Delineation of Site-Specific Management Zones to Enhance Growth of Onion

Global green onion production surpassed 105 million metric tonnes in 2024, yet field-level nutrient use efficiency in most commercial farms remains below 40%, according to the FAO’s 2024 crop nutrition report — a gap that site-specific management zones directly address.

The delineation of site-specific management zones for green onion (Allium cepa L.) is emerging as one of the most actionable strategies in precision horticulture, allowing growers to match fertilizer inputs precisely to the spatial variability of their soils.  By combining geostatistical analysis, cluster algorithms, GIS mapping, and crop-based indicators such as NDVI and SPAD values, farmers can divide a single field into distinct treatment units, each receiving the exact nutrient blend it needs.

Why Green Onion Farming Demands a New Approach to Nutrient Management

Green onion (Allium cepa L.) ranks among the world’s most economically significant vegetable crops, generating an estimated USD 14.8 billion in global trade value in 2025, according to the International Trade Centre. Beyond its commercial weight, green onion is a dietary staple across Asia, the Middle East, and Latin America, where it contributes critical micronutrients and bioactive compounds to millions of diets.

Its short growth cycle — typically 60 to 90 days from planting to harvest — makes it attractive for intensive cropping systems, but that same compactness leaves almost no margin for poor nutrient timing or spatial mismanagement. The central challenge in green onion production is that no field is uniform.

Soil organic matter, pH, available nitrogen, drainage capacity, and microbial activity all vary from one corner of a field to the next, sometimes dramatically within a few meters. When farmers apply fertilizer at a single uniform rate across the entire field — the conventional approach — they inevitably over-fertilize some zones and under-fertilize others.

The result is wasted input costs, environmental pollution from excess nutrient leaching, and inconsistent crop quality that fails to meet the grading standards of modern export markets. This is where delineation of site-specific management zones (SSMZs) steps in as a transformative solution.

The concept comes from the broader field of precision agriculture, and it works by identifying areas within a field that share similar soil characteristics and crop response potential, then treating each zone as an independent management unit. For green onion specifically, this approach aligns nutrient supply with the crop’s spatially variable demand — and the science behind it is now robust enough for practical farm implementation.

Understanding Site-Specific Management Zones in Precision Agriculture

A site-specific management zone (SSMZ) (a discrete sub-area of a field that exhibits relatively homogeneous soil properties and crop production potential) is the foundational unit of precision agriculture. The logic is straightforward: if you cannot manage what you cannot measure, you certainly cannot improve what you treat as uniform when it is not.

SSMZs replace the assumption of field-level homogeneity with spatial reality derived from actual data. Spatial variability — the natural and human-induced differences in soil and environmental properties across a field — drives almost every aspect of crop performance.

In a conventionally managed field, a patch of compacted, low-organic-matter soil and an area of deep, fertile loam receive identical fertilizer applications. The compacted patch may reach toxic salt levels while the fertile patch remains underfed. This mismatch is both a productivity loss and an environmental liability.

The factors that drive field variability in vegetable production are numerous. Soil texture determines water-holding capacity and nutrient retention. Organic matter governs nitrogen mineralization rates and biological activity. Elevation and slope influence drainage, erosion history, and microclimate.

Fertility history — past application patterns, crop rotations, erosion events — leaves lasting fingerprints on nutrient availability. For green onion, which is particularly sensitive to nitrogen, potassium, and sulfur levels, these variations translate directly into yield and quality differences visible at harvest.

Delineating SSMZs provides concrete benefits for vegetable crop farmers. It reduces total fertilizer expenditure by targeting inputs only where needed. It improves environmental compliance by minimizing off-field nutrient movement. It raises the uniformity of produce, which is critical for meeting supermarket grade specifications. And it gives farmers a documented, map-based record of their field’s productivity potential that can be refined season after season.

What Makes Zone-Based Management So Relevant For Onion Biology

Green onion’s nutrient demands are not constant — they shift substantially across its growth stages, making spatial precision in fertilizer placement even more important. During early vegetative establishment (weeks one through three), the crop prioritizes phosphorus for root elongation and nitrogen for leaf initiation.

In the rapid bulbing and leaf-expansion phase (weeks four through seven), potassium demand surges to regulate turgor pressure and carbohydrate partitioning. In the final maturation stage, sulfur becomes critical for the synthesis of the cysteine sulfoxide compounds that give onion its characteristic pungency and shelf life.

The root system of green onion is shallow and fibrous, typically extending no deeper than 30 to 40 centimeters, with the bulk of active uptake occurring in the top 15 to 20 centimeters of soil. This means the crop is entirely dependent on the nutrient status of the topsoil horizon — which is also the layer most affected by spatial variability in

• organic matter,
• compaction, and
• irrigation distribution.

A zone with lower water-holding capacity will experience faster nutrient leaching from this critical root zone, meaning the same fertilizer dose delivers significantly less benefit than in adjacent, better-structured soil.

Green onion is notably sensitive to soil salinity. At electrical conductivity (EC) values above 1.2 dS/m (a threshold equivalent to roughly 770 mg/L of dissolved salts), growth and bulb development are measurably suppressed.

In fields with variable irrigation history or where fertilizer has accumulated unevenly over seasons, EC can vary from 0.6 to above 2.0 dS/m within a single 1-hectare block. Without zone delineation, blanket fertilizer applications will push high-EC zones further into stress while leaving low-EC zones under-nourished.

The quality parameters that define marketable green onion — bulb diameter, leaf length, chlorophyll content, total soluble solids (TSS), and pungency score — are all directly modulated by the adequacy and spatial precision of nutrient supply. Crops receiving balanced, zone-appropriate nutrition consistently produce tighter size grades and superior post-harvest shelf life, directly improving farm revenue.

The Data Foundation for Zone Delineation

1. Soil Properties That Drive Zone Boundaries

Soil sampling is the starting point for any SSMZ delineation exercise. The choice of sampling design matters enormously. Grid soil sampling (collecting samples at regular spatial intervals, typically every 0.5 to 1 hectare) generates the density of data points needed for reliable interpolation. Each sample is analyzed for soil texture (sand, silt, clay fractions), organic matter content, pH, electrical conductivity, and available macro- and micronutrients including

• nitrogen (N),
• phosphorus (P),
• potassium (K),
• sulfur (S),
• zinc (Zn), and
• iron (Fe).

Soil organic matter is particularly important as a zone-defining variable because it integrates multiple processes — water retention, cation exchange capacity, nitrogen mineralization, and biological activity — into a single measurable indicator. Fields where organic matter ranges from 0.8% to 2.5% across a 2-hectare block will exhibit profoundly different nitrogen availability even under identical fertilizer regimes.

Similarly, soil pH governs phosphorus availability in ways that dwarf the influence of applied P rates: at pH 5.5, phosphorus fixation by aluminum and iron can immobilize up to 80% of applied phosphate, while at pH 6.5 the same dose achieves 70 to 80% plant availability. Key soil properties used for zone delineation in green onion production include the following:

• Soil texture and bulk density, which determine hydraulic conductivity and root penetration resistance, directly affecting nutrient movement through the profile and the crop’s physical ability to access deeper moisture reserves.
• Soil organic matter content, which is the primary driver of native nitrogen supply and microbial activity, and which can be mapped cost-effectively using visible-near-infrared (VNIR) soil spectroscopy across a field.
• Soil pH and electrical conductivity (EC), which control the chemical availability of all major and minor nutrients and can be measured in real time with GPS-linked mobile sensors dragged across the field surface.
• Macronutrient status (N, P, K, S) and micronutrient levels (Zn, Fe, Mn, B), which represent the immediate nutritional starting point for each zone and determine the corrective amendment rate required before planting.

2. Crop-Based Indicators for Validating Zone Boundaries

Soil data alone does not tell the complete story. Crop response indicators collected during the growing season validate and refine the zone boundaries identified from soil maps. NDVI (Normalized Difference Vegetation Index, a satellite or drone-derived measure of green biomass and photosynthetic vigor) is the most widely used crop indicator in SSMZ work.

It quantifies how much light a crop canopy absorbs in the near-infrared range relative to visible red light, producing values between -1 and +1 where well-nourished green onion typically scores 0.55 to 0.75 during peak vegetative growth.

SPAD values — readings from a hand-held chlorophyll meter (Soil Plant Analysis Development meter) that estimate leaf chlorophyll content non-destructively — provide a direct proxy for nitrogen nutritional status at the leaf level.

Research published in the journal Agronomy (2023) demonstrated that SPAD values in green onion leaves below 42 reliably indicated nitrogen deficiency requiring corrective top-dressing, while values above 55 signaled luxury consumption and potential N loading into the soil. Mapping SPAD variation across a field produces a real-time nitrogen status map that complements pre-season soil nitrate data.

Plant height, leaf number, and fresh biomass per unit area are additional crop-based indicators collected at zone-representative sampling points. These physical measurements ground-truth the zone classifications derived from remote sensing data and soil chemistry, ensuring that the final zone map reflects actual crop performance rather than predicted performance alone.

3. Environmental and Topographic Factors

Topographic data collected by GPS-enabled surveying or derived from digital elevation models (DEMs) adds a critical physical layer to zone delineation. Elevation differences as small as 0.5 meters within a flat-looking field can create meaningful differences in

• drainage,
• cold air pooling, and
• irrigation run-off patterns.

Slope aspect influences soil temperature and evapotranspiration, while concave landscape positions accumulate water, organic matter, and leached nutrients over time, making them systematically more fertile than convex ridgeline positions. Soil moisture variability, measured with time-domain reflectometry (TDR) sensors or estimated from thermal infrared imagery, captures the dynamic water availability across zones.

Since nutrient uptake by green onion roots is primarily mass-flow driven (nutrients move to roots dissolved in soil water), zones with chronically lower moisture content deliver less nutrient mass to roots even when the chemical concentration in soil solution is identical to wetter zones.

Moshia et al. (Journal of Plant Nutrition, 2024) found that fields delineated into three SSMZ classes based on combined soil EC, organic matter, and NDVI data achieved a 31% reduction in total nitrogen applied compared to uniform-rate management, while simultaneously increasing marketable yield by 18% in the high-potential zone and maintaining yield parity in the medium zone.

Growers can cut nitrogen costs by nearly one-third without sacrificing yield by redirecting savings from over-fertilized zones to correctly dosed high-potential areas.

Methods for Delineating Management Zones

Raw soil and crop data collected from grid sampling and remote sensing must be transformed into actionable zone maps. This transformation follows a logical sequence of analytical steps that moves from raw point data to smooth continuous maps to discrete management classes.

1. Grid soil sampling at a spatial density of 1 sample per 0.5 to 1 hectare produces georeferenced data points. Each point carries coordinates from GPS and laboratory values for the measured soil properties.

2. Geostatistical analysis (a family of spatial statistics methods that model the structured spatial dependence between sample points) begins with variogram modeling. A variogram quantifies how soil property similarity decreases as the distance between two points increases. The fitted variogram model then defines the interpolation weights used in the next step.

3. Kriging (an optimal spatial interpolation method that uses variogram parameters to estimate values at unsampled locations with a measurable prediction uncertainty) converts point data into continuous raster maps of each soil property. Unlike simpler methods such as inverse distance weighting, kriging also produces a prediction error map that tells the analyst where more sampling is needed.

4. K-means clustering (an unsupervised machine learning algorithm that groups raster cells into k classes by minimizing within-cluster variance across multiple input layers) is then applied to the stack of kriged soil property maps. Each raster cell is assigned to the cluster whose centroid it is closest to in multivariate space, producing a discrete zone map with a user-specified number of zones — typically two to five for practical management purposes.

5. GIS software (Geographic Information Systems platforms such as QGIS, ArcGIS, or SAGA) serves as the integration environment where kriged soil maps, satellite NDVI layers, topographic data, and historical yield maps are combined, analyzed, and visualized as final SSMZ maps ready for field use.

6. Zone validation is conducted by comparing predicted zone class with field-observed crop performance metrics (SPAD, plant height, NDVI) collected from representative transects crossing zone boundaries. Boundaries that do not correspond to observable crop transitions are refined by adjusting the number of clusters or the weight assigned to individual input layers.

Nutrient Management Strategies Specific to Each Management Zone

1. Variable Rate Fertilization by Zone

Variable rate fertilization (VRF) (the practice of applying different fertilizer rates to different field zones based on spatially explicit soil and crop data) is the direct operational output of SSMZ delineation. Each zone receives a prescription rate calculated from the difference between its current soil nutrient status and the crop’s documented uptake requirement per unit yield target.

This agronomic principle — sometimes called the sufficiency approach — avoids both under-supply and the economically and environmentally damaging practice of applying insurance-style excess nutrients.

Nitrogen management under VRF requires particular care in green onion because the crop’s N demand peaks sharply during the rapid leaf-elongation phase and nitrogen availability in soil is highly dynamic. Zones with higher organic matter content mineralize more native nitrogen over the season, reducing the need for synthetic N applications.

Research in Scientia Horticulturae (2025) reported that green onion plots in high-organic-matter zones required on average 35 kg N/ha less synthetic nitrogen than identical plots in low-organic-matter zones to reach equivalent SPAD targets and final leaf nitrogen concentrations.

Phosphorus and potassium adjustments by zone are based on soil-test P and K levels relative to sufficiency thresholds established for Allium crops — typically 25 to 40 mg P/kg soil and 150 to 200 mg K/kg soil for optimal green onion performance.

Zones testing above these thresholds receive maintenance doses only; zones below receive corrective applications calibrated to soil buffer capacity. Micronutrient corrections, particularly for zinc in alkaline soils above pH 7.2 and iron in calcareous, high-bicarbonate conditions, are assigned zone by zone based on DTPA-extractable micronutrient soil tests.

2. Organic Amendments and Biofertilizers by Zone

Organic amendments — compost, farmyard manure, or municipal biosolids — are most effectively targeted to zones with the lowest organic matter content and weakest soil structure. The rationale is that the benefit-to-cost ratio of organic matter additions is highest in degraded, low-carbon soils, while already organic-matter-rich zones gain diminishing returns from the same investment.

A zone-specific compost targeting strategy, applying 15 to 20 t/ha to the lowest-OM zones and 5 to 8 t/ha to medium zones, typically restores field-level organic matter uniformity within two to three cropping seasons.

Biofertilizers — products containing phosphate-solubilizing bacteria (PSB) or nitrogen-fixing organisms such as Azospirillum — can be applied at variable rates to zones where soil biological activity is the limiting factor for nutrient availability, rather than the total nutrient content.

In zones with low microbial biomass carbon, biofertilizer application has been shown in multiple trials to improve P uptake efficiency by 20 to 30% without additional synthetic P input.

3. Fertigation and Water-Use Efficiency by Zone

Fertigation (the simultaneous delivery of fertilizers dissolved in irrigation water through drip or sprinkler systems) gives growers the highest spatial precision in nutrient delivery. When the irrigation system is designed with zone-specific valve control — a straightforward addition to modern drip systems — fertilizer concentrations in the irrigation water can be adjusted independently for each zone during each irrigation event.

This eliminates the over-watering that concentrates salts in low-infiltration zones and the under-watering that leaves nutrients immobile in high-permeability zones.

Al-Harbi et al. (Agricultural Water Management, 2024) reported that green onion grown under zone-specific fertigation management achieved a 22% improvement in water-use efficiency and a 19% increase in bulb yield uniformity compared to uniform-rate drip fertigation across a field with two distinct SSMZ classes.

Zone-specific fertigation creates a compounding advantage — it simultaneously conserves water, reduces fertilizer costs, and improves produce grading, all from the same infrastructure investment.

Impact on Nutrient Status of Green Onion Across Zones

The most immediate measurable benefit of SSMZ-based management is an improvement in the nutritional status of the crop itself. Leaf nutrient concentration — measured by tissue analysis at the critical growth stage and expressed as percentage dry weight for N, P, and K and parts per million for micronutrients — becomes more uniform across the field when zones receive tailored inputs rather than a blanket rate.

Precision nutrient management does not add more fertilizer to the best zones — it removes the waste from the worst-managed ones, and that difference is where both profit and environmental protection are found.

Nutrient uptake efficiency (NUpE, defined as the total nutrient absorbed by the crop divided by the total nutrient applied) increases under zone-based management for a simple mechanistic reason: fewer nutrients are applied to zones that already have adequate supply, reducing the denominator of the efficiency ratio while maintaining or improving uptake.

Studies reviewed in Frontiers in Plant Science (2024) found that NUpE for nitrogen in Allium species increased from an average of 42% under uniform management to 61 to 67% under SSMZ-based variable rate management — a gain that directly reduces the nitrate load available for leaching into groundwater.

Effects on Green Onion Growth Parameters

Zone-specific nutrient management produces measurable improvements in plant height, leaf area index, and biomass accumulation. The mechanism is straightforward: when each zone receives the nitrogen dose matched to its supply-demand gap, nitrogen is neither diluted by luxury application nor limiting in deficient zones, and the crop allocates carbon to above-ground growth rather than to compensatory root exploration for scarce nutrients.

In field trials conducted in Egypt’s Nile Delta region (published in the Journal of Horticultural Science and Biotechnology, 2023), green onion plots managed under a three-zone SSMZ regime showed statistically significant improvements in growth metrics.

• Plant height in the high-potential zone increased by 14.3% over the field-average height recorded under uniform management, attributed to optimized nitrogen delivery during the rapid vegetative growth phase.
• Leaf area index at 45 days after transplanting was 18% higher in the medium-potential zone under zone-specific management compared to the same zone under uniform management, because the corrected phosphorus application improved root development and water uptake capacity.
• Total above-ground fresh biomass at harvest was 12.7% greater in the SSMZ-managed field compared to the conventionally managed control, primarily driven by improvements in the previously under-fertilized low-potential zone.

Root development improvements are harder to measure destructively at scale, but rhizotron studies show that zone-appropriate potassium nutrition increases root hair density and elongation, improving the physical contact surface between roots and soil particles where mass-flow nutrient delivery is most critical.

Effects on Yield and Quality of Green Onion

Yield improvements from SSMZ management in green onion accrue from two distinct pathways. First, zones that were previously over-fertilized — typically the high-organic-matter, naturally fertile patches — are protected from salinity stress and luxury nutrient toxicity, which can reduce yields even in inherently productive soils.

Second, zones that were previously under-fertilized receive corrective rates that lift their performance toward their genetic yield potential, raising the field average without requiring additional total fertilizer expenditure. The key quality parameters that improve under zone-based management tell a commercially important story:

1. Bulb diameter and uniformity improve because zone-specific potassium supply ensures consistent carbohydrate partitioning to the bulb across the entire field, rather than only in the areas that happened to have adequate native K availability.

2. Chlorophyll content at harvest — measured by SPAD or destructive extraction and expressed as mg chlorophyll per gram fresh weight — is higher and more uniform in SSMZ-managed crops, producing the deep green leaf color that commands premium prices in fresh markets and export chains.

3. Total soluble solids (TSS), a direct indicator of sugar accumulation and flavor intensity, increase by 8 to 12% under zone-optimized potassium and sulfur management, according to data published in the Journal of the Science of Food and Agriculture (2024).

4. Pungency score — quantified as pyruvic acid concentration (mmol/100g fresh weight), the accepted biochemical marker of onion pungency intensity — responds directly to adequate sulfur nutrition. Zone-specific sulfur application in sulfur-deficient zones has been shown to increase pyruvic acid content by 15 to 22%, improving both flavor profile and the shelf-stable sulfur compounds that extend post-harvest life.

EcoEnvironmental Implications of Zone-Based Management

The economic case for SSMZ adoption in green onion production is anchored in the cost-benefit structure of precision input management. The upfront investment includes soil sampling (typically USD 12 to 25 per hectare for grid sampling), laboratory analysis, GIS mapping software (with open-source QGIS available at no cost), and variable rate application equipment.

For a 10-hectare commercial green onion enterprise, total setup costs range from USD 800 to 2,500 depending on sampling density and equipment choices. Against this investment, growers can expect measurable financial returns. Fertilizer savings from eliminating over-application in high-fertility zones typically range from 15 to 25% of total fertilizer expenditure.

Premium-grade yield improvements — the proportion of harvest meeting export or supermarket grade specifications — increase by 10 to 20%, which commands price premiums of 20 to 35% per kilogram in premium vegetable markets. Combined, these benefits produce a return on SSMZ investment of 2.5 to 4.5 times the setup cost within a single growing season for commercial-scale producers.

The environmental implications are equally significant. Nitrate leaching into groundwater, the principal environmental externality of intensive vegetable production, is reduced by 40 to 60% under zone-specific nitrogen management compared to uniform blanket applications, according to a meta-analysis published in the European Journal of Agronomy (2024).

Phosphorus runoff, which drives eutrophication of surface water bodies, decreases proportionally as over-applied P in high-fertility zones is eliminated. The reduction in total synthetic fertilizer use also lowers the carbon footprint of the production system, since synthetic nitrogen manufacture accounts for approximately 1.5 kg CO2-equivalent per kg of urea produced.

Challenges and Limitations That Growers Should Anticipate

SSMZ delineation is not without practical barriers, and honest recognition of these limitations is essential for realistic adoption planning.

i. Data collection costs represent the primary barrier for smallholder producers. Grid soil sampling at sufficient density for reliable kriging interpolation requires 15 to 30 samples per hectare in highly variable fields, and laboratory analysis for a full nutrient profile can cost USD 30 to 80 per sample. For a 1-hectare smallholder plot, this single cost item may exceed the entire input budget.

ii. Technical expertise in geostatistics, GIS software operation, and variable rate equipment calibration is not widely available in most vegetable-producing regions. Extension services rarely cover spatial data analysis, and private agronomic consultants with SSMZ competency charge premium fees that are accessible only to larger operations.

iii. Smallholder applicability is structurally limited by plot size. Kriging interpolation requires a minimum of 10 to 15 sample points per variable to generate reliable maps, setting a practical lower limit of approximately 2 to 3 hectares for cost-effective SSMZ work with conventional soil sampling. Below this threshold, directed composite sampling by visible field zones is a more pragmatic alternative.

iv. Temporal variability of soil properties — particularly nitrate nitrogen, which can change by 50% or more within a single month depending on rainfall and temperature — means that zone maps derived from pre-season sampling may not accurately reflect conditions at the time of in-season top-dressing decisions. Crop sensor technologies (NDVI drone flights, real-time SPAD readings) are necessary to update nutrient prescriptions within the season.

Future Perspectives: Where SSMZ Science Is Heading

The next generation of SSMZ science for vegetable crops is converging on three technological frontiers that will substantially reduce the cost and increase the accuracy of zone delineation.

Drone-based multispectral and hyperspectral imaging is replacing time-intensive manual soil sampling as the primary data source for rapid SSMZ delineation. A single drone flight at 30 to 50 meters altitude can capture canopy reflectance data at 5 to 10 cm spatial resolution across an entire farm in under an hour.

When calibrated with targeted soil samples at representative points, drone imagery can generate NDVI, red-edge chlorophyll index, and canopy temperature maps that identify zone boundaries with accuracy comparable to dense grid sampling at a fraction of the cost.

Machine learning algorithms — particularly random forest classifiers and neural networks trained on multi-year datasets of soil properties, yield history, and satellite imagery — are transforming zone delineation from a single-season snapshot into a dynamic, predictive system.

Models trained on five or more years of field data can predict zone boundaries for the upcoming season before any new soil sampling is conducted, allowing prescription maps to be prepared weeks before planting and reducing the season-start time pressure on growers.

Climate-smart nutrient management represents the conceptual frontier of SSMZ work. As seasonal temperature and precipitation patterns become less predictable, the ability to adjust zone-specific fertilizer prescriptions in response to real-time weather forecasts — reducing N applications in zones facing waterlogging risk before a heavy rainfall event, or increasing K in heat-stressed zones during a dry spell — will become a core function of farm management systems.

Integration with cloud-based decision support platforms that combine weather data, crop models, soil sensor readings, and market price signals is already underway in early-adopter farming enterprises in the Netherlands, Israel, and Australia.

Conclusion

The delineation of site-specific management zones for green onion (Allium cepa L.) is no longer a research curiosity — it is a commercially validated strategy for improving nutrient status, growth uniformity, and produce quality while simultaneously reducing input costs and environmental impact. The evidence base reviewed demonstrates that SSMZs, when properly delineated using combined soil chemistry, geostatistical analysis, crop-based sensors, and GIS integration, consistently outperform uniform management across the metrics that matter most to commercial producers: nitrogen use efficiency, marketable yield, bulb grade uniformity, and post-harvest shelf life. For agronomists and crop consultants advising on green onion enterprises, the practical recommendations are clear. Begin with grid soil sampling at 1 sample per hectare minimum, prioritizing pH, organic matter, EC, and available NPK as the primary zone-defining variables.