Guano-Based Organic Fertilizer Shows a Sweet Spot for Restoring Degraded Soils

Soil degradation now touches roughly 40 percent of the world’s agricultural land, and the consequences reach far beyond the farm gate. When soils lose their physical, chemical, and biological integrity, they lose the capacity to cycle nutrients, regulate water, and sustain the microbial communities that underpin crop productivity. Research cited in a new study suggests that crop yields can fall by as much as 70 percent on severely degraded soils, a figure that underscores why scientists are racing to find amendments that can rebuild fertility without introducing new problems. A team at the University of Fort Hare in South Africa has now tested one such product under tightly controlled laboratory conditions, and their results reveal a surprising pattern: the soil itself, not the dose of fertilizer, may be the decisive factor in whether organic amendments deliver on their promise.

The study, published in the journal Discover Soil, examined Guanoboost, a commercially available organic fertilizer built around seabird and bat guano and blended with biochar, kraal manure, lime, molasses, and gypsum. Guano has long been prized for its exceptional concentrations of nitrogen, phosphorus, and potassium, and the manufacturer positions the product as an environmentally sustainable alternative to synthetic fertilizers. Yet farmers have reported inconsistent performance across different fields, and the researchers wanted to know why. Their hypothesis was that the answer lies not in the fertilizer itself but in the starting condition of the soil it is applied to.

To test this, the team collected topsoil from three contrasting sites in South Africa’s Eastern Cape: a Katspruit Gleysol from the University of Fort Hare Research Farm, a Hutton Humic Ferralsol from Hogsback, and a Klapmuts Haplic Cambisol from Upper Gqumashe. Based on pH, nutrient levels, and heavy metal concentrations, the soils were classified as slightly, moderately, or severely degraded respectively. Each soil was then mixed with Guanoboost at five rates: an untreated control, 200, 400, 600, and 800 kilograms per hectare, spanning half to double the manufacturer’s recommended field rate for potatoes. The mixtures were placed in sealed containers, moistened to 70 percent of water-holding capacity, and incubated at around 30 degrees Celsius for 84 days, with destructive sampling at 21-day intervals.

The experimental design was deliberately rigorous. With five fertilizer rates, three soil types, and four sampling periods, replicated three times, the study generated 180 experimental units arranged in a randomized complete block design. The researchers measured soil pH, macronutrients including nitrogen, phosphorus, potassium, calcium, magnesium, and sodium, micronutrients including iron, manganese, zinc, and copper, and the heavy metals aluminium and chromium. Nutrient extraction involved microwave digestion with nitric acid and hydrogen peroxide, followed by analysis with inductively coupled plasma mass spectrometry, while nitrate and phosphate were quantified after water extraction and centrifugation. Statistical analysis used three-way analysis of variance with Tukey’s honestly significant difference test for mean separation.

The first striking result was what did not matter. The three-way interaction between fertilizer rate, soil degradation level, and incubation time was not statistically significant for any measured parameter, meaning the combined effect of all three factors did not shape soil chemistry in any meaningful way. Instead, the main effects of soil degradation level and incubation time dominated, both highly significant across nearly every variable. Fertilizer rate played a comparatively minor role, appearing as a significant interaction only for calcium and, in a separate analysis, for iron. This finding challenges the intuitive assumption that simply applying more organic fertilizer will proportionally improve soil fertility.

Soil degradation level proved to be the strongest predictor of macronutrient availability. The moderately degraded soil consistently showed the highest concentrations of nitrogen at 7.52 milligrams per kilogram, phosphorus at 1.54, potassium at 8.62, calcium at 28.37, and sodium at 2.81. The slightly degraded soil, despite its name, recorded the lowest levels of all macronutrients, particularly potassium at 5.53 and calcium at 13.09, a pattern the researchers attribute to the nutrient-leaching effect of acidity. The severely degraded soil fell in between, comparable to the moderately degraded soil for calcium but lower for nitrogen and phosphorus, likely because low soil carbon limited its capacity to retain nutrients.

Temporal dynamics added another layer of insight. Across all soils, most macronutrients peaked around day 63 of incubation, a pattern consistent with microbial decomposition of the organic amendment. Calcium, which ranged between 15 and 30 milligrams per kilogram, declined initially between days 21 and 42 before rising at day 63, suggesting dissolution and re-adsorption cycles as the amendment broke down. Nitrogen and potassium showed their steepest increases between days 42 and 63, pointing to this window as the period of most active microbial mineralization. Phosphorus and sodium remained relatively stable throughout, implying limited mobilization or strong adsorption to soil particles. The researchers interpret this as a timed biological rhythm rather than a linear release curve, one that could theoretically be synchronized with crop nutrient demand.

The response to fertilizer rate followed a non-linear pattern that reinforces the importance of moderation. Nitrogen concentrations increased with amendment rate up to 600 kilograms per hectare, reaching a peak of 7.85 milligrams per kilogram, then declined at the highest rate of 800 kilograms per hectare. This diminishing-returns curve suggests that beyond a certain threshold, microbial immobilization or leaching reduces the efficiency of nutrient mineralization. The finding aligns with previous research showing that excessive organic amendment rates can actually reduce nutrient availability compared to moderate applications, and it positions the 600-kilogram rate as the sweet spot for maximizing nitrogen accumulation while minimizing losses.

Micronutrient behavior revealed the powerful influence of soil chemistry on metal solubility. The slightly degraded soil, with its acidic pH, accumulated the highest iron at 407.09 milligrams per kilogram and zinc at 365.67, consistent with enhanced metal solubility under low-pH conditions. The severely degraded soil recorded the highest manganese at 6836.80 milligrams per kilogram but the lowest copper and zinc, likely reflecting redox-driven dissolution of manganese oxides in carbon-deficient environments. The moderately degraded soil maintained the most balanced micronutrient profile. Correlation analysis confirmed these relationships, showing a strong negative association between iron and pH, with a coefficient of negative 0.64, demonstrating that iron availability is directly governed by acidity.

Perhaps the most consequential finding concerned heavy metals. Aluminium and chromium concentrations were highest in the slightly degraded soil, at 460.87 and 700.33 milligrams per kilogram respectively, and decreased progressively as soil pH increased. Aluminium in the acidic soil fluctuated from 437.41 at day 21 to over 500 by day 84, while chromium rose from 656 to approximately 700 in early stages and exceeded 1000 by the end of incubation. In contrast, the moderately and severely degraded soils, with their higher pH, maintained substantially lower metal concentrations throughout. This inverse relationship between pH and metal mobility suggests that the lime component of Guanoboost, or the pH-buffering effect of organic matter decomposition, may actively reduce the bioavailability of toxic metals, offering a pathway to rehabilitate acidic and contaminated soils.

The authors conclude that soil-specific characteristics, rather than uniform application rates, should guide organic amendment strategies. Moderately and severely degraded soils benefited most from moderate application rates, while the slightly degraded soil may require prior pH correction before amendment can deliver its full benefit. They recommend aligning fertilizer application with the period of maximum microbial activity, integrating Guanoboost into soil health rehabilitation programs for its pH-stabilizing and metal-immobilizing properties, and promoting bio-based fertilizers through extension and policy frameworks. Future research, they note, should focus on long-term field validation and microbial enzyme dynamics to fully unravel the biochemical mechanisms underlying nutrient release and heavy metal stabilization. For now, the message is clear: the soil you start with matters as much as the fertilizer you add.

Subject of Research: Effects of guano-based organic fertilizer application rates on nutrient availability and heavy metal dynamics in soils with different degradation levels

Article Title: Influence of organic fertilizer rates on nutrients and heavy metals dynamics in degraded soils

Article References: Manyaku, A., Chiduza, C., Dzvene, A. R., Mthunzi, F., & Witbooi, H. (2026). Influence of organic fertilizer rates on nutrients and heavy metals dynamics in degraded soils. Discover Soil, 3(1), Article 189. https://doi.org/10.1007/s44378-026-00319-5

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00319-5

Keywords: soil degradation, organic fertilizer, guano, soil pH, nutrient mineralization, heavy metals, aluminium toxicity, chromium, soil incubation, soil fertility, micronutrients, sustainable agriculture