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Article

Evaluation of Biochar and Humus Amendments and Early-Season Insect Netting on Soil Properties, Crop Yield, and Pest Management in Organic Vegetable Production in Maine †

by
Robert P. Larkin
New England Center for Sustained Soil and Water Health, Agricultural Research Service, United States Department of Agriculture, Orono, ME 04469, USA
†
Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture.
Agronomy 2025, 15(11), 2567; https://doi.org/10.3390/agronomy15112567
Submission received: 23 September 2025 / Revised: 4 November 2025 / Accepted: 5 November 2025 / Published: 7 November 2025
(This article belongs to the Section Pest and Disease Management)
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Abstract

Effective implementation and optimization of organic amendments and other management practices is essential for sustainable organic vegetable production, yet needed information is lacking on the effects and benefits of different organic matter amendments and pest management approaches under Northeastern USA production conditions. The impacts of soil amendments of biochar or humus (soluble humate complex) in conjunction with compost, as well as the presence or absence of an early-season insect netting row cover (mesotunnels), were evaluated on soil chemical and biological properties, crop development and yield, and disease and pest issues in organic vegetable production, as represented by legume (green snap bean), cucurbit (green zucchini squash), and amaranth (garden beet) vegetable crops, in a three-year field trial in Maine. Composted cow manure and a commercial organic fertilizer alone were included as controls. All plots were either covered or not covered with a permeable insect netting row cover from the time of planting until flowering. All compost-based amendments increased soil pH; organic matter; microbial activity; crop yields; and K, Mg, and Ca content relative to a fertilizer-only treatment. Biochar amendments further increased soil pH, CEC, and Ca content above those of compost alone and also resulted in the overall highest yields of bean and zucchini but were not significantly greater than with compost amendment alone. Humus amendments did not improve soil characteristics, with some indications of potential reductions in emergence and yield. Insect netting substantially improved yield of zucchini (by 59%) and somewhat improved bean yield (by 11%), in addition to improving plant emergence and reducing insect leaf damage, but it did not reduce powdery mildew on zucchini or provide any significant benefits for beets. These results help define specific management practices to improve organic vegetable production and provide useful information and options for growers.

1. Introduction

Numerous daunting production challenges impact organic vegetable growers, including building and maintaining adequate soil health and fertility, managing various pests and diseases, and balancing the needs and special requirements of multiple different vegetable crops, all without many of the tools (such as synthetic chemical fertilizers and pesticides) used in conventional agriculture [1]. The continued growth in consumer demand for, and sales of, organic vegetables in recent years emphasizes the need for improvement and optimization of organic production practices [2]. Effective implementation, utilization, and integration of beneficial soil and crop management strategies are critical for establishing successful and sustainable organic vegetable production [1,3].
Soil organic matter is the key component to soil health, as it impacts all aspects of soil physical, chemical, and biological properties; supplies food for the soil biota; stabilizes soil structure and water relations; and increases soil fertility [3,4]. Organic matter amendments are especially important in organic agriculture for improving nutrient availability, water retention, soil structure, and microbial biomass and activity [5]. But the type and form of organic amendments also determine the specific properties and characteristics. Compost amendments can supply larger amounts of organic matter to soil than other types of organic amendments, and in a more available form than crop residues or manure. In addition to supplying some basic nutrition, compost amendments have been shown to increase microbial biomass, activity, and diversity; reduce soilborne diseases; and improve soil structure by reducing bulk density and increasing aggregate stability, infiltration, and water-holding capacity, among other benefits [6,7,8,9,10,11]. For organic vegetables in the Northeast, compost amendments were previously shown to improve soil properties and plant nutrition, as well as increase crop yields in multiple vegetable crops relative to organic fertilizer alone [12,13].
Biochar, which is a porous, carbon-rich material produced through the thermal decomposition of biomass, such as plant residues, agricultural waste, or wood, under controlled oxygen-limited conditions (pyrolysis), has been receiving much attention as a soil amendment for its impacts on soil properties and crop production, as well as remediation and carbon sequestration [14,15,16]. Biochar is very stable and may provide benefits for extended periods of time. Biochar amendments have been used to improve soil structure and fertility, increase pH and water-holding capacity, increase crop yield, and reduce plant diseases and availability of heavy metals [14,15,16,17,18,19,20,21,22,23]. However, it has not yet been established whether biochar amendments provide benefits superior to those provided by other organic amendments, and research on their use and efficacy in organic vegetable production under the conditions of the Northeastern US is scarce.
Humus and humate substances refer to the breakdown products from decomposition of organic matter, mainly consisting of humic and fulvic acids and humin [24]. These compounds are known to improve soil structure, increase fertilizer utilization, and act as plant growth enhancers [25,26]. Although these are naturally produced from compost and other organic matter, additional amendments of these compounds may act as a biostimulant to hasten and increase organic matter benefits [27]. Humus amendments are already fully decomposed, stable forms of organic matter.
Management of insect pests can be especially difficult for organic growers, as most chemical pesticides are not compatible with organic production, and even those compounds that are derived from natural materials and are allowed in organic production are generally not considered desirable or consistent with the organic rationale, and they are only used very sparingly or as a last resort [28]. Thus, other types of management practices, such as use of physical barriers, where applicable, are considered more appropriate for use in organic production. The use of fine mesh insect netting row covers (sometimes referred to as mesotunnels) can provide such a physical barrier, protecting the early growth of organic vegetable plants [29]. The netting provides air and water exchange but shields the developing plants from insect attacks. Although the netting needs to be removed at the time of flowering (to enable pollination), the protection provided during those early weeks of growth may be sufficient for effective management, resulting in more robust growth and increased fruit and vegetable production. The efficacy of early-season insect netting has already been demonstrated in several cucurbit crops [29,30,31,32] but has not yet been fully assessed or utilized in other crops, particularly under the growing conditions of the Northeast U.S.
Another factor when multiple crop management practices are implemented relates to their compatibility and complementarity, in that the different practices need to function well together to provide beneficial results. Thus, how different factors affect each other, especially the interaction between the soil amendments and the use of insect netting, also needs to be assessed. The purpose of this research was to assess the efficacy of early-season insect netting as a viable option for pest management for multiple vegetable crops, as well as determine the potential benefits and interactions that biochar and humus soil amendments, in conjunction with compost, can provide to the system, in relation to compost or organic fertilizer alone. Thus, in this research, crops with and without insect netting and biochar or humus soil amendments were assessed for their effects on soil properties, crop growth and yield, and pest and disease issues in three different organic vegetable crops (zucchini squash, snap beans, and garden beets) in an organic vegetable production system over three full cropping seasons in central Maine. This research provides new and useful information on the effects and benefits of different organic matter amendments and an insect netting treatment for organic vegetables under Northeastern U.S. production conditions. The overall goal of these and related trials is to develop and optimize improved production systems for organic vegetables that maximize sustainability and productivity.

2. Materials and Methods

2.1. Field Design and Management

Field trials were conducted at an organic research site located in St. Albans, ME (44°53′ N, 69°25′ W), over three consecutive growing seasons (2022–2024). The trial was established with a split-block design (with insect netting as the main plot [11.0 m × 6.1 m] and soil amendment treatments as sub-plots [6.1 m × 1.8 m]) and had four replicate blocks and random assignment of treatments for each of the three different vegetable crops in a field layout that has been previously described and illustrated [13]. Details of the field site, including soil type, soil texture, slope, and cropping history, have also been previously described [12]. The field was tilled using a Perfecta field cultivator prior to planting (Unverferth Manufacturing Co., Kalida, OH, USA).
The water needs of the crops were met by natural rainfall, with supplemental watering provided only when dry conditions threatened crop health, with approximately 2500 L/ha being applied. An on-site weather station (Spectrum Technologies, Plainfield, IL, USA) was used to monitor environmental conditions (air temperature, relative humidity, and rainfall), and data were used to determine daily, weekly, and monthly average conditions throughout the cropping season.

2.2. Insect Netting Treatments

Early-season insect netting was established on half the plots in each block shortly after planting using a polyamide-based fine-knitted mesh (mesh size 0.35 mm × 0.35 mm) fabric (ProtekNet, Johnny’s Seeds, Winslow, ME, USA), which was laid over steel hoops to protect the plants from various insects and other pests, including squash bugs (Anasa tristis) and cucumber beetles (Acalymma vittatum) on zucchini and Mexican bean beetles (Epilachna varivestis) on beans, as well as Japanese beetles (Popillia japonica) and flea beetles, etc., during the early growth stages. Netting remained in place until flowering (approximately 6 weeks after planting), when it was removed to allow for pollination and fruit development. Uncovered plots received no treatment for insect control other than occasional physical removal by hand.

2.3. Soil Amendment Treatments

The soil amendments consisted of four different treatments: (1) commercial organic fertilizer alone (as a non-amendment control), (2) composted cow manure, (3) compost plus humate complex, and (4) compost plus biochar. The fertilizer-only plots received 1000 kg/ha of an organic fertilizer (Fertrell Feed-n-Grow, Fedco seeds, Clinton, ME, USA) which contains 3-2-3 NPK content and was applied to the soil prior to mulch covering and planting [13]. The rest of the treatments received an initial application of 500 kg/ha of the commercial fertilizer, plus the additional soil amendments. Composted cow manure was added at a rate of 60 m3/ha (~18 Mg/ha dry weight) and tilled into the soil prior to planting. The average compost composition (dry weight basis) was 26.1% C, 1.9% N, 0.9% K, and 1.0% P, with a pH of 7.6 and a C:N ratio of 17.5. Biochar amendments were made in the form of horticultural-grade raw biochar made from North American hardwood (Fedco Seeds, Clinton, ME, USA), applied at a rate of 6.7 Mg/ha prior to planting. Because raw biochar needs to be activated prior to application, biochar was mixed with compost (at the same rate as the compost amendment) and allowed to equilibrate prior to application. The humus amendment consisted of a soluble humate complex (containing fulvic and humic acids) and soluble seaweed extract on a base of raw Leonardite and kelp meal (Hum-Amend Max, Fedco Seeds, Clinton, ME) and was also added in conjunction with compost and applied at a rate of 400 kg/ha prior to planting. In all plots, a permeable and reusable woven polypropylene fabric ground cover (DeWitt Sunbelt, Sikeston, MO, USA), was used as mulch, as previously described [12].

2.4. Soil Chemical and Biological Properties

Soil samples were collected from each plot in the Spring of the first year (2022) after soil amendments were applied but prior to vegetable planting, as well as in the fall of each subsequent year (2022–2024) after harvest, to evaluate treatment effects on soil properties. Eight soil cores (2.5 × 15 cm) were collected from within each plot and mixed together into one composite sample per plot. Measured soil properties included pH, organic matter content, nutrient concentrations, and cation exchange capacity. Potentially available N content, including nitrate (NO3−) and ammonium (NH4+) content, was determined using cold water bath KCl extractions [33]. Soil concentrations of P, K, Ca, Mg, Al, B, Fe, Mn, Na, and Zn were estimated using Modified Morgan extraction procedures [34] and analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES), performed by the University of Maine Analytical Lab (Orono, ME, USA). The microbial biomass and activity of the soil samples was estimated through CO2 respiration using the Solvita CO2− burst assay [35] in 2023 and 2024.

2.5. Vegetable Crops

The three organic vegetable crops grown were green zucchini squash (Cucurbita pepo L., variety ‘Dunja’), green snap beans (Phaseolus vulgaris L., variety ‘Provider’), and striped garden beets (Beta vulgaris L., variety ‘Chioggia’), as representative examples of cucurbit, legume, and amaranth vegetable crops. All crops were direct-seeded, and all vegetable seeds were obtained from organically grown sources (Johnny’s Selected Seeds, Winslow, ME, and High Mowing Organic Seeds, Wilcott, VA, USA). The planting dates for all three crops were 26 May and 23 May in 2022 and 2024, respectively. In 2023, due to wet conditions throughout early spring, planting was delayed until 12–13 June. Due to wet soils and some poor germination in 2023 and 2024, some replants of squash and beets were needed on 28 June and 10 June in 2023 and 2024, respectively. Emergence (as percentage of emerged seedlings relative to total seeds planted) was assessed for all crops periodically through the first 30 days after planting (DAP). The same plots were used each year, but vegetable crops were rotated, with blocks 1 and 3 following a bean–squash–beet rotation and blocks 2 and 4 following a bean–beet–squash rotation. Following each cropping season, a cover crop of winter rye was planted over the entire field. All seed, products, equipment, inputs, and methodologies used throughout these trials were certified organic and/or approved for use in organic production.

2.6. Yield, Pest, and Disease Evaluations

Crops were monitored for symptoms of foliar and soilborne diseases, as well as insect or other pest infestations and damage throughout the field season. Vegetables were harvested by hand as they ripened to maturity and were weighed, with data recorded by row. Zucchini was harvested 3 times each week (due to fast growth), and beans and beets were harvested once each week. Harvest for all crops covered an 8–10-week period from mid-July to mid-September, except for 2023, which, due to the delayed planting, ran from late July to late September. Yield was determined as the total weight of harvested vegetables per each plot row (6.1 m) at each harvest date, as well as the total for all harvest dates, and expressed as t/ha. For beets, in addition to the beets themselves, beet greens were also harvested and weighed at the same time as the beets, and yields were also expressed as t/ha. Visual assessments of symptoms of powdery mildew, leaf spots, or other plant diseases present, as well as insect infestations and damage, were recorded (as the percentage of leaves affected or total plot leaf area affected) during the latter half of the growing season.

2.7. Statistical Analysis

Standard analysis of variance (ANOVA) for a split-block design with factor interactions was conducted for all soil property, yield, and pest/disease assessment data. Data were analyzed for each crop year separately, as well as with all years combined (with year and interactions as additional factors), to assess overall effects over the course of the study. Significance was evaluated at p < 0.05 for all tests. Mean separation was accomplished with Fisher’s protected LSD test. All analyses were conducted using Statistical Analysis Systems ver. 9.4 (SAS Institute, Cary, NC, USA).

3. Results

3.1. Environmental Conditions

Environmental conditions, as measured by temperature and rainfall, varied during the vegetable growing seasons over the three years of the study. Daily temperatures averaged above normal throughout the summer for all three years, with notably higher temperatures in June 2023 and 2024 compared with the long-term (30-year) averages for the area (Table 1). Rainfall was variable from month to month and year to year, with 2022 having a dry spring but wet summer, whereas 2023 was a particularly wet year, with every month of the field season (except September) showing higher than normal rainfall, leading to wet fields for most of the summer. In 2024, a dry spring (May–June) and August was interspersed with a wetter than normal July and September. Overall, 2022 and 2024 received close to normal rainfall for the summer season, whereas in 2023, there was a rain surplus of 23.5 cm compared to long-term average conditions (Table 1).

3.2. Treatment Effects on Soil Properties

Soil amendment treatments had significant effects on many soil properties, whereas insect netting had no major effects on measured soil properties, and interactions between the two factors were not significant for any parameter. Similar results were observed each year (no significant year by amendment or year by netting interactions), so soil properties are presented by amendment as averaged over all three years (Table 2). The biochar treatment resulted in significantly higher pH and calcium contents than all other treatments, whereas the fertilizer-only treatment had lower pH and Ca content than all other treatments (Table 2). Fertilizer-only treatment also resulted in the lowest OM, K, and Mg concentration compared to all other amendments, as well as lower P content compared to the biochar-amended soils. Cation exchange capacity was higher in the biochar- and humus-amended soils than the compost and fertilizer-only soils. Microbial biomass and activity, as represented by soil microbial respiration (CO2 evolution), was similar among all three compost-amended soils, being higher than that in the fertilizer-only treated soil.
Some effects were also observed on other minor elements, with biochar-amended soils resulting in lower available concentrations of the metal elements Al, Fe, and Zn compared to all other treatments, whereas fertilizer-only treatment tended to result in higher concentrations of these elements (Table 3). Fertilizer-only treatment also tended to show lower concentrations of Na, Mn, and B than the compost-amended soils.

3.3. Treatment Effects on Crop Growth and Yield

Variable yearly conditions affected emergence and crop growth, with wet soil conditions in 2023 substantially reducing emergence of zucchini (as well as beets) compared to the other two years (Table 4). Although replants were made, emergence remained uncharacteristically low, which also affected subsequent growth and yield for both zucchini and beets. Both factors, netting and amendment, significantly affected emergence in some years and when averaged over all years, but there was no significant interaction between the factors. Plots with insect netting resulted in greater emergence in two out of the three years, as well as when averaged over all three years, for both zucchini and beans, with netting increasing emergence by an average of 14% and 6% for zucchini and beans, respectively (Table 4). Due to the nature of beet seeds being a conglomerate containing one-to-several individual plant seeds, it was not possible to accurately quantify emergence for beets, but based on observation, beets were also affected similarly with netting. Soil amendment did not significantly affect emergence of zucchini or beans in 2023 or 2024, but humus amendment reduced zucchini emergence relative to fertilizer-only treatment in 2022, as well as over all three years (by 9%) when averaged. Fertilizer-only and humus amendment showed lower bean emergence in 2022 relative to the biochar- and compost-amended soils, and fertilizer treatment showed lower bean emergence than biochar when averaged over all three years (Table 4).
Early-season growth also improved in all three vegetable crops with insect netting versus without, with larger, more robust vegetation through the first several weeks of growth (Figure 1). Presence of the early-season insect netting resulted in greater total yields in two of the three years for bean and beets, as well as in all three years and to a much greater degree for zucchini, with increases of 47% to over 100% (Table 5). When averaged over all years, netting resulted in an increase in yield of 11% for beans and 59% for zucchini (Figure 2). These increases were observed in the early yield totals (first 4 weeks), as well as in the later and final totals (Figure 2). However, the effect of insect netting on beets was inconsistent, showing a reduction in yield in 2022 and no overall significant effect over all three years (Figure 2).
Amendments also resulted in significant yield effects, with all compost-amended treatments (compost, biochar, and humus) out-yielding the fertilizer treatment in all three years for beans and the biochar treatment also resulting in higher yields than the humus treatment in 2022 and the compost treatment in 2024 (Table 5). The biochar treatment increased bean yield by 40% and 12% relative to the fertilizer and humus treatments, respectively, in 2022, and it led to increases of 55% and 10% relative to the fertilizer- and compost-only treatments in 2024 (Figure 2A). Averaged over all three years, all three compost amendment treatments increased bean yield by 34–42% relative to fertilizer treatment, and the biochar treatment also increased bean yield relative to the humus treatment by 15%, but the addition of biochar did not significantly increase yield relative to the compost-only amendment.
For zucchini, the biochar and compost amendments increased squash yield relative to the fertilizer treatment, by 26–35%, but only in 2022, with no significant yield effects observed in the other two years (Table 5). Zucchini yields were substantially reduced in 2023 relative to the other two years, due to the wet conditions. However, when averaged over all three years, biochar- and compost-only amendments increased zucchini yield by 27% and 19%, respectively, relative to fertilizer-only treatment, and biochar amendment increased yield by 17% relative to the humus treatment but was not significantly greater than the compost-only treatment (Figure 2B).
Beet yields were less affected by soil amendments, with the only significant effects observed in 2024, with the biochar and compost amendments resulting in yield increases of 15–28% relative to the humus and fertilizer treatments (Table 5). As with zucchini, beet yields were much lower in 2023 than the other years due to the wet conditions. When averaged over all three years, only the compost amendment showed an increase in beet yield relative to fertilizer treatment (by 16%) (Figure 2C). For beet greens, yields ranged from an average of 3.2 t/ha in 2023 to 6.6 t/ha and 7.4 t/ha in 2022 and 2024, with similar differences among treatments, as observed with the main beet crop. For all crops, similar effects on early yields as seen on later and final yields were observed (Figure 2).

3.4. Treatment Effects on Pests, Diseases, and Other Issues

Occurrence of insects and other pests were numerous and variable throughout the study, and typical of the region. On zucchini, cucumber beetles and squash bugs were consistently present and caused notable damage throughout the season, but particularly on young developing uncovered plants. Insect netting did provide substantial reduction in insect damage throughout the early season (Figure 3). On beans, leafhoppers, Japanese beetles, and cucumber beetles were observed throughout the season but were not a major issue, whereas Mexican bean beetles (Epilachna varivestis) did result in substantial damage to some plants but were limited in scope, and infestations varied by year. Insect netting substantially reduced the infestation and incidence of plant damage due to bean beetle larvae throughout the season, with netting reducing the severity of leaf symptoms by 41 to 95% in each year and an average of 69% over all three years (Table 6). However, although infestations varied among amendments, there was no significant effect of amendment on damage due to beetle infestation (Table 6). Beets did have some leaf injury due to beetles and leafhoppers later in the season but were generally minor.
In general, crop disease issues were minor, with the only consistently observed foliar disease being powdery mildew (Podosphaera xanthii) on zucchini plants. Powdery mildew occurred late in the season (beginning in late August) and varied by year (lowest levels in 2023 and highest in 2022), and although the disease increased until season’s end, due to the late occurrence, this had little effect on yield. Both netting and amendment treatments significantly affected the development of powdery mildew disease in each year and when averaged over all years. Differences among treatments were best expressed in 2022, when conditions were most favorable for disease. The fertilizer-only treatment showed the highest disease incidence throughout the season, significantly greater than observed for all other amendments, and the biochar amendment resulted in the lowest disease incidence, reducing disease incidence by 51% relative to fertilizer treatment and 22–28% relative to the other compost amendments at the end of the season (Figure 4A). However, in this case, presence of the early-season insect netting resulted in higher incidence of powdery mildew than no netting throughout the growing season, with an increase of 65% (Figure 4B).
One other disease on zucchini was only observed in 2024. Plectosporium blight, caused by the fungus Plectosporium tabacinum, began showing symptoms—white, scaly lesions on leaves, stem, and fruit—in mid-August 2024. The disease progressed rapidly, with lesions coalescing and covering wider areas and more plants through the end of the season, ultimately affecting around 40% of the plants and causing considerable destruction of foliage (Figure 5). However, because the disease appeared and progressed late in the season, damage to fruit and harvests were not severe. Although the disease developed across all treatments, the biochar amendment resulted in the lowest overall disease incidence of all amendments, with a reduction of 22% relative to the fertilizer treatment at the end of the season (Figure 5A). Insect netting also affected disease development, with the presence of the early-season netting resulting in increased disease development, accounting for an increase in incidence of 43% compared to no netting by the end of the season (Figure 5B).
One other foliar symptom (not directly related to any specific disease) that was observed in both zucchini and beans each year was late-season senescence, consisting of leaf yellowing (chlorosis) or plant decline that occurred in the final two weeks of the season, which may indicate early senescence due to a variety of factors. This late-season senescence was affected by netting as well as amendment treatments, with presence of insect netting leading to a greater degree of yellowing by the end of the season in each year (averaging an increase of 54% for zucchini and 21% for beans (Table 7). This yellowing was also affected by amendment, with the fertilizer treatment resulting in a higher degree of yellowing than all other treatments in all years for beans and in two out of the three years (2022 and 2023) for zucchini (Table 7).

4. Discussion

In this research, the usefulness of early-season insect netting for management of insect pests and the addition of biochar or humus amendments in conjunction with compost were assessed on multiple organic vegetable crops under Maine production conditions. The differing conditions each year, particularly regarding rainfall, enabled assessment of these treatments under varying environmental conditions (dry vs. wet periods, etc.), and overall, treatment effects were consistent regardless of year or conditions. Although the extremely wet season in 2023 negatively affected emergence and yield totals, overall treatment effects were similar across all three years.
Overall, presence of the insect netting for the first 6 weeks after planting increased emergence and early-season growth for all crops, as well as increasing yield and reducing insect damage for zucchini and beans. However, insect netting also led to increases in the development of the plant diseases powdery mildew and Plectosporium blight on zucchini. Soil amendment treatments also significantly affected most soil and crop parameters, but there was no significant interaction between the netting and amendment factors. Thus, there was no direct impact of the insect netting on amendment effects, or vice versa.
As was observed in a previous study [13], all of the compost-based amendments (biochar, humus, compost) improved soil properties (pH, OM, nutrient concentrations, and respiration) and increased yield relative to the fertilizer-only treatment. This is consistent with other studies showing improvements in physical–chemical soil properties such as aggregate stability, porosity, water retention, and nutrient availability due to compost and other organic amendments [9,10,11]. Although specific soil microbial characteristics were not assessed in this study, the increase in soil respiration suggests increased microbial biomass and activity as well, as has also been documented in other studies, often leading to crop yield increases [36,37,38,39]. Yield increases due to compost-based amendments were comparable to those observed in a previous study under similar conditions [13], but additions of biochar or humus to the compost amendment did not consistently increase overall yields compared to compost alone for any crop over the course of the study. The compost-based amendments were also associated with reductions in powdery mildew on squash relative to the fertilizer-only treatment, as well as less incidence of late-season senescence in all three crops.
Biochar amendment did further increase pH and Ca content above that of compost amendment alone, and both biochar and humus amendments increased CEC relative to compost alone. Various biochar amendments have shown significant improvement in basic soil properties related to soil structure, bulk density, porosity, aggregate stability, pH, CEC, and nutrient availability, as well as microbial characteristics [40,41,42]. However, it is not always clear how these effects compare with other organic amendments. In limited studies that involved both biochar and compost amendments, for most soil properties, biochar amendments were comparable and did not significantly improve upon results observed with compost alone; however, biochar’s effects were longer-lasting [43,44,45]. Biochar was used in conjunction with compost in this study because raw biochar needs to be activated or ‘charged’ with nutrients and colonizing microorganisms prior to incorporation into soil to be most effective, as raw biochar can immobilize nutrients (make unavailable to plants) and interfere with microbial processes if not previously colonized [46]. Biochar amendment also reduced the development of powdery mildew and Plectosporium blight on zucchini more than any other treatment. According to recent reviews, reduction of plant diseases by biochar has been attributed to such mechanisms as induction of systemic resistance, enhancement of rhizosphere competence of the microbial community, and sorption of phytotoxic compounds of plant and/or microbial origin [18,21,47].
The specific characteristics and properties of biochar vary depending on how it is produced and the organic materials used, with the main factors being feedstock raw material, pyrolysis temperature and time, gas pressure, and additives [48]. Different types of biochar may produce different results on soil properties and crop growth. For this study, we purposely chose a commercial horticulture-grade biochar readily available to growers to demonstrate feasibility. It is conceivable that other biochar types may provide improved results and could be assessed in future testing. Currently, there is much interest and activity in developing biochar with characteristics especially suited for specific tasks and roles [49,50].
In general, the application of humus with compost did not improve soil or crop characteristics relative to compost alone and, in some cases (reduced emergence, lower yield), may have had slightly negative or inhibitory effects. Humic substances are the most active and stable form of organic matter and are known to improve soil structure, increase water retention, stimulate microbial activity, and enhance root architecture and nutrient acquisition, resulting in improved yield and tolerance to plant stress [25,26,51,52]. The lack of any additional benefits beyond those observed for compost alone in this study may indicate that sufficient humic substances were provided by the compost amendment, and additional application of humus was not needed. Other studies have noted that application of humic substances above optimal levels can result in negative effects related to nutrient availability, plant growth, or microbial communities [53,54]. Under the conditions of the current study, there was no benefit provided by the application of humus in addition to compost.
The usefulness of early-season insect netting had previously been documented for cucurbits, such as cucumber, melons, and squash [29,30,31,32]. However, the extent and degree of improvement had not previously been well established, and the utility for other vegetable crops had not been assessed, particularly under the conditions of Maine and the Northeastern U.S. Significant effects were observed with all three crops tested, but the greatest benefits were observed with zucchini, where the drastic reduction in feeding effects by cucumber and squash beetles provided improved growing conditions, increased size, and improved yield, whereas effects were less substantial for bean and beet crops. With beans, early-season yields (through the first month of harvests) were improved by about 25% compared to no netting, but as the season progressed, the non-netted plants showed comparable later season yields, resulting in a more modest final improvement of 11%. This was probably due to minor early-season slowing and disruption of growth of the bean plants due to insect feeding, but plants were able to compensate with more robust growth as the season progressed, as damage due to insect feeding was less severe on beans than on zucchini. With beets, the minor foliar damage due to early insect feeding did not appear to have much effect on the development of the beet tuber, as variable results and no overall significant effect on yield were observed (lower yield in 2022 and slightly higher yields in 2023 and 2024 for netted plots).
Although the insect netting did reduce insect problems, netted plots resulted in greater development of powdery mildew disease, as well as Plectosporium blight (in 2024 only), in zucchini. These diseases occurred primarily late in the season, so they did not have much effect on yield, but significant increases in these diseases were observed. This appeared to primarily be a result of the increased growth of the netted plants, producing more lush and dense foliage, which resulted in increased humidity, less air movement, and more shaded leaves within the canopy. As well as serving as a physical barrier, such insect netting is known to affect various microclimate conditions, including increasing temperature and humidity [55], which also can increase early-season crop growth. Powdery mildew thrives in high-humidity, low-ventilation, and low-light conditions [56], and Plectosporium blight, which is a more recent threat in this area, prefers warm, humid, wet, and less-ventilated conditions [56]. Thus, in this case, the increased growth and dense foliage provided by the insect netting also led to increased disease development as well. In a recent study with acorn squash, in addition to reductions in insect damage and pest abundance and increased marketable yield, Fiske et al. [31] observed that mesotunnel row covers reduced the incidence of powdery mildew relative to uncovered treatments and attributed the decreases to alteration of the microclimate as well as acting as a physical barrier for spore dispersal. In the current study, the alterations made to microclimate due to more robust, dense foliage (increased humidity and shading and reduced ventilation) resulted in higher disease incidence rather than lower incidence. In a study that combined the use of row covers and compost amendments, both the row cover and amendments improved yield of organic cucumbers, and the effects were additive [55].
The limitations of this study include that it was conducted in a single location over a limited period of time (3 seasons) and that only a single type of compost, biochar, and humus amendment at a specific rate were used. Thus, the results are most applicable to organic systems in the Northeastern U.S. under similar growing conditions. However, the concepts and feasibility of these approaches are applicable to much more diverse and widespread systems. Additional studies assessing different specific types and rates of organic amendments in other systems under different conditions and other locations can expand upon this work to provide recommendations for more extensive implementation in various systems.

5. Conclusions

This research assessed the potential benefits of early-season insect netting (mesotunnel row covers), as well as biochar or humus in conjunction with compost amendments, within a diversified organic vegetable production system. Early-season insect netting using mesotunnel row covers was confirmed as an effective practice for managing insect pests and increasing yield in cucurbits but showed lesser effects on other crops. Although insect damage was reduced on the foliage of all three crops, yields were not affected by presence of the netting for the beet crop, and a relatively small increase in yield was observed for the beans. Due to the increased costs associated with the materials and labor involved in establishing the mesotunnels, it would not be a recommended practice for crops such as beets and only practical for beans if there were known serious insect issues affecting early-season growth.
Organic amendments containing compost provided significant improvements in multiple soil properties, including pH, organic matter, nutrient concentrations, and microbial respiration, as well as increased vegetable yield, for all three crops relative to an organic fertilizer-only treatment. The addition of a commercially available wood-based biochar to the compost amendment provided additional improvements to some soil properties (pH, Ca, CEC) and reduced disease development of powdery mildew and Plectosporium blight on zucchini, but it did not consistently improve yield more than compost alone for any crop over the three years of this study. However, the full benefits of the biochar amendments may develop over many years. The addition of humus to the compost amendment did not provide consistent or significant benefits relative to compost alone, perhaps because additional humus substances were not needed and would not be recommended in addition to compost under these conditions. These results emphasized the importance of additions of organic matter, and especially compost, for providing many benefits beyond those supplied by fertilizer alone. The biochar treatment used here may provide some additional benefits for improving soil properties and enhancing crop performance over extended periods of time. Additional studies assessing different types and characteristics of biochar for their potential to further improve soil quality, fertility, and microbial processes for enhanced crop growth and yield in organic vegetable production are warranted.

Funding

This research received no external funding.

Data Availability Statement

The raw data supporting the conclusions of this article will be made available by the authors upon request.

Acknowledgments

Special thanks to Jim Hunt, Alex Introne, and Ethel Champaco for their technical support and roles in establishing, maintaining, and conducting these trials. Thanks to our summer workers—Connor Albertson, Cameron Barone, Quin Daly, Aldous Hofmann, Everett Pietila, Ollie Rice, and Juliana Rusnak—for all their help with planting, maintaining, harvesting, and distributing the vegetable crops, and a very special thanks to Christina Gee for all her help and cooperation in continuing to make the field site available for this work.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Early-season (6 weeks) crop growth of (A) zucchini, (B) green beans, and (C) beets when grown without insect netting cover (left side) or with insect netting cover (right side).
Figure 1. Early-season (6 weeks) crop growth of (A) zucchini, (B) green beans, and (C) beets when grown without insect netting cover (left side) or with insect netting cover (right side).
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Figure 2. Average total yield and split between early (first 4 weeks) and late (final 4–5 weeks) of harvest for (A) green snap beans, (B) zucchini squash, and (C) garden beets over three consecutive growing seasons (combined data for 2022–2024) and as affected by different soil amendments and the presence or absence of early-season insect netting. Bars topped by the same letter for each vegetable and factor are not significantly different based on Fisher’s protected LSD test (p < 0.05). Compost = composted cow manure treatment; Biochar = biochar-amended compost treatment; Humus = humus-amended compost treatment; Fert = organic fertilizer-only (no compost); Net = early-season insect netting treatment; Non = no insect netting treatment. No significant interaction between netting and amendment factors was found (p > 0.05).
Figure 2. Average total yield and split between early (first 4 weeks) and late (final 4–5 weeks) of harvest for (A) green snap beans, (B) zucchini squash, and (C) garden beets over three consecutive growing seasons (combined data for 2022–2024) and as affected by different soil amendments and the presence or absence of early-season insect netting. Bars topped by the same letter for each vegetable and factor are not significantly different based on Fisher’s protected LSD test (p < 0.05). Compost = composted cow manure treatment; Biochar = biochar-amended compost treatment; Humus = humus-amended compost treatment; Fert = organic fertilizer-only (no compost); Net = early-season insect netting treatment; Non = no insect netting treatment. No significant interaction between netting and amendment factors was found (p > 0.05).
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Figure 3. Example of early-season insect feeding damage on zucchini plants when either (A) not covered or (B) covered by a protective insect netting through the first several weeks of growth.
Figure 3. Example of early-season insect feeding damage on zucchini plants when either (A) not covered or (B) covered by a protective insect netting through the first several weeks of growth.
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Figure 4. Development of powdery mildew disease incidence (% foliage affected) on zucchini plants in 2022 growing season over time and as affected by (A) soil amendment treatments and (B) early-season insect netting treatments. Season endpoints followed by the same letter are not significantly different based on Fisher’s protected LSD test (p < 0.05).
Figure 4. Development of powdery mildew disease incidence (% foliage affected) on zucchini plants in 2022 growing season over time and as affected by (A) soil amendment treatments and (B) early-season insect netting treatments. Season endpoints followed by the same letter are not significantly different based on Fisher’s protected LSD test (p < 0.05).
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Figure 5. Development of Plectosporium blight incidence (% foliage affected) on zucchini plants in 2024 growing season over time and as affected by (A) soil amendment treatments and (B) early-season insect netting treatments. Season endpoints followed by the same letter are not significantly different based on Fisher’s protected LSD test (p < 0.05).
Figure 5. Development of Plectosporium blight incidence (% foliage affected) on zucchini plants in 2024 growing season over time and as affected by (A) soil amendment treatments and (B) early-season insect netting treatments. Season endpoints followed by the same letter are not significantly different based on Fisher’s protected LSD test (p < 0.05).
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Table 1. Average daily temperature and total rainfall for the months of May through September at the research site in St. Albans, ME, for 2022, 2023, and 2024 compared with long-term (30-year) average (LTA) conditions.
Table 1. Average daily temperature and total rainfall for the months of May through September at the research site in St. Albans, ME, for 2022, 2023, and 2024 compared with long-term (30-year) average (LTA) conditions.
Average Daily Temperature (°C)Rainfall (cm)
Treatment202220232024LTA202220232024LTA
May14.012.114.212.04.311.26.99.5
June16.916.818.817.06.416.013.39.9
July20.822.322.219.87.716.712.58.7
August20.318.719.618.913.917.211.18.6
September15.517.515.814.213.38.72.99.6
Season avg17.517.518.116.49.114.09.49.3
Table 2. Selected soil properties (expressed as mg/kg soil except for pH or as otherwise noted) as affected by soil amendments and measured each fall and averaged over three cropping years (2022–2024).
Table 2. Selected soil properties (expressed as mg/kg soil except for pH or as otherwise noted) as affected by soil amendments and measured each fall and averaged over three cropping years (2022–2024).
AmendmentpHOM wNO3NH4PKMgCaCEC xCO2 y
Biochar6.76 a z4.44 a12.7 ab1.50 a168.6 a188.3 a203.9 a5271 a9.29 a72.9 a
Humus6.37 b4.59 a14.8 ab2.07 a154.2 ab169.1 a208.9 a4618 b8.87 a76.3 a
Compost6.32 b4.36 a15.3 a1.55 a153.6 ab181.5 a205.6 a4435 b8.38 b71.7 a
Fertilizer6.19 c4.02 b12.3 b1.55 a113.4 b115.1 b129.5 b4032 c8.00 b60.8 b
LSD0.090.272.60.7549.428.017.63700.476.4
w OM = organic matter content (%). x CEC = cation exchange capacity (meq/100g). y CO2 = soil microbial respiration measured as CO2 evolution over a 24 h period (mg/kg soil). z values within columns followed by the same letter for each parameter are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05).
Table 3. Soil concentrations (ppm) of trace elements as affected by soil amendments, as measured each fall and averaged over three cropping years (2022–2024).
Table 3. Soil concentrations (ppm) of trace elements as affected by soil amendments, as measured each fall and averaged over three cropping years (2022–2024).
AmendmentAlFeZnNaMnB
Biochar24.6 c z4.4 c1.3 b24.6 b3.4 a0.58 a
Humus35.8 b5.3 ab1.8 a27.3 a3.6 a0.55 a
Compost39.8 ab5.4 ab1.6 a25.0 b3.3 ab0.56 a
Fertilizer47.6 a6.2 a1.7 a22.3 c3.1 b0.44 b
LSD8.00.80.32.10.30.07
z values within columns followed by the same letter for each parameter are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05).
Table 4. Effect of insect netting and soil amendments on 21 DAP seedling emergence (%) for green bean and zucchini crops in the 2022, 2023, and 2024 cropping seasons and averaged over all seasons.
Table 4. Effect of insect netting and soil amendments on 21 DAP seedling emergence (%) for green bean and zucchini crops in the 2022, 2023, and 2024 cropping seasons and averaged over all seasons.
Zucchini Green Bean
Treatment202220232024Avg202220232024Avg
Netting
Netted85.6 a z52.2 a89.1 a75.6 a78.6 a88.8 a85.6 a84.3 a
Not Netted76.3 b36.0 b85.6 a66.3 b77.5 a84.6 b77.3 b79.8 b
LSD7.64.95.43.54.23.66.63.3
Amendment
Biochar81.3 ab41.9 a90.6 a71.3 ab82.4 a86.6 a86.1 a85.0 a
Humus73.8 b45.0 a86.3 a67.5 b74.4 b85.7 a81.6 a80.5 ab
Compost83.1 ab43.1 a83.8 a70.8 ab82.2 a87.7 a80.8 a83.5 ab
Fertilizer85.6 a48.1 a88.8 a74.2 a73.3 b86.7 a77.4 a79.1 b
LSD10.86.97.85.05.95.09.44.7
z values within columns followed by the same letter for each factor are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05). Interaction between netting and amendment factors was not significant (p > 0.05).
Table 5. Total vegetable yield (t/ha) of green bean, zucchini, and beets as affected by insect netting and soil amendment treatments in the 2022, 2023, and 2024 cropping seasons.
Table 5. Total vegetable yield (t/ha) of green bean, zucchini, and beets as affected by insect netting and soil amendment treatments in the 2022, 2023, and 2024 cropping seasons.
Green BeanZucchiniBeet
202220232024202220232024202220232024
Netting
Netted18.3 a z18.8 a22.5 a68.7 a21.1 a52.0 a9.4 b5.7 a10.1 a
Not Netted17.5 a16.8 b19.5 b46.6 b7.9 b34.7 b11.1 a3.8 b9.6 b
LSD1.11.31.27.84.36.30.81.20.5
Amendment
Biochar19.9 a18.8 a24.0 a65.6 a15.3 a48.1 a10.1 a4.4 a10.5 a
Humus17.8 b18.5 a22.7 ab55.0 ab12.4 a42.8 a10.5 a5.6 a9.1 b
Compost19.6 a19.6 a21.8 b61.4 a16.1 a43.4 a10.6 a5.0 a11.1 a
Fertilizer14.3 c14.1 b15.5 c48.6 b14.4 a39.2 a9.9 a4.1 a8.7 b
LSD1.61.81.711.16.18.91.21.80.8
z values within columns followed by the same letter for each factor are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05). Interaction between netting and amendment factors was not significant for any crop or year (p > 0.05).
Table 6. Effect of insect netting and soil amendments on incidence (%) of plant infestation of Mexican bean beetle on green bean in the 2022, 2023, and 2024 cropping seasons and averaged over all seasons.
Table 6. Effect of insect netting and soil amendments on incidence (%) of plant infestation of Mexican bean beetle on green bean in the 2022, 2023, and 2024 cropping seasons and averaged over all seasons.
Treatment202220232024Avg
Netting
Netted1.5 b z1.0 b18.8 b7.3 b
Not Netted31.5 a8.0 a32.0 a23.8 a
LSD4.53.06.53.5
Amendment
Biochar19.25.329.818.0
Humus16.52.527.315.5
Compost17.07.324.316.3
Fertilizer13.53.320.512.3
LSDNSNSNSNS
z values within columns followed by the same letter for netting treatment are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05). Values among amendments were not significantly different (NS) in any year.
Table 7. Effect of insect netting and soil amendments on late-season senescence (represented by % yellowing plants) of green bean and zucchini crops in the 2022, 2023, and 2024 cropping seasons and averaged over all seasons.
Table 7. Effect of insect netting and soil amendments on late-season senescence (represented by % yellowing plants) of green bean and zucchini crops in the 2022, 2023, and 2024 cropping seasons and averaged over all seasons.
ZucchiniGreen Bean
Treatment202220232024Avg202220232024Avg
Netting
Netted19.9 a z25.4 a25.3 a23.6 a20.3 a21.7 a30.3 a24.1 a
Not Netted9.8 b20.0 b16.3 b15.3 b16.6 b18.1 b25.0 b19.9 b
LSD1.91.91.41.92.62.12.51.9
Amendment
Biochar12.4 b19.6 c20.6 a17.5 b16.5 b15.0 b23.8 b18.4 b
Humus12.4 b29.4 a20.2 a20.6 a17.8 b16.1 b23.1 b19.0 b
Compost13.4 b17.5 c21.9 a17.6 b16.5 b15.9 b25.0 b19.1 b
Fertilizer21.3 a24.4 b20.6 a22.1 a23.1 a32.5 a38.7 a31.5 a
LSD2.72.72.02.73.63.03.52.7
z values within columns followed by the same letter for each factor are not significantly different from each other based on ANOVA and Fisher’s protected LSD test (p < 0.05).
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Larkin, R.P. Evaluation of Biochar and Humus Amendments and Early-Season Insect Netting on Soil Properties, Crop Yield, and Pest Management in Organic Vegetable Production in Maine. Agronomy 2025, 15, 2567. https://doi.org/10.3390/agronomy15112567

AMA Style

Larkin RP. Evaluation of Biochar and Humus Amendments and Early-Season Insect Netting on Soil Properties, Crop Yield, and Pest Management in Organic Vegetable Production in Maine. Agronomy. 2025; 15(11):2567. https://doi.org/10.3390/agronomy15112567

Chicago/Turabian Style

Larkin, Robert P. 2025. "Evaluation of Biochar and Humus Amendments and Early-Season Insect Netting on Soil Properties, Crop Yield, and Pest Management in Organic Vegetable Production in Maine" Agronomy 15, no. 11: 2567. https://doi.org/10.3390/agronomy15112567

APA Style

Larkin, R. P. (2025). Evaluation of Biochar and Humus Amendments and Early-Season Insect Netting on Soil Properties, Crop Yield, and Pest Management in Organic Vegetable Production in Maine. Agronomy, 15(11), 2567. https://doi.org/10.3390/agronomy15112567

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