B.H. Janssen

A system for quantitative evaluation of the fertility of tropical soils (QUEFTS).

Janssen, B.H., Guiking, F.C.T., van der Eijk, D., Smaling, E.M.A., Wolf, J. and van Reuler, H., 1990. A system for quantitative evaluation of the fertility of tropical soils (QUEFTS). Geoderma, 46: 299-318. A system is described for a quantitative evaluation of the native fertility of tropical soils, using calculated yields of unfertilized maize as a yardstick. The system is applicable to well drained, deep soils, that have a pH(H20) in the range 4.5-7.0, and values for organic carbon, P-Olsen and exchangeable potassium below 70 g/kg, 30 mg/kg and 30 mmol/kg, respectively (0-20 cm). Soil fertility is interpreted as the capacity of a soil to provide plants with nitrogen, phosphorus and potassium, but the methodology allows for including other nutrients. The procedure consists of four successive steps. First the potential supplies of nitrogen, phosphorus and potassium are calculated, applying relationships between chemical properties of the 0-20 cm soil layer and the maximum quantity of those nutrients that can be taken up by maize, if no other nutrients and no other growth factors are yield-limiting. In the second step the actual uptake of each nutrient is calculated as a function of the potential supply of that nutrient, taking into account the potential supplies of the other two nutrients. Step 3 comprises the establishment of three yield ranges, as depending on the actual uptakes of nitrogen, phosphorus, and potassium, respectively. Next, these yield ranges are combined in pairs, and the yields estimated for pairs of nutrients are averaged to obtain an ultimate yield estimate (Step 4). The relationships used in Steps 1 and 3 were derived from empirical data of field trials in Suriname and in two strongly different agro-ecological zones in Kenya. The equations developed for Steps 2 and 4 were mainly based on theoretical considerations. The equations used to calculate the potential supplies of nutrients (Step 1 ) should be applied only to soils with the indicated properties. The other equations are more generally applicable. Examples are given to elucidate the procedure. QUEFFS may be a very useful tool in quantitative land evaluation. Its principles may be applied to other crops, soils, nutrients and agro-ecological regions than those involved in this study.

Distribution of mycorrhizal fungal spores in soils under agroforestry and monocultural coffee systems in Brazil

Deep-rooting trees in agroforestry systems may promote distribution of spores of arbuscular mycorrhizal fungi (AMF) at deeper soil levels. We investigated the vertical distribution of AMF spores in Oxisols under agroforestry and monocultural (unshaded) coffee systems in on-farm experiments (Brazil). The number of AMF spores was considered as an indicator of mycorrhiza incidence in soil. Spores were extracted from 0–1, 2–3, 5–7.5, 10–15, 20–30, 40–60 cm soil-depths in agroforestry and monocultural coffee systems, of three different age groups (young, medium-aged and old), using centrifugation methods, and counted. Fine roots were collected and dry-weighed from 0–30 cm in young and old systems and from several depths in medium-aged systems. Soils were characterised with respect to texture, pH, organic matter, calcium, magnesium, phosphorus and potassium. Agroforestry had a higher percentage of spores (12–21% of the total number of spores) and roots (on average 1.5 g L−1 soil) in deeper layers (20–60 cm), and a lower percentage (79–88%) closer to the surface (0–15 cm) than the monocultural fields (respectively 3–12%, 0.6 g L−1 soil and 88–97%). Greater numbers of spores in the deeper soil layers may be explained by greater amounts of roots and may be an indicator of greater incidence of mycorrhiza in agroforestry than in monocultural coffee systems. Greater mycorrhizal incidence at deeper soil layers in the agroforestry system may change the dynamics of phosphorus cycling in soil, making this nutrient more available to plants.

Inorganic nitrogen dynamics in fallows and maize on an Oxisol and Alfisol in the highlands of Kenya

Abstract Fallows with naturally regenerated or planted vegetation are important in many subsistence agricultural systems of tropical regions, but the underlying soil processes in fallows are not properly understood. We investigated N dynamics under different fallow vegetation on a Kandiudalfic Eutrudox (2372-mm rain in 16 months) and a Kandic Paleustalf (1266-mm rain in 15 months) in the Kenyan highlands. The treatments, which extended for three cropping seasons (15–16 months), were Zea mays (maize), natural regrowth of vegetation (natural fallow), planted Sesbania sesban (sesbania fallow) and uncultivated soil without vegetation (bare fallow). Inorganic N (nitrate+ammonium-N) to 2-m depth under bare fallow increased by 242 kg N ha −1 year −1 on the Oxisol and 54 kg N ha −1 year −1 on the Alfisol, indicating that N mineralization exceeded N losses. Subsoil inorganic N (0.5–2.0 m) remained relatively unchanged after three crops of unfertilized maize, which produced limited total biomass because of P deficiency. Inorganic N decreased during natural and sesbania fallows, and both fallows similarly depleted subsoil inorganic N. The fallows depleted inorganic N at 0.5–2.0 m by 75–125 kg N ha −1 year −1 down to a minimum N content between 40 and 80 kg N ha −1 . After slashing sesbania and incorporating the above-ground biomass with 154–164 kg N ha −1 , soil inorganic N increased within 2 months by 136 kg N ha −1 on the Oxisol and 148 kg N ha −1 on the Alfisol. Inorganic N decreased after cropping the bare fallow on the Oxisol with maize, indicating that inorganic N was prone to leaching during heavy rains when the maize was small. A considerable part of the N in biomass of the natural fallow was recycled. Much of the total N accumulated by the sesbania fallow was removed with the wood and the amount of N recycled was similar on the Oxisol and Alfisol. We conclude that sesbania fallows can retrieve considerable subsoil inorganic N on deep soils with high subsoil N and effectively cycle this N through its rapidly decomposable biomass to subsequent crops.

Relationship between substrate initial reactivity and residues ageing speed in carbon mineralization

A desk study was conducted on the general relationship between substrate initial reactivity and the speed of ageing of residues in carbon mineralization. Totally 306 sets of experimental data were collected from 36 studies, covering a wide range of substrates and soil and environmental conditions. A model was used as a framework, which treats a substrate and subsequent residues as a whole, and describes the carbon mineralization with two parameters: the initial average rate coefficient (R) and the speed of ageing of residues (S). While both R and S were affected by substrate properties as well as by soil and environmental conditions, they were positively related to each other. In other words, the more quickly mineralization goes in the beginning, the more quickly the residues age. As a result, the initial differences in substrate reactivity would disappear over time, but the differences in residues quantity and carbon loss rate could last much longer, with substrates initially less reactive having higher carbon loss rates. The implications of the relationship between R and S was discussed with respect to dynamics of residues reactivity, quantity and carbon loss rate in relation to effects of substrate differences and impacts of external conditions.

Nutrient fluxes in the shifting cultivation system of south-west Côte d'Ivoire

At two sites, one with a 4-year-old (4-Y) secondary vegetation and the other with a 20-year-old (20-Y) vegetation, the influence of burning slashed vegetation on crop performance was studied during three seasons. In the first season after clearing, also the influence on weed growth was studied. At both sites, burning significantly decreased the number of weed seedlings. The lowest number of seedlings was found on the burnt plots of the 20-Y site. Burning increased yield and nutrient uptake significantly in the first and second season after clearing. In the third season after burning, only at the 4-Y site a significantly higher yield and nutrient uptake were found. At the 20-Y site the effect had disappeared. Calculations of efficiency of utilization of absorbed N, P and K indicated that P was the least available nutrient, also after burning. At both sites three consecutive crops absorbed approximately 40% of P applied in ash, while the cumulative recovery of K was at least 36% at the 4-Y site and at least 59% at the 20-Y site. On non-burnt plots, yields were not lower in the third season than in the first season after clearing, thus indicating that the inherent soil fertility did not decrease. Hence, yield decline on the burnt plots could be ascribed to ash depletion. It was concluded that in the local shifting cultivation system, the combination of ash depletion and infestation of weeds are the main reasons for abandoning the fields.

Analysis of phosphorus by 31PNMR in Oxisols under agroforestry and conventional coffee systems in Brazil

Abstract Phosphorus (P) is the primary limiting nutrient for crop production in highly weathered tropical soils. The deficiency is mainly caused by strong adsorption of H2PO4− to Al- and Fe-(hydr)oxides, which turns large proportions of total P into a form that is unavailable to plants. Soil management modifies P dynamics. Some plants, including trees used in agroforestry systems, are known to accelerate P cycling. The objective of this paper was to use phosphorus 31 nuclear magnetic resonance (31PNMR) to evaluate the inorganic (Pi) and organic P (Po) compounds in Oxisol from two agroforestry (15 and 19 years old) and two conventional (full-sun, monoculture, ca. 15–20 and 20–24 years) coffee systems at three different depths (2–3, 10–15 and 40–60 cm). We hypothesised that the amounts of (1) organic P and (2) diester are higher in agroforestry fields than in conventional coffee fields and (3) the organic P and the diester decrease less with depth in the agroforestry systems than in the conventional systems. The soils were sampled from on-farm experiments in the Atlantic Coastal Rainforest, Brazil. The soil P was extracted with NaOH 0.5 M+EDTA 0.1 M. Resin chelex-X100 was used to remove the paramagnetic ions. The total P in the NaOH–EDTA extract was measured through ICP and the Pi by the ammonium molybdate–ascorbic acid method. Po was calculated as the difference between total P and Pi. The amount of Po was higher, the decrease of Po with depth was more sharp and the Po/total P was lower in the conventional systems than in the agroforestry systems. Based on literature and standards, 31PNMR signals were interpreted as inorganic orthophosphate, orthophosphate monoester (inositol phosphates and mononucleotides), orthophosphate diester (phospholipids, nucleic acids and teichoic acid) and pyrophosphates. The proportion of organic P (Po) was on average 47%, consisting of monoester (95%) and diester (5%). The amounts of diester phosphates did not differ between systems, but the proportion of diester to total spectra areas was higher and the decrease of diester with depth was less in the agroforestry than in the conventional systems. The proportions of inorganic P to total P consisted on average of 45% orthophosphate and 8% pyrophosphate. Our results suggest that agroforestry systems influence the dynamics of P through the conversion of part of the inorganic P into organic P. The effect was higher in deeper layers. Because the rate of cycling is higher for organic P than for inorganic P and for diester than for monoester, and because the P in deep layers is normally less available to crop plants, agroforestry would maintain larger fractions of P available to agricultural crops, thereby reducing P losses to the unavailable pools. The rate and the impacts of these changes on P cycling and efficiency of P use of the crops in the long-term need to be further examined and understood, for full evaluation of the importance of agroforestry in soil P utilisation.

Differential Access to Phosphorus Pools of an Oxisol by Mycorrhizal and Nonmycorrhizal Maize

Abstract This study investigated whether arbuscular mycorrhizal fungi (AMF) could take up phosphorus (P) from pools that are normally considered unavailable to plants. An aluminum (Al) resistant maize variety, inoculated with three species of Glomus or uninoculated, supplied with nutrient solution without P, was cultivated (90 days) in the A and B horizons of a P‐fixing Oxisol. Plant uptake of P was calculated by assessing P content of shoots and roots and correcting for seed P. Soil P fractionation was done prior to and at the end of the experiment. Phosphorus in the A and B soil horizons (∼270 mg soil kg−1) was differently distributed among the pools. Nonmycorrhizal plants did not acquire any P from the soil, and all P found in the plants was from the seeds. Mycorrhizal plants depleted the inorganic Resin‐P and NaHCO3‐P, used part of the inorganic NaOH‐P, and used neither the recalcitrant inorganic P nor the organic P fractions. Changes in plant P content matched changes in the soil P pools. Mechanisms by which maize through the mycorrhizal association acquires P are discussed. In the cultivar used, the mechanisms to cope with P deficiency and Al excess are different.

Influence of time of application on the performance of gliricidia prunings as a source of n for maize

Asynchrony between nitrogen (N) released by organic materials and N demand by the crop leads to low N use efficiency. Optimizing the time of application could increase the N recovery. A field experiment was designed to determine the effects of time of application of Gliricidia sepium prunings and of the addition of small doses of inorganic N fertilizer on N recovery and yield of maize. Six split applications of gliricidia prunings (in October, December and February) were compared. The prunings were incorporated into the soil while fresh. The application in October was done four weeks before planting the maize. Higher N uptake and maize yields were obtained when gliricidia prunings were applied in October than when applied in December and February. The corresponding substitution values were 0.66, 0.32 and 0.20. Split applications of prunings prolonged mineral N availability in the soil until March but did not increase N uptake and maize grain yield compared to a sole application in October. Combinations of gliricidia prunings and inorganic fertilizer increased N uptake and maize yield over prunings alone but the effect was only additive. We concluded that application of gliricidia prunings in October was more efficient than application in December and February

Effects of improved drainage and nitrogen source on yields, nutrient uptake and utilization efficiencies by maize (Zea mays L.) on Vertisols in sub-humid environments

Nitrogen is the most limiting plant nutrient in Vertisols in Kenya. Soil properties, climatic conditions and management factors as well as fertilizer characteristics can influence fertilizer nitrogen (N) use efficiency by crops. Vertisols, characterized by low-basic water infiltration rate, are prone to waterlogging under sub-humid and humid conditions. We determined effects of drainage, N source and time of application on yields, nutrient uptake and utilization efficiencies by maize grown on Vertisols in sub-humid environments. Treatments comprised two furrows (40 cm and 60 cm deep) and a check (i.e., no furrow), calcium nitrate to furnish NO3-N, ammonium sulphate to supply NH4-N at 100 kg N ha−1, a control (i.e., no fertilizer N), and fertilizer N application at sowing, 40 days after sowing, and split (i.e., half the rate at sowing and half 40 days after sowing). A split-plot design was used in which drainage formed the main plots and N source × time of N application formed the sub-plots. Higher grain and total dry matter yields, harvest index, leaf N content, uptake of N, P and K, as well as N agronomic (NAE) and recovery (NRE) efficiencies were obtained from drained compared to undrained plots. The increase ingrain yields as a result of drainage varied from 31 to 45% for control, 35 to 43% for NO3-N, and 16 to 21% for NH4-N treatments. Drainage resulted in total N uptake increases from 50 to 80 kg N ha−1 in control plots, 80 to 130 kg N ha−1 in NO3-N treated plots, and 90 to 130kg N ha−1 in NH4-N treated plots. Ammonium-N source was superior to NO3-N source in terms of higher yields, NAE, and NRE in undrained plots, but the two N sources behaved similarly in drained plots. Delayed or split NO3-N application gave higher yields, NAE and NRE than when all N was applied at sowing in undrained plots. There was no difference between 40 cm and 60 cm deep furrows in terms of crop yields and nutrient use efficiencies. Thus, draining excess water with furrows at least 40 cm deep is essential for successful crop production in these Vertisols under sub-humid conditions.

Intercropping affects the rate of decomposition of soil organic matter and root litter

AimsIntercropping increases aboveground and belowground crop productivity, suggesting potential for carbon sequestration. Here we determined whether intercropping affects decomposition of soil organic matter (SOM) and root litter.MethodsWe measured in the laboratory and the field the breakdown of SOM, root litter of maize, wheat, or faba bean, litter mixtures, and a standard substrate (compost) in soils from a long term intercropping experiment.ResultsSoil organic matter from intercrop plots decomposed faster than SOM from monocrop plots, but compost decomposed at similar rates in different soils. Faster SOM decomposition was associated with lower soil C:N ratio. Root litter mixtures of maize and wheat decomposed as expected from single litters, but litter mixture of maize and faba bean decomposed faster than expected, both in the laboratory and in the field. Root litter decomposed slowly in maize/wheat intercrop soil compared to the two monocropped soils in the laboratory, but the effect was absent in the field.ConclusionsIntercropping increases SOM decomposition, presumably through reduced SOM recalcitrance resulting from lower C:N ratio, higher litter input and better N retention. Depending on the crop combination, also non-additive effects of root litter mixing can enhance organic matter decomposition in intercropping soils.