How Do Apple Tree Rootstocks and Cultivars Affect Soil Health in Southern Minnesota Orchards?

Materials and Methods

For this study, the two most widespread and commercially successful apple varieties (Honeycrisp and Zestar) in North America were selected from an apple orchard in Lake Crystal, MN (Figure 1). The study site soil was characterized as silt loam (sand: 16.3%; silt: 76.6%; and clay: 7.1%) with 3.1% total carbon (C) and 0.3% total nitrogen (N).

In fall 2021, soils were collected from four randomly selected apple trees in each cultivar with same‐sized rootstock and age (ranging from 5 to 20 years old), and thus, we sampled from 16 trees (two cultivars × two rootstocks × four replications). The two rootstock sizes were dwarf and semidwarf. At each sampling tree, a metal soil hand probe with a 1‐inch diameter was used to collect six random soil cores composited from the 0‐ to 6‐inch depth at the position where there is 2 ft radius from the base of the apple tree trunk (Figure 1). Composited field moist soil samples were stored in a cooler while transported to the laboratory for refrigeration at 39.2 °F until analyses. After passing through a 2‐mm sieve, some soils were kept at the field moist for soil biological analyses, and the remaining portion was air‐dried for soil physical and chemical analyses. Soils were analyzed for selected physical (e.g., bulk density, maximum water‐holding capacity, and slake score), chemical (e.g., total carbon, nitrogen, phosphorus, potassium, calcium, magnesium sulfur, iron, manganese, zinc, copper, and aluminum), and biological (e.g., soil respiration, and microbial biomass carbon and nitrogen) soil health indicators following standard procedures.

We used a two‐way analysis of variance (ANOVA) to determine the main and interactive effects of cultivar and rootstock on response variables (i.e., physical, chemical, and biological soil health indicators). Tukey’s honest significant difference (HSD) tests were performed to determine significant differences among treatment means. All statistical analyses were done at the 5% significance level (α = .05) with the Statistical Analysis Software (SAS) version 9.4 (SAS Institute Inc.).

Physical Soil Health Indicators

While bulk density is an important indicator of soil physical properties, such as soil compaction, porosity, and root penetration resistance, the maximum water‐holding capacity can be a proxy for soil water storage and is often related to the available water that the plant can use. The rootstock sizes, cultivars, and their interaction did not significantly change bulk density and maximum water‐holding capacity (Table 1), suggesting poor treatment sensitivity. In contrast, slake score appeared to be a sensitive measure of the soil’s physical health because both interactions and main effects of cultivars and rootstock sizes were significant.

Zestar, a semidwarf cultivar, had the highest aggregate stability among the four treatments (Figure 3). The slake test measured the aggregate stability through a simple experiment where three pea‐sized (0.08–0.59 inches) soil aggregates, a petri dish of water, and a smartphone ran the free SLAKES application to distinguish the treatment differences (Figure 2). Good soil aggregation in an apple orchard is essential in resisting soil erosion, protecting organic matter, and improving soil fertility and productivity. Apart from indicating management differences, slake test is one of the most inexpensive and accessible procedures that anyone with a smartphone can use.

Note. NS represents not significant.
*Significant at the .05 probability level.
**Significant at the .01 probability level.
***Significant at the .001 probability level.

Chemical Soil Health Indicators

It is crucial to maintain optimal soil fertility levels in our orchards. Based on soil testing, orchard farmers can decide how much fertilizer apple trees need during the growing season. The soils in Minnesota are rich in organic matter (∼5–6%), which not only acts like storage for macro‐ and micronutrients, but also supplies nutrients to the apple trees over time. Although rootstocks had significant main effects on inorganic N, Ca, Mg, S, and Mn, cultivars did not significantly affect them (Table 1). Cultivars significantly affected total K and available Fe. The two‐way interaction between cultivars and rootstocks was significant for total C, N, K, and available Al and highlights the importance of Al and macronutrients (C, N, and K) in both the sizes of rootstocks (dwarf and semidwarf) and cultivars (Honeycrisp and Zestar) when planning a commercial orchard. Apart from Al, Zestar semidwarf consistently had higher concentrations of three macronutrients and partially supported our hypothesis (Figure 3). Overall, no chemical soil health indicators were sensitive to both interactions and main effects. This suggests that soil chemical indicators need to be monitored to maintain high soil fertility levels for more apple production even though they fail to detect changes due to different‐sized rootstocks and cultivars.

Biological Soil Health Indicators

Microbial biomass, a biological soil health indicator, represents the living mass (mostly of bacteria and fungi) of soil organic matter (SOM) with only a tiny fraction (<6%) of the total soil C and N but is highly sensitive to management changes. The microbial biomass decomposes SOM and plant litter to release carbon dioxide (CO₂) gas (i.e., soil respiration) and plant‐available nutrients. Our study measured soil microbial activity and SOM decomposition level as soil respiration. Surprisingly, no biological soil health indicators were sensitive to two‐way interaction with cultivars and rootstocks, but only rootstocks significantly affected microbial respiration rates and microbial biomass C (Table 1). However, both Honeycrisp and Zestar semidwarf trees had 6.2 to 17.3% and 26 to 28% higher microbial respiration rates and microbial biomass C, respectively, than the dwarf trees (although non‐significant; Figure 3). The increased biomass and biological activity were likely due to larger trees with more root structure and biomass and nutrient addition through falling leaves and apple litter for more than 15 years. Additionally, semidwarf trees probably adapted well to the living mulches with the grass growing in the tree rows and the drive‐rows by developing deeper root structures to access water and nutrients below grass feeder roots and producing carbon‐based exudates, rhizodeposits, and antimicrobials (Atucha et al., 2011).

Conclusions

The rootstocks impacted the soils in this study the most as was indicated by significant effects on slake score; inorganic N; total Ca, Mg, S, and Mn; microbial respiration; and microbial biomass C. Cultivars had significant main effects only on slake score, total K, and available Fe. A two‐way interaction on slake score, total C, N, K, and Al highlights the importance of both the rootstock sizes and cultivars. Out of all soil health indicators, slake score showed the most promise as a sensitive soil physical health indicator. Thus, apple orchard farmers may want to consider using slake score, an inexpensive and accessible procedure that anyone with a smartphone can use to indicate any management practices’ ability to improve soil health.

The soils under Zestar semidwarf had significantly higher aggregate stability, macronutrients, and microbial biomass and activity than Honeycrisp semidwarf and thus partially supported our hypothesis. However, dwarf apple cultivars are easy to maintain and have a higher yield efficiency in relation to their size than semidwarf (Barritt et al., 1997). The Minnesota weather dictates that the dwarf varieties must be propped up by a wired system that hangs horizontally over the orchard, known as a trellis. Thus, they can grow upwards and not break due to the wind or other inclement weather. Further research is needed on apple rootstock–cultivar interactions in different locations/farms in Minnesota with more treatments and replications to better understand soil health discrepancies and establish more robust correlations among the tested soil health indicators and yield.