Exploring Low-Phosphorous Wool Pellets as Fertilizer and Soil Conditioner for Vegetables

Due to a shift in personnel after one key personnel left UVM and another retired (but stayed on the project part-time), this project shifted into an undergraduate student-centered service learning project. Data in fall 2021 were collected by a team of students and summarized in December 2021. In January-February 2022, a team of student interns completed full statistical analysis of all project data for publication in a scientific journal . This research was published in May 2022 as:

Bradshaw, T. and Hagen, K. Wool Pellets Are a Viable Alternative to Commercial Fertilizer for Organic Vegetable Production. Agronomy. 2022; 12(5):1210. https://doi.org/10.3390/agronomy12051210.

Results presented in this report are taken from that paper.

General weather conditions during the growing season included relatively dry conditions at both sites for the majority of the growing season. Temperatures in summer were generally above normal at both locations, with 14 days at CCF and 10 days at HREC recording temperatures in excess of 30 °C during June and July, based on on-site weather monitoring. Pest incidence was low for spinach at both sites and for tomatoes at HREC, but Colorado Potato Beetles decimated the plants at CCF despite multiple applications of organic-certified pesticides. As a result, there was total crop loss at that site, and no results are presented in this report.

There were no differences in soil fertility at harvest for either crop or site, except for a biologically insignificant difference in aluminum at CCF. At HREC, there was a clear dose response for nitrogen and spinach crop yield, where increasing nitrogen was positively associated with increased yield. That response was not as clear at CCF- the lower rate wool treatment was similar to the non-treated control, and the other two treatments were similar but not statistically different. All yields were 2-3x greater at CCF than at HREC.

Few differences in mineral nutrient composition of spinach were observed at CCF. At HREC, differences were observed for several mineral nutrients, but those results were inconsistent across treatments and likely related as much to baseline soil conditions as much as to fertilizer program. One important result observed was that  phosphorous levels in spinach grown in wool fertilization were lower than that grown in the control or commercial fertilizer treatments. This may indicate a reduced ability of the plants to uptake phosphorous, or a dilution of the P that was uptaken by the larger biomass that was produced, particularly in the highest-rate wool pellet treatment (WL-25).

Statistically significant differences in spinach yield among the treatments were few, but the design of the experiment and general soil conditions may not have been conducive to detecting differences. At HREC, the mean spinach yields from GS and WL-25 were 61% and 47% greater than NTC, despite no differences detected. The magnitude of these differences would be economically important, and an experiment with a greater number of replications may detect differences among these treatments’ populations. All of the fertility treatments ranked higher than NTC, with an overall mean increase of 47–149% in yield over the non-fertilized treatment.

This is likely due to the short season for spinach production at both sites, with 41 and 43 days of field growth from transplanting to harvest for HREC and CC, respectively. Raw wool is a relatively slow-release nitrogen material that requires microbial degradation in order to convert complex proteins and other nitrogen-containing compounds into plant-available forms. At CC, the base soil fertility may have been sufficient to provide adequate nitrogen for plant growth even without fertilization, but the lower SOM at HREC likely provided limited nitrogen that prevented adequate crop growth without supplemental fertilization.

The productivity of the tomatoes was affected by the fertilizer treatments, with the higher application of nitrogen generally relating to increased plant growth and crop yield. The total weight of the harvested fruit per plant was greatest for the WL-25 treatment compared to the NTC (86% increase), and the GS nitrogen treatments, whether from wool or commercial fertilizer, were intermediate but different from neither other treatment. However, the number of fruits per plant did not differ among the fertilizer treatments. The fruit weight was highest for the WL-GS treatment and lowest for the NTC and GS, and the WL-25 was intermediate and did not differ from the others. The plant dry matter weight was different among the treatments, with the GS treatment having a greater (122%) dry mass than the NTC-treated plants, and the wool-treated plants were 58–69% larger than the NTC, but not statistically different in this study. Quality characteristics, including the proportion of split or rotten fruit and fruit grade (i.e., red vs green fruit), were not affected by the fertilizer treatment. The chemistry of the tomato juice did not differ among the fertility treatments for most of the measured parameters. However, the juice from NTC plants had higher total polyphenols than WL-25.

In this experiment, tomatoes served as a longer-season crop than spinach, and a positive effect of fertilization was observed on their crop yield and plant growth. Similar to spinach at HREC, there was a dose-response observed with the increasing nitrogen application, although only the NTC and WL-25 treatments were statistically different from one another. It is important to note that when the target nitrogen application of 130 kg × ha⁻¹ was applied, the wool pellets were comparable to standard commercial fertilizer. This is similar to the results seen with spinach, at least at HREC, where there was no difference between the nitrogen-dose equivalent GS and the WL-GS treatments for crop yield. In tomato, the crop yield did not appear to be a function primarily of fruit number, as there were no statistical differences among treatments for that parameter, but all fertilized treatments did have 25–35% more fruit than the NTC. Fruit size was likely a significant determinant in total crop yield, and WL-GS had a larger fruit weight than both non-wool treated treatments, and WL-25 ranked second-largest but was not different from any other treatment.

In contrast to crop yield and fruit size, tomato plant growth, as measured by dry matter weight of aboveground plants post-harvest, was increased by commercial fertilizer in the GS treatment compared to the NTC and the wool treatments were intermediate. Many of the nitrogen courses included in Pro-Booster are rapidly-available forms, including natural soda nitrate, which is chemically similar to many fast-acting synthetic nitrogen sources.

Soil moisture data were collected in this experiment, but the equipment used was unreliable, so the results are not included in this report. Further study should include the meticulous collection of soil moisture data, either continuously via electroconductivity-based soil moisture meters or irrometers, to better elucidate the effects of wool pellets on the soil moisture conditions.