The fundamental biophysical cause of stagnant per capita food production in Africa is soil fertility depletion. Because mineral fertilizers cost two to six times as much as those sold worldwide, a soil fertility replenishment approach has been developed based on naturally available resources: nitrogen-fixing leguminous tree fallows that accumulate 100 to 200 kg N ha, indigenous rock phosphate applications, and biomass transfers of the nutrient-accumulating shrub
 Tithonia diversifolia. Tens of thousands of farmers in East and Southern Africa are becoming food secure with these technologies. Soil fertility depletion must be addressed before other technologies and policies can become effective in overcoming hunger in Africa.

https://science.sciencemag.org/content/sci/295/5562/2019.full.pdf

Soil Fertility and Hunger in Africa

http://directory.umm.ac.id/Data%20Elmu/jurnal/A/Agriculture,%20Ecosystems%20and%20Environment/Vol83.Issue1-2.Jan2001/1692.pdf

Organic inputs for soil fertility management in tropical agroecosystems: Application of an organic resource database

Household-level determinants of adoption of improved natural resources management practices among smallholder farmers in western Kenya

In the highlands of Western Kenya, we investigated the reversibility of soil productivity decline with increasing length of continuous maize cultivation over 100 years (corresponding to decreasing soil organic carbon (SOC) and nutrient contents) using organic matter additions of differing quality and stability as a function of soil texture and inorganic nitrogen (N) additions. The ability of additions of labile organic matter (green and animal manure) to improve productivity primarily by enhanced nutrient availability was contrasted with the ability of stable organic matter (biochar and sawdust) to improve productivity by enhancing SOC. Maize productivity declined by 66% during the first 35 years of continuous cropping after forest clearing. Productivity remained at a low level of 3.0 t grain ha-1 across the chronosequence stretching up to 105 years of continuous cultivation despite full N–phosphorus (P)–potassium (K) fertilization (120–100–100 kg ha−1). Application of organic resources reversed the productivity decline by increasing yields by 57–167%, whereby responses to nutrient-rich green manure were 110% greater than those from nutrient-poor sawdust. Productivity at the most degraded sites (80–105 years since forest clearing) increased in response to green manure to a greater extent than the yields at the least degraded sites (5 years since forest clearing), both with full N–P–K fertilization. Biochar additions at the most degraded sites doubled maize yield (equaling responses to green manure additions in some instances) that were not fully explained by nutrient availability, suggesting improvement of factors other than plant nutrition. There was no detectable influence of texture (soils with either 11–14 or 45–49% clay) when low quality organic matter was applied (sawdust, biochar), whereas productivity was 8, 15, and 39% greater (P < 0.05) on sandier than heavier textured soils with high quality organic matter (green and animal manure) or only inorganic nutrient additions, respectively. Across the entire degradation range, organic matter additions decreased the need for additional inorganic fertilizer N irrespective of the quality of the organic matter. For low quality organic resources (biochar and sawdust), crop yields were increasingly responsive to inorganic N fertilization with increasing soil degradation. On the other hand, fertilizer N additions did not improve soil productivity when high quality organic inputs were applied. Even with the tested full N–P–K fertilization, adding organic matter to soil was required for restoring soil productivity and most effective in the most degraded sites through both nutrient delivery (with green manure) and improvement of SOC (with biochar).

/pdf/reversibility-of-soil-productivity-decline-with-organic-2vgsnt2fyz.pdf

Reversibility of Soil Productivity Decline with Organic Matter of Differing Quality Along a Degradation Gradient

Mycorrhizas and tropical soil fertility

Tithonia diversifolia, a shrub in the family Asteraceae, is widely distributed along farm boundaries in the humid and subhumid tropics of Africa. Green biomass of tithonia has been recognized as an effective source of nutrients for lowland rice (Oryza sativa) in Asia and more recently for maize (Zea mays) and vegetables in eastern and southern Africa. This paper reviews the potential of tithonia green biomass for soil fertility improvement based on recent research in western Kenya. Green leaf biomass of tithonia is high in nutrients, averaging about 3.5% N, 0.37% P and 4.1% K on a dry matter basis. Boundary hedges of sole tithonia can produce about 1 kg biomass (tender stems + leaves) m−1 yr−1 on a dry weight basis. Tithonia biomass decomposes rapidly after application to soil, and incorporated biomass can be an effective source of N, P and K for crops. In some cases, maize yields were even higher with incorporation of tithonia biomass than with commercial mineral fertilizer at equivalent rates of N, P and K. In addition to providing nutrients, tithonia incorporated at 5 t dry matter ha−1 can reduce P sorption and increase soil microbial biomass. Because of high labor requirements for cutting and carrying the biomass to fields, the use of tithonia biomass as a nutrient source is more profitable with high-value crops such as vegetables than with relatively low-valued maize. The transfer of tithonia biomass to fields constitutes the redistribution of nutrients within the landscape rather than a net input of nutrients. External inputs of nutrients would eventually be required to sustain production of tithonia when biomass is continually cut and transferred to agricultural land.

/pdf/tithonia-diversifolia-as-a-green-manure-for-soil-fertility-3osn0nqhu0.pdf

A. I. Niang

Papers

Tithonia diversifolia as a green manure for soil fertility improvement in western Kenya: A review

Soil fertility, small-farm intensification and the environment in Africa.