André Bationo

Research on nutrient flows and balances in west Africa: state-of-the-art

West Africa is poorly endowed when it comes to soil fertility. Unlike for example the Rift Valley area, west African soils never enjoyed volcanic rejuvenation. At low agricultural intensity, this does not matter as nutrients cycle through the soil and the natural vegetation and losses are few. However, the past decades have shown high population increases, the breakdown of traditional shifting cultivation systems, and a rapid decline of land productivity and soil fertility in particular. The present review paper shows how much is known about the severity of this process and the technologies at hand that can stop it. The information provided shows that on the technical side much is known now, but research output still is poorly integrated into development efforts.

Role of manures and crop residue in alleviating soil fertility constraints to crop production: With special reference to the Sahelian and Sudanian zones of West Africa

In the West African semi-arid tropics (WASAT), continuous cultivation leads to drastically reduced levels of soil organic matter. Such reductions in the level of soil organic matter have resulted in decreased soil productivity. The addition of organic materials either in the form of manures or crop residue has beneficial effects on the soils chemical and physical properties. For many of the countries in this region, the amounts of nutrients in crops and crop residue are often several orders of magnitude higher than the quantity of the same nutrients applied as fertilizers. The return of the crop residue for soil fertility improvement cannot be overstressed. It is essential that more information on the rates of organic matter decomposition as well as the many reactions between products of organic matter decomposition and the soil under WASAT conditions be made available.

Cereal/legume rotation effects on cereal growth in Sudano-Sahelian West Africa: soil mineral nitrogen, mycorrhizae and nematodes.

Yield increases of cereals following legumes in rotation have been previously reported for West Africa, but little progress has been made to explain the mechanisms involved. At four sites in Niger and Burkina Faso, field trials with pearl millet (Pennisetum glaucum (L.) R. Br.), cowpea (Vigna unguiculata (L.) Walp), sorghum (Sorghum bicolor (L.) Moench) and groundnut (Arachis hypogaea L.) were conducted from 1996 to 1998 to investigate the role of soil mineral nitrogen (Nmin), native arbuscular mycorrhizae (AM) and nematodes in cereal/legume rotations. Grain and total dry matter yields of cereals at harvest were increased by legume/cereal rotations at all sites. Soil Nmin levels in the topsoil were consistently higher in cereal plots previously sown with legumes (rotation cereals) compared with plots under continuous cereal cultivation. However, these rotation effects on Nmin were much larger with groundnut than with cowpea. Roots of rotation cereals also had higher early AM infection rates compared to continuous cereals. The dominant plant-parasitic nematodes found in all experiment fields were Helicotylenchus sp., Rotylenchus sp. and Pratylenchus sp. In sorghum/groundnut cropping systems, nematode densities were consistently lower in rotation sorghum compared to continuous sorghum. Continuous groundnut had the lowest nematode densities indicating that groundnut was a poor host for the three nematode groups. In millet/cowpea cropping systems with inherently high nematode densities, crop rotations barely affected nematode densities indicating that both crops were good hosts. These results suggest that on the nutrient poor Sudano-Sahelian soils of our study, total dry matter increases of rotation cereals compared with continuous cereals can be explained by higher Nmin and AM infection levels early in the season. The site-specific magnitude of these effects may be related to the efficiency of the legume species to suppress nematode populations and increase plant available N through N2-fixation.

Alleviating soil fertility constraints to increased crop production in West Africa: The experience in the Sahel

Lack of moisture limits crop production in semi-arid west Africa but poor soil fertility is a more serious constraint in the long run. Work done by IFDC and ICRISAT showed that phosphorus is the most limiting nutrient although response by millet to nitrogen when moisture and P are non-limiting can be substantial. A summary of the results of field trials in Niger showed that the sufficiency level for P estimated by regression analysis on the basis of Bray 1 extractable P is 7.9 μg P/g of soil for 90% of maximum yield of millet. Application of 15–20 kg P/ha was usually adequate for optimum yields. Matam phosphate rock (PR) from Senegal, Tilemsi PR from Mali and Tahoua PR from Niger which are medium reactive were found to be suitable for direct application while partial acidulation (50% with sulfuric acid) of the less reactive phosphate rocks resulted in products with similar agronomic effectiveness as commercial superphosphates. Tests conducted by farmers showed that millet yields can be increased by more than 250% by the use of fertilizers. The need for constant monitoring of the crop needs for sulfur and potassium under continuous cultivation was highlighted.

Effect of crop residues on root growth and phosphorus acquisition of pearl millet in an acid sandy soil in Niger

The effect of long-term (1983–1988) applications of crop residues (millet straw, 2–4 t ha-1 yr−1) and/or mineral fertilizer (30 kg N, 13 kg P and 25 kg K ha-1 yr-1) on uptake of phosphorus (P) and other nutrients, root growth and mycorrhizal colonization of pearl millet (Pennisetum glaucum L.) was examined for two seasons (1987 and 1988) on an acid sandy soil in Niger. Treatments of the long-term field experiment were: control (−CR−F), mineral fertilizer only (−CR+F), crop residues only (+CR−F), and crop residues plus mineral fertilizer (+CR+F).In both years, total P uptake was similar for +CR−F and −CR+F treatments (1.6−3.5 kg P ha-1), although available soil P concentration (Bray I P) was considerably lower in +CR−F (3.2 mg P kg-1 soil) than in −CR+F (7.4) soil. In the treatments with mineral fertilizers (−CR+F; +CR+F), crop residues increased available soil P concentrations (Bray I P) from 7.4 to 8.9 mg kg-1 soil, while total P uptake increased from 3.6 to 10.6 kg P ha-1. In 1987 (with 450 mm of rainfall), leaf P concentrations of 30-day-old millet plants were in the deficiency range, but highest in the +CR+F treatment. In 1988 (699 mm), leaf P concentrations were distinctly higher, and again highest in the +CR+F treatment. In the treatments without crop residues (−CR−F; −CR+F), potassium (K) concentrations in the leaves indicated K deficiency, while application of crop residues (+CR−F; +CR+F) substantially raised leaf K concentrations and total K uptake. Leaf concentrations of calcium (Ca) and magnesium (Mg) were hardly affected by the different treatments.In the topsoil (0–30 cm), root length density of millet plants was greater for +CR+F (6.5 cm cm-3) than for +CR−F (4.5 cm cm-3) and −CR+F (4.2 cm cm-3) treatments. Below 30 cm soil depth, root length density of all treatments declined rapidly from about 0.6 cm cm-3 (30–60 cm soil depth) to 0.2 cm cm-3 (120–180 cm soil depth). During the period of high uptake rates of P (42–80 DAP), root colonization with vesicular-arbuscular mycorrhizal (VAM) fungi was low in 1987 (15–20%), but distinctly higher in 1988 (55–60%). Higher P uptake of +CR+F plants was related to a greater total root length in 0–30 cm and also to a higher P uptake rate per unit root length (P influx). Beneficial effects of crop residues on P uptake were primarily attributed to higher P mobility in the soil due to decreased concentrations of exchangeable Al, and enhancement of root growth. In contrast, the beneficial effect of crop residues on K uptake was caused by direct K supply with the millet straw.

Long- and short-term effects of crop residues on aluminum toxicity, phosphorus availability and growth of pearl millet in an acid sandy soil*

In a long-term field experiment millet straw application (+CR) increased soil pH and base saturation and strongly improved pearl millet (Pennisetum glaucum L.) growth on acid sandy soils. Aluminum (Al) toxicity may be responsible for poor millet growth in plots without crop residues (−CR). Laboratory experiments were conducted to verify this assumption. The concentrations of labile Al (8-hydroxyquinoline, 15 sec) in equilibrium soil solutions of top soil samples from field plots were 14.0 and 0.6 μM in unfertilized samples of −CR and +CR soil, respectively. The corresponding values for labile Al in fertilized (NPK) samples were 51.8 and 11.0 μM, respectively. A short-term (14 days) incubation of −CR soil with ground millet straw (0.1% w/w) increased soil solution pH and decreased total and labile Al in the soil solution by more than 44%. In a water-culture experiment with increasing concentrations of Al (0–60 μM) pearl millet proved to be very Al-tolerant compared to cowpea, peanut and soybean. A short-term (12 days) pot experiment with the incubated soil showed that root growth of pearl millet is not restricted by Al toxicity in the acid soils from Niger, but that after millet straw incubation root growth is considerably enhanced. Phosphorus (P) concentration in the soil solution was about three times higher in +CR (1.75 μM) than in −CR (0.52 μM) top soil. Since P is the most growth-limiting nutrient in those soils, the beneficial effect of crop residues on pearl millet is likely due to improvement of P nutrition by both increase in P mobility in the soil and enhancement of root growth.

The effect of crop residue and fertilizer use on pearl millet yields in Niger

A field study was conducted over a 4-year period in Niger, West Africa, to determine the effects of crop residue (CR), fertilizer, or a combination of crop residue and fertilizer (CRF) on yields of pearl millet (Pennisetum glaucum [L.] R. Br.). Despite a decline in yields of control plots (initial yields were 280 kg grain ha−1 declining to 75 kg grain ha−1 over 4 years), yields of fertilizer plots were maintained at 800–1,000 kg grain ha−1. Continued application of CR slowly augmented yields to levels similar to those of the fertilized plots. The effects of CR and fertilizer were approximately additive in the CRF plots. Addition of CR and fertilizer increased soil water use over the control by 57 mm to 268 mm in an average season and helped trap wind-blown soil. These plots tended to exhibit slightly higher soil pH and lower Al saturation than did the fertilized treatments. Return of CR to the soil resulted in significantly reduced export of most plant nutrients, especially Ca, Mg, and K.

A farm-level evaluation of nitrogen and phosphorus fertilizer use and planting density for pearl millet production in Niger

Mineral fertilizer use is increasing in West Africa though little information is available on yield response in farmers fields. Farmers in this region plant at low density (average 5,000 pockets ha−1, 3 plants pocket−1), which can affect fertilizer use efficiency. A study was conducted with 20 farmers in Niger to assess the response of pearl millet [Pennisetum glaucum (L.) R. Br.] to phosphorus and nitrogen fertilizers under farm conditions. In each field, treatments included control, single superphosphate (SSP) only, SSP plus N (point placed near plant), and either SSP or partially acidulated phosphate rock (PAPR) plus N broadcast. N and P were applied at 30 kg N ha−1 and 30 kg P2O5 ha−1. Farmers were allowed to plant, weed, etc., as they wished and they planted at densities ranging from 2,000 to 12,000 pockets ha−1. In the absence of fertilizer, increasing density from 2,000 to 7,000 pockets ha−1 increased yield by 400%. A strong interaction was found between fertilizer use and density. Farmers planting at densities less than 3,500 pockets ha−1 had average yields of 317 kg grain ha−1 while those planting at densities higher than 6,500 pockets ha−1 showed average yields of 977 grain ha−1. Though phosphate alone increased yields significantly at all densities, little response to fertilizer N was found at densities below 6,000 pockets ha−1. Significant residual responses in 1987 and 1988 were found to P applied in high-density plots in 1986. Depending on fertilizer and grain prices, analysis showed that fertilizer use must be be combined with high plant density (10,000 pockets ha−1) or no economic benefit from fertilizer use will be realized.

Farmers' perceptions and attitudes towards introduced soil-fertility enhancing technologies in western Africa

Soil fertility enhancing technologies (SFETs) have been promoted in the West African Semi-Arid Tropics (WASAT) for many years with limited success. Using a qualitative approach of focus group discussions, long, open-ended interviews and observations from field visits, this paper explores with farmers their beliefs and rationales behind the adoption or non-adoption of SFETs. Farmers are knowledgeable about, and practise SFETs of rock phosphate application, crop residue and farm yard manure, chemical fertilizer and crop rotation to combat soil fertility decline. Their attitudes to and rationales behind adoption decisions are influenced by the availability and use policies of land and labour resources, food security concerns, perceived profitability, contribution to sustainability and access to information. Some of the factors are beyond farmers control and require a broad and integrated effort from research, extension and government to promote the use of the SFETs in the region.

Short- and long-term effects of crop residues and of phosphorus fertilization on pearl millet yield on an acid sandy soil in Niger, West Africa

Abstract Field experiments were conducted during the rainy seasons of 1990 and 1991 on an acid sandy soil (Luvic Arenosol) in Niger, to assess long-term (since 1986) and short-term (since 1990) effects of millet straw (crop residues) at different amounts (2 t and 6 t ha−1) and modes of application (incorporation, mulching and burning), and of phosphorus (P) fertilization on dry matter yield of pearl millet [Pennisetum glaucum (L.) R. Br.]. Especially long-term, but also short-term application of crop residues increased dry matter yield of pearl millet by more than 60%, whereas their omission decreased yield immediately. Compared to mulching or burning, higher yields were obtained with incorporation of crop residues. When crop residues were applied as mulch, dry matter yield was increased to the same extent by 2 t and 6 t ha−1. Similar dry matter yields were achieved with crop residues (-P) to those with P fertilizer only. An additional yield increase could be obtained by simultaneous application of crop residues and P. Without crop residues, potassium (K) concentrations in the shoot dry matter indicated K deficiency. With crop residues considerable amounts of K (15 kg t−1) were provided and raised the K concentrations in the plants to the sufficiency range. Although with crop residues some P was provided (1.5 kg t−1), the beneficial effects were primarily attributed to increased P acquisition by the millet plants, which was reflected in enhanced root growth in the topsoil (0–10 cm).