Evaluating the soil block technique for organic vegetable transplant production

Results

Growing media properties

pH and EC. The pH increased on average by 0.8 in 2022 and 0.9 in 2023 throughout the weeks of sampling (WOS) (Table 3). Lab displayed a higher pH than the other growing media in all six WOS in both years, except Cowsmo had a similar pH to Lab in all WOS in 2022. In 2022, Purple and Vermont had the lowest pH in 1 and 2 WOS, and in 6 WOS Vermont (6.1) and BLP (6.1) had the lowest pH. In 2023, BLP and Vermont consistently had the lowest pH.

The EC of all media decreased on average by 5.03 mS·cm-1 in 2022 and 4.76 mS·cm-1 in 2023 throughout the six WOS (Table 3). In 2022, Vermont exhibited a higher EC than the other media from one WOS (7.82 mS·cm-1) until five WOS (2.69 mS·cm-1). In 2022, the EC of Purple reached 1.64 mS·cm-1 in three WOS, while the other growing media did not reach an EC below 2.00 mS·cm-1 until five WOS (BLP, Cowsmo, and Lab) or six WOS (Vermont). In 2023, all growing media reached an EC below 2.00 mS·cm-1 in six WOS, except Cowsmo, which had an EC of 3.19 mS·cm-1 in the final WOS. 

Bulk density. Growing media bulk density was between 76% and 111% greater in soil blocks than in flats (Table 2). In soil blocks, Purple displayed the greatest bulk density (0.46 g·cm-3), while BLP had the lowest (0.21 g·cm-3). In flats, all media displayed a similar bulk density.   

Water holding capacity. Water holding capacity differed between growing media. BLP (680%) exhibited the greatest water holding capacity, while Lab (287%) exhibited a lower water holding capacity compared to BLP and Vermont (Table 2).

Cucumber and pepper emergence and plant count

Cucumbers. In 2022, cucumbers seeded in Cowsmo, Lab, and Vermont showed between 42% and 46% higher emergence in flats than in soil blocks (Table 5). In soil blocks, cucumber emergence was 23% greater in BLP and 32% greater in Purple than in Cowsmo, Lab, and Vermont. In flats, cucumber emergence was similar between media. Cucumber plant count was higher in flats compared with soil blocks with Vermont, Lab, and Cowsmo. No differences were found in cucumber emergence and plant count between treatments in 2023.

Peppers. In 2022, peppers seeded in Lab and Purple showed a 45% and 26% higher emergence in flats than soil blocks, respectively (Table 5). Pepper plant count was similar between flats and soil blocks for all media in 2022, except Lab produced a 38% lower plant count in soil blocks than flats. Across all media in 2023, pepper emergence was 33% higher in flats than in soil blocks, while no differences between the media were found. Pepper plant count was similar between all treatments in 2023.

Transplant growth parameters

Dry weight. Cucumber dry weight was greater in flats than soil blocks by 20% in 2022 (Fig. 1) and 38% in 2023 (Fig. 2). All media produced cucumbers with similar dry weight in 2022, except Cowsmo grown cucumbers exhibited between 90% and 103% lower dry weight than all other media. In 2023, cucumbers grown with Lab and Cowsmo exhibited a lower dry weight than cucumbers grown with BLP, Purple, and Vermont.

In 2022, pepper dry weight was between 50% and 130% greater in soil blocks than flats for all media, except Cowsmo. Cowsmo grown peppers had a similar dry weight in both methods in 2022 (Fig. 3), but dry weight was consistently lower than all other treatments in both years. In 2022, Purple in soil blocks (1.22 g) and Vermont in soil blocks (1.18 g) produced the largest pepper dry weights. In 2023, the largest pepper dry weights were found in BLP in soil blocks (0.55 g), Purple in soil blocks (0.59 g), and Vermont in flats (0.58 g) (Fig. 4). In 2023, no differences in dry weight were found between peppers grown in flats and soil blocks.

Plant height. In 2022, cucumber plants were 13% taller in flats than in soil blocks (Fig. 1). In 2023, Cowsmo and Vermont grown cucumber plants were 39% and 30% taller in flats than soil blocks, respectively, while all other media produced plants with a similar height in both methods (Fig. 2). Cucumber plants grown in BLP (68 mm) and Purple (71 mm) were the tallest in 2022. In 2023, the tallest cucumber plants were found in BLP in flats (78 mm) and soil blocks (85 mm) and Vermont in flats (90 mm). Cucumber plants in Cowsmo were 48% to 96% shorter than all other media in both years. 

In 2022, pepper plants were between 47% and 74% taller in soil blocks than flats in all media, except Cowsmo, which produced peppers with a similar height in both methods (Fig. 3). In 2023, BLP and Purple produced taller peppers in soil blocks than in flats, by 18%, and 30%, respectively (Fig. 4). Pepper plants were tallest with Vermont, Purple, and BLP in soil blocks, and, in 2023, Vermont in flats as well. Cowsmo in soil blocks consistently produced peppers that were between 97% and 166% shorter than all other media in either method.

Stem diameter. Cucumber stem diameter was 7% larger in flats than in soil blocks in 2022 (Fig. 1). In 2023, Cowsmo and Vermont produced cucumbers with 20% and 14% larger stem diameters in flats than in soil blocks, respectively, while all other media produced a similar stem diameter in both methods (Fig. 2). In 2023, Vermont in flats displayed a larger cucumber stem diameter than all other treatments. Cowsmo in soil blocks consistently produced cucumbers with a smaller stem diameter than all other media in either method.

In 2022, peppers in soil blocks exhibited a 26% larger stem diameter than in flats (Fig. 3). In 2023, peppers grown with BLP, Lab, and Purple had larger stem diameters in soil blocks than in flats (Fig. 4). The largest pepper stem diameters were found in BLP, Vermont, and Purple in 2022, and with those same media in soil blocks in 2023. Cowsmo in soil blocks consistently produced smaller pepper stem diameter than all other media in either method.

Root surface area. Cucumber root surface area was 92% and 118% larger in flats than in soil blocks in 2022 (Fig. 1) and 2023 (Fig. 2), respectively. Cowsmo in flats and soil blocks produced between 171% and 193% smaller pepper root surface area than all other treatments. Pepper root surface area with Purple produced 52% to 193% larger root surface area than all other treatments (Fig. 3). In 2023, no differences between media were found in cucumber root surface area. 

Discussion

Peppers grown in soil blocks tended to have lower emergence at 15 DAS than those in flats, seen across all media in 2023 and with Lab and Purple in 2022 (Table 5). Although, pepper plant count between methods was not different at the time of plant count data collection (36 DAS), apart from Lab in 2022. The lack of continued disparity from emergence (15 DAS) to plant count (36 DAS) between methods indicates a delay in pepper emergence in soil blocks, as opposed to a reduction in germination. The seedling environment as determined by the transplant method, specifically the growing medium temperature and bulk density, may help explain the delay in emergence found in peppers in soil blocks. 

Flats used were made of black plastic, which conducts heat from artificial lighting and solar radiation into the growing medium. The growing media within flats was on average 3˚C higher as compared with soil blocks, 21.3˚C in flats and 18.5˚C in soil blocks. This increase in medium temperature is similar to what is found in containerized production in black pots (Ingram et al. 2003; Markham et al. 2011). An increase in growing medium temperature, without exceeding an ideal range, can lead to more rapid germination and seedling emergence (Cantliffe 1998). Research found ‘Dasher II’ cucumbers to reach 80% germination two DAS at 15˚C, 1.5 DAS at 20˚C, and 0.75 DAS at 25˚C (Jennings and Saltveit 1994). This reduction in hours to germination in temperatures above 20˚C has also been found in hot peppers (Capsicum baccatum var. pendulum), reaching 50% germination after 10 DAS at 15˚C, eight DAS at 20˚C and 7.5 DAS at 25˚C (Silva et al. 2012). Therefore, the faster emergence of peppers in flats as compared to soil blocks may be partly explained by the higher medium temperature with that method.

An additional factor which may have impacted pepper emergence rates was the difference in growing medium bulk density between methods. The bulk density of the growing media were between 137% to 237% higher in soil blocks as compared to the same medium in flats. The compression of moistened growing media into the blocker tool, required to make soil blocks, helps explain the higher bulk density. A higher bulk density imparts an increase in overall compaction and a reduction in available oxygen, as pore space is reduced, and medium solids make up a higher proportion of the block after compression (Hanks and Thorp 1956; Sabahy et al. 2015; Gruda et al. 2013). Without adequate pore space for air and water, root respiration and root metabolic activity are reduced, and overall root growth is inhibited (Green 1976). Furthermore, research has shown pepper seedling emergence to be slowed by three to five days in field plots with higher compaction (Fawusi 1978). Research has also shown an oxygen reduced environment, as found with a higher bulk density, increased the number of days required to reach 50% germination in Brassicaceae, Apiaceae, Asteraceae, and Amaranthaceae seeds (Yasin and Andreasen 2016), as most seeds require an aerobic environment to germinate (Cantliffe 1998). The greater bulk density in soil blocks, with a proportionate decrease in the available air and water pore space, may have contributed to a slower emergence of peppers in soil blocks as compared to flats. 

As research on soil blocks is limited, bulk density comparison between this study and others is not possible. Further, soil block making is highly variable, using a hand-tool and varying levels of compression. Considering the importance of bulk density for root growth and seed germination, an evaluation of appropriate block making technique and ideal block bulk density has the potential to optimize soil blocks for transplant production. Future research should compare transplants grown in flats and soil blocks with the same bulk density to better elucidate differences found in transplant performance and seedling emergence.

Although peppers grown in soil blocks experienced a delay in emergence, at 50 DAS peppers grown in soil blocks often outperformed peppers grown with the same media in flats. In 2022 soil block grown peppers with BLP, Lab, Purple, and Vermont displayed a greater plant height, stem diameter, and dry weight than in flats with the same media (Fig. 3). And, in 2023, peppers in soil blocks displayed a greater plant height (BLP and Purple), dry weight (Purple), and stem diameter (BLP, Lab, and Purple) than those grown in flats (Fig. 4). 

In contrast to what was found in peppers, cucumbers showed no difference in emergence between methods in 2023 (Table 5). Additionally, cucumbers grown in flats had a larger dry weight than those in soil blocks (Fig. 2). An emergence delay, resulting in a slower growth rate and final dry weight, may have occurred in cucumbers in soil blocks, but not be represented in the data. Cucumber emergence data was collected at 10 DAS, while cucumbers are known to germinate and emerge in two DAS (Jennings and Saltveit 1994). Collecting emergence data at 10 DAS may not have accurately captured a delay. Therefore, it is possible that a reduction in growing medium temperature and increased bulk density in soil blocks may have resulted in a slower cucumber growth rate which could not be overcome before destructive sampling at 22 DAS.

The overall study findings showed peppers performing better in soil blocks and cucumbers performing better in flats. This may partly be explained by a delay in emergence in soil blocks, which peppers were able to overcome with a longer transplant growth period, while cucumber were not. The research available on soil blocks in vegetable production is mostly limited to evaluations of the in-field performance and yield from soil block and flat grown transplants. Therefore, we are unable to compare our results to other work. 

The second aim of this study was to evaluate the suitability of four commercially available certified organic growing media. Many differences were found between growing medium nutrient concentrations and physical properties. The results of this investigation show that BLP and Purple performed similarly to Vermont, while Cowsmo stunted cucumber and pepper plant growth, and Lab performed better only than Cowsmo.

BLP displayed the lowest bulk density when used in both flats and soil blocks (Table 2), but still increased by 250% in soil blocks compared with in flats. Additionally, BLP exhibited the highest water holding capacity out of the media evaluated (680%). The companies which developed the products evaluated in this study list medium ingredients without proportions. BLP was the only medium that did not contain compost and appeared to have a higher proportion of peat than the other mixes. Peat is highly porous, with a low bulk density and a high water holding capacity (Kubota et al. 2013). When comparing soilless medium mixes, studies have found that an increase in the proportion of compost in the mix correlates with an increase in bulk density (Chrysargyris et al. 2019; Wilson et al. 2002). Additionally, higher proportions of compost in relation to peat in a mix can lower the water holding capacity of the medium (Papafotiou et al. 2005). Our study supports the previous findings. Yet, properties are variable by type of compost used and, more specifically, particle size (Carlile et al. 2019). Growing medium for the soil block method is most often recommended to possess a high water holding capacity and fibrous materials, such as peat, to help hold the block form together (Coleman 1995; Kasten 2019). BLP meets this criterion. The other media evaluated also held together well in block form, although longevity of block cohesion was not within the parameters of this study.

The EC of all the growing media was far above the general guideline of 2 mS·cm-1 for optimum plant growth (Ozores-Hampton et al. 1998), and often above 4 mS·cm-1, which can limit plant growth (Ozores-Hampton et al. 1998; Willumsen 1997). One of the challenges with soilless medium mixes containing compost is the risk of high EC, but great variability exists depending on the components and process used to make the compost (Balliu et al. 2017). The EC of all the growing media reduced each week of sampling, as irrigation leached excess salts and the growing plants took up nutrients. Even though most of the growing media had an EC outside the optimum range (<4.0 mS·cm-1) in the first two to three weeks of pepper growth, peppers exhibited healthy growth and no signs of salinity stress. This finding may help inform growers and researchers that guidelines should be revisited for EC limits to plant health in soilless mixes. 

Cowsmo performed poorly, with transplants displaying lower plant height, stem diameter, and dry weight in both cucumbers and peppers compared with almost all other media with either method. Visible stunting occurred in cucumbers and peppers grown in Cowsmo. Despite similar bulk density and water holding capacity with the other growing media, Cowsmo displayed 177% to 193% lower total plant available nitrogen (nitrate-N and ammonium-N) concentration than the other media in 2022 and 63% to 177% lower in 2023 (Table 1). Nitrogen is one of the most essential elements for plant growth, contributing to photosynthesis, protein formation, and overall growth. Symptoms of nitrogen deficiency in seedlings include stunting and chlorosis (Uchida 2000). Tissue analysis confirmed a lower amount of plant available nitrogen in Cowsmo, with cucumber plant tissue containing up to 105% lower percentage of nitrogen when grown in Cowsmo than in all other media (Table 4). 

The media did not perform consistently in flats and soil blocks, resulting in a significant method by media interaction for most pepper growth parameters. While peppers grown with BLP, Purple, and Lab in soil blocks performed better than in flats across most metrics in both years, Vermont performed better in flats than in soil blocks across some metrics. Additionally, Purple performed well in soil blocks, with the greatest plant height, stem diameter, and dry weight in 2023 (Fig. 4). But, in flats, peppers grown with Purple had the smallest plant height, stem diameter, and dry weight in both years and smallest root surface area in 2022, excluding Lab and Cowsmo. These inconsistencies in pepper performance, depending on both medium and method, led to an inability to parse out a clear method or media affect. As such, the data is presented for each method by media variable when an interaction was found, in contrast to the whole plot and subplot factor presented independently in other parameters.

BLP, Purple, and Vermont can all be considered suitable for growing certified organic vegetable transplants in both soil blocks and flats, while Cowsmo should undergo further investigation before continued usage. Our results are contrary to what was found in an on-farm trial, which found Cowsmo grown broccoli and tomato transplants compared similarly or better to Vermont and a different growing media from Beautiful Land Products (Liddle 2021). Unfortunately, no information regarding growing media analysis from the trial is available to compare with the products used in this study. Although new media was purchased each year during this experiment, variability has been reported within retail potting mix brands and many commercially available products (Wiberg et al. 2006). 

The claims surrounding soil block grown transplants assert that there is an increase in the vegetable transplant root system, due to increased lateral root growth and a lack of root circling (Coleman 1995; Pill and Stubbolo 1986; Tresemer 1983). Our findings do not support these claims, as we did not see a general increase in root surface area in transplants grown with the soil block method. Instead, root growth of peppers and cucumber correlated with shoot growth, irrespective of method. Cucumbers had a greater root surface area in flats in both years, corresponding with a greater plant height, stem diameter, and dry weight in flats. Additionally, in 2022, pepper root surface area did not show differences between methods, except for Purple, which showed a greater root surface area in soil blocks as compared to flats. Further research should look specifically at the architecture of the root system to further understand the impact the soil block method has on vegetable transplant root development.

The lack of physical barriers between soil blocks is claimed to reduce root circling by allowing for air-pruning (Coleman 1995; Pill and Stubbolo 1986; Tresemer 1983). We did not witness air pruning, as the air separating blocks became filled with surrounding media during watering. If underwatering was used, instead of the traditional overhead watering which we performed, the air gaps between blocks may be preserved and allow for air-pruning. The presence of air gaps may also limit the tendency of transplants to grow into neighboring soil blocks. If this occurs, separating the blocks for transplanting would result in a large amount of root damage, leading to reduction in yields. Others have indicated this tendency as a challenge with soil blocks (Greer and Adam 2005). To prevent this from occurring, establishing the correct size of block and time to transplant is essential. Many favor transplanting relatively young transplants (Balliu et al. 2017), which may be encouraged by the need to stop transplant growth before roots outgrow their individual block. 

A natural progression of this work is to analyze the performance of soil block grown vegetable transplants in the field. Our research was limited to the greenhouse phase and data on in-field performance would be revelatory, allowing for an investigation of the claims of a reduction in transplant shock and increased yields when using this method.

Making soil blocks is a time-consuming and labor-intensive process, especially when compared with the relative ease of filling a flat. The benefit of on-farm plastic reduction can be a significant driver for many growers looking for alternatives to flats (FAO 2021), but the labor cost of using soil blocks on a commercial scale should be investigated further. Additionally, by compressing growing media when soil blocks are made, more growing media is used per block than per cell. This additional cost and resource use should be considered, especially as peat continues to be a major component of most growing media. Peat is a non-renewable resource under threat of over-exploitation (Nesse et al. 2019), therefore, any reduction in the amount of media used can result in resource conservation and environment benefit.

Conclusion

Our work demonstrated that pepper transplants performed better in soil blocks, while cucumber transplants performed better in flats. A significant advantage to the root system of transplants grown in soil blocks was not found, although future research should examine the differences in root system morphology produced between flat and soil block grown transplants. More work needs to be done to examine the in-field performance of soil block grown vegetable transplants and assess transplant shock. Cowsmo stunted pepper and cucumber growth, while BLP, Purple, and Vermont produced healthy and vigorous transplants. Our findings are relevant for certified organic vegetable transplant producers in the Midwestern U.S. looking to use regionally produced growing media, potentially limiting shipping costs and carbon footprint. Very limited research is available regarding the soil block method, even as it maintains popularity amongst small and medium-sized growers. More information and continued research on soil blocks would be of great value, establishing optimal bulk density parameters and improving understanding of the impact this method has on vegetable transplant performance.