Integrated soil fertility management practices for improved crop production in smallholder farming systems of semi-arid areas: A synthetic review

Abstract

Soil fertility degradation is one of the major constraints in smallholder farming environments especially in the semi-arid region of Sub-Saharan Africa. This has been worsened by poor farming methods, lack of technical knowledge and inadequate application of nutrient sources by farmers. The use of mineral fertiliser alone has been cost ineffective and causes land degradation. Although several soil fertility management (SFM) practices have been put forward, knowledge about their benefits, access and adoption by smallholder farmers is still limited. Synthetic review of published articles obtained from Web of Science, Google Scholar and Scopus which cover semi-arid regions across Africa were retrieved and synthesied. Farmers have been applying low organic manure (<5 t ha⁻¹) against the recommended rate of 20–40 t ha⁻¹ depending on soil texture and this caused low crop yields. This call for farmers to adopt integrated soil fertility management (ISFM) practices to improve nutrient availability and crop yields. The use of ISFM has been reported to increase maize yields by 15–145% in semi-arid areas. Maize yields varies from 4200 to 5500 kg ha⁻¹ and sorghum from 690 to 3500 kg ha⁻¹ under ISFM. Yield increment for various crops with the use of cattle manure integrated with other nutrient sources was reported but there are still few studies where effects compost, farmyard manure and poultry manure were combined with inorganic fertiliser were evaluated. Therefore, this review paper seeks to synthesise the effects of ISFM practices available for smallholder farming systems in semi-arid areas of sub-Saharan Africa to improve crop production.

Keywords:

• soil fertility
• nutrient management
• semi-arid areas
• crop production, smallholder farming areas

PUBLIC INTEREST STATEMENT

The paper reviews the effects of integrated soil fertility management (ISFM) on crop production across Africa especially in semi-arid areas in Sub-Saharan Africa. The review results indicated that combining organic and inorganic fertiliser improve crop yields under smallholder farming systems. Application of small quantities of mineral fertiliser and organic manure creates a sustainable agriculture for smallholder farmers to increase crop production.

1. Introduction

Integrated soil fertility management (ISFM) has been a call by many researchers and farmers across semi-arid areas. Soil fertility management (SFM) is a key issue in improving crop production, poverty alleviation and food security across the globe. Farmers in Sub-Saharan Africa (SSA) are overusing mineral fertiliser and poor SFM practices which causes a decline in crop yields (Tsujimoto et al., 2021). There is need for smallholder farmers to adopt ISFM to reduce cost of production, improve nutrient availability, reduce toxicity caused by metallic accumulation and increase microbial population responsible for decomposition (Gram et al., 2020; Kugedera et al., 2022). The use of ISFM can be a major option for farmers in semi-arid areas to increase crop yields, food security and reduce poverty (Kimaru-Muchai et al., 2021; Mugwe et al., 2019) and must be cost effective for farmers to maximise profit (Tsujimoto et al., 2021). It is paramount to use interventions which are profitable to farmers and maximize grain yield per unit of nutrient source use.

Several SFM practices are available to farmers in semi-arid areas include conservation agriculture (Mugandani & Mafongoya, 2019), agroforestry (Kugedera et al., 2022, 2022a, 2022b; Nyamadzawo et al., 2008), use of organic manure (Gram et al., 2020), mulching (Masaka et al., 2019) and mineral fertiliser (Kubiku et al., 2022; Kugedera et al., 2022). Intercropping cereals with legumes are another SFM practice available for smallholder farmers to improve crop yields and this can be integrated with water conservation techniques to maximise yields and farm profit (Kimaru-Muchai et al., 2021; Nyamadzawo et al., 2015). The use of animal manure is a major technology in smallholder farming areas but most of them apply less than 5 t ha⁻¹ against the recommended rate of 20–40 t ha⁻¹ (Nyamangara et al., 2005). Increasing quantities of decomposed animal manure has the potential of increasing soil fertility and this can be mixed with compost and lime to regulate soil pH (Workineh et al., 2022). Farmers can use crop residues as mulch allowing them to reduce soil erosion and decompose releasing nutrients. Furthermore, mulching improves infiltration of water making it available in the plant root zone for effective nutrient absorption.

Agroforestry is another key SFM strategy that farmers can adopt and use to improve crop production, reduce soil degradation and increase farm profitability. Agroforestry systems such as alley cropping allow farmers to incorporate prunnings, harvest biomass and transfer to other fields. Alley cropping improves cation exchange capacity (CEC), reduce leaching of nutrients, improve soil structure and increase nutrient availability for crop growth and development leading to improve crop yields (Kugedera et al., 2022; Mugwe, 2007; Nair, 2007; Nyamadzawo et al., 2008). Agroforestry systems used across several countries were reported to increase soil fertility and crop yields for example in countries like Burkina Faso (Coulibaly, 2015), Kenya (Kimaru-Muchai et al., 2021; Kwesiga et al., 2003; Mugendi et al., 2003; Mugwe, 2007), Zambia (Mafongoya et al., 2007) and Zimbabwe (Kugedera et al., 2022, 2022a, 2022b; Nyamadzawo et al., 2008, 2012). Several agroforestry species are available for use, for example Leucaena leucocephala, Tithonia diversifolia, Sesbania sesban and Acacia have the potential capability to increase soil fertility and crop production across semi-arid areas globally. Proper management of agroforestry technologies is needed to avoid tree-crop competition but improve crop productivity.

Compositing is another strategy to maintain soil fertility which farmers can adopt to improve soil fertility and crop production although takes time to be produced. Quality of crop residues is another key factor to consider in compositing since most plant residues from cereal crops are lignified. Lignification reduce decomposition because decomposers use more mineral N from crop residues for their survival (Gonda, 2015; Nyamadzawo et al., 2008). There is need for technical experts to teach farmers how to produce high quality compost manure using legume plant residues which contain high N content. Gonda (2015) reported that the use of well decomposed compost manure improves pearl millet yields and food security in Niger. Cattle and farm yard manure are other nutrient sources which are also used by farmers to improve soil fertility. These nutrient sources improve soil structure, nutrient content, water retention and soil porosity as well as crop yields (Kilasara et al., 2015; Kubiku et al., 2022; Mucheru-Muna et al., 2014; Mugwe et al., 2019). These nutrient sources are not available to every farmer and can be applied in small quantities which are affordable, for example, ≤5 t ha⁻¹ to allow continual future supply (Sileshi et al., 2019). Reports from several countries have shown that the use of cattle manure improved crop yields with more than 20% and has the potential to reduce food insecurity and improve farm profitability (Kilasara et al., 2015; Kugedera et al., 2022a; Mucheru-Muna et al., 2007).

Soil infertility generally cause food insecurity and poverty due to increase in human population which is projected to double in the next decades (Bado et al., 2022). This contributes to land shortage hence creating food crisis. There is need to come up with ISFM and review their effects in improving soil fertility and crop yields to meet food demand. Smallholder farmers tried ISFM but used it inappropriately hence poor results are being observed. Therefore, this review paper seeks to synthesise various SFM strategies available for smallholder farming systems in semi-arid areas of SSA to improve crop production. This review help to answer the following research questions:

1. What are the effects of ISFM on crop yields?

2. Which SFM strategy can be easily adopted by smallholder farmers to improve food security?

3. What improvements can be done on recently used ISFM in future to increase crop yields to meet food demand?

2. Materials and methods

2.1. Data collection

Comprehensive literature search was done in May-June 2022 from Web of Science, Google Scholar and Scopus, for publications on SFM strategies used to improve crop production in semi-arid regions across Africa. Our search was restricted to SFM options in semi-arid areas of Africa and the published paper fulfilling the following criteria; (i) published in accredited journals, (ii) an experiment done in semi-arid area in African country, (iii) gone through under peer review and show experimental season(s) and (iv) published from January 1990 to December 2022. The search was done using the following keywords; soil fertility management, soil fertility strategies in Africa*crop yields, soil fertility*crop production*Africa and improving crop production*Africa, “soil fertility management” (Topic) or “soil fertility strategies” (Topic) or “ISFM (Topic) and “smallholder farmers” (Topic) and “semi-arid regions”. The review mainly focused on SSA countries where SFM strategies were used to improve crop production in smallholder farming environments. The review put more emphasis on researches where soils of low fertility (sandy, loam and sandy loam or loamy sands) were improved with nutrient amendments. Major effects were reviewed on main staple crops such as maize, pearl millet and sorghum. Most of the researches reviewed were from countries such as Zimbabwe, Kenya, Tanzania, Zambia, Ethiopia, Burkina Faso, Niger and Mali. These countries were chosen because they have climatic conditions which are more similar and most of reviewed crops are grown in these countries. In these countries smallholder farmers have adopted agroforestry, use of cattle manure, farmyard manure, compost and combination of these with mineral fertiliser. This creates a great base for reviewing and coming up with good conclusions.

A total of 1200 articles were retrieved from this search. We employed the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRIMSA) to come up with publications for inclusion (Figure 1). The selected papers were evaluated and duplicate articles were removed, leaving a total of 1015 articles. These articles were further screened where 920 articles were removed as they were not meeting the required criteria. Conference papers, articles published in non-peer review and duplicates were also removed. A total of 95 articles were assessed for eligibility and 60 articles were excluded due to unmatching treatments, averaged seasonal data and it was also difficult to extract data from graphs. After eligibility, a total of 35 articles were included in the synthesis.

Figure 1. Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRIMSA) flow chart showing how data searching, identification and selection were done.

3. Results and discussion

3.1. Soil fertility management strategies for improving crop production

Traditionally smallholder farmers across Africa have been using cattle manure and mineral fertilisers as sole SFM strategies with few combining the two to improve crop yields. However, combining cattle manure and mineral fertiliser by most smallholder farmers proved to improve yields but their application rates were too low compared to the recommended rates of manure and 300–400 kg ha⁻¹ mineral fertiliser. The major issue was based on the quality of cattle manure used by farmers which was not fully decomposed and poor-quality feeds given to animals also contributed to low nutrient content (Mafongoya et al., 2007). Farmers mainly combined NPK fertilisers with cattle manure and this did not fully solve the issue of low total nitrogen in soils. This contributed to continual land degradation due to monoculture as farmers think that continual growing same crop in piece of land, they apply cattle manure can solve problem of low N content. Due to this, several researchers bring in other strategies such as agroforestry with legumes, use of compost manure, combining cattle manure with nitrogen fertiliser, alley cropping, improved fallows, biomass transfers and crop rotation as well as intercropping. SFM strategies discussed below include use of agroforestry practices, cattle manure + mineral fertiliser, cattle manure alone, farm yard manure and compost manure. These are major strategies adopted by smallholder farmers in SSA countries.

3.1.1. Agroforestry systems for improving soil fertility and crop production

Agroforestry is a collective system where leguminous trees are grown together with field crops or in rotation. Agroforestry systems such as alley cropping, improved fallow, biomass transfer and fertiliser tree systems can be used to improve soil fertility in semi-arid areas and improve crop yields. Biomass transfer is when nutrient rich leaves are cut and transferred to another field for use as fertiliser (Kugedera et al., 2022; Kwesiga et al., 2003; Mugendi et al., 2003). Biomass transfer is done in areas which have high capacity of losing soil nutrient faster than other areas for example in sandy soils (Kugedera et al., 2022; Murovhi & Materechera, 2006). Biomass transfer of Leucaena leucocephala +25 kg N ha⁻¹ produced 4500 kg ha⁻¹ of maize in Kenya during 1994–1996 cropping seasons (Mugendi et al., 1999).

Improved fallow is a system where land is left uncultivated for 2–3 years with legume trees, such as Leucaena leucocephala, Cajanus cajan and Sesbania sesban are grown to improve soil fertility and reduce land degradation (Table 1). The use of improved fallow improves soil hydraulic conductivity, infiltration rates, aeration, CEC and soil structure (Kugedera et al., 2022b; Mafongoya et al., 2006; Mugwe, 2007; Mugwe et al., 2019; Nyamadzawo et al., 2008). These systems have the potential of increasing soil fertility in degraded lands and improve crop production for example alley cropping increased N mineralisation and its availability to maize crops (Katanga et al., 2007). On average farmers are getting 300–600 kg ha⁻¹ without using any nutrient amendment in maize and sorghum production (Kugedera et al., 2022; Twomlow et al., 2008). Agroforestry systems used in maize production have been reported to increase maize yields by more than 500 kg ha⁻¹ showing high potential of reducing food insecurity and poverty in semi-arid areas (Table 1). Significant increases in crop production were observed from all results in Table 1 with lowest maize grain yield of 700 kg ha⁻¹ in Zimbabwe after using Sesbania sesban + conventional tillage (Nyamadzawo et al., 2012) and a highest grain yield of 5500 kg ha⁻¹ after using biomass transfer of Tithonia diversifolia in Kenya (Mucheru-Muna et al., 2007).

Table 1. Effects of sole agroforestry systems and in combination with mineral fertiliser and cattle manure on crop yields

Treatments	Country/region	Experimental year	Soil type	Crop	Yield (kg ha^−1)	References
Biomass transfer (Tithonia diversifolia)	Ethiopia	2017, 2018	Nitosol	Maize	6,594	Endris (2019)
Tephrosia candida (provenance 02970)	Zambia	2002	Loam	Maize	900	Mafongoya et al. (2006)
Cajanus cajan	Zambia	2002	Loam	Maize	800	Mafongoya et al. (2006)
Biomass transfer of Leucaena	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,900	Mucheru-Muna et al. (2007)
Biomass transfer of Calliandra	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,500	Mucheru-Muna et al. (2007)
Biomass transfer of Tithonia	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	5,500	Mucheru-Muna et al. (2007)
Biomass transfer of Leucaena + 30 kg N ha^−1	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,800	Mucheru-Muna et al. (2007)
Biomass transfer of Tithonia + 30 kg N ha^−1	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	5,400	Mucheru-Muna et al. (2007)
Biomass transfer of Calliandra +30 kg N ha^−1	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,900	Mucheru-Muna et al. (2007)
Biomass transfer of Leucaena leucocephala	Kenya	1994–1996	Clay	Maize	4,500	Mugendi et al. (1999)
Biomass transfer of Leucaena + 30 kg N ha^−1	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,800	Mucheru-Muna et al. (2007)
Biomass transfer of Tithonia + 30 kg N ha^−1	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	5,400	Mucheru-Muna et al. (2007)
Biomass transfer of Calliandra + 30 kg N ha^−1	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,900	Mucheru-Muna et al. (2007)
Biomass transfer of Calliandra calothyrsus + 25 kg N ha^−1	Kenya	1994–1996	Clay	Maize	4,500	Mugendi et al. (1999)
Leucaena leucocephala (5–30 t ha^−1)	Zimbabwe	2018–2020	Sandy loam	Sorghum	776–1,083	Kugedera et al. (2022b)
Leucaena/cattle manure combination (5–30 t ha^−1)	Zimbabwe	2018–2020	Sandy loam	Sorghum	690–1,200	Kugedera et al. (2022a)
Leucaena biomass + NPK fertiliser	Zimbabwe	2018–2020	Sandy loam	Sorghum	650–1,100	Kugedera et al. (2022)
Acacia angustissima + conventional tillage	Zimbabwe	2000/01	Sandy clay loam	Maize	1,800	Nyamadzawo et al. (2012)
Acacia angustissima	Zimbabwe	2002/03	Sandy clay loam	Maize	2,000	Nyamadzawo et al. (2012)
Tithonia diversifolia+ Zai pits	Kenya	2013–2014	Ferrasols	Sorghum	4,300	Kimaru-Muchai et al. (2021)
Conventional +Tithonia diversifolia+ 30 kg N ha^−1	Kenya	2013–2014	Ferrasols	Sorghum	3,790	Kimaru-Muchai et al. (2021)
Tithonia diversifolia+ Zai pits +30 kg N ha^−1	Kenya	2013–2014	Ferrasols	Sorghum	3,960	Kimaru-Muchai et al. (2021)
Tithonia diversifolia	Kenya	2002–2003	Sandy clay	Maize	1,300–5,000	Mugwe (2007)
Tithonia +30 kg N ha^−1	Kenya	2002–2003	Sandy clay	Maize	2,800–3,700	Mugwe (2007)
Calliandra calothyrsus	Kenya	2002–2003	Sandy clay	Maize	2,200–4,100	Mugwe (2007)
Calliandra +30 kg N ha^−1	Kenya	2002–2003	Sandy clay	Maize	1,700–4,300	Mugwe (2007)
Leucaena trichandra	Kenya	2002–2003	Sandy clay	Maize	1,800–4,700	Mugwe (2007)
Leucaena +30 kg N ha^−1	Kenya	2002–2003	Sandy clay	Maize	2,100–4,200	Mugwe (2007)

Alley cropping is one of the most important agroforestry practices which smallholder farmers can adopt to improve soil fertility and crop yields. The practices provide mulch through foliage which conserves moisture, increase CEC, improve soil hydraulic conductivity and mine leached nutrients through deep capture (Katanga et al., 2007). Smallholder farmers can adopt alley cropping but the major challenge is on reducing arable land for agriculture, some tree species habours pests and pathogens which reduce crop production and increase production cost. This makes it difficult for smallholder farmers to adopt alley cropping in favour of biomass transfer, improved fallow and their combination with mineral fertilisers or cattle manure. Agroforestry practices takes time for farmers to enjoy the benefits since trees need time to establish and there is competition between trees and field crops during first years. This creates problems for smallholder farmers to adopt alley cropping, scattered trees in farmlands and adopt biomass transfer where trees and crops will no longer in one field. Biomass transfer reduces loss of land, outbreak of pests and pathogens as tree leaves and twigs will be directly incorporated in the soil (Kugedera et al., 2022). Kugedera et al. (2022b) reported sorghum grain yields ranging from 650–1146 kg ha⁻¹ after applying 5–30 t ha⁻¹ of Leucaena biomass in Zimbabwe. However, most researchers were concerned on why farmers prefer biomass transfer and evaluated the effects of biomass transfer on crop yields (Table 1).

3.1.2. Effects of sole cattle manure and in combination with other nutrient sources as soil fertility management strategy on crop yields

Cattle manure is one of the widely used nutrient sources across semi-arid areas of Africa. Farmers use cattle manure at varied rates depending on its availability to the farmer. The quality of cattle manure depends on the management by farmers and quality of feeds provided to animals (Mafongoya et al., 2007). Most farmers in semi-arid areas use poor quality manure due to unavailability of high-quality feeds and this affects soil fertility and crop yields even farmers add same quantity of manure. High quality cattle manure improves N mineralisation and its availability to crops. Poor quality cattle manure contains a lot of crop residues which takes time to be decomposed hence reduce N mineralisation and yields. Besides that, cattle manure improves soil structure, porosity, phosphorous content and population of soil organisms which are responsible for decomposition (Kubiku et al., 2022; Tapiwa et al., 2020). Application of cattle manure increases crop yields because it releases nutrients throughout the season and conserves moisture which will be available during grain filling (Kimaru-Muchai et al., 2021; Kubiku et al., 2022). In addition, Nyamangara et al. reported increase in maize grain yield from 2590 kg ha⁻¹ to 4950 kg ha⁻¹ and 5470 kg ha⁻¹ with application of 12.5 and 37.5 t ha⁻¹ of cattle manure. Cattle manure can be combined with mineral fertiliser to make nutrients readily available at the start of season (Kubiku et al., 2022) and combined with legume prunings such as Leucaena biomass to increase crop yields (Kugedera et al., 2022a). Integrating cattle manure with mineral fertiliser causes ISFM-add for example 37.5 t ha⁻¹ cattle manure +120 kg N ha⁻¹ produced 8040 kg ha⁻¹ maize grain yield in Zimbabwe (Nyamangara et al., 2003), manure +30 kg N ha⁻¹ increased maize grain yield in Kenya from 1200 kg ha⁻¹ to 6500 kg ha⁻¹ with an average of 4700 kg ha⁻¹ after seven cropping seasons (Mucheru-Muna et al., 2007) and Mwadalu et al. (2022) reported 5120 kg ha⁻¹ of sorghum after use manure + N fertiliser in Tanzania. Increasing cattle manure quantity increases nutrient availability, conserves more soil moisture and improve crop production (Table 2).

Table 2. Crop yields (kg ha⁻¹) response to sole cattle manure application or in combination with other fertilizer sources in deferent countries and soil types

Treatments	Country/region	Experimental year	Soil type	Crop	Yield (kg ha^−1)	References
2.5 t ha^−1 cattle manure	Zimbabwe	2019	Sandy loam	Potato	21,000	Rumbidzai et al. (2022)
5 t ha^−1 cattle manure	Zimbabwe	2019	Sandy loam	Potato	24,830	Rumbidzai et al. (2022)
2.5 t ha^−1 cattle manure + 25 kg ha^−1 Potash	Zimbabwe	2019	Sandy loam	Potato	22,610	Rumbidzai et al. (2022)
2.5 t ha^−1 cattle manure + 50 kg ha^−1 Potash	Zimbabwe	2019	Sandy loam	Potato	26,800	Rumbidzai et al. (2022)
5 t ha^−1 cattle manure	Zimbabwe	2017/18; 2018/19	Sandy loam	Maize	2,340; 2,420	Tapiwa et al. (2020)
5 t ha^−1 cattle manure + 50 kg ha^−1	Zimbabwe	2017/18; 2018/19	Sandy loam	Maize	2,210; 2,270	Tapiwa et al. (2020)
5 t ha^−1 cattle manure + 100 kg ha^−1	Zimbabwe	2017/18; 2018/19	Sandy loam	Maize	3,120; 3,260	Tapiwa et al. (2020)
Cattle manure + 30 kg N ha^−1	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,700	Mucheru-Muna et al. (2007)
Cattle manure	Kenya (Chuka)	2000–2003	Sandy clay loam	Maize	4,500	Mucheru-Muna et al. (2007)
5 t ha^−1 cattle manure	Nigeria	2020	Sandy loam	Maize	1,240	Eleduma et al. (2020)
10 t ha^−1 cattle manure	Nigeria	2020	Sandy loam	Maize	1,540	Eleduma et al. (2020)
15 t ha^−1 cattle manure	Nigeria	2020	Sandy loam	Maize	1,700	Eleduma et al. (2020)
20 t ha^−1 cattle manure	Nigeria	2020	Sandy loam	Maize	2,040	Eleduma et al. (2020)
Farmyard Manure	Tanzania	2010–2011	Loamy sand	Sorghum	2,573–2,789	Kilasara et al. (2015)
Farmyard manure	Tanzania	2010–2011	Sandy clay loam	sorghum	523–1,320	Kilasara et al. (2015)
5 t ha^−1 cattle manure	Zimbabwe	2018	Loam	Maize	4,760	Motsi et al. (2019)
5 t ha^−1 cattle manure + 50 kg N ha^−1	Zimbabwe	2018	Loam	Maize	5,390	Motsi et al. (2019)
5 t ha^−1 cattle manure + 100 kg N ha^−1	Zimbabwe	2018	Loam	Maize	5,700	Motsi et al. (2019)
Cattle manure	Kenya	2013–2014	Ferrasols	Sorghum	180–3,710	Kimaru-Muchai et al. (2021)
Cattle manure + 30 kg N ha^−1	Kenya	2013–2014	Ferrasols	Sorghum	180–3,920	Kimaru-Muchai et al. (2021)
5 t ha^−1 Farmyard manure	Kenya	2012/13	Sandy loam	Sorghum	1,970; 4,715	Mwadalu (2014)
10 t ha^−1 Farmyard manure	Kenya	2012/13	Sandy loam	Sorghum	1,886; 4,463	Mwadalu (2014)
5 t ha^−1 Farmyard manure + 50 kg ha^−1NP	Kenya	2012/13	Sandy loam	Sorghum	1,864; 5,187	Mwadalu (2014)
10 t ha^−1 Farmyard manure + 50 kg ha^−1NP	Kenya	2012/13	Sandy loam	Sorghum	1,566; 5,393	Mwadalu (2014)
5 t ha^−1 Cattle manure	Zimbabwe	2016/17–2018/19	Sandy loam	Sorghum	400	Kugedera et al. (2022)
5 t ha^−1 Cattle manure + 50 kg N ha^−1	Zimbabwe	2016/17–2018/19	Sandy loam	Sorghum	1,180	Kubiku et al. (2022)
5 t ha^−1 Cattle manure + 170 kg N ha^−1	Zimbabwe	2016/17–2018/19	Sandy loam	Sorghum	1,390	Kubiku et al. (2022)
2.5 t ha^−1 Cattle manure	Mali	2012/13	Sandy loam	Pearl millet	1,166	Coulibaly (2015)
2.5 t ha^−1 Cattle manure	Mali	2013/14	Sandy loam	Pearl millet	1,603	Coulibaly (2015)
2.5 t ha^−1 Cattle manure + 20.5 kg N ha^−1 + 23 kg P2O5 ha^−1	Mali	2012/13	Sandy loam	Pearl millet	940	Coulibaly (2015)
2.5 t ha^−1 Cattle manure + 20.5 kg N ha^−1 + 23 kg P2O5 ha^−1	Mali	2013/14	Sandy loam	Pearl millet	1,728	Coulibaly (2015)
2.5 t ha^−1 Cattle manure + 41 kg N ha^−1 + 46 kg P2O5 ha^−1	Mali	2012/13	Sandy loam	Pearl millet	1,370	Coulibaly (2015)
2.5 t ha^−1 Cattle manure + 41 kg N ha^−1 + 46 kg P2O5 ha^−1	Mali	2013/14	Sandy loam	Pearl millet	1,716	Coulibaly (2015)

3.1.3. Effects of sole compost and in combination with other nutrient sources as soil fertility management strategy on crop yields

Crop production has been negatively affected by soil infertility caused by poor farming methods such as monoculture where crops mine nutrients from the same zone. This created nutrient deficiencies and negatively change soil structure leading to poor crop yields. This resulted in soil quality and health decline especially in semi-arid areas where smallholder farmers occupy the lands. Application of compost has the potential to benefit farmers through adding nutrients in the soils (Tejada et al., 2001), improve water retention ability, increase soil organic matter (SOM), lower bulk density (Gonda, 2015) and increasing population of beneficial soil organisms (Kugedera et al., 2022b; Mafongoya et al., 2007). Applying compost does not only improve soil fertility but contribute immensely towards better crop production as a result of improved crop growth (Table 3). Application of 2.5 t ha⁻¹ compost +75 kg ha⁻¹ DAP in Niger increased cowpea yield from 95 kg ha⁻¹ to 306 kg ha⁻¹, and increasing cattle manure to apply 5 t ha⁻¹ compost +75 kg ha⁻¹ DAP produced 389 kg ha⁻¹ (Gonda, 2015). This indicate that increasing organic manure by 100 % may not have same effect on yield increment. Applying more than 5000 kg ha⁻¹ may lower nutrient use efficiency, with most being lost through leaching (Sileshi et al., 2019). Compost reduces stickiness of the soil during ploughing and this reduce energy requirement from draught animals (Asaye et al., 2022). Compost improves infiltration rates, root penetration and supply nutrients in the plant root zone (Asaye et al., 2022; Rady et al., 2016). Application of 30 t ha⁻¹ compost + 50 % NPK fertiliser has been reported to increase yield of field beans from 5500 to 9300 kg ha⁻¹ in Egypt (Rady et al., 2016). This promotes protein synthesis, increase photosynthetic rate and improve yields. Compost have better phosphorous and potassium supply which promote tuber development in potatoes (Asaye et al., 2022; Kugedera & Kokerai, 2019; Rumbidzai et al., 2022) and this increases yields. Sufficient quantities of compost applied at the right time has the potential to improve soil fertility, soil structure and aggregation which stimulate root growth and crop yields (Asaye et al., 2022; Takeda et al., 2009). Combining compost with mineral fertiliser contributes towards fast release of nutrients during seedling stage and this promotes effective growth and nutrient utilisation causing higher yields (Rady et al., 2016). This combination also reduces nutrient stress, salinity stress and even moisture stress since compost improves infiltration rates causing higher water retention ability (Asaye et al., 2022).

Table 3. Effects of sole compost and in combination with other sources on crop yields

Treatments	Country/Region	Experimental year	Soil type	Crop	Yield (kg ha^−1)	References
6 t ha^−1 Compost + Mineral fertiliser	Ethiopia	2021	Sandy loam	Maize	4,990	Asaye et al. (2022)
12 t ha^−1 compost + Mineral fertiliser	Ethiopia	2021	Sandy loam	Maize	5,500	Asaye et al. (2022)
6 t ha^−1 Compost + Mineral fertiliser	Ethiopia	2021	Sandy loam	Potato	12,900	Asaye et al. (2022)
12 t ha^−1 compost + Mineral fertiliser	Ethiopia	2021	Sandy loam	Potato	13,850	Asaye et al. (2022)
Compost	Ethiopia	2006	Loam	Maize	3,748	FAO ()
Compost	Ethiopia	2006	Loam	Barley	2,349	FAO (2007)
Compost	Ethiopia	2006	Loam	Finger millet	2,652	FAO (2007)
Compost	Ethiopia	2006	Loam	Sorghum	2,497	FAO (2007)
Compost	Ethiopia	2000–2006	Loam	Faba bean	2,857	FAO (2007)
10 t ha^−1 compost + 50 % NPK fertiliser	Egypt	2011/12	Clay loam	Field beans	7,140	Rady et al. (2016)
20 t ha^−1 compost + 50 % NPK fertiliser	Egypt	2011/12	Clay loam	Field beans	8,910	Rady et al. (2016)
10 t ha^−1 compost + 50 % NPK fertiliser	Egypt	2012/13	Clay loam	Field beans	6,980	Rady et al. (2016)
20 t ha^−1 compost + 50 % NPK fertiliser	Egypt	2012/13	Clay loam	Field beans	9,100	Rady et al. (2016)
30 t ha^−1 compost + 50 % NPK fertiliser	Egypt	2012/13	Clay loam	Field beans	9,420	Rady et al. (2016)
2.5 t ha^−1 compost	Niger	2012/13	Sand	Pearl millet	1,527	Gonda (2015)
2.5 t ha^−1 compost	Niger	2013/14	Sand	Pearl millet	431	Gonda (2015)
5 t ha^−1 compost	Niger	2012/13	Sand	Pearl millet	737	Gonda (2015)
5 t ha^−1 compost	Niger	2013/14	Sand	Pearl millet	492	Gonda (2015)
2.5 t ha^−1 compost +100 kg ha^−1 NPK	Niger	2012/13	Sand	Pearl millet	1,225	Gonda (2015)
2.5 t ha^−1 compost +100 kg ha^−1 NPK	Niger	2013/14	Sand	Pearl millet	544	Gonda (2015)
5 t ha^−1 compost +100 kg ha^−1 NPK	Niger	2012/13	Sand	Pearl millet	1,347	Gonda (2015)
5 t ha^−1 compost +100 kg ha^−1 NPK	Niger	2013/14	Sand	Pearl millet	716	Gonda (2015)
2.5 t ha^−1 compost	Niger	2012/13	Sand	Cowpeas	299	Gonda (2015)
2.5 t ha^−1 compost	Niger	2013/14	Sand	Cowpeas	95	Gonda (2015)
5 t ha^−1 compost	Niger	2012/13	Sand	Cowpeas	221	Gonda (2015)
5 t ha^−1 compost	Niger	2013/14	Sand	Cowpeas	136	Gonda (2015)
2.5 t ha^−1 compost +75 kg ha^−1 DAP	Niger	2013/14	Sand	Cowpeas	201	Gonda (2015)
5 t ha^−1 compost +75 kg ha^−1 DAP	Niger	2012/13	Sand	Cowpeas	225	Gonda (2015)

4. Lessons and future direction

Soil fertility management strategies are the main drivers in improving crop yields, food security and reduce poverty in smallholder farming environments. To be successful in the use of SFM strategies, there is need to augment organic strategies with mineral fertilisers which supply ready nutrients. Review results show that agroforestry is one of the best SFM strategies and can be combined with cattle manure, compost and low (micro dose) quantities of mineral fertiliser to maximize crop yields. This can be a better and cheaper option to increase crop yields. In future there is need to evaluate these combinations and evaluate their effects on crop yields to come up with least cost combination which is profitable for smallholder farmers since they are resource poor and unable to buy large quantities of mineral fertiliser. Since mineral fertilisers which supply nitrogen are expensive, there is need to focus more on the use of legume agroforestry species though biomass transfer or improved fallows to increase nitrogen content and reduce use of mineral fertiliser which may affect soil structure and microbial population. Surveys need to be done in smallholder farming environments to come up with first hand information on what farmers are experiencing on the use of ISFM and the quantities they are using.

5. Conclusion

Crop production in semi-arid areas can be improved with the use of various SFM strategies can be a solution of improving crop yields in smallholder farming environments. Soil management strategies such as agroforestry not only improve soil fertility and crop production but also creates micro-climatic conditions which positively alter hydrological cycle, conserve biodiversity and provide fodder for livestock. Option such as alley cropping makes degraded lands productive through restoring their values and increase crop yields. Cattle manure and compost can also be used to improve crop production but the quality of these nutrient sources affect crop yields. High quality manure and compost increase N mineralisation, CEC, regulate pH and supply large quantities of micro elements which play an important role in improving crop production. Smallholder farmers are resources poor and cannot afford to buy adequate amounts of mineral fertiliser hence integrate small quantities of mineral N, P and K to make ISFM a success story.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

Andrew Tapiwa Kugedera

Andrew Tapiwa Kugedera is a PhD student in Agricultural Sciences at Bindura University of Science Education. He is a part time lecturer at Zimbabwe Open University and Great Zimbabwe University in Masvingo. His research interest is in exploring the effects of climate smart agriculture, ISFM and rainwater harvesting techniques on crop productivity.

Taruvinga Badza

Taruvinga Badza is an experienced Agricultural Scientist with a demonstrated history in working in the agricultural sector and institutes of higher learning. A strong professional researcher with a PhD in Agronomy. His research interest are sustainable agriculture, Agronomy and soil nutrient cycling.

Alfred Oduor Odindo

Alfred Odindo is an Associate Professor (Crop Science/Agronomist) in the School of Agricultural, Earth and Environmental Science, University of KwaZulu Natal (UKZN) in South Africa. He focuses on understanding the interactions between engineering, agriculture and social systems and their application in seeking practical solutions to the global challenges of water, energy, food security and environmental pollution.