“Maintaining Soil Fertility” in the April/May issue of Rural Heritage featured a soil test report for Field 4 from Waypoint Analytical. It showed the level of nutrients in the soil, determined by chemical analysis. A more complete picture of soil quality would include the biological and physical aspects of the soil, as well as key indicators of soil management.
In 2018, we were invited to participate in the PASA Soil Health Benchmark Study. (PASA is the Pennsylvania Association of Sustainable Agriculture.) Each farm in the PASA study selected three fields representative of the rotation, and maintained records of all fieldwork, planting/harvest dates and soil amendments.
PASA personnel collected soil samples in October and shipped them to the Cornell Soil Health Lab for analysis. Over the winter, PASA’s Soil Institute director, Dr. Franklin Egan, and assistant, Sarah Nawa, compiled the data and calculated the benchmarks, including the median, maximum and minimum soil health scores for all of the fields from the 35 organic vegetable farms in the project. A preliminary summary of the soil management results, along with the Cornell soil health ratings, can be found in the chart at the end of this article.
On our farm, we rotate the fields between vegetable production and a full year of cover crops, the exception being that in one of the 13 fields we grow vegetables for two years in a row. For the PASA study, we chose three adjacent fields which represented all three scenarios.
The photos provide a brief overview of the soil management in Field 5 (garlic and spring produce) and Field 6 (a fallow year of cover crops). Field 4 was the designated second-year vegetable field in 2018. We planted early potatoes on the south side and fall vegetables on the north, as described in “Know-Till and Vetch,” (February/March Rural Heritage).
The overall soil health score for the three fields was excellent, right at the PASA median. We thought that was pretty good, considering our relatively low level of organic inputs. On the other hand, knowing that half of the fields in the study were rated better than ours made us think there is a lot of room for improvement.
Our first priority is aggregate stability. Our fields ranked at the very bottom for this important indicator of soil health. Granted, the median for all of the vegetable growers was not much better. The low rating was a big drop from 2017 when the median was in the excellent range. Excessive rainfall in 2018 was probably the major reason for the significant change in aggregate stability.
According to the explanation of soil health indicators that came with our Cornell test results, “aggregate stability is a measure of how well soil aggregates, or crumbs, hold together under rainfall or other rapid wetting stresses … Good aggregate stability helps prevent crusting, runoff and erosion, and facilitates aeration, infiltration and water storage, along with improving seed germination and root and microbial health.”
During our first years on the farm, soil crusting and poor seed germination were major problems. Switching from moldboard plowing to shallow incorporation of cover crops and compost minimized these issues by concentrating organic matter in the surface of the soil. However, this change in practice has not been as effective at improving rainfall infiltration.
An explanation for this may be found in the tillage index. Although our shallow tillage techniques are less intensive than moldboard plowing or rotovating, we make many more passes over the field with our odd assortment of horsedrawn tools, adding up to a higher intensity of tillage than any of our PASA peers. Cornell recommends reducing tillage to improve aggregate stability.
The Cornell soil health team also suggest adding fresh organic materials, manures and mulches, using shallow-rooted cover crops and rotating with sod crops to increase the percentage of water stable aggregates in the soil. The prescription is pretty much the same for the other physical and biological soil health indicators, but with a slightly different emphasis.
For example, more emphasis is made on adding fresh materials and green manures to improve soil respiration and active carbon while stable organic materials and high biomass cover crops are advised for increasing organic matter and available water capacity. To enhance soil protein, the emphasis shifts to adding nitrogen-rich material and growing nitrogen-fixing cover crops. Unique to soil respiration is the recommendation “to maintain plant cover throughout the season.”
Our land-extensive system makes it possible to rotate between shallow-rooted, nitrogen-fixing and high biomass cover crops. However, we do not maintain living vegetation year-round for the simple reason that we do not use irrigation. Cover crops take a lot of moisture out of the soil, so we find it necessary to kill them at least six weeks before planting a cash crop to ensure enough time for at least one moisture-replenishing rain. Consequently, there are extended periods of the growing season when the soil may be completely, or partially, covered with dead cover crop residue, but no live roots in the ground, preventing us from maximizing the days in living cover.
Two of our fields ranked below the median for soil respiration. “A direct biological activity measurement, respiration is an indicator of the biological status of the soil community … Soil biological activity influences key physical characteristics, like organic matter accumulation and aggregate formation and stabilization.”
Our fields averaged just above the median for available water capacity, soil protein and active carbon. “Available water capacity is an indicator of the amount of plant-available water the soil can store, and, therefore, how crops will fare in droughty conditions.”
Soil protein “represents the large pool of organically bound nitrogen in the soil organic matter, which microbial activity can mineralize and make available for plant intake … Protein content influences the ability of the soil to make nitrogen available by mineralization and has been associated with soil aggregation and water movement.”
Active carbon is “the small portion of organic matter that can serve as an easily available food source for microbes, thus helping maintain a healthy food web … A healthy and diverse microbial community is essential to maintain disease resistance, nutrient cycling, aggregation and many other important functions.”
We noticed that the definitions of soil respiration, protein and active carbon shared one thing in common of particular relevance to the health of our soil: good soil aggregation. Organic matter is also linked to aggregate stability, and the organic matter of our fields tested below the PASA median. One reason for our soil plateauing at 3.5 to 4 percent Overview of Field 5 on June 27 with garlic, salad mix, strawberries, spinach, peas, and beets. We planted a cover crop of rye after the garlic was harvested, and interseeded the cultivated vegetables with hairy vetch on June 6. June/July 2019 31 organic matter is our philosophical decision to rely solely on the manure from our team of work horses to supply all of the compost for the market garden.
A benefit of this decision is soil phosphorus levels are still optimal despite applying horse manure compost to the market garden for 37 years. Some of the vegetable fields in the PASA study received very low phosphorus ratings due to very high levels of this mineral. Cornell gives low scores for both deficient and excessive phosphorus because “excessively high phosphorus values indicate a risk of adverse environmental impact through runoff and contamination of surface waters.”
We were surprised that Field 4 received the lowest score in the study for potassium. The Waypoint soil test report in the last issue showed a very high level of this element for the north side of the field the beginning of September. Our only explanation for the discrepancy is the potato crop on the south side may have removed so much potassium that, when the whole field was sampled for the Cornell Soil Health Lab in October, a small deficiency was detected. We are hesitant to increase compost inputs to correct this minor shortfall since our fields usually test too high in this cation.
So what is our action plan? The Cornell soil health ratings and PASA soil management indicators have encouraged us to act on a couple of ideas we have been discussing for years, namely, cover crop cocktails and sod. With a little tweaking of the rotation, we should be able to include a two-year, or even three-year, grass-legume sod every six years beginning in 2020. This change should increase aggregate stability and other soil health measures by significantly reducing tillage and increasing the days of continuous living cover over the course of the rotation. However, adding a multi-year sod to the rotation would require a return to the moldboard plow and regularly planting vegetables two or more years in a row, a possible case of two steps forward, one step back, for soil health.
Cover crop cocktails, by contrast, would not require a change in tillage or the cropping sequence. Last fall, we planted a six-way mix of seeds we had on hand: oats, rye, sorghum-sudangrass, forage peas, hairy vetch and crimson clover. This year we are trialling an eight-way blend since advocates of cover crop cocktails claim at least seven or eight species are necessary to make rapid improvement in aggregate stability.
We have only skimmed the surface of the PASA Soil Health Benchmark Study in this article. For more background on this citizen-science research project, including introductory webinars and case studies of several vegetable farms, go to the resources section of the PASA website (www.pasafarming.org). The PASA Soil Institute is also investigating soil health benchmarks for grain growers and dairy grazers, financial benchmarks for diversified vegetable farms and the footprints of grazing dairy farms versus confinement operations.
1 Days in living cover refers to planted vegetation, such as produce and cover crops, not weed growth or winterkilled residue.
2 The tillage index is based on the NRCS Soil Tillage Intensity Rating (STIR) database for different types of farm equipment. Needless to say, our unusual horsedrawn tools are not in the database, so a little creative substitution was necessary.
3 Organic inputs include compost, manure, mulch and other organic materials, but not organic fertilizers, rock minerals, and lime. Since we spread compost for Field 4 (approx. 5.5 tons per acre) in December 2017, this application did not get recorded in the 2018 organic inputs.
For more details on the Cornell soil health test results, recommendations and scoring methods, we highly recommend the Comprehensive Assessment of Soil Health manual. Hard copies can be ordered from Aaron Ristow, (607) 745-7165, firstname.lastname@example.org, or a PDF file downloaded here. The Cornell Soil Health Team’s website is http://soilhealth.cals.cornell.edu