Murray, M. S., Integrated Pest Management Specialist, Utah State University Extension

ABSTRACT

Utah’s commercial vegetable industry is concerned about improving yield of crops grown in arid and alkaline soils, and reducing losses due to diseases caused by soil-borne pathogens. This project investigated whether biochar improves yield of 'Hamson' tomato and 'Sleeping Beauty' melon fruits, and/or increases plant resistance to crown rot caused by Phytophthora spp. Biochar is a carbon-rich material similar to charcoal that has been shown to improve soil qualities and plant growth. For this study, crops were field-planted with and without biochar, and yield (harvest weights) was compared between treatments. Greenhouse trials evaluated the effect of biochar on susceptibility of tomato and melon to crown rot. In the field trials, the use of biochar increased the three-year average yield of tomato over no fertilizer application, but not over fertilizer alone. There were no differences in the three-year melon yield among the treatments. In the crown rot trials, the addition of biochar to the planting media had no significant effect on disease incidence.

Introduction

Biochar is a product resulting from the slow-burning of organic solids in the absence of oxygen, called pyrolysis. It is similar to charcoal, but does not contain petroleum, is made sustainably from biowaste products, and has many potential beneficial uses, including in agriculture (Phillips and Trippe 2020; Wang and Wang 2019).

Soil-applied biochar has been shown to improve soil quality and crop yield by improving soil nutrient and water retention, increasing aeration, and improving the efficacy of fertilizers (Agegnehu et al., 2017; Ding et al., 2016; Sun Cha et al., 2016). Studies have also shown that growth of beneficial rhizobacteria and mycorrhizal fungi are greater in biochar soils, which have helped defend plants against soilborne diseases (Bonanomi et al., 2015).

To address yield-improvement and plant disease concerns raised by Utah’s vegetable producers, we investigated whether biochar as a soil amendment could improve crop yield and prevent crown rot caused by Phytophthora spp. in melon and tomato. The recommendation is that biochar is applied to the soil just once, acting as a conditioner, and that nutrient amendments should continue to be applied as needed (Brick 2010).

Methods

This project consisted of two trials. Trial 1 was a field-planted trial to evaluate yield and trial 2 was a greenhouse trial to evaluate resistance to crown rot.

TRIAL 1 – TOMATO, MELON YIELD

The biochar used for this study was produced from beetle-killed pine pyrolized at 375°C and masticated to 0.6 cm size (Confluence Energy, Colorado). The 375°C production temperature results in a product with a higher yield of biochar-to-wood at a lower temperature (Phillips and Trippe 2020) and the influence of biochar particle size on nutrient contents in soil is relatively small (Borchard et al., 2012).

To determine a per-acre application rate, a greenhouse study of seed-propagated, pot-grown Parris Island Cos romaine lettuce was conducted, where a rate of 2% (by volume) biochar produced the healthiest plants and highest yield after 6 weeks of growth. This rate equates to 10 tons of biochar per acre.

Trial 1 was conducted on a ½-acre plot at the Utah Agricultural Experimental Research Farm in Kaysville, UT. The plot was a randomized block design with four replications. The three treatments were:

1. Biochar plus fertilizer
   • 10 tons biochar mixed with 60 lb, 4-4-4 fertilizer per acre (Jobe’s Organic All-Purpose)
   • Biochar was applied once in year 1 (2016)
   • Fertilizer was applied each year (2016, 2017, and 2018)
2. Fertilizer
   • 60 lb fertilizer per acre rate, applied each year (2016, 2017, and 2018)
3. Control (untreated)

After spreading biochar (Figure 1) and before the first planting, all rows were tilled using a forward-rotating mini gas rototiller (Honda FG110) to a depth of 6 inches at a single pass. Each replicate was planted with six 'Sleeping Beauty' melon and six 'Hamson' tomato plants per treatment in May of 2016, 2017, and 2018, watered with drip irrigation, and covered in weed fabric (Figure 1).

At the end of each season, yield was determined by weighing harvestable fruit.  Yield data was analyzed by comparing means using one-way ANOVA (Microsoft Excel; 0.05 significance level). Soil samples were collected prior to the project (spring of 2015) and after final harvest (fall of 2018). Soil was tested for pH, salinity, organic matter content, and nutrient content by the USU Analytical Lab (Logan, UT). Unfortunately, the biochar was not tested prior to soil application.

 

Figure 1. Biochar was applied in the treated rows in a band pattern and then tilled in (left). Afterward, 'Sleeping Beauty' melon and 'Hamson’ tomato plants were planted in a randomized block design into untreated soil, fertilizer soil, and biochar plus fertilizer soil (right).
 

TRIAL 2 – CROWN ROT GREENHOUSE TRIAL

At the USU research greenhouses (Logan, UT), 40 'Sleeping Beauty' melon and 40 'Hamson' tomato transplants were planted in 3-gallon pots, where 10 plants of each were randomly assigned one of the following four treatments:

1. Uninoculated, untreated potting media (Pro-Mix General Purpose)
2. Uninoculated potting media plus biochar
3. Inoculated, untreated potting media
4. Inoculated potting media plus biochar

Initially, half the plants were grown in plain potting media and half were grown in media plus biochar (using a rate of 2% biochar by volume) for six weeks under normal watering and fertilization (Figure 2). After this time, the pre-assigned plants were inoculated with a mix of Phytophthora nicotianae, P. megasperma, and P. capsici using the protocol described by Benson and Parker (2015). All pots were then placed in a water-filled saucer for 12 hours, followed by normal irrigation and fertilization for eight weeks.

At the end of the trial, each plant was rated on a symptom scale of 1 (no symptoms) to 5 (dead). Rating means were compared using one-way ANOVA (Microsoft Excel; 0.05 significance level). We also collected random and symptomatic root tissue and tested for the presence of Phytophthora spp. with a diagnostic kit (Phytophthora ImmunoStrip®, Agdia, Inc.). This entire process was repeated three times.

 

Figure 2. Biochar was mixed into typical greenhouse potting media (left) and tomatoes and melons were planted in media with or without biochar (right).

Results

TRIAL 1 – TOMATO, MELON YIELD

The addition of the biochar resulted in a slight numeric increase in crop yield in some years, but there were no significant differences in the average three-year yield between plants grown in biochar plus fertilizer or fertilizer alone for both melon (p=0.82) and tomato (p=0.34). For both crops, yield actually decreased from year 1 to year 3 across all treatments (Figure 3), likely due to the fact that the crops needed to be grown in the same location each year and were not rotated.
 

Melon

Over the three-year project, the highest yield of melon fruit occurred in year 1 in the biochar soil, at 32.6 lb/plant (Figure 3). Although this yield was significantly greater than the control soil (p=0.03), it was similar to the fertilizer soil (p=0.29). The lowest yield of the three years occurred in year 3, also in the biochar soil, at 13.2 lb/plant. Although the average three-year yield was numerically highest in the biochar treatment (4% over fertilizer alone), yield was statistically similar across all three treatments (p=0.82; Figure 3).
 

Tomato

For tomatoes, the only year that fruit yield in the biochar treatment was significantly greater than fertilizer alone was in year 2, at 22% greater (p=0.03). In the third year, yield was highest in the biochar treatment again (12.8 lb/plant), but this was not statistically different from the fertilizer-alone treatment (11.8 lb/plant). As with the melons, when yield data was averaged across the three years, there was no significant difference in the biochar plus fertilizer and fertilizer treatments (p=0.34; Figure 3), but tomato yield in the biochar plus fertilizer treatment was significantly greater (25% greater) than yield in the control soil (p=0.04).

 

Figure 3. Average yield for melon (left) and tomato plants (right) grown for three years in soil amended with biochar plus fertilizer, fertilizer alone, or with no amendment in Kaysville, UT. Although the biochar treatment did not significantly increase the 3-year average yield of either crop over fertilizer alone, we found that tomato yield responded best to biochar.
 

An economic analysis is presented below for tomato using Washington State University’s enterprise budget for a fresh‐market tomato crop (Galinato et al., 2011a). This analysis includes the results shown in this study, where the three-year yield of biochar plus fertilizer was 25% greater than the control (no fertilizer) and yield of biochar plus fertilizer was 11% greater than fertilizer alone (although not statistically significant). The comparison includes a one-year application of biochar plus yearly applications of fertilizer, and compares this to using no fertilizer and using just fertilizer yearly.

The results show that the use of biochar provides a net return over a three-year period of $25,103/acre more than using no fertilizer and $16,783/acre more than yearly fertilizer applications.

Assumptions:

• $2/lb return
• No-fertilizer application of one acre of production provides 26,000 lb/acre yield (5,500 plants/acre)
  • No-fertilizer net return is $27,353 per year
• Fertilizer-only application (yearly) of one acre of production provides 30,360 lb/acre yield (5,500 plants/acre)
  • Fertilizer-only net return is $34,737 per year

Biochar cost for one acre, one application = $2,785

• One application = $2,625
  • 10 tons/acre, banded into rows
  • 22 rows at 4-feet wide with 5 feet between rows
  • $525/ton (Bushell 2018)
• Labor to apply biochar: $160 (Williams and Arnott 2010)

Returns for one acre over No-fertilizer:

• Based on the results of our study, the addition of biochar increased the yield from the assumed no-fertilizer amount (26,000 lb/acre) in season 1 by 4% (27,040 lb/acre), in season 2 by 25% (31,500 lb/acre), and in season 3 by 44% (37,440 lb/acre).
• The net returns in seasons 1, 2, and 3 combined are:
  • $82,059 with no-fertilizer
  • $107,162 with one biochar application and yearly fertilizer
• The net return after three years with biochar is $25,103 more than no-fertilizer

Returns for one acre over Fertilizer-only:

• Based on the results of our study, the addition of biochar increased the yield from the assumed fertilizer-only amount (30,360 lb/acre) in season 1 by 4% (31,574 lb/acre), in season 2 by 22% (37,039 lb/acre), and in season 3 by 8% (32,789 lb/acre).
• The net returns in seasons 1, 2, and 3 combined are:
  • $104,211 with fertilizer-only
  • $120,994 with one biochar application and yearly fertilizer
• The net return after three years with biochar is $16,783 more than fertilizer alone

Soil Characteristics

After three years, biochar did not significantly improve soil condition over the pre-biochar soil, and the biochar soil was similar to the fertilizer-only soil (Table 1). In fact, the soil tests showed that while the values of some soil properties improved (salinity slightly decreased, organic matter increased, and zinc increased), others became worse (nitrogen, iron, and sulfur levels declined). Phosphorus content increased likely due to the yearly addition of fertilizer, and the high levels suggest that a fertilizer with lower phosphorus content would have been a better choice.
 

Table 1. Soil properties at the Kaysville, UT study site before biochar application, and three years later. (No analysis was conducted of the biochar alone).  Testing conducted by USU Analytical Lab (Logan, UT).

Property	Soil pre-Biochar, 2015	Fertilizer-only Soil, 2018	Biochar plus Fertilizer Soil, 2018
pH	7.4	7.4	7.5
Salinity (dS/M)	0.67	0.57	0.51
Phosphorus (mg/kg)	31	108	104
Potassium (mg/kg)	209	244	278
Nitrogen (mg/kg)	15.64	9.97	10.40
Zinc (mg/kg)	1.08	4.49	3.82
Iron (mg/kg)	19.27	11.10	10.00
Copper (mg/kg)	1.63	2.78	2.66
Manganese (mg/kg)	4.45	4.52	3.59
Sulfur (mg/kg)	18.2	5.5	4.9
Organic Matter (%)	2.0	4.9	4.3

TRIAL 2 – CROWN ROT GREENHOUSE TRIAL

The healthiest plants in this study were the uninoculated plants grown in the biochar-amended soil, which was to be expected. For the inoculated plants, we hoped to see less incidence of disease on plants grown in the biochar media, but this was not the case. Averaging across all three replicated trials, both the inoculated melons (p=0.66) and tomatoes (p=0.92) showed no significant difference in the average symptom rating of plants whether grown in soil with or without biochar (Figure 4) or in the number of diseased plants.

 

Figure 4. Average health ratings for melon (left) and tomato plants (right), where 1 represented a plant with no symptoms, and 5 represented a dead plant. The graphs show ratings for plants that were not inoculated and grown in plain media (Uninoc-untreated) and media with biochar (Uninoc-char), and plants that were inoculated with Phytophthora spp. and grown in media (Inoc-untreated) and media with biochar (Inoc-char).
 

Conclusions

These experiments testing biochar as a soil amendment to improve yield of tomato and melon, and to reduce root rot disease incidence, did not show significant findings of a biochar application over standard growing practices.

• For the tomato field trial, the greatest increase in yield over plants grown in fertilizer alone was in year 2. The increase in yield of plants grown in biochar was significantly greater than plants grown without fertilizer, with a 25% increase.
• An economic analysis revealed that with our study findings, if a grower applied biochar plus yearly fertilizer applications, they could earn $25,103/acre more over a 3-year period than using no fertilizer at all.
• For the field-grown melons, the plants in the biochar plots had the highest yield in the first year, but that improvement was not seen again in subsequent years, and the average 3-year yield was no different across all three treatments.
• For the crown rot greenhouse trials, the addition of biochar to plant growth media improved the growth of uninoculated plants over a 14-week period, but did not improve resistance of plants that were inoculated with Phytophthora spp.

Indications suggest that biochar could play a role in improving sustainable agriculture, but the challenges of initial cost, variability in biochar types and application rates, and how biochar application can work with other soil health practices such as no-till, cover cropping, manuring, and mulching, still need to be addressed (Clough et al., 2013; Galinato et al., 2011b). Certainly, improved recommendations for agricultural uses will still be investigated.

Literature Cited

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