Effect of straw application on rice yields and nutrient availability on an alkaline and a pH-neutral soil in a Sahelian irrigation scheme

Like elsewhere in the Sahel, actual rice yields (3-5 t ha-1) are far below yield potential (±8 t ha-1) in an irrigation scheme in central southern Mauritania. Earlier studies showed that yields are especially low on alkaline soils due to N and P deficiency. We investigated the potential of rice straw application as a mean to improve yields and fertilizer efficiency on an alkaline soil (pH 8.2) and a pH-neutral soil (pH 6.2). Application of 5 t straw ha-1 increased yields by 1.1 t ha -1 on average, independent of soil type and fertilizer dose. Contrary to our study, similar studies in Asia showed little short-term effects of straw on yield and N uptake. Straw application improved N availability, but not P availability. The improved N availability was attributed to N mineralized from the straw, from increased mineralization of soil organic matter (SOM) with a low C:N ratio (<7.2) and from increased mineral fertilizer N (urea) recovery efficiency. We deduced that improved N fertilizer recovery upon straw application was due to reduced nitrification-denitrification losses. On the alkaline soil, volatilization was important, but that process seemed unaffected by straw application. We hypothesize that the positive effects of straw application at our study site are due to low soil C content (<43 g kg -1) and low C:N ratio compared to most lowland rice soils in Asi

Soil quality and rice productivity problems in Sahelian irrigation schemes

In irrigation schemes in theSahel, rice yields and cropping intensity are still far from their potential and some of the 10-20 year old irrigation schemes experience declining yields. The large investments in irrigation infrastructure, made to improve food security, to generate income, and to reduce rice imports, are currently at stake. Many farmers and researchers suggested that the rice productivity problems might (partly) be due to salt-related soil degradation. Sahelian irrigation waters contain little salt, but are relatively rich in carbonates (RA calcite > 0). In the hot and dry Sahelian climate, irrigation with such waters could lead to the formation of an alkaline (high pH) and sodic (high sodium content) soil, which has a low productivity. We investigated the causes of declining rice productivity in the irrigation schemes of Foum Gleita (Mauritania) and theSourouValley(Burkina Faso). We found that rice productivity problems in Foum Gleita were primarily caused by N and P deficiency, while Zn deficiency prevailed in theSourouValley. These nutrient deficiencies were caused by no or insufficient application of N, P and Zn fertilizers, in combination with low plant available N, P and Zn in the soil. The alkaline-calcareous nature of the studied soils likely contributed to the low availability of N through volatilization, and low availability of P and Zn through precipitation of or adsorption onto carbonate minerals. Application of ample N, P and Zn fertilizer increased yields at the study sites from 3-4 t ha -1 to 5-6.5 t ha -1 . However, the required chemical fertilizers are often expensive or not available on the localmarkets, which hampersadoption of improved fertilizer recommendations by the resource-poor Sahelian rice farmers. Application of organic amendments (a.o. rice straw) at a rate of 5 t ha -1 often improved yields at the study sites by 1-2 t ha -1 . In Foum Gleita, the yield increasing effect was attributed to improved N availability, while in theSourouValley, organic matter amendments improved Zn uptake.Although rice productivity problems correlated with the alkaline-calcareous nature of the soil, we found no evidence that irrigated cropping increased the alkalinity of these soils. On the contrary, this study and literature sources indicate that alkalinity problems were inherited from the parent material and/or from the prolonged cultivation of non-flooded crops. At the study sites inMauritaniaandBurkina Faso, concentration factors of irrigation waters are small (< 6). Moreover, detailed study in Foum Gleita showed that clayey rice soils seem to have a large buffer capacity against alkalinization. Adsorbed Ca 2+ at the exchange complex is desorbed with increasing salt concentrations in the soil solution, which leads to a decrease in soil solution alkalinity through precipitation of calcite. In addition, irrigated rice largely prevents the build-up of alkaline salts in the soil root zone through proton excretion. In a soil column trial, using soils from Foum Gleita and the Office du Niger (Mali), we found that irrigated rice cropping can be used to reclaim alkaline-sodic soils, provided some (natural) drainage is possible. These findings are in line with recent literature on changes in soils under flooded crops inMaliandNiger. Rice straw amendments accelerated the reclamation processes on soils that contained calcite.Anaerobic decomposition of straw increased pCO 2 , dissolution of calcite, and consequently Ca 2+ in the soil solution.The Ca 2+ exchanged with adsorbed Na + and the latter was leached together with (bi)-carbonates. Reduction of iron hydroxides could theoretically lead to ferrolysis and acidification of the upper horizons, but we found that the process was of little importance to the short term alkalinity changes of the studied soils

The agronomic and economic benefits of fertilizer and mulch use in highland banana systems in Uganda

Banana is the most important food crop in Uganda. However, there has been a decline in productivity, attributed to declining soil fertility, drought, pests and diseases and crop management factors. This study aimed to explore the possibility of increasing yields through the use of fertilizer and mulch, and to evaluate the benefits of these inputs across the major banana producing regions in Uganda. This study was carried out in 179 smallholder plots in Central, South, Southwest and East Uganda in 2006/7. Half of the plots were 'demonstration plots' of an agricultural development project, while the other half were neighboring farmer plots that acted as 'control'. Demonstration plots received mineral fertilizer (100% of plots), averaging 71 N, 8 P, 32 K kg ha-1 yr-1 and external mulch from grass and crop residues (64% of plots), whereas control plots received no mineral fertilizer and little external mulch (26% of plots). Demonstration plots had significantly (P [less-than-or-equals, slant] 0.05) higher yields than control plot in Central, South and Southwest, but average yield increases varied from 4.8 t ha-1 yr-1 (Southwest) to 8.0 (Central), and 10.0 (South). Average weevil corm damage (3%) and nematode-induced root necrosis (7%) was low and similar for both plot types, so yield increases could only be explained by the use of fertilizer and mulch. The highest demonstration plot yield increases were observed where fertilizer addressed key nutrient deficiencies identified using the compositional nutrient diagnosis approach. Farm gate bunch prices declined from 0.17 (Central Uganda) to 0.07 USD kg-1 (Southwest Uganda). Consequently, average marginal rate of return (MRR) of fertilizer and mulch use ranged from 0.1 (Southwest) to 5.8 (Central). The technologies were likely to be acceptable to farmers (MRR [greater-or-equal, slanted] 1.00) up to 160 km away from the capital. Fertilizer use is likely to be acceptable in all regions (MRR = 0.7-9.4) if local fertilizer prices of 2006/7 (average USD 0.56 kg-1 of fertilizer) declined by 50%. Doubling of fertilizer prices is likely to make fertilizer use unacceptable beyond 100 km away from the capital. The study concludes that there is scope for increased input use in banana systems in Uganda, but that regional variations in crop response, input/output prices, and price fluctuations have to be taken into account.Distance to market Farm gate prices Fertilizer Mulch Nutrient deficiencies Profitability

Increasing land pressure in East Africa: The changing role of cassava and consequences for sustainability of farming systems

Increasing land pressure during the past three to four decades has transformed farming systems in the mid-altitude zone of East Africa. Traditional millet-, cotton-, sugarcane- and/or banana-based farming systems with an important fallow and/or grazing component have evolved into continuously cultivated cassava or cassava/maize-based systems. Within three to four decades, cassava cultivation increased from 1¿11 to 16¿55% of cropped fields in our six study sites. Declining soil fertility, and not labour or food shortage, was apparently the primary trigger for this transformation. The land use changes have increased nutrient offtakes and reduced nutrient recycling rates. Cassava and maize now account for 50¿90% of nutrient removal. Whereas single-season fallows were the most important source of nutrient recycling on cropped fields in the past, currently cassava litterfall and maize stover contribute roughly 70% of nutrient recycling, with 50¿70% of N, P and K recycled in cassava litterfall. This may explain why many farmers reason that cassava `rests¿ the soil. With increasing land use pressure farmers progressively use cassava as an `imitation fallow¿ throughout their farm. Farmers increasingly target cassava to poor fertility fields characterized by low pH and available P. High cassava intensities are nonetheless maintained on more fertile fields, probably to guarantee regeneration of soil fertility on all fields. Once cassava is targeted to poor fertility soils, farmers have run out of low-input management options and need to intensify management to maintain system productivity. As cassava is now used by more farmers and on a larger acreage than fallowing in the studied farming systems, cassava cropping could perhaps serve as an excellent entry point to strengthen system sustainability

Increasing land pressure in East Africa: The changing role of cassava and consequences for sustainability of farming systems

Increasing land pressure during the past three to four decades has transformed farming systems in the mid-altitude zone of East Africa. Traditional millet-, cotton-, sugarcane- and/or banana-based farming systems with an important fallow and/or grazing component have evolved into continuously cultivated cassava or cassava/maize-based systems. Within three to four decades, cassava cultivation increased from 1¿11 to 16¿55% of cropped fields in our six study sites. Declining soil fertility, and not labour or food shortage, was apparently the primary trigger for this transformation. The land use changes have increased nutrient offtakes and reduced nutrient recycling rates. Cassava and maize now account for 50¿90% of nutrient removal. Whereas single-season fallows were the most important source of nutrient recycling on cropped fields in the past, currently cassava litterfall and maize stover contribute roughly 70% of nutrient recycling, with 50¿70% of N, P and K recycled in cassava litterfall. This may explain why many farmers reason that cassava `rests¿ the soil. With increasing land use pressure farmers progressively use cassava as an `imitation fallow¿ throughout their farm. Farmers increasingly target cassava to poor fertility fields characterized by low pH and available P. High cassava intensities are nonetheless maintained on more fertile fields, probably to guarantee regeneration of soil fertility on all fields. Once cassava is targeted to poor fertility soils, farmers have run out of low-input management options and need to intensify management to maintain system productivity. As cassava is now used by more farmers and on a larger acreage than fallowing in the studied farming systems, cassava cropping could perhaps serve as an excellent entry point to strengthen system sustainability

Drought is a major yield loss factor for rainfed East African highland banana

Although drought stress has been identified among the production constraints of East African highland bananas (Musa spp., AAA-EA genome), no quantitative data were available to support this assumption. This study uses data from three on-station fertilizer trials (5-6 cycles) in Central and Southwest Uganda to quantify the effect of drought stress on banana production and explore possible interactions with nutrient availability. Production data were collected at individual plant basis from 1996 to 2002 in one trial and from 2004 to 2009 in two trials. Cumulative rainfall in the 12 months before harvest (CRF12) was computed per plant from daily rainfall measurements. Average bunch weight ranged from 8.0 to 21.9 kg between trials and cycles and was 8-28% less in dry (CRF12Banana Boundary line analysis Cumulative annual rainfall Drought stress Physiology Soil fertility Uganda

Phenological development of East African highland banana involves trade-offs between physiological age and chronological age

The phenology of East African highland banana (Musa acuminata AAA-EA, hereafter referred to as ‘highland banana’) is poorly understood. We tested three hypotheses: (1) the physiological age at flowering is independent of site effects, (2) there is no difference in threshold size at flowering between sites with different growth potential, and (3) morphological and physiological components of highland banana relative growth rate (RGR) contribute equally to mitigate growth reduction in response to limiting supply of water, K or N. The physiological age of highland banana plants from field trials at Kawanda (central Uganda) and Ntungamo (south-western Uganda) was computed from daily temperature records. Growth analysis was conducted using RGR, net assimilation rate (NAR), specific leaf area (SLA) and leaf mass ratio (LMR) estimated from allometry. Growth response coefficients were used for quantifying the relative contribution of NAR, SLA and LMR to RGR. Physiological age at flowering was delayed by 739 °C d at Kawanda compared with that at Ntungamo whose chronological age at flowering was in turn 51 d older. At both sites a threshold total dry mass of 1.5 kg per plant was required for flowering. Faster absolute growth rate and NAR fostered by wet conditions, K input and cooler temperatures enabled plants at Ntungamo to attain the threshold total dry mass sooner than those at Kawanda, hence the phenotypic plasticity in age at flowering. Net assimilation rate contributed at least 90% to RGR increase due to wet conditions at both sites. The contribution of NAR to RGR increase in response to K at Kawanda reduced to 38% while that for SLA increased to 49%. Net assimilation rate contributes more to highland banana RGR modulation than SLA except when warmer conditions reduce NAR. Differences in crop growth rate cause phenotypic plasticity in highland banana rate of phenological development