Linkages between soil carbon, soil fertility and nitrogen fixation in Acacia senegal plantations of varying age in Sudan

Background Over the last decades sub-Saharan Africa has experienced severe land degradation and food security challenges linked to loss of soil fertility and soil organic matter (SOM), recurrent drought and increasing population. Although primary production in drylands is strictly limited by water availability, nutrient deficiencies, particularly of nitrogen (N) and phosphorus (P), are also considered limiting factors for plant growth. It is known that SOM (often measured as soil organic carbon (SOC)) is a key indicator of soil fertility, therefore, management practices that increase SOM contents, such as increasing tree cover, can be expected to improve soil fertility. The objectives of this study were to investigate the effect of Acacia senegal (Senegalia senegal) trees on soil nitrogen, phosphorus and potassium (K) in relation to SOC, the potential of A. senegal for N2 fixation, and to identify possible N and P ecosystem limitations. Methods Soil nutrient (total N, P, K and available P and exchangeable K) concentrations and stocks were determined for the 0–10, 10–20,20–30 and 30–50 cm layers of A. senegal plantations of varying age (ranging from 7 to 24-years-old) and adjacent grasslands (reference) at two sites in semi-arid areas of Sudan. At both sites, three plots were established in each grassland and plantation. The potential of A. senegal for N2 fixation in relation to plantations age was assessed using δ15N isotopic abundances and nutrient limitations assessed using C:N:P stoichiometry. Results Soil concentrations of all studied nutrients were relatively low but were significantly and directly correlated to SOC concentrations. SOC and nutrient concentrations were the highest in the topsoil (0–10 cm) and increased with plantations age. Acacia foliage δ15N values were >6‰ and varied little with plantations age. Soil C:N and C:P ratios did not differ between grassland and plantations and only 0–10 cm layer N:P ratios showed significant differences between grassland and plantations. Discussion The results indicated that soil fertility in the Sahel region is strongly related to SOM contents and therefore highlighting the importance of trees in the landscape. The higher mineral nutrient concentrations in the topsoil of the plantations may be an indication of ‘nutrient uplift’ by the deeper roots. The high foliar δ15N values indicated that N2 fixation was not an important contributor to soil N contents in the plantations. The accretion of soil N cannot be explained by deposition but may be related to inputs of excreted N brought into the area annually by grazing and browsing animals. The soil C:N:P stoichiometry indicated that the plantations may be limited by P and the grasslands limited by N.

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Mulching with pruned fronds promotes the internal soil N cycling and soil fertility in a large-scale oil palm plantation

Biogeochemistry ◽

10.1007/s10533-021-00798-4 ◽

2021 ◽

Author(s):

Greta Formaglio ◽

Edzo Veldkamp ◽

Muhammad Damris ◽

Aiyen Tjoa ◽

Marife D. Corre

Keyword(s):

Microbial Biomass ◽

Soil Fertility ◽

Oil Palm ◽

Large Scale ◽

Management Practices ◽

N Cycling ◽

Organic N ◽

Soil N ◽

Total N ◽

Organic C

AbstractIntensive management practices in large-scale oil palm plantations can slow down nutrient cycling and alter other soil functions. Thus, there is a need to reduce management intensity without sacrificing productivity. The aim of our study was to investigate the effect of management practices on gross rates of soil N cycling and soil fertility. In Jambi province, Indonesia, we established a management experiment in a large-scale oil palm plantation to compare conventional practices (i.e. high fertilization rates and herbicide weeding) with reduced management intensity (i.e. reduced fertilization rates and mechanical weeding). Also, we compared the typical management zones characterizing large-scale plantations: palm circle, inter-row and frond-stacked area. After 1.5 years of this experiment, reduced and conventional management showed comparable gross soil N cycling rates; however, there were stark differences among management zones. The frond-stacked area had higher soil N cycling rates and soil fertility (high microbial biomass, extractable C, soil organic C, extractable organic N, total N and low bulk density) than inter-row and palm circle (all p ≤ 0.05). Microbial biomass was the main driver of the soil N cycle, attested by its high correlation with gross N-cycling rates (r = 0.93–0.95, p < 0.01). The correlations of microbial N with extractable C, extractable organic N, soil organic C and total N (r = 0.76–0.89, p < 0.01) suggest that microbial biomass was mainly regulated by the availability of organic matter. Mulching with senesced fronds enhanced soil microbial biomass, which promoted nutrient recycling and thereby can decrease dependency on chemical fertilizers.

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Awareness of Integrated Soil Fertility Management Practices in the Savelugu Municipal of the Northern Region of Ghana

Rural Sustainability Research ◽

10.2478/plua-2020-0005 ◽

2020 ◽

Vol 43 (338) ◽

pp. 35-41

Author(s):

Ammal Abukari ◽

Rahamatu Abukari

Keyword(s):

Soil Fertility ◽

Management Practices ◽

Household Size ◽

Sub Saharan Africa ◽

Inorganic Fertilizer ◽

Small Scale ◽

Soil Fertility Management ◽

Appropriate Use ◽

Integrated Soil Fertility Management ◽

Fertility Management

AbstractIn sub-Saharan Africa intensifying small-scale farming is essential in addressing poverty related issues in rural communities and the degradation of natural resources. Integrated Soil Fertility Management (ISFM) are the best practices used to improve the productivity of crops whilst maximizing agronomic efficiency of inputs applied and hence contributing to sustainable intensification. ISFM usually include the appropriate use of inorganic fertilizer and organic resources, good agronomic practices and appropriate use of germplasm. The survey was carried-out on the awareness of Integrated Soil Fertility Management practices in the Savelugu Municipal of Northern Ghana to study the awareness of integrated soil fertility management practices amongst farmers through the administration of questionnaires. A multistage method of sampling was used in selecting thirty (30) respondents randomly from five (5) selected communities namely Jana, Yapalsi, Diari, Nabogu and Gushie to make up a total sample size of 150 respondents. Frequency distribution and percentages were used to represent the data. Correlation analysis was used to test for the relationship between awareness, educational level and household size. The survey showed that majority of the respondents at ages between 21 and 30 years were married and majority with household size of 3 to 5 as well as primary and secondary education. Farming activities were carried-out by hand (80%). Majority of the respondents (43.3%) were informed about ISFM through demonstrations and 20% of the farmers apply inorganic fertilizer. About 85.5% of the respondents were aware of ISFM. The research also revealed that ISFM improves production and supports finances of respondents. The assessment of respondents’ perception of ISFM revealed a positive agreement of the effect of ISFM on soil health as well as improved production. In conclusion, it is thus suggested that it is needful for the involvement of the government on the adoption of ISFM via Non-Governmental Organisations (NGOs) locally and or internationally for a suitable advancement and to guarantee a sustainable environment with a world-wide corporation for improvement.

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Management of sugarcane harvest residues: consequences for soil carbon and nitrogen

Soil Research ◽

10.1071/sr06080 ◽

2007 ◽

Vol 45 (1) ◽

pp. 13 ◽

Cited By ~ 83

Author(s):

Fiona A. Robertson ◽

Peter J. Thorburn

Keyword(s):

Microbial Biomass ◽

Soil Fertility ◽

Microbial Activity ◽

Soil N ◽

Soil Organic C ◽

Total N ◽

Organic C ◽

Soil Microbial ◽

C And N

The Australian sugar industry is moving away from the practice of burning the crop before harvest to a system of green cane trash blanketing (GCTB). Since the residues that would have been lost in the fire are returned to the soil, nutrients and organic matter may be accumulating under trash blanketing. There is a need to know if this is the case, to better manage fertiliser inputs and maintain soil fertility. The objective of this work was to determine whether conversion from a burning to a GCTB trash management system is likely to affect soil fertility in terms of C and N. Indicators of short- and long-term soil C and N cycling were measured in 5 field experiments in contrasting climatic conditions. The effects of GCTB varied among experiments. Experiments that had been running for 1–2 years (Harwood) showed no significant trash management effects. In experiments that had been running for 3–6 years (Mackay and Tully), soil organic C and total N were up to 21% greater under trash blanketing than under burning, to 0.10 or 0.25 m depth (most of this effect being in the top 50 mm). Soil microbial activity (CO2 production) and soil microbial biomass also increased under GCTB, presumably as a consequence of the improved C availability. Most of the trash C was respired by the microbial biomass and lost from the system as CO2. The stimulation of microbial activity in these relatively short-term GCTB systems was not accompanied by increased net mineralisation of soil N, probably because of the greatly increased net immobilisation of N. It was calculated that, with standard fertiliser applications, the entire trash blanket could be decomposed without compromising the supply of N to the crop. Calculations of possible long-term effects of converting from a burnt to a GCTB production system suggested that, at the sites studied, soil organic C could increase by 8–15%, total soil N could increase by 9–24%, and inorganic soil N could increase by 37 kg/ha.year, and that it would take 20–30 years for the soils to approach this new equilibrium. The results suggest that fertiliser N application should not be reduced in the first 6 years after adoption of GCTB, but small reductions may be possible in the longer term (>15 years).

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DETERMINANTS OF THE DECISION TO ADOPT INTEGRATED SOIL FERTILITY MANAGEMENT PRACTICES BY SMALLHOLDER FARMERS IN THE CENTRAL HIGHLANDS OF KENYA

Experimental Agriculture ◽

10.1017/s0014479708007072 ◽

2009 ◽

Vol 45 (1) ◽

pp. 61-75 ◽

Cited By ~ 38

Author(s):

JAYNE MUGWE ◽

DANIEL MUGENDI ◽

MONICAH MUCHERU-MUNA ◽

ROEL MERCKX ◽

JONAS CHIANU ◽

...

Keyword(s):

Soil Fertility ◽

Management Practices ◽

Smallholder Farmers ◽

Household Head ◽

Sub Saharan Africa ◽

Full Time ◽

Soil Fertility Management ◽

Integrated Soil Fertility Management ◽

Fertility Management ◽

Central Highlands

SUMMARYDeclining soil fertility is a major cause of low per capita food production on smallholder farms of sub-Saharan Africa. This study attempted to provide an empirical explanation of the factors associated with farmers' decisions to adopt or not to adopt newly introduced integrated soil fertility management (ISFM) technologies consisting of combinations of organics and mineral fertilizer in Meru South district of the central highlands of Kenya. Out of 106 households interviewed, 46% were ‘adopters’ while 54% were ‘non-adopters’. A logistic regression model showed that the factors that significantly influenced adoption positively were farm management, ability to hire labour and months in a year households bought food for their families, while age of household head and number of mature cattle negatively influenced adoption. The implication of these results is that the adoption of ISFM practices could be enhanced through targeting of younger families where both spouses work on the farm full-time and food insecure households. It is also important to target farmers that lack access to other sources of soil fertility improvement. Examples include farmers that do not own cattle or those owning few and who, therefore, have limited access to animal manure.

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Integrated soil fertility management in sub-Saharan Africa: unravelling local adaptation

SOIL ◽

10.5194/soil-1-491-2015 ◽

2015 ◽

Vol 1 (1) ◽

pp. 491-508 ◽

Cited By ~ 111

Author(s):

B. Vanlauwe ◽

K. Descheemaeker ◽

K. E. Giller ◽

J. Huising ◽

R. Merckx ◽

...

Keyword(s):

Soil Fertility ◽

Local Adaptation ◽

Management Practices ◽

Crop Productivity ◽

Sub Saharan Africa ◽

Soil Fertility Management ◽

Integrated Soil Fertility Management ◽

Farm Level ◽

Fertility Management ◽

Sub Saharan

Abstract. Intensification of smallholder agriculture in sub-Saharan Africa is necessary to address rural poverty and natural resource degradation. Integrated soil fertility management (ISFM) is a means to enhance crop productivity while maximizing the agronomic efficiency (AE) of applied inputs, and can thus contribute to sustainable intensification. ISFM consists of a set of best practices, preferably used in combination, including the use of appropriate germplasm, the appropriate use of fertilizer and of organic resources, and good agronomic practices. The large variability in soil fertility conditions within smallholder farms is also recognized within ISFM, including soils with constraints beyond those addressed by fertilizer and organic inputs. The variable biophysical environments that characterize smallholder farming systems have profound effects on crop productivity and AE, and targeted application of agro-inputs and management practices is necessary to enhance AE. Further, management decisions depend on the farmer's resource endowments and production objectives. In this paper we discuss the "local adaptation" component of ISFM and how this can be conceptualized within an ISFM framework, backstopped by analysis of AE at plot and farm level. At plot level, a set of four constraints to maximum AE is discussed in relation to "local adaptation": soil acidity, secondary nutrient and micronutrient (SMN) deficiencies, physical constraints, and drought stress. In each of these cases, examples are presented whereby amendments and/or practices addressing these have a significantly positive impact on fertilizer AE, including mechanistic principles underlying these effects. While the impact of such amendments and/or practices is easily understood for some practices (e.g. the application of SMNs where these are limiting), for others, more complex processes influence AE (e.g. water harvesting under varying rainfall conditions). At farm scale, adjusting fertilizer applications to within-farm soil fertility gradients has the potential to increase AE compared with blanket recommendations, in particular where fertility gradients are strong. In the final section, "local adaption" is discussed in relation to scale issues and decision support tools are evaluated as a means to create a better understanding of complexity at farm level and to communicate appropriate scenarios for allocating agro-inputs and management practices within heterogeneous farming environments.

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Integrated soil fertility management in sub-Saharan Africa: unravelling local adaptation

SOIL Discussions ◽

10.5194/soild-1-1239-2014 ◽

2014 ◽

Vol 1 (1) ◽

pp. 1239-1286 ◽

Cited By ~ 20

Author(s):

B. Vanlauwe ◽

K. Descheemaeker ◽

K. E. Giller ◽

J. Huising ◽

R. Merckx ◽

...

Keyword(s):

Soil Fertility ◽

Local Adaptation ◽

Management Practices ◽

Crop Productivity ◽

Sub Saharan Africa ◽

Soil Fertility Management ◽

Integrated Soil Fertility Management ◽

Farm Level ◽

Fertility Management ◽

Sub Saharan

Abstract. Intensification of smallholder agriculture in sub-Saharan Africa is necessary to address rural poverty and natural resource degradation. Integrated Soil Fertility Management (ISFM) is a means to enhance crop productivity while maximizing the agronomic efficiency (AE) of applied inputs, and can thus contribute to sustainable intensification. ISFM consists of a set of best practices, preferably used in combination, including the use of appropriate germplasm, the appropriate use of fertilizer and of organic resources, and good agronomic practices. The large variability in soil fertility conditions within smallholder farms is also recognised within ISFM, including soils with constraints beyond those addressed by fertilizer and organic inputs. The variable biophysical environments that characterize smallholder farming systems have profound effects on crop productivity and AE and targeted application of limited agro-inputs and management practices is necessary to enhance AE. Further, management decisions depend on the farmer's resource endowments and production objectives. In this paper we discuss the "local adaptation" component of ISFM and how this can be conceptualized within an ISFM framework, backstopped by analysis of AE at plot and farm level. At plot level, a set of four constraints to maximum AE is discussed in relation to "local adaptation": soil acidity, secondary nutrient and micro-nutrient (SMN) deficiencies, physical constraints, and drought stress. In each of these cases, examples are presented whereby amendments and/or practices addressing these have a significantly positive impact on fertilizer AE, including mechanistic principles underlying these effects. While the impact of such amendments and/or practices is easily understood for some practices (e.g., the application of SMNs where these are limiting), for others, more complex interactions with fertilizer AE can be identified (e.g., water harvesting under varying rainfall conditions). At farm scale, adjusting fertilizer applications within-farm soil fertility gradients has the potential to increase AE compared with blanket recommendations, in particular where fertility gradients are strong. In the final section, "local adaption" is discussed in relation to scale issues and decision support tools are evaluated as a means to create a better understanding of complexity at farm level and to communicate best scenarios for allocating agro-inputs and management practices within heterogeneous farming environments.

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Soil Nutrient Dynamics under Old and Young Cocoa, Coffee and Cashew Plantations at Uhonmora, Edo State, Nigeria

Journal of Tropical Soils ◽

10.5400/jts.2014.v19i2.75-80 ◽

2015 ◽

Vol 19 (2) ◽

pp. 75 ◽

Cited By ~ 1

Author(s):

Rotimi Rufus Ipinmoroti ◽

Joseph Sunday Ogeh

Keyword(s):

Soil Fertility ◽

Cover Crops ◽

Nutrient Dynamics ◽

Management Practices ◽

Soil Nutrient ◽

Soil Fertility Management ◽

Total N ◽

Nutrient Contents ◽

Fertility Management ◽

Edo State

A study was conducted to assess nutrient dynamics of soils under old and young cocoa, coffee and cashew plantations and the leaf nutrient contents of the crops at Uhonmora, Edo State, Nigeria for proper cultural and soil fertility management of the plantations. Soil and crop leaf samples were collected from each plantation using a random sampling technique. The samples were analyzed using standard procedures for sand, silt, clay, pH (H2O), electrical conductivity (EC), total N, available P, K, Ca, Mg, Na, and Effective Cation Exchange Capacity (ECEC). Leaf samples were analyzed for N, P, K, Ca, Mg and Na. Data were compared with the corresponding soil and foliar critical nutrient values for each crop. Results indicated that the soils were texturally sandy clay loam and acidic. The soils varied in their nutrient contents, with soil P for the old cocoa, young coffee and cashew plantations far below critical values. The young cashew plot was low in N content but adequate for other plots. However, the soil ECEC increased with the increasing of calcium contents. Leaf N was below critical for all the crops. Leaf K was low for cocoa and coffee plants, leaf Ca was low for the young cashew plants, while leaf Mg was low for the young cocoa and old cashew. The high soil Mg/K ratio of 8.7- 22.3 as against the established value of 2.0 might have resulted in gross nutrient imbalance which must have affected the absorption and utilization of other nutrients. Hence, adequate soil N did not translate the same availability to the crops. The ECEC showed that the soil needs to be improved upon for sustainable productivity. Soil nutrient content variation across the plantations with age of establishment will necessitate the need for consistent routine soil nutrient assessment for proper and balanced soil nutrient supply to the crops, for healthy crop growth and optimum yield. Management practices of soil surface mulching using organic wastes and cover crops under compatible cropping systems are needed for successful plot establishment and better growth performance of the young seedlings.Key words: Nutrient dynamics, plantation crops, rehabilitation, soil fertility management [How to Cite: Rotimi RI and JS Ogeh. 2014. Soil Nutrient Dynamics under Old and Young Cocoa, Coffee and Cashew Plantations at Uhonmora, Edo State, Nigeria. J Trop Soils 19(2): 85-90. Doi: 10.5400/jts.2014.19.2.85] [Permalink/DOI: www.dx.doi.org/10.5400/jts.2014.19.2.85]

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Dynamics of N-P-K demand and uptake in cassava

Agronomy for Sustainable Development ◽

10.1007/s13593-020-00649-w ◽

2020 ◽

Vol 41 (1) ◽

Author(s):

Joy Geraldine Adiele ◽

Antonius G. T. Schut ◽

Kodjovi S. Ezui ◽

Pieter Pypers ◽

Ken E. Giller

Keyword(s):

Management Practices ◽

Growth Cycle ◽

Raw Material ◽

Growth And Yield ◽

Nutrient Concentrations ◽

Total Biomass ◽

Storage Roots ◽

Total N ◽

And Storage ◽

Dilution Curves

AbstractFertilizers are required to improve productivity of cassava and meet the increasing demand for cassava as food, feed, or raw material for processing industries. Our objective was to develop nutrition indices for N, P, and K to provide quantitative insight in the dynamics of nutrient demand and uptake of cassava. On-farm experiments were conducted at six locations in Nigeria from 2016 to 2018, across the major cassava growing agro-ecologies of West Africa. Nitrogen, P, and K were applied at different rates. Uptake of nutrients was measured in leaves, stems, and storage roots at 4, 8, and 12 or 14 months after planting (MAP) and used to construct NPK dilution curves and nutrition indices. About 67, 61, and 52% of total N, P, and K were taken up at 4 MAP, with a maximum uptake rate of 0.21, 0.03, and 0.12 g/m2/d for N, P, and K, respectively. Nutrient concentrations in stems and storage roots declined gradually, in contrast to concentrations in the leaves that fluctuated within narrow ranges. Dilution curves and nutrition indices for N, P, and K were established for the first time in cassava. Dilution curves of N, P, and K in the crop for the highest NPK application treatment were described as Nc = 82DM−0.61, Pc = 7.4DM−0.54, and Kc = 43DM−0.54, when total biomass was between 5 and 57 t/ha dry matter (DM). The nutrition indices were linearly related to relative crop biomass. Insight into the nutrient uptake and dilution patterns during the growth cycle can help to understand the temporal nutrient demands of cassava and identify sustainable management practices. Initial ample supply of N and P and moderate K, with extra K top-dress during the second growth phase, will benefit cassava growth and yield. Furthermore, such information provides a basis to develop a dynamic model to simulate nutrient-limited growth of cassava.

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Estimating Nitrogen Flows and Nitrogen Footprint for Agro-Food System of Rwanda Over the Last Five Decades: Challenges and Measures

Frontiers in Environmental Science ◽

10.3389/fenvs.2021.778699 ◽

2021 ◽

Vol 9 ◽

Author(s):

Barthelemy Harerimana ◽

Minghua Zhou ◽

Muhammad Shaaban ◽

Bo Zhu

Keyword(s):

Food Insecurity ◽

Management Practices ◽

Food System ◽

Crop Yields ◽

N Uptake ◽

N Fertilizer ◽

Soil N ◽

N Losses ◽

Total N ◽

Per Capita

This study presents the first detailed estimate of Rwanda’s nitrogen (N) flows and N footprint for food (NFfood) from 1961 to 2018. Low N fertilizer inputs, substandard production techniques, and inefficient agricultural management practices are focal causes of low crop yields, environmental pollution, and food insecurity. We therefore assessed the N budget, N use efficiency (NUE), virtual N factors (VNFs), soil N mining factors (SNMFs), and N footprint for the agro-food systems of Rwanda with consideration of scenarios of fertilized and unfertilized farms. The total N input to croplands increased from 14.6 kg N ha−1 yr−1 (1960s) to 34.1 kg N ha−1 yr−1 (2010–2018), while the total crop N uptake increased from 18 kg N ha−1yr−1 (1960s) to 28.2 kg N ha−1yr−1 (2010–2018), reflecting a decline of NUE from 124% (1960s) to 85% (2010–2018). Gaseous N losses of NH3, N2O, and NO increased from 0.45 (NH3), 0.03 (N2O), and 0.00 (NO) Gg N yr−1 (1960s) to 6.98 (NH3), 0.58 (N2O), and 0.10 (NO) Gg N yr−1 (2010–2018). Due to the low N inputs, SNMFs were in the range of 0.00 and 2.99 and the rice production, cash-crop production, and livestock production systems have greater SNMFs in Rwanda. The weighted NFfood per capita that presents the actual situation of fertilized and unfertilized croplands increased from 4.0 kg N cap−1 yr−1 (1960s) to 6.3 kg N cap−1 yr−1 (2010–2018). The NFfood per capita would increase from 3.5 kg N cap−1 yr−1 to 4.8 kg N cap−1 yr−1 under a scenario of all croplands without N fertilizer application and increase from 6.0 to 8.7 kg N cap−1 yr−1 under the situation of all croplands receiving N fertilizer. The per capita agro-food production accounted for approximately 58% of the national NFfood. The present study indicates that Rwanda is currently suffering from low N inputs, high soil N depletion, food insecurity, and environmental N losses. Therefore, suggesting that the implementation of N management policies of increasing agricultural N inputs and rehabilitating the degraded soils with organic amendments of human and animal waste needs to be carefully considered in Rwanda.

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Nitrogen Level and Form Affect Taro Growth and Nutrition

HortScience ◽

10.21273/hortsci.38.1.36 ◽

2003 ◽

Vol 38 (1) ◽

pp. 36-40 ◽

Cited By ~ 10

Author(s):

N.W. Osorio ◽

X. Shuai ◽

S. Miyasaka ◽

B. Wang ◽

R.L. Shirey ◽

...

Keyword(s):

Leaf Area ◽

Nitrogen Concentration ◽

Maximum Growth ◽

Quadratic Model ◽

Mineral Nutrient ◽

Nutrient Concentrations ◽

Solution Ph ◽

Total N ◽

N Level ◽

N Form

Nitrogen (N) is often the most limiting mineral nutrient for taro growth. Two experiments were carried out under hydroponics conditions to determine the effects of varying solution N levels and N form on taro (Colocasia esculenta L. Schott cv. Bun Long) growth and foliar nutrient concentrations for 42 days. In the first experiment, taro plants were grown at six NH4NO3 levels (0, 0.25, 0.5, 1.0, 2.0, and 4.0 mm N). In the second experiment, taro plants were grown at a total N level of 3 mm with five nitrate (NO3-): ammonium (NH4+) percent molar ratios (100:0, 75:25, 50:50, 25:75, and 0:100). In the N level experiment, dry matter and leaf area increased up to 2 mm N and then decreased at the highest N level. The reduced growth of taro at the highest N level was attributed in part to a high NH4+ level that reduced uptake or translocation of cations, such as Ca2+, Mg2+, and Mn2+. Nitrogen concentration in leaf blades increased with increasing N levels. The critical foliar N concentration that coincided with 95% of maximum growth based on a quadratic model was 40.4 g·kg-1 (dry weight basis). In the N form experiment, NO3-: NH4+ ratios of 75:25 or 100:0 favored greater plant growth compared to other treatments. Taro plants grown in NH4+-rich solutions drastically acidified the solution pH, and had retarded growth and smaller leaf area compared to those grown in NO3--rich solutions.

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