An available source of phosphorus for organic orchard soils?
A view of the “under-sod” condition, right after applying fertilizer and replacing the removed sod.
Summary
A major challenge in revitalizing fruit trees in decline is stimulating root growth. Research has shown that although phosphorus (P) is a key nutrient in this process, it must be biologically available. Even though soil P levels may be adequate by analysis, if the P is chemically bound it may not be available for root development.
In summer 2021, in collaboration with Bill McPhee (Plant Pathologist and Tree Fruit Consultant), we carried out a small-scale, controlled trial to test whether a new (to us) organic fertilizer (Rebuilder Organic 0-17-0-12) would be effective at stimulating the growth of feeder roots in high-density apple trees within a single growing season. Crucially, this product is formulated to release P in an available form. While our sample size was small and our results must be interpreted as preliminary, there did appear to be a larger mid-season growth response in root systems treated with Rebuilder Organic 0-17-0-12 as compared with untreated controls. The growth response was slightly smaller but had a quicker onset when Rebuilder Organic 0-17-0-12 was applied to the soil as a top-dressing, compared to when it was incorporated 6 inches into the soil at half the rate. A possible reason for this unexpected result is discussed below.
The problem
In the early spring of 2021, our orchard experienced drought. Trees did not take up enough water early in the season and became stressed, to the point where some trees suddenly began manifesting visible signs of nutrient deficiency. These signs were most pronounced in one high-density pink lady block (super spindle, M9 rootstock) that grows on a steep, east-facing hill. Trees in this zone showed reduced vigour, a reddish tinge to their bark, and yellow-green leaves, symptoms indicative of nitrogen and (perhaps) sulfur deficiency. Along with phosphorus, low levels of these nutrients in our soil were also confirmed by laboratory tests.
Bill McPhee, a Plant Pathologist and Tree Fruit Consultant, was invited to our property to take a look. He swiftly discovered that our stressed-looking trees differed substantially from our healthy-looking trees in the degree of new growth in their root systems. In particular, he found that the root systems of stressed trees had very few feeder roots, which are fundamental for absorbing nutrients and water from the soil. In order to help these trees in the immediate term, Bill recommended finding a way to stimulate their root systems.
Phosphorus in the soil is known to play an especially important role in stimulating growth in apple root systems (Slykhuis and Li 1985). Crucially, however, not just any phosphorus in the soil will do - it must be in a form that plants can take up. For conventional orchards, synthetic fertilizers exist which can supply a quick dose of available phosphorus to tree roots (e.g. Monoammonium phosphate, a.k.a. MAP). However, since synthetic fertilizers are not allowed in organic production systems, we needed a different solution.
Luckily, Bill knew of a phosphorus-containing product that is OMRI-certified, hence usable in an organic system. The product, called Rebuilder Organic 0-17-0-12, is a granular fertilizer composed of rock phosphate (P205), sulfur, activated compost, and various micro-nutrients. Whereas Bill has seen conventional products like MAP stimulate root growth within as little as a matter of weeks after application, we wondered whether Rebuilder Organic 0-17-0-12 would succeed at stimulating feeder roots in our stressed trees within a similarly short period. This product had not, to our knowledge, been experimentally tested in tree fruit orchards previously.
Stressed trees show reduced vigour, reddish bark, and yellow-green leaves.
The experiment
In total, 12 panels of high density pink lady trees (each panel containing 12 trees) were designated as our “experimental zone”. All 12 panels were located on the east-facing hill described above, and consisted of trees which were manifesting the most severe complex of symptoms already described. The 12 panels were each assigned to one the following three conditions:
In 4 panels, Rebuilder Organic 0-17-0-12 was incorporated into the soil @6 inches deep, at a rate of 13 lbs/panel (“under-sod condition”).
In 4 panels, Rebuilder Organic 0-17-0-12 was applied to the top of the soil, at a rate of 26 lbs/panel (“over-sod condition”).
The remaining 4 panels were designated as controls and received no treatment (“control condition”).
The under-sod condition was accomplished by removing the top 6 inches of sod (including vegetation), applying Rebuilder Organic 0-17-0-12 evenly over the exposed soil layer, and then replacing the removed sod. This process was very labour intensive and would not be practical at the scale of a commercial orchard; we carried it out, nevertheless, to see whether direct contact with the roots was necessary. In the over-sod condition, a higher rate was used in order to increase the chances we would see any response.
To control for some of the variability on our hill, we attempted to counterbalance the 12 panels across the three conditions in terms of (1) how severe the visible tree symptoms were in each panel, as well as (2) with respect to each panel’s physical location on the hill.
Devon and Katie applied Rebuilder 0-17-0-12 on June 18, 2021. Bill then visited the experimental zone three times over the summer/autumn to assess growth within the trees’ root systems. Measurements were made using an assessment scale developed by Bill, where the presence of new feeder roots in a root sample is assigned a value between 0 and 10. Measurements were taken at multiple locations within each panel and on both sides of the tree row (north and south).
Results
Given the small sample size, high variability of the site, and preliminary nature of this trial, we have not carried out a statistical analysis on our measurement data. We report our results simply as averaged values in order to avoid either overstating or understating the strength of our conclusions.
A summary of the average feeder root scores for each assessment date is provided below.
At the time of the first assessment, around July 2, 2021, feeder root growth in the over-sod condition was more pronounced than in the under-sod condition. The growth response in both treatment conditions was larger than in the control condition.
At the time of the second assessment, around August 3, 2021, feeder root growth in the under-sod condition had overtaken growth within the under-sod condition. Growth in both treatment conditions continued to exceed growth in the control condition.
At the time of the third assessment, around September 24, 2021, the under-sod condition continued to show the most overall feeder root growth. By this time, feeder root development in the over-sod and control conditions had evened out.
Discussion
It is normal for the root systems of apple trees to undergo a flush of growth in the autumn, so we were not surprised to see feeder root growth in the control treatment eventually catch up to growth in the treatment conditions. The point of the experiment was to see whether we could stimulate feeder root growth quickly, since our interest was in helping trees that were stressed early in the season obtain as many nutrients as possible within the same growing season. Thus, despite our small sample size and the preliminary nature of our results, we conclude that Rebuilder Organic 0-17-0-12 does appear to stimulate feeder root development in apple trees within a short period of time. We hope to see further investigations of Rebuilder Organic 0-17-0-12 (or equivalent products) in organic tree fruit orchards, both in order to assess its value in treating stressed trees and in order to assess its potential for use during tree replant, when establishing strong and healthy root systems is especially critical.
We were somewhat surprised that the over-sod condition initially showed a faster growth response than the under-sod condition, since we had expected the sod layer to act as a barrier to fertilizer penetration. This initial faster response likely occurred because the process of digging up the sod damaged the roots, resulting in a delayed response to Rebuilder 0-17-0-12 while the roots recovered. In any case, we are relieved that it does not seem to be necessary to incorporate Rebuilder 0-17-0-12 into the soil to get results, since the process of doing so would be impractical at any commercial scale.
Kaleidoscope Fruit Ranch extends its gratitude to Bill McPhee and Tamara Richardson for collaboration and consultation on this project.
Post written by Katie Sardinha and Bill McPhee
References
Slykhuis, John T. & Thomas S. C. Li. 1985. Responses of apple seedlings to biocides and phosphate fertilizers in orchard soils in British Columbia. Canadian Journal of Plant Pathology 7. 294-301.
