Sesbania tree fallows on phosphorus-deficient sites: Maize yield and financial benefit

Rotation of Sesbania sesban (L.) Merr., a fast-growing N2-fixing tree, with maize (Zea mays L.) has potential for increasing fertility of tropical soils, where fertilizer use by resource-poor farmers is limited. At two sites in Kenya (Ochinga, with a Kandiudalfic Eutrudox soil, and Muange, with a Kandic Paleustalf), we compared maize yields and financial returns for (i) sesbania grown for three or four seasons followed by three maize crops (sesbania fallow), (ii) one maize crop followed by natural regrowth of vegetation for three seasons and then three maize crops (natural fallow), and (iii) maize monoculture for seven seasons. After the fallows, plots were split with and without added P. Maize responded to P at both sites. Cumulative grain yields for seven seasons of maize monoculture were 8.4 Mg ha1 at Ochinga and 5.6 Mg ha1 at Muange. They were comparable to cumulative maize yields for sesbania fallow (Ochinga, 10.6 Mg ha1 Muange, 4.5 Mg ha1) and natural fallow (Ochinga, 7.7 Mg ha1; Muange, 4.2 Mg ha1), even though maize was grown for only three or four seasons in the fallow treatments. Sesbania fallow was financially attractive at Ochinga (500 mm rain in each season) but not at Muange, where low rainfall (<300 mm in each posffallow season) limited maize yield. Phosphorus fertilization of maize at Ochinga increased (P < 0.2) net benefit for sesbania fallow. Improved fallows have potential to supply nutrients to crops, but they are unlikely to eliminate the need for P fertilizers on P-deficient soils.

Soil fertility management for reclamation of imperata grasslands by smallholder agroforestry

Imperata cylindrica grasslands are widely believed to indicate poor soil fertility. Soil fertility improvement may have to be an important component of a reclamation strategy. Data for Sumatra, Indonesia indicate, however, that Imperata occurs on a broad range of soil types and is not confined to the poorest soils. A direct role of Imperata in soil degradation cannot be ascertained. In many instances, however, Imperata soils are low in available P and effective N supply. The use of rock phosphate in combination with erosion control (‘fertility traps’) and legume cover crops can be effective in restoring soil fertility. Case studies for a number of sites in Sumatra have confirmed the practical possibility of reclaiming grasslands for food and tree crops.

Phosphorus and liming effects on early growth of selected plant species grown on an ultisol

A series of pot experiments were conducted to test the effect of phosphorus (P) application and liming on early growth performance of Zea mays, Mucuna pruriens and nine woody species (Albizia ferruginea, Cajanus cajan, Dactyladenia barteri, Dalbergia sissoo, Enterolobium cyclocarpum, Flemingia macrophylla, Gliricidia sepium, Leucaena leucocephala, and Pentacletra macrophylla) on an Ultisol. Low reponse to liming and high response to P application were observed in this trial. Early top growth of the eleven species was correlated to seed size and seed nitrogen (N), P, and calcium (Ca) contents. Highest correlation was observed with seed P content (r = 0.896). Large seeded size species (D. barteri, E. cyclocarpum, M. pruriens, and P. macrophylla) showed the least reponse to P application, while the remaining small seeded size species showed significant responses to P application. The large seeded species also showed relatively less increases in N, P and Ca uptakes with P application and liming. The pot test can be used in prescreening of woody species for use on low P and acid soils. The results also emphasize the need for seed size consideration in multipurpose trees and shrubs selection for establishment in low P and on degraded soils

Soil fertility replenishment in sub-saharan Africa: a new techniques and the spread of their use on farms

In farmed parts of su-saharan Africa where population pressure is high, soil fertility has been declining as fallows become shorter and cropping gets more intense. Loss of nutrients from the soil is now a fundamental problem for farmers in eastern and southern Africa. Scientists are studying on-farm as well as on-station experimentation in western Kenya and eastern Zambia in order to find ways of retaining and restoring soil fertility so that the land can continue to supply farmers with food and livelihood. This article presents the trials being conducted in these areas.

Modelling root architecture and phosphorus uptake in agroforestry

A modified version of the WaNuLCAS model [Water, Nutrient and Light Capture in Agroforestry Systems, which models water, nitrogen and light interactions in agroforestry systems] is outlined. The modified model includes phosphorus as a second nutrient, with a range of phosphorus (P) acquisition mechanisms for crops and trees, and interactions via recycling and rhizosphere modification

Soil phosphorus availability after slash-and-burn fires of different intensities in rubber agroforests in Sumatra, Indonesia

Land clearing fires in Sumatra, Indonesia, caused enormous environmental problems for southeast Asia in 1997, but rubber farmers as well as large rubber and oil palm plantation owners continued to slash-and-burn due to the lack of an alternative that is equally quick, cheap, and capable of improving soil fertility. A partial alternative may be found in the reduction of the fuel load before the burn by harvesting and selling a larger fraction of the wood and, thus, changing the intensity of the fire and reducing particulate and greenhouse gas emission. An adequate phosphorus (P) supply is critical to crop production in Sumatra. Thus, it is important to understand the effects of such a reduction in fire intensity on crop P availability. Field and laboratory experiments were conducted to: (1) determine P sorption capacities and affinity constants for a forest soil exposed to different fire intensities in a controlled oven experiment (heat effects only); (2) compare the sorption characteristics of oven-burned soil with field-burned soil (heat and ash addition effects); and (3) determine the effects of fire-induced changes in soil properties on indices of P availability. Sorption experiments using oven-heated forest soil showed an increase in both the maximum amount of P it can hold (P sorption capacity, Xm) and the strength with which this P is retained to soil particles (affinity constant, K) with heating to 450 °C. Field burning resulted in similar increases in Xm but reduced the affinity constants in the surface 15 cm soil. The increase in Xm was related to a fire-induced increase in specific surface area of the mineral fraction. The addition of ash in field burns appeared to be responsible for the observed decrease in K. The increase in Xm is expected to enlarge the need for P fertilizers as soon as solution P declines to pre-burn levels. These mineralogy-based changes in P sorption characteristics of surface soil are expected to be long-term and could reduce the time period for sustained annual crop production, establishment of new rubber plantations from seeds, as well as affect soil loss and sustained rubber production. In managing the intensity of slash-and-burn fires, the farmer, thus, has to balance between short-term gains in P availability and long-term costs in increased P sorption.

Use of calliandra–Napier grass contour hedges to control erosion in central Kenya

Contour hedgerow systems consisting of various combinations of tree and grass species can be used on sloping lands to minimize erosion, restore fertility, and improve crop productivity, but there is need to evaluate the effectiveness of each system for its suitability at any locality as effective erosion control. The objectives of this study were to determine the amount of soil conserved by contour calliandra (Calliandra calothyrsus)–Napier grass (Pennisetum purpureum) hedgerows, and then develop a support practice P-subfactor for conservation planning in central Kenya. As a benefit beyond soil conservation, biomass yield and N and P retention by the hedgerows were determined. Cumulative data for five cropping seasons from 1997 to 1999 indicated that the contour hedges on 20% slope conserved more soil (168 Mg ha1) than on the 40% slope (146 Mg ha1) compared to the control plots. For both slopes, this was equivalent to a 0.7 P-subfactor for use by the Revised Universal Soil Loss Equation (RUSLE) model in predicting soil erosion. The N and P losses between the hedges and control were statistically significant only on the 20% slope (P=0.05). Combined biomass yield from the calliandra–Napier grass hedges were 12 and 9 Mg ha1 per year and 40% slopes, respectively. This soil conservation technology may be used by small-scale farmers that use mixed farming systems in the highlands of central Kenya and similar ecoregions as a step towards sustainable farming.

Phosphorus availability and maize response to organic and inorganic fertilizer inputs in a short term study in western Kenya

The use of organic materials as P sources is of considerable interest in smallholder farming systems in tropical Africa, mainly because of their potential as alternatives to inorganic P fertilizers. Field studies conducted in a Nitisol of western Kenya in 1995 (crop 1) and 1996 (crop 2) compared effects of organic and inorganic fertilizers on resin extractable P availability (Pext) and maize (Zea mays L.) yield (MY). Leaf biomass and small twigs of Tithonia diversifolia, Croton megalocarpus, Lantana camara L., Senna spectabilis, Calliandra calothyrsus, and Sesbania sesban, were applied at 5 Mg ha1 (DW), supplying an estimated 9–15 kg P ha1 and 30–212 k N ha1. The inorganic fertilizer was triple superphosphate (TSP), applied at 0 (control), 10, 25, 50, and 150 kg P ha1 with each plot receiving 120 kg N ha1 as urea. All plots received a blank application of 100 kg K ha1 as potassium chloride. Between 92 and 98% (P<0.001) of the variation in Pext was explained by the P added (Padd) from the amendments. Response of MY to Padd was best described by logarithmic function as: Full-size image (<1 K) (R2=0.91) for crop 1, and Full-size image (<1 K) (R2=0.90) for crop 2. Response of MY to Pext was best described by a linear function with R2 ranging from 0.84 to 0.89 for crop 1, and 0.76 to 0.81 for crop 2. Effects of Tithonia and Croton on MY were similar to effects of 50 kg P ha1+120 kg N ha1 as inorganic fertilizer. Although the confounding effects of nutrients other than P in the organic materials on MY were isolated, it appears that reasonable MY can be achieved if adequate amounts of high quality organic materials such as Tithonia and Croton are used as P sources. A term, ‘phosphorus availability index’ (PAI), calculated as: (Pext treatmentsPext control)/P added, was introduced to describe the P availability capacity of the amendments. The PAI values suggested a greater propensity for net P mineralization following addition of all organic materials (except S. spectabilis) than for the inorganic amendments. The PAI results indicate that improvements of soil Pext can come from either P released from organic inputs or increased availability of native soil P following addition of organic and inorganic fertilizers.

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

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) m1 yr1 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 ha1 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.

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