Identifying Soil Health Indicators for Central Washington Orchards

Soil health assessment has been recognized as a critical soil testing tool. But what does soil health mean in perennial orchards in the irrigated west? Our group set out to identify a set of soil health indicators that are useful to track in Central Washington orchards. Specifically, we were challenged to track which factors may be limiting to yield and fruit quality.

Soil health refers to the ability of soil to perform key ecosystem functions including sustaining plant growth, minimizing erosion, regulating and partitioning water flow, and buffering and filtering of potentially toxic materials. When it comes to our goal of crop productivity some of the functions that sustain plant growth are root health, water entry and movement, competition and protection from pathogens and provision of nutrients. The challenge is there are many, many measurements of soil functions. Soil health testing labs test for microbial respiration, water extractable carbon, organic nitrogen, wet aggregate stability, autoclave-citrate extractable protein, available water capacity and more. Scientists have more than twenty additional indicators they can measure. Our goal was to narrow down the list to identify which if any soil health indicators are most important in our cropping system and our region.

Useful soil health assessment tools should be regional, cropping system and management goal specific. To date the majority of soil health investigations have been in annual crops in the Midwest and East. Tillage and long fallow periods in annual crops make minimizing erosion a key driver for soil health in annual systems. In contrast grass alleyways and perennial trees have little erosion potential. In our growing region low rainfall and irrigation pose different challenges than in the rainfed East.

This study measured twenty-one soil health indicators in 101 Central Washington apple orchards. To determine the relationship between soil health indicators and fruit yield and quality we used 30 sets of matched sites with high and low productivity orchards of the same or similar variety, rootstock, age, and training system. We measured indicators of water availability: available water capacity (AWC), water infiltration, and % sand; indicators of root health: apple root health rating, bean root health rating, Pratylenchus spp. nematodes; indicators of soil structure: surface and subsurface penetration resistance (PR), bulk density (BD), and wet aggregate stability (WAS); chemical fertility factors: P, K, Mg, Ca, Fe, Mn, Zn, pH; microbially available fertility factors: autoclaved citrate-available protein (ACE), potentially mineralizable N (PMN); and OM and biological activity indicators: organic matter (OM), permanganate oxidizable active carbon (POXC), microarthropods, soil food web structure and enrichment indices (SI, EI), and respiration. Fruit yield and packout were determined using two-to-four year grower averages and fruit measurements from five representative trees per orchard.

The soil health indicators we measured had a wide range across Washington orchards surveyed but overall organic matter, available water capacity, and wet aggregate stability were lower, and % sand higher than soils measured in other Midwest, Mid-Atlantic and Northeastern studies. Water related factors (available water capacity and % sand) had a significant relationship with yield according to linear mixed model analysis and root health factors (Pratylenchus nematode and bean root health rating) had consistent but not significant relationships. A high percentage of sites with subsurface compaction and high organic nitrogen content suggest these factors are important to track Washington orchards.

Root health and available water were the most common limiting factors in the orchards we studied. Almost half of the soils sampled had coarse soil texture with an average of 66% sand. Available water capacity is a measure of the porosity of soil and indicates the amount of plant available water a soil can hold. Below 0.15 g/g available water is considered moderately to severely limited. Of sites surveyed 11% had available water capacity indicating moderate water limitation and 5% levels indicating severe water limitation. For example, consider matched Granny Smith on M9.337 rootstock blocks where the high productivity block yielded 64 bin/A and the low yielded 34 bin/A on average. Available water capacity was 19 g/g (56% sand) in the high yielding block compared to 15 g/g in the low yielding block (75% sand).

It is not surprising that root health was an important factor in Central Washington orchards. Plant pathogens Phytophthora and Pythium, Ilyonectria robusta, Rhizoctonia solani as well as the lesion nematode Pratylenchus penetrans are known to negatively impact growth and production in young apple trees. Root health ratings measured negative impacts in 29% of orchards surveyed according to bean root health ratings and 15% based on Pratylenchus nematode counts. The impacts of poor root health can be significant. For example, in two matched Gala on M9 rootstock orchards, the low yielding orchard with 33 bin/A had high lesion nematodes numbers (129 per 500 cc) well over the 80 per 500 cc threshold.

Soils with high bulk density and compaction limit root growth and root access to water and nutrients. Twenty-six of the surveyed orchards had high subsurface penetration resistance indicating compaction and limited rooting area. Five of the matched sets of orchards had higher compaction in low-yielding compared to high-yielding sites. For example, in Ultima gala on Nic.29 rootstock orchards planted the same year with the same training system the orchard yielding 15 bin/A less (55 bin/A vs 70 bin/A) had a deep compaction layer at the 18 inch-depth.

Many orchards surveyed had high organic N content. Including a measurement of organic nitrogen in the minimum dataset for soil health assessment in orchards could be critical to avoid nitrogen over applications. For example, the average PMN for sites measured was 21 µ g^-1 week^-1 which would supply 2.45 lb/A per week reducing N needs by 49 lbs/A over the 20 week season. Assuming an 80 bin/A yield goal and N recommendations of 70 lb/A per season the N needs may be only 21 lbs/A. Unfortunately, while extractable organic N fractions are generally positively correlated with mineralizable N, they often only partially explain the variation in mineralizable N and there is disagreement about which test provides more usable information.

Developing a minimum dataset of soil health indicators for Central Washington Orchards will be an iterative process. Our survey suggests that the minimum dataset of soil health indicators for Central Washington orchards should include measurements of water availability (AWC, % sand) and of root health (bean root health rating, Pratylenchus nematodes) as well as fertility indicators to meet stakeholder management goals. High levels of mineralizable N in some orchards indicate the need to include a measurement of organic N availability in the minimum data set. With more than 25% of surveyed orchards with high subsurface penetration resistance values, a measurement of compaction should be included. While OM and active carbon were not correlated with the stakeholder management goal of productivity, soil organic matter influences multiple soil functions including microbial activity, nutrient cycling, soil carbon accumulation and water relations, and as such should be included in the minimum dataset as indicators of environmental health.

We look forward to the next stage of the project working with soil health testing labs to provide testing opportunities to growers and refine the Washington Orchard Soil Health test. We are lucky to have twenty years of research from the Cornell Soil Health lab and others to inform our strategy in Washington. Collaboration with WSU soil scientist Deirdre Griffin, the Washington Soil Health Initiative, and the WSDA will allow us to contribute to the soil health assessment conversation and survey across commodities. Plant-soil health relationships are complicated. For a high level of confidence that we have the best set of indicators we will need more than a thousand robust orchard sites in the database. Collaboration with soil testing labs and you as growers will be key to success.

Contact

Tianna DuPont

Tree Fruit Extension Specialist

Washington State University

(509) 293-8758

[email protected]

For more information

DuPont, S.T., Kalcsits, L, Kogan, C. Soil Health Indicators for Central Washington Orchards. Soil Science Society of America. Submitted October 2020.

DuPont, S.T., Granatstein, D., Sallato, B. Soil Health in Orchards. Washington State University. EM120E

Sallato, B., DuPont, S.T., Granatstein, D. Tree Fruit Soil Fertility and Plant Nutrition in Cropping Orchards in Central Washington. Washington State University. 2020. EM119E.

DuPont, S.T. Soil Biota in Orchards. Washington State University. 2019. FS315E.