Live-Microbial Fertilizers: A Literature Review of Soil-Health, Yield, and Carbon Outcomes

Executive summary

Live-microbial fertilizers — products that deliver living bacteria, fungi, and their metabolites to soil, typically alongside or in place of fraction of the conventional NPK regime — have moved from a niche subject in agronomy journals to a substantial empirical literature. This review surveys peer-reviewed evidence on their effects across five outcome categories: yield, soil-microbiome composition and function, water-use efficiency, soil-carbon dynamics, and farm-level economics. The synthesis covers meta-analyses, region-specific field trials, mechanistic studies, and long-term plot data from temperate, Mediterranean, and arid systems.

The evidence is strongest, and most consistent, in three areas. First, microbial inoculants — particularly arbuscular mycorrhizal fungi (AMF), nitrogen-fixing rhizobia, and plant growth-promoting rhizobacteria (PGPR) — produce yield gains in the 10–25 percent range on average, with high variance and a clear dependence on baseline soil condition (Schütz et al., 2018; Rubin et al., 2017). Second, live-microbial inputs paired with reduced or partially substituted NPK can match conventional yields while lowering synthetic fertilizer use by 25–50 percent in many cropping systems (Mitter et al., 2021; Adesemoye et al., 2009). Third, drought and salinity tolerance is reliably modulated by AMF and certain PGPR consortia, particularly in arid and Mediterranean systems (Augé, 2001; Aroca et al., 2013; Ruiz-Lozano et al., 2012).

The evidence is weaker, and warrants conservative claims, in two areas. Soil-carbon sequestration outcomes from microbial inputs are smaller and noisier than industry communications often imply. Measurement challenges — depth, time horizon, spatial heterogeneity — make many short-term claims unfalsifiable in the time scale at which they are made (Schlesinger and Amundson, 2019; Powlson et al., 2014; Lal, 2004). And long-term economic outcomes, while broadly positive, depend heavily on regional input prices, water cost, and the trajectory of a given farmer's soil prior to intervention.

This review concludes with the gaps that most constrain confident recommendation: the scarcity of long-term replicated trials in the Gulf and North Africa, the weakness of inoculum quality standards globally, and the difficulty of generalizing across crop-soil-climate combinations. Practitioners can act on the strongest signals while remaining honest about the rest.

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1. Scope and methodology

This review covers peer-reviewed literature published between approximately 1990 and 2025, with emphasis on work from the past fifteen years. Search strategies included Web of Science, Scopus, and Google Scholar queries built on combinations of "microbial inoculant," "biofertilizer," "PGPR," "arbuscular mycorrhiza," "soil organic carbon," "water-use efficiency," and "yield response." Reference lists of major meta-analyses and review articles were mined for primary studies. FAO and IPCC reports were consulted as authoritative grey literature for soil resource and climate context.

Inclusion criteria favored controlled field trials, replicated greenhouse studies, and meta-analyses with explicit effect-size methodology. Single-farm anecdotes and unblinded vendor-sponsored trials were noted but de-weighted. Where industry communications and peer-reviewed evidence diverge — as they do in some carbon sequestration claims — the review states the divergence rather than papering over it.

The term "live-microbial fertilizer" is used here to mean any product whose intended mode of action depends on a population of living microorganisms surviving application and establishing — at least transiently — in the rhizosphere or bulk soil. This includes AMF inocula, rhizobial seed coats, PGPR consortia, compost teas, and live-microbe-rich liquid extracts derived from manured or fermented sources, including the live microbial extract derived from fish manure and plant biomass that is the topical reference for this review series. Products labelled "biofertilizer" but consisting only of dead microbial cells, microbial metabolites, or humic acids without living organisms are out of scope, although they appear in the literature alongside live products and are sometimes conflated.

Five outcome categories structure the synthesis: yield (Section 2), region-specific responses in arid and Mediterranean systems (Section 3), soil carbon (Section 4), water-use efficiency (Section 5), and economics (Section 6). Section 7 catalogues gaps. The reference list is long because the audience for this review is expected to fact-check it; readers should treat any claim made here as inviting verification against the primary source cited.

A note on framing: live-microbial fertilizers are not a substitute for nitrogen, phosphorus, and potassium when those are limiting. The strongest peer-reviewed work treats them as complements that allow synthetic NPK to be reduced — not eliminated — while maintaining or improving yield (Bender et al., 2016; Mitter et al., 2021). Throughout this review, claims about live-microbial inputs are paired with the NPK context in which they were studied. Marketing claims that imply microbial inputs alone can replace mineral nutrition are not supported by the consensus literature.

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2. Soil microbiome and yield outcomes — evidence survey

The single most-cited recent meta-analysis on biofertilizer yield response is Schütz et al. (2018), which synthesized 171 published studies across cereals, legumes, and vegetables. Aggregate biofertilizer effect sizes on yield were positive and statistically robust: mean grain-yield increase of approximately 16 percent in cereals, with larger effects in legumes (where rhizobial co-inoculation drove the response) and a wide confidence interval reflecting heterogeneous trial conditions. Crucially, the analysis identified soil organic carbon and baseline P availability as moderators — inoculants performed better on organic-carbon-poor soils and in P-limited systems, exactly the soils where smallholder and arid agriculture is most concentrated.

An earlier and equally influential meta-analysis, Rubin, Stiling, and van Diepen (2017), examined the response of plant growth and yield to microbial inoculants across more than 700 study cases. Their headline finding — that mycorrhizal inoculation produced significant biomass and yield increases on average, but with effect sizes shrinking under high-fertility conditions — has become canonical in the field and underlies a now-standard agronomic intuition: microbial inputs add the most where the soil starts most degraded.

Veresoglou et al. (2012) and Lehmann et al. (2017) document complementary patterns for AMF specifically. AMF inocula are most effective in P-deficient soils, in non-irrigated or water-limited systems, and where soil disturbance has reduced the indigenous mycorrhizal network. In high-input European cereal rotations, AMF effects can be small or absent — not because the fungi are ineffective in principle, but because indigenous AMF populations are already saturating the available niche.

Mechanistic work helps explain the variance. PGPR effects on yield operate through several pathways: nitrogen fixation (Vessey, 2003), phosphate solubilization (Khan et al., 2009), phytohormone production, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity that lowers stress-induced ethylene (Glick, 2014). AMF effects operate through extension of the root absorptive surface via hyphal networks, improved P acquisition, and water-relations modulation (Smith and Read, 2008). When the limiting factor in a given trial matches the mode of action of the inoculum, response is large; when it does not, response is small. This is intuitive and yet routinely missed by general-purpose marketing.

A central honest finding from the meta-analyses: variance in outcomes is high, and a non-trivial fraction of trials report no significant yield effect. Bashan et al. (2014) catalogues persistent commercialization failures linked to low cell viability at point of use, lack of carrier-to-soil compatibility, and incompatible co-formulated chemistries. The evidence supports yield benefit on average; it does not support a guarantee in any specific season-soil-crop combination.

Recent work has begun to use community-level metrics — 16S rRNA and ITS amplicon profiles, shotgun metagenomics — to characterize how inoculants reshape resident microbiomes. Mitter et al. (2021) reviews the rapidly evolving "synthetic microbial community" (SynCom) approach, in which engineered consortia are designed to fill multiple functional niches simultaneously. Early SynCom field trials report yield gains comparable to single-strain inoculants with greater consistency, but the literature is still young and replication outside of high-input greenhouse contexts is limited.

For agronomists and consultants, the take-away from the yield literature is summarized in three propositions. First, expect mean yield gains in the 10–25 percent range when live-microbial inputs are deployed on degraded or moderately degraded soils alongside reduced NPK. Second, expect substantial year-to-year and field-to-field variance. Third, do not expect inoculants to add yield on top of already-saturated high-input systems with intact indigenous microbial communities; the more degraded the starting point, the larger the response. (See /blog/whats-actually-alive-in-your-soil for a plain-language treatment of these mechanisms.)

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3. Live-microbial inputs in arid and Mediterranean agriculture

Arid and Mediterranean systems are where live-microbial inputs perform most consistently, and the reason is structural. Arid soils typically combine low organic carbon, depleted indigenous microbial biomass, water scarcity, and varying degrees of salinity — a combination that creates exactly the conditions under which microbial inoculation has the largest documented effect (FAO, 2015; FAO and ITPS, 2020).

Augé's (2001) foundational review on AMF and plant water relations, with more than 200 cited studies, established that AMF symbiosis improves leaf water status, stomatal conductance, and transpiration efficiency under drought across a wide range of host species. Subsequent work by Ruiz-Lozano et al. (2012) and Aroca et al. (2013) refined the mechanism: AMF-colonized plants accumulate compatible solutes more rapidly, regulate aquaporin expression more favorably under stress, and recover root function faster after drought episodes. These effects translate into measurable yield protection in arid trials, not just physiological measurements in growth chambers.

Field evidence from Mediterranean systems is consistent. Querejeta et al. (2007, 2008) documented improved water uptake and reduced plant water stress in AMF-treated olive and almond systems in semi-arid Spain. Estaún et al. (2010) reported yield maintenance under deficit irrigation in AMF-treated horticultural crops. In Israel and Jordan, work on PGPR consortia in tomato and pepper under saline irrigation has shown yield protection of 15–30 percent versus uninoculated controls under matched water budgets (Mayak et al., 2004; Egamberdiyeva, 2007).

Gulf-region literature is sparser but growing. Studies from the United Arab Emirates and Saudi Arabia on date palm, alfalfa, and protected horticulture report positive responses to AMF and Bacillus-based PGPR under saline groundwater irrigation (Al-Karaki, 2006; Hashem et al., 2016). Sample sizes are smaller and replication weaker than in Mediterranean Europe, and this asymmetry — much more long-term data from Spain than from the UAE — is the single largest evidence gap for Gulf practitioners. Section 7 returns to it.

The aggregate effect of live-microbial inputs in arid systems is not just yield protection but yield protection at lower water use. The trial-level evidence is consistent: AMF and PGPR-treated plants typically achieve 80–100 percent of conventional yield with 20–30 percent less applied irrigation, especially under deficit irrigation regimes (Augé, 2001; Aroca et al., 2013; Querejeta et al., 2008). For a Gulf or North African farmer paying for desalinated or pumped groundwater, this is the headline outcome.

A caveat on regional generalization. Mediterranean trial data — Spain, Italy, Greece, Israel, parts of North Africa — is much richer than data from the inner Arabian Peninsula or sub-Saharan Sahel. Conclusions drawn from Mediterranean systems and applied to a UAE container greenhouse, an Egyptian wheat field, or a Senegalese groundnut plot should be treated as informed extrapolation rather than direct evidence. Section 7 catalogues this as a primary gap. (See /blog/water-that-stays-where-you-put-it for the irrigation-economics implications.)

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4. Soil carbon sequestration — what the evidence supports

Soil carbon sequestration is the area where industry messaging and peer-reviewed evidence diverge most sharply, and a literature review for a sophisticated audience must be candid about that gap.

Lal (2004), in his canonical Science paper, estimated a global soil carbon sequestration potential of 0.4–1.2 Gt C per year through improved management — including conservation tillage, cover cropping, residue retention, and biological soil amendments. This estimate has been the bedrock for two decades of climate-smart agriculture policy. It has also been challenged, refined, and in places contradicted by subsequent measurement work.

The most important refinement comes from Lehmann and Kleber (2015), whose Nature paper articulates the modern view of soil organic matter (SOM) as a continuum of decomposing fragments rather than a stable "humus" pool. Under this view — now mainstream in soil biogeochemistry — much of the SOM increase observed in short-term studies is biologically labile and will return to the atmosphere on decadal time scales unless management is sustained. The implication is uncomfortable for some sequestration claims: short-term carbon increases measured in the topsoil after a few years of biological inputs are not equivalent to durable atmospheric removals.

Schlesinger and Amundson (2019) and Powlson et al. (2014) have been the most prominent critical voices. Their argument is methodological: many soil carbon sequestration claims rely on shallow sampling (top 0–15 cm or 0–30 cm only), short time horizons (under 5 years), absent measurement of bulk density changes, and inadequate accounting for spatial variability. When sampled to 1 m, integrated over decades, and corrected for bulk density and erosion, the apparent stocks change in ways that can substantially reduce — and in some cases reverse — the headline sequestration figure.

For live-microbial inputs specifically, the peer-reviewed evidence on direct soil-carbon impact is mixed and modest. Rillig (2004) and Rillig et al. (2015) document the role of AMF and glomalin-related soil proteins in aggregate stabilization, which in principle protects occluded SOM from microbial mineralization. Six et al. (2002, 2004) developed the now-standard framework for SOM stabilization through aggregate occlusion and mineral association. These mechanisms are real. But the quantitative carbon flux they support, at field scale, on the time scales available in typical agricultural studies, is smaller than the climate-smart-marketing literature often claims.

Honest framing for a 2026 audience: live-microbial inputs paired with reduced synthetic NPK and integrated organic residue return are part of a genuine soil-carbon-friendly system. They are not, on their own, a high-confidence verifiable carbon offset at multi-tonne-per-hectare-per-year scales. Claims of that magnitude, when not paired with deep, long-term, bulk-density-corrected measurement and durable practice change, fail the scrutiny of papers like Schlesinger and Amundson (2019) and should be treated skeptically by ESG analysts and consultants.

What the evidence does support, conservatively: sustained live-microbial input regimes coupled with cover cropping, reduced tillage, and residue retention can build measurable topsoil organic carbon over 5–10 year horizons, on the order of 0.1–0.4 t C ha⁻¹ yr⁻¹ in many temperate and Mediterranean systems (van Groenigen et al., 2017; Minasny et al., 2017). These are real but modest numbers. They support a soil-health and resilience narrative more confidently than they support a stand-alone carbon-finance narrative. Practitioners and analysts should act accordingly.

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5. Water-use efficiency outcomes

Water-use efficiency (WUE) outcomes from live-microbial inputs are well-documented and operate through three distinct but compounding mechanisms: physiological (plant water relations), structural (soil aggregate stability and porosity), and ecological (root architecture and microbiome-mediated stress signaling).

The physiological pathway, reviewed in Section 3, is best characterized for AMF. Plants colonized by AMF show altered stomatal behavior, improved leaf turgor under deficit, and faster recovery after drought (Augé, 2001; Ruiz-Lozano et al., 2012). PGPR with ACC deaminase activity reduce stress-induced ethylene, allowing root growth to continue under conditions where uninoculated plants stall (Glick, 2014; Belimov et al., 2009).

The structural pathway operates at the soil scale. Rillig (2004), Rillig and Mummey (2006), and Bedini et al. (2009) document the contribution of fungal hyphae, glomalin-related proteins, and bacterial extracellular polysaccharides to macroaggregate formation. Stable macroaggregates increase porosity, improve infiltration, and raise soil water holding capacity at field-relevant tensions. Reported water holding capacity improvements in well-managed live-microbial input regimes range from 5 percent to 25 percent over 2–5 year horizons, with effect size strongly dependent on baseline soil organic matter and clay content (Lehmann and Kleber, 2015; Bardgett and van der Putten, 2014).

The ecological pathway is more recent in the literature but increasingly well-supported. Rhizosphere microbiomes mediate root-derived signaling under drought, including hormonal cross-talk that affects whole-plant water use. Synthetic-community work by Niu et al. (2017) and Carlström et al. (2019) demonstrates that defined microbial consortia can shift host stress response phenotypes in measurable ways. The mechanisms are complex; the outcomes — improved WUE per unit water applied — are reproducible across multiple plant-microbe systems.

For practitioners, the operational signal is that live-microbial inputs, sustained over multiple seasons, produce measurable improvements in three field-level metrics: infiltration rate (single-ring), volumetric water holding capacity at -33 kPa, and crop water productivity (yield per cubic meter applied). Reported improvements across the literature fall in the ranges of 10–40 percent for infiltration, 5–25 percent for water holding capacity, and 10–30 percent for crop water productivity (Rillig, 2004; Augé, 2001; Querejeta et al., 2008; Bender et al., 2016).

These ranges are wide because the underlying systems are heterogeneous. A consultant or extension agent recommending live-microbial inputs as a water-savings intervention should design the trial to measure these three metrics directly rather than relying on generic vendor claims, and should expect realized savings to land somewhere in the lower half of the published range in the first season, with improvement over subsequent seasons as the soil structural response compounds.

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6. Long-term economic outcomes

The economic literature on live-microbial fertilizers is smaller than the agronomic literature but consistent in its broad conclusion: when deployed alongside partially substituted NPK, live-microbial inputs typically produce favorable economics on a 2–5 year horizon, with greater favorability the further the system is from a saturated high-input baseline.

Pretty et al. (2018) survey "Sustainable Intensification" outcomes across 85 projects in low- and middle-income countries, documenting consistent yield and income increases when biological inputs were combined with reduced synthetic fertilizer and improved water management. Mean yield increase of 79 percent across the surveyed cases is partly an artifact of the very low baseline yields of those starting systems — but the directional finding is robust: biological inputs paired with reduced inputs improved farm-level economics in the great majority of cases tracked.

In high-input contexts, the economic signal is more nuanced. Adesemoye et al. (2009) showed that combined PGPR + AMF inoculation in tomato systems allowed 25 percent reduction in synthetic fertilizer with no yield penalty — a finding which directly translates into fertilizer cost savings, though without yield gain to expand the gross margin. The economic crossover point — the breakeven where the cost of inoculum is balanced by NPK savings — depends sharply on the relative price of synthetic fertilizer versus inoculum and on the substitution rate that can be achieved without yield loss.

Long-term plot data is rare and especially valuable. The Rodale Institute's Farming Systems Trial in the United States, with more than three decades of side-by-side comparison of organic and conventional grain rotations, has consistently shown competitive yields and superior long-term net returns for organically managed plots after the third year (Rodale Institute, 2020 update; multiple peer-reviewed reports including Pimentel et al., 2005). The "transitional risk" — the temporary yield dip that often appears in the first 2 years after sharply reducing synthetic inputs — is a real and documented phenomenon (Liebman et al., 2013; Pimentel et al., 2005). It is the single largest economic obstacle to adoption.

For arid and Mediterranean farmers paying high water and imported fertilizer costs, the economic calculus shifts in favor of biological inputs more sharply than in temperate, low-water-cost systems. Where water is the binding constraint, the WUE improvements documented in Section 5 translate directly into either reduced pumping cost or expanded planted area at fixed water budget. Studies on date palm and protected horticulture in the Gulf, while less numerous than in Mediterranean Europe, point in the same direction (Al-Karaki, 2006; Hashem et al., 2016).

For ESG analysts and consultants, two cautions on the economic literature are worth emphasizing. First, vendor-sponsored economic studies routinely understate transitional risk and overstate first-season returns; the peer-reviewed and extension-service literature tells a more honest story of a 2–3 year payback in well-managed systems. Second, regional input prices vary dramatically — the same agronomic intervention is much more economically attractive in the UAE in 2026, where imported NPK and desalinated water are both expensive, than in a low-input rainfed European system. Generic ROI claims should be regionalized before being trusted. (See /blog/water-that-stays-where-you-put-it for the irrigation-economics framing relevant to Gulf farms.)

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7. Gaps in the literature

A literature review that does not name its gaps is not honest. Five gaps stand out in the live-microbial fertilizer literature, and they shape how confidently any specific claim can be made.

Long-term replicated trials in the Gulf and inner arid Arabia are scarce. Most arid-system data comes from Mediterranean Europe (Spain, Italy, Greece), Israel, parts of North Africa, and the U.S. Southwest. Direct, multi-year, replicated field trial data from the United Arab Emirates, Saudi Arabia, Oman, and Qatar — and from inner Egypt and Sudan — is limited (Al-Karaki, 2006; Hashem et al., 2016 are among the cited exceptions). For Gulf practitioners and policymakers, this means region-specific extrapolation from Mediterranean evidence is currently the best available guidance, and locally-replicated trials remain a high-priority research need.

Inoculum quality standardization is weak globally. Commercial inoculant products vary enormously in cell viability at point of sale, in carrier compatibility, and in survival under field storage conditions (Bashan et al., 2014). Regulatory standards — where they exist — vary across jurisdictions. Two products labeled identically can differ by orders of magnitude in delivered live cell count. A non-trivial share of "no-effect" trial outcomes in the meta-analysis literature is plausibly attributable to product quality failures rather than to mode-of-action failures. Consultants should not assume products are interchangeable.

Cross-crop and cross-soil generalization is risky. A trial showing 20 percent yield gain in tomato in a P-deficient sandy loam in Spain does not safely predict the same gain in wheat in a saline-affected silty clay in Egypt. Mode of action varies with both crop and soil, and the published meta-analyses are clear about the moderating role of baseline conditions. Confident application of live-microbial inputs to a new crop-soil combination should be preceded by site-specific validation, ideally with a small replicated strip trial in season one before scaling.

Soil carbon measurement remains methodologically contested. Section 4 catalogued the divergence between industry sequestration claims and the more conservative peer-reviewed view exemplified by Schlesinger and Amundson (2019), Powlson et al. (2014), and the Lehmann-Kleber (2015) reframing of SOM. Until measurement protocols routinely include deep sampling, bulk-density correction, and decadal time horizons, headline carbon claims should be discounted heavily.

Microbiome-level mechanism studies are advancing faster than agronomic field trials. Sequencing-based work — 16S, ITS, shotgun metagenomics — is producing a rich picture of how inoculants reshape soil and rhizosphere communities (Mitter et al., 2021; Trivedi et al., 2020). Agronomic field trials, with their longer time scales and harder logistics, are not keeping pace. The result is a literature in which mechanistic claims sometimes outrun the field evidence they are based on. This is a normal phase in a maturing field; it argues for treating mechanism reviews as hypothesis-generating and field trials as the actual evidence.

For research priorities, the gaps suggest a clear ordering. First, multi-year, multi-site replicated trials in Gulf, North African, and sub-Saharan systems. Second, inoculum quality standardization with transparent live-cell labeling. Third, deep-sampling, decadal-horizon soil carbon studies that can move the carbon-sequestration claim out of contested territory. Fourth, site-specific validation protocols that allow practitioners to cheaply test inoculants on their own soil-crop combination before committing.

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8. Conclusion and field implications

The peer-reviewed literature supports a measured but real conclusion: live-microbial fertilizers, deployed in combination with — not in place of — partially reduced synthetic NPK, produce measurable improvements in yield, water-use efficiency, and soil structural metrics across a wide range of cropping systems. The evidence is strongest where the soil starts most degraded; the evidence is weakest where industry claims are most extravagant, particularly around stand-alone carbon sequestration credit.

For a sustainability consultant or ESG analyst evaluating a live-microbial product or program, the literature supports three operational stances.

First, treat live-microbial inputs as a yield-and-resilience intervention with strong evidence and as a carbon-credit instrument with weak-to-modest evidence. The first claim is well-supported by Schütz et al. (2018), Rubin et al. (2017), and the meta-analytic literature; the second is undercut by the methodological critique of Schlesinger and Amundson (2019) and the SOM reframing of Lehmann and Kleber (2015). Allocate confidence accordingly.

Second, expect substantial variance and design programs that measure rather than assume. Field-by-field replication, baseline measurement, and multi-year tracking are not optional in a literature this heterogeneous. Single-season vendor case studies, no matter how flattering, are not evidence at the standard a sophisticated client should accept.

Third, pair the agronomic claim with the regional economic context. In high-water-cost, high-imported-fertilizer-cost systems — the Gulf, much of North Africa, parts of South and Southeast Asia — the documented WUE and fertilizer-substitution effects translate into farm economics more favorably than in low-cost temperate systems. The same intervention has a different economic footprint in Al Ain than in Iowa.

For practitioners — extension agents, agronomists, farm managers — the actionable conclusions are simpler. Live-microbial inputs paired with reduced NPK can plausibly deliver 10–25 percent yield response on degraded soils, 10–30 percent water productivity improvement, and modest topsoil carbon improvement on multi-year horizons. The first season is rarely the headline; the second and third seasons typically are. Plan accordingly, measure honestly, and read the gaps in this literature as honestly as the findings.

For the companion field protocol, see /soil-science. For plain-language treatments of microbiome and irrigation outcomes, see /blog/whats-actually-alive-in-your-soil and /blog/water-that-stays-where-you-put-it.

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