Project: Nutrient Availability

Project manager:
Renee Hadley
Walla Walla County Conservation District
District Manager

325 N. 13th Ave.
Walla Walla, WA 99362
(509) 522-6340 ext. 110

Project Goal
To determine the amount of bio-available nutrient levels within a crop’s effective rooting zone as a measure of soil health and crop productivity. The working hypothesis is that nutrient analysis for bio-availability is an accurate indicator of crop health and a more accurate way to determine how much fertilizer to add and when.

Project Update Fall 2018
The Walla Walla County Conservation District (WWCCD), with the support of local area farmers, conducted soil sampling study using UNIBEST Ion-Exchange Resin Capsules (IERC’s) to measure soil nutrient bioavailability at six study sites. Study sites included three crops (winter wheat, seed alfalfa, and wine grapes) distributed across three rainfall zones representative of the county (20 to 30, 13 to 19, and less than 12 inches per year) (Figure 1). Sampling occurred monthly from October 2017 through June 2018. An exception was the Foothills location which was only sampled April through June 2018. Two (2) soil samples were collected at each site: in-field and at the edge of field buffer areas (Native soils). Soil samples were collected from 0 – 12 inches at each site and for each location (in-field and buffer area) to evaluate soil bioavailable nutrient levels between cultivated and “native” soils. The soil samples were composited at the UNIBEST Laboratory to create a uniform field-moist soil sample representing each sample location. Available nutrient analysis was conducted using UNIBEST IERC’s.
For the purpose of this study “native soils” are defined as no commercial cropping in the year of sampling. Historical uses of the “native” area of the property are unknown. Edge of field or “native” in this study is considered a zone rather than sharp line, especially if fertilizer is applied by broadcasting, spraying, or fertigation. The Edge of field or “native” sample area was as far from the cultivated field as possible to avoid any fertility additions that may have been applied to the cropping system. Existing cover and type of “native” vegetation can influence nutrient uptake from native soils. For most locations in this study, the “native” was defined by perennial and annual grass stands with fibrous rooting systems.

Native soils in the study region were found to cycle available Phosphorus and Potassium close to adequate levels for most of the study soils as it relates to growing a commercial agricultural crop. Nitrogen levels were low to responsive in the native soil areas for wheat and alfalfa. If the native soil areas were to produce a commercial crop they would require nitrogen additions as well as supplemental P & K in some locations for some specific crops.

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