From reports «Role of humic acid for climate change adaptation measures to boost up sustainable agriculture and soil health: A potential review«, from Uttam Biswas Antu, Tusar Kanti Roy, Taslima Islam Kulsum, Puja Rani Mitu, Zulhilmi Ismail, Mehenaz Arifin, Mitu Datta, Sk Arafat Hossain, Md. Saiful Islam, Nor Aida Mahiddin, Abdullah Al Bakky, Shahin Hossin, Safiqul Islam, and Abubakr M. Idris, published in Science direct, «Enhancing Potato Productivity and Nutritional Status Under Drought Stress: The Role of Humic Acid in Climate-Resilient Agriculture«, from Hany Kamal Koheal; Yasser Eldamarawy; Mona El-Azab; Intsar Essa; and Fatma S. Aboud, published in Egiptian journal of agronomy, «Mitigation of water stress in broccoli by soil application of humic acid«, from Ehab A. Ibrahim, Noura E. S. Ebrahim and
Gehan Z. Mohamed, published at Nature, and «Mitigation of drought stress in maize and sorghum by humic acid: differential growth and physiological responses«, from Mohamed E. Abu-Ria, Eman M. Elghareeb, Wafaa M. Shukry, Samy A. Abo-Hamed, and Farag Ibraheem, published at BMC Plant biology.
Climate‑change‑driven aridification is reshaping soil–plant interactions across agroecosystems, intensifying the frequency and duration of drought episodes and thereby constraining crop productivity. In all four studies—maize, sorghum, potato, and broccoli—water deficit triggers a coherent suite of morphophysiological disruptions: reduced cell expansion and division, impaired stomatal regulation, depressed CO₂ assimilation, pigment degradation, and destabilization of redox homeostasis. These responses are particularly acute in sandy or low‑organic‑matter soils, where limited water‑holding capacity and low cation‑exchange capacity exacerbate plant stress. The potato experiment conducted in the semi‑arid Nubaria region exemplifies this dynamic: high temperatures (20.5–30.8 °C), minimal rainfall (10–15 mm), and elevated ET₀ create a hydrologically hostile environment that sharply reduces tuber size, starch accumulation, and total yield.
Humic acids (HAs) emerge across the four studies as multifunctional biostimulants capable of modulating drought responses through both edaphic and physiological pathways. Their phenolic and carboxylic functional groups enhance soil aggregation, porosity, and water retention, particularly in coarse‑textured Entisols such as the Typic Torripsamments of the potato study. By increasing soil organic matter inputs and improving the soil’s capacity to retain nutrients (notably NH₄⁺, K⁺, and micronutrients), HAs buffer plants against the nutrient‑uptake disruptions characteristic of drought.
| Mechanism | Drought effect | Humic acid effect | Crops where observed |
|---|---|---|---|
| Soil water retention | Sharp decline, especially in sandy soils; reduced capillary continuity | Increases WHC via aggregation, porosity, hydrophilic functional groups | Potato (sandy Typic Torripsamments), broccoli |
| Cation exchange capacity (CEC) | Lower effective CEC under dry conditions; reduced nutrient mobility | HA carboxyl/phenolic groups increase CEC and nutrient buffering | Potato, broccoli, review article |
| Nutrient availability | P fixation, K depletion, micronutrient immobility | Chelation, complexation, improved ion diffusion | Potato (DRIS/NBI), broccoli |
| Microbial activity | Declines due to low moisture and substrate diffusion | HA stimulates microbial biomass and enzymatic activity | Review article |
In maize and sorghum, HA seed priming improves relative water content, chlorophyll and carotenoid concentrations, and gas‑exchange parameters, while reducing oxidative stress markers such as H₂O₂, MDA, and electrolyte leakage. These effects reflect an up‑regulation of both enzymatic (CAT, POD, PPO, APX) and non‑enzymatic antioxidant systems, stabilizing cellular metabolism under water deficit.
The potato study adds a strong edafological dimension through its use of the Diagnosis and Recommendation Integrated System (DRIS), which quantifies nutrient balance via dual nutrient ratios (N/P, P/N, K/P). Water‑stressed plants exhibit more extreme DRIS indices and higher Nutrient Balance Index (NBI) values, indicating pronounced nutritional disequilibrium under drought. Humic acid application modulates these imbalances: although its effects are not uniformly corrective across all nutrients, HA generally improves NBI—especially under stress—and aligns nutrient ratios more closely with high‑yielding DRIS norms. The quantitative yield response is clear: under unstressed conditions, tuber yield increases from 35.4 to 44.4 Mg ha⁻¹ with HA; under drought, from 31.7 to 35.1 Mg ha⁻¹. These improvements reflect enhanced root architecture, increased membrane permeability, and improved nutrient acquisition, all of which are critical in sandy soils with low inherent fertility (OM 0.8%, Olsen‑P 3 mg kg⁻¹, NH₄OAc‑K 98 mg kg⁻¹).
The broccoli study reinforces the cross‑crop consistency of HA‑mediated drought mitigation. HA increases plant height, leaf area, nutrient concentrations, and chlorophyll content under both irrigation regimes, while reducing proline accumulation under drought—an indicator of reduced stress intensity.
| Mechanism | Drought effect | Humic acid effect | Crops |
|---|---|---|---|
| Chlorophyll & carotenoids | Degradation; reduced light harvesting | HA increases pigment stability and synthesis | Maize, sorghum, broccoli |
| Gas exchange (gs, A, Ci) | Stomatal closure → reduced CO₂ assimilation | HA improves stomatal conductance and CO₂ fixation | Maize, sorghum |
| Relative water content (RWC) | Declines sharply | HA increases RWC via improved root water uptake | Maize, sorghum |
| Oxidative stress | ↑ H₂O₂, MDA, electrolyte leakage | HA enhances CAT, POD, PPO, APX; reduces ROS | Maize, sorghum, broccoli |
Importantly, HA enhances water‑use efficiency, a key agronomic parameter in regions facing chronic water scarcity. These responses parallel those observed in cereals and tubers, underscoring the capacity of HA to stabilize photosynthetic performance and nutrient economy across diverse plant functional types.
CLIMATE CHANGE → DROUGHT → MULTILEVEL STRESS
↓
SOIL degradation → ROOT dysfunction → LEAF metabolic collapse
↓
WHOLE‑PLANT instability → YIELD loss
HUMIC ACID → MULTILEVEL MITIGATION
↓
SOIL improvement → ROOT efficiency → LEAF stability
↓
WHOLE‑PLANT resilience → YIELD recovery