Soil quality does not predict plant nutrition available to white-tailed deer

Morphometric variation of white-tailed deer (Odocoileus virginianus; hereafter, deer) is caused by multiple environmental factors, including temperature, precipitation, soil quality, and forage availability. The effects of temperature and precipitation on plant growth are well documented, but it is unclear how soil mineral concentration affects deer morphometrics. Although it is recognized that deer have larger morphology and greater productivity in areas of the United States with mineral-rich soils, the question remains as to whether this trend is driven by increased food availability in areas with mineral-rich soils because of land use (i.e., agriculture) or if nutrient concentration of plants is greater in soils with greater mineral concentration. We collected plant tissue from 40 species commonly selected as forage by deer at 36 sites in 16 states across the eastern United States, representing a wide range of soil quality conditions over a broad geographical area. We categorized plant sampling locations into 5 soil quality classes based on a soil productivity index and analyzed plant nutrients to assess if soil productivity index classifications could be used to predict deer forage quality. We then made direct soil-plant comparisons by pairing each plant sample with a soil sample collected around the base of the plant to determine if soil nutrient concentration explained variation in plant nutrient concentration. We tested soil samples for phosphorus, potassium, and calcium. We analyzed plant samples for crude protein, phosphorus, potassium, and calcium for the soil productivity classification comparisons, and we analyzed phosphorus, potassium, and calcium for the direct site-specific analyses, as these macronutrients are critical to deer body mass, productivity, and antler growth. We analyzed forages by plant type (i.e., forb, semi-woody, shrub, or tree) and plant tissue age (i.e., young or older tissue). Nutrient concentration varied by plant type and tissue age. Plant type explained the majority of variation in plant nutrients, with forbs containing greater concentrations of all nutrients than the other plant types. Young plant material contained more crude protein and phosphorus, whereas older plant material contained more calcium. The soil productivity index was a poor predictor of deer forage quality. Calcium was the only mineral that differed by soil productivity class, but calcium was not limiting for deer at any site, as plant calcium levels exceeded maximum requirements for deer across all soil productivity classes. Site-specific soil phosphorus, potassium, and calcium explained little variation in plant mineral concentration (partial R² = 0.00). Our results provide evidence that naturally occurring plants, especially forbs, can provide nutrition sufficient for maximum deer body weight, reproduction, and antler size, within a given area, across the eastern United States regardless of the dominant soil type. As such, the availability of naturally occurring native forbs may be a primary limiting factor regulating age-specific deer body mass and antler size, not limitations in plant nutrients resulting from soil mineral concentration. Land managers should be encouraged to influence plant composition toward greater forb coverage where increased forage quality for deer is desired, even in areas with soils low in mineral concentration.