N uptake for both soil N and organic residue N supply and then dividing by the application rate of N fertilisers. When these operations are performed across all rates of fertiliser addition separately for treatments receiving maize stover at Kasonga and Songani, N fertiliser use efficiency was estimated to be 0.25 and 0.75, respectively. These findings again suggest N immobilisation at Kasonga. The same approach could not be conducted for treatments receiving soybean residues because adjustments for soil and residue N supplies accounted for most of the N uptake.
Strategies of crop residue management. One of the widely recognised benefits of crop rotation is that cereals often require less fertiliser N when grown following legumes (Lory et al., 1995). Findings of this study are supported by those of other researchers. Vanotti and Bundy (1995) reported that soybean production contributes significantly to N supply of the following maize crop and mean maize N uptake was higher in soybean-maize than maize-maize rotation. Soybean in rotation contributed to soil N and consequently increased the succeeding grain sorghum yield for 2 years where soil N was limiting (Bagayoko et al., 1992). Graham and Temple (1984) and Mughogho and Kumwenda (1991) reported that under appropriate conditions, N returned to the soil in residues can enhance the yield of subsequent crops under Malawian smallholder conditions.
Maize grown after Ocepara 4 gave higher yields at all fertiliser levels except at 60 kg N ha-1 in Kasonga (Fig. 2A). Addition of inorganic fertiliser N and crop residues improved the yield of subsequent maize grown after soybean from 0.5 t ha-1 up to 1.3 t ha-1. Addition of soybean residues increased unfertilised maize yields and the yield increased with each increase in mineral N up to yields of 4.2 t ha-1. It is important to note that the highest rate of fertiliser N application is below the 96 kg N ha-1 that is currently recommended by the Malawian Ministry of Agriculture through local extension. The results of this experiment agree with the reports of Peterson and Varvel (1989) and Bundy et al. (1993) that yields are often maximised with lower N rates of fertilisers in soybean-maize than in maize-maize rotations. These results suggest that an adjustment in recommendations for maize following soybeans is needed in some soybean-maize rotation systems to maximise N use from limited fertiliser resources in conjunction with use of crop residue. In addition, yields are often maximised with lower N rates in soybean-maize than in maize-maize system (Baldock et al., 1981; Crookson et al., 1991; Bundy et al., 1993).
Large yield differences were observed between Kasonga and Songani (Fig.1). This may be due to loss of N during cropping through leaching and erosion as the soils in the experimental plots at Kasonga had sandier surface horizons and were located on steeper slopes than those in Songani. Bundy et al. (1993) reported that soybean provides little N to subsequent crops on sandy soils and this was attributed to leaching of nutrients prior to use by the following crop. In addition the higher clay contents of soils in Songani are likely to result in higher inherent soil fertility, higher moisture and cation exchange capacities and have more favourable tillage and management characteristics that result in moisture conservation and erosion control.
The increases in maize yield in plots where soybean trash was returned into the soil indicate that the residues were beneficial to the following crop but a larger yield resulted from response to fertiliser application. However, soil fertility trials elsewhere indicate that use of mineral fertilisers in absence of organic inputs or recycling of residues is unable to sustain long-term soil productivity but that maize stover alone also results in yield decline (Kapkiyai et al., 1998). Farmers confronting soil fertility decline should be encouraged to try soybean in rotation with maize as it contributes organic inputs to the following maize crop. Understandably, soybean yields are likely to be of greater priority to farmers than simply the residual benefits of soybean residues. This consideration will determine farmers choice of cultivar as soybean production becomes more popular in Malawi.
The authors are grateful for assistance of the 17 farm households for participating in this experiment and to Paul L. Woomer for helpful comments during preparation of this manuscript. Funds were provided for this study by The Rockefeller Foundation under the Forum for Agricultural Resource Husbandry.
©1999, African Crop Science Society
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