African Crop Science Journal African Crop Science Society ISSN: 1021-9730 EISSN: 2072-6589 Vol. 7, Num. 4, 1999, pp. 423-431

African Crop Science Journal, Vol. 7. No. 4,  pp. 423-431, 1999          

Use of wheat straw, soybean trash and nitrogen fertiliser for maize production in the Kenyan highlands

J.R. Okalebo, C.A. Palm¹, M. Gichuru1, J.O. Owuor, C.O. Othieno, A. Munyampundu²,  R. M. Muasya and P.L. Woomer³
Faculty of  Agriculture, Moi University, P.O. Box 1125, Eldoret, Kenya
¹Tropical Soil Biology and Fertility Programme, TSBF, P.O. Box 30592, Nairobi, Kenya
²Department of Forestry, Moi University, P.O. Box 1125, Eldoret, Kenya
³Department of Soil Science, University of Nairobi,  P.O. Box 30197, Nairobi, Kenya

Code Number: CS99033

ABSTRACT

Making best use of available crop residues is an important component of integrated nutrient management.  A field study was conducted over two seasons (1997 and 1998) in Kenya that examined use of wheat straw, soybean trash and nitrogen fertiliser as nutrient inputs for maize (Zea mays L.) production.  The organic inputs were applied at the rate of  2 t ha^-1 per season and urea was added at rates of 0, 20, 40, 80 and 100 kg N ha^-1 in an incomplete factorial treatment structure that also included a complete control (no inputs) and 80 kg N ha^-1 as urea without organic inputs.  Maize grain yield ranged between 751 and 6836 kg ha^-1 with lowest yields observed in the treatment receiving wheat straw alone and higher yields associated with soybean residue incorporation and during the second, wetter growing season.  The 1998 crop  benefited from more favourable rainfall, providing grain yield increase of 141% above control treatment  as a result of combining 2 t ha^-1 soybean trash and 100 kg N ha^-1 urea.  The generally high yields from soybean trash are explained in terms of its higher quality, faster decomposition and nutrient release compared to the lower quality wheat straw.  A positive effect in increases of soil pH, C, N and P status as a result of cumulative use of crop residues was observed. Larger yields were obtained when organic and inorganic inputs were applied to soils, particularly when soil moistue was adequate and the organic inputs higher in mineralisable nutrients.  Early indications of additional longer-term benefits through soil quality improvement were also measured.  These findings suggest that better use may be made of crop residues than the burning following harvest as is currently practiced by many farmers in this area of western Kenya.

Key Words: Crop residues, East African highlands, integrated nutrient management, smallhold farming systems, soil fertility, urea

RÉSUMÉ

La bonne utilisation de résidus des cultures disponibles est une composante importante de la gestion intégrée des éléments nutritifs.  Une étude en champs a été conduite pendant deux saisons (1997 et 1998) au Kenya pour examiner l’utilisation de la paille de blé, les fanes de soja et l’engrais azoté comme intrants pour la production du maïs (Zea mays L.).  Les intrants organiques étaient appliqués à un taux de 2 t ha^-1 par saison et l’urée était ajoutée à des taux de 0, 20, 40, 80 et 100 kg N ha dans une structure de traitement factoriel incomplet qui comprenait aussi un contrôle complet (pas d’intrant) et 80 kg N comme urée sans intrant organique.  Le rendement en grains du maïs variait entre 751 et 6856 kg ha^-1, le plus faible rendement était observé dans le traitement ayant reçu la paille de blé seule et les plus haut rendements étaient associés avec l’incorporation des résidus de soja durant la seconde saison pluvieuse.  La culture de la saison 1998 a bénéficie des pluies favorables aboutissant à une augmentation de rendement en grains de 141% plus que le traitement témoin résultant de la combinaison de 2 t ha^-1 de fanes de soja et de 100 kg N ha^-1 d’urée. Les rendements générallement élevés obtenus à partir des fanes de soja sont expliqués en termes de qualité, de décomposition plus rapide et de libération des éléments nutritifs en comparaison de la pauvre qualité de la paille de blé.  Un effet positif dans l’augmentation du pH du sol, de l’état du C, N et P résultant de l’utilisation cumulative des résidus des cultures a été observé.  Des rendements élevés étaient obtenus quand les intrants organiques, et inorganiques étaient appliqués aux sols, en particulier quant l’humidité du sol était adéquate et les intrants organiques élevés en éléments minéralisables.  Des indications précoces des bénéfices additionels à long-terme via l’amélioration de la qualité du sol ont été aussi mésurées.  Ces résultats suggèrent que la meilleure utilisation peut tre faite des résidus de cultures plus que le brélage après recolte comme il est présentement pratiqué par la majorité des agriculteurs dans cette région ouest du Kenya.

Mots Clés:   Résidus de culture, hautes terres de l’Afrique de l’Est, gestion intégrée des éléments nutritifs, systèmes de cultures des petits fermiers, fertilité du sol, urée

Introduction

Many countries in  sub-Saharan Africa continue to require increasing amounts of food aid (World Bank, 1996). As a result, donor countries question their abilities to meet the growing food deficits.  Some reasons for this situation include  frequent droughts,  political unrest, rapid population growth, and unaffordale costs of external farm inputs while  increased food supplies are needed to feed the growing populations in Africa.  This requirement has contributed to the abandonment of traditional methods of land fallow that were important in soil fertility conservation. Many croplands are now continuously cultivated with little nutrient additions (Makken, 1993), resulting in diminishing productivity and declining per capita food production (Woomer  and   Muchena, 1996; Woomer et al., 1997).  A fundamental constraint to crop production in African smallholder agriculture is soil nutrient depletion. Sanchez et al. (1997)  reported annual losses of 660, 75 and 450 kg ha^-1 of nitrogen  (N), phosphorus (P) and potassium (K), respectively, during the past three decades, in  about 200 million hectares cultivated in 37 African countries.  Apart from nutrient mining from continuous cultivation practices, nutrient depletion also results from erosion, leaching and dentrification.   Smaling et al.  (1997) estimated that farming systems in the East African Highlands lose nutrients at  rates of 130 kg  N, 5 kg P and 25 kg K ha^-1 y^-1.

Past studies in western Kenya strongly suggest that nitrogen and phosphorus are the two most widespread nutrient limitations to crop growth (FURP, 1994; Shepherd et al., 1995; Woomer et al., 1997). Many farmers in western Kenya are aware of the need to apply N and P fertilisers to increase crop yields, but the costs of fertilisers are prohibitive.  However, alternative options for soil fertility improvement have been identified,  and are better suited to  smallhold farmers’conditions.  One option is the use of organic resources such as manure and composts and the return of crop residues and tree prunings to soils (Probert et al., 1992; Palm, 1995).  The combination of crop residues and N and P fertilisers improves the  availability of these two nutrients, particularly when low quality organic resources, such as  maize stover, or wheat straw are retained (Palm et al., 1997).  Agroforestry systems  enhance nutrient recycling as short-term fallows.  The  fast growing nitrogen-fixing Sesbania sesban offers particular promise in western Kenya (Buresh et al., 1997).

Limitations exist regarding the use of organic resources as a  sole means to improve soil fertility.  In central Kenya,  crop residues are mainly fed to cattle (Ikombo et al., 1994).  But in Uasin Gishu district, crop residues (4 to 6 t ha^-1 per season) are burnt to facilitate land clearing and ploughing for the subsequent crop (Muasya, 1995).  The quality of organic resources targeted for soil fertility improvement  is highly variable (Probert et al., 1992).  This quality factor plays  an important role in the decomposition and nutrient release patterns from organic materials.  The materials with carbon to nitrogen  ratios above 25 decompose and release nutrients slowly, and  materials with low lignin and polyphenolic contents decompose more quickly (Palm et al., 1997).  This paper reports findings in which high (soybean trash) and low (wheat straw) quality crop residues were studied in combination with  nitrogen fertiliser to enhance N availability and nutrient uptake by maize.

Materials and methods

Chepkoilel site. A field experiment was conducted at the Chepkoilel Campus of Moi University, Uasin Gishu District, Kenya.  The site is located 35º18' E  and 0º30' N at an elevation of 2140 m above sea level.  Precipitation ranges from 900 to 1300 mm with an annual mean of 1124 mm.  This rainfall occurs during one long season from March to September.  Mean annual temperature is 23ºC with a minimum of 10ºC.  The soils are underlain by tertiary  volcanic rocks (phenolites) and murram, acidic (pH 4.5-5.0), dark red, friable and  classified as Rhodic Ferralsols (FAO/UNESCO) or oxisols (USDA).  Uasin Gishu lies in a highland plateau, which forms a large wheat and maize growing area in Kenya, mainly within the  agroecozone known as the wheat, barley zone (Jaetzold and  Schmidt, 1983).

Maize field experiment. Two different crop residues were tested in the field experiment; wheat straw and soybean trash.  These organic materials differ in nutrient status  (Table 1) and are available in large quantities (2-7 t ha^-1)  following harvests (Muasya, 1995).

Table 1. Chemical characteristics of crop residues in a field trial at Chepkoilel, Kenya 1997/98

Treatments consisted of sun-dried and chopped (2 to 4 cm) wheat straw and soybean trash applications at a uniform rate of 2 t ha^-1 for each material.  Fertiliser N was combined with these organics at the rates of 0, 20, 40, 80 and 100 kg N ha^-1 as urea.  These treatments were applied at maize planting, first in March 1997, and were repeated in the following season (March 1998).  To eliminate P and K limitations, 100 kg P ha^-1 as single superphosphate and 100 kg K ha^-1 as muriate of potash were applied.  All inputs were incorporated into the seedbed by hand tillage.  The experiment was arranged as a randomised complete block with  4 replicates.  Starter N for all nitrogen treatments was applied at 20 kg N ha^-1 at planting.

Two maize seeds (hybrid 614D) were planted into each hole of about 10 cm depth, spaced at 30 cm within and 75 cm between rows.  Plots were  6 m x 3.75 m, accommodating 5  rows per plot.  Plants were thinned to one per "hill" four weeks after planting, resulting in a maize population of 4.4 plants m ^-2.  At this stage, nitrogen was topdressed along maize rows as urea.  Weeding was done by hoe and insects were controlled using depterex granules.  In 1998, maize heights were measured from 6 plants per plot biweekly.  At harvest, cobs and stover were separated.  Sub-samples of these components were dried (40ºC) and weighed to obtain yield measurements.  The samples were ground (20 mesh), analysed for N, P and K (Okalebo et al., 1993) and nutrient uptake and removal by each treatment calculated.

Soil sampling, preparation and anlysis. Before treatment application in 1997, surface (0 - 20 cm) soils were sampled  at random.  This field had been under continuous wheat cropping for over 5 years with modest diammonium phosphate applications.  Thirty auger borings were made and the soils from these sampling points were bulked, mixed thoroughly and a 1.0 kg sub-sample  air-dried and sieved (2 mm) for laboratory analysis.  At the end of the second consecutive maize cropping season (October 1998), soils  (0-20 cm) were obtained from each plot by bulking, mixing and sub-sampling 9 cores made at random across each plot.  All soil samples were analysed for pH  (2.5:1 H₂O), total carbon and nitrogen and extractable phosphorus (Olsen) following the methods outlined in Okalebo et al.  (1993).

Results and discussion

Maize growth.  Seedling establishment at two weeks after planting was uniform across all plots during both years of maize cropping at Chepkoilel.  This was attributed to favourable soil moisture   necessary for seed germination, emergence and establishment.  Uniform emergence of seedlings also resulted from use of quality seed in this study.

In Table 2, as expected from good rainfall of 1998, maize heights increased to  295 cm 112 days after planting.    During  early growth, the roots from young plants were likely not fully developed to exploit nutrients from the larger  soil volume.   However, from 44 days after planting  until maize maturity, heights increased with N level but soybean trash-N combinations provided taller plants than wheat straw-N combinations (Table 2).  The comparison of soybean-N incorporation and wheat straw-N incorporation is also presented, combining the heights for each organic input source across all N application rates.  It is possible that maize obtained an ‘extra’ supply of N  in plots receiving soybean  trash with a higher N content (Table 1).  In addition, N release in soils may have been favoured by rapid decomposition of the legume residue compared to the more recalcitrant cereal residue (Palm et al., 1997).

Table 2.  Effect of organic and inorganic nutrient sources on maize height increases in 1998 season at Chepkoilel

Maize yield. In 1997, grain yield ranged from 875 to 1876 kg ha^-1 while stover yield was from 3696 to 6028 kg ha^-1 (Table 3).  This grain yield is rather low for the  croplands of Uasin Gishu District under the average levels of nutrient inputs of  60 kg N ha^-1 plus 22 kg P ha^-1 (Muasya, 1995).  The low yield is partly explained in terms of low and poor rainfall distribution from August to September.  Total rainfall received in this period at Chepkoilel in 1997 and 1998 was 232 and 380 mm, respectively, and this low rainfall did not favour  soil moisture and  nutrient availability, resulting in sub-optimal maize nutrition, grain fill and low yield.  Nevertheless, many treatments demonstrated significant increases  in  maize  yield (P<0.05), particularly from 2  t  ha^-1  wheat straw and soybean trash combined with fertiliser N above 80 kg N ha^-1 (Table 3).  Grain yields for 1998 were between 2832 and 6836 kg ha^-1, whereas stover yields ranged between 10390 and 13950 kg ha^-1 (Table 3).  Favourable rainfall and its distribution in that year contributed to larger maize yields.  Again higher yields were obtained from soybean trash and N fertiliser applications above 80 kg ha^-1.  This response may be due to an increased N input of about 20 kg N ha^-1 from soybean trash compared to that of only 13  kg N ha^-1 from wheat straw (Table 1).  Past work has identified a nitrogen limitation in Chepkoilel soils.  In the present study, high maize yields were  found  from  fertiliser N  addition above at 80 kg N ha^-1 and from soybean trash combined with nitrogen above 80 kg ha^-1 (Fig. 2) in two seasons.

Table 3. Effect of combined crop residues and nitrogen fertiliser on maize yield (kg ha^-1) in Chepkoilel soils

aWS = Wheat straw, SYT = Soybean trash
N = Nitrogen applied as urea at 0,20,40,80 and 100 kg N ha^-1

Total uptake of nitrogen and phosphorus. The total nitrogen accumulated in grain and stover obtained at harvest in 1997 ranged from 27 to 58 kg N ha^-1, while the phosphorus uptake  was 3.9 to 7.1 kg P ha^-1  (Table 4).  These rather low N and P uptake figures are implied from low maize yields.  However, some treatments resulted in  significantly greater N and P uptakes in the grain plus stover.  Wheat straw at 2 t ha^-1 with  N combinations above 40 kg ha^-1 were  associated with larger N and P accumulations.  These nutrients may have been released from the wheat straw slowly, extending  availability for uptake  compared to soybean trash. 

Total uptake of N by maize in 1998 was between 53 and 130 kg N ha^-1 while the phosphorus accumulated in the two components ranged from 4.3 to 8.7 kg P ha^-1 (Table 4).  Higher N and P accumulation in this second season is attributed to favourable rainfall and  increased maize yields.  Again, N and P accumulation increased with levels of fertiliser N incorporated with organic residues.  Figure 3 shows the N accumulation in the treatments.  Less N uptake from straw may be indicative of N immobilisation in soils, where soil microrganisms assimilate nitrogen and other nutrients from the residues, reducing their availability to plant roots (Palm et al., 1997).

Table 4. Effects of organic and inorganic nutrient inputs on total uptake of nitrogen and phosphorus (kg ha^-1) by maize at Chepkoilel, Kenya

^aWS = Wheat straw, SYT = Soybean trash
N = Nitrogen applied as urea at 0,20,40,80 and 100 kg N ha^-1

Changes in soil properties. Table 5 summarises data for soil pH, C, N and available P obtained in surface (0-20 cm) soils sampled before application of treatments in March 1997 and soils obtained soon after harvesting the second maize crop in October 1998.  There were positive changes from organic inputs to increase the levels of these four parameters in soils.  Marked increases were realised in  N and P levels.  These are due to their  accumulation from consecutive organic and mineral nutrient additions made in 1997 and 1998 cropping seasons.   High Olsen P levels possibly occurred from an initial blanket application of single superphosphate at 100 kg P ha^-1.   Additional P is likely to have originated from the decay and release of the organically-bound P held in organic materials applied as previously described by Russell (1973).

Table 5. Changes in soil (0-20 cm) properties as influenced by cumulative incorporation of crop residues and urea- N at Chepkoilel, Kenya during two seasons of maize cropping

^aWheat straw and soybean trash data include all treatments receiving urea at 0, 20, 40, 80 and 100 kg N ha^-1

This study has demonstrated the positive effect of incorporating crop residues with N fertiliser into the seedbed to improve their decomposition and nutrient release characteristics. The two forms of residues tested (wheat straw and soybean trash) were  each  applied at  a  uniform rate of 2 t  ha^-1.  It is quite feasible that this rate of residue be retained in cropland even while alternative requirements  (e.g. fuel, feed) are also being met (Table 3). Comparisons of different rates of residue are suggested to obtain responses to both residues and N fertiliser rates. Organic matter  fractionation  needs to be done after cropping to determine the dynamics of important labile fractions.  Investigations of the economics of combined organic and mineral nutrient sources will provide useful information on crop residue management.

Acknowledgement

We acknowledge the financial assistance  of the  Kenya Agricultural Research Institute (KARI).  We are also grateful to  G. Nandwa, J. Wesonga and R. Njoroge  for their  assistance in field and laboratory tasks.  The  Rockefeller Foundation FORUM Programme provided funds that allowed for these findings to be presented at the 4th African Crop Science Congress in Morocco. 

References

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©1999, African Crop Science Society

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