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Seasonal fate of phosphorus in vineyard soils


Growing premium wine grapes requires the right soil and climate. An ideal combination occurs in parts of California where the majority of the wine grapes in the United States are produced. As an agricultural system, wine grapes are a specialty crop, and there is limited research on the soils and nutrient dynamics that occur in vineyards.

“There’s a huge need for a better understanding of nutrient cycling and soil fertility in vineyards,” says Stewart Wilson, lead author of a recent Soil Science Society of America Journal article titled, “Seasonal Phosphorus Dynamics in a Volcanic Soil of Northern California.” According to Wilson, a Ph.D. candidate at the University of California–Davis (UC-Davis), research presented in the article was initiated when growers in Lake County, CA, approached scientists at UC-Davis about fertilizer use in vineyards.

The study site is located on volcanic soils, which are highly weathered. “Volcanic and weathered soils are largely thought to be P fixing,” Wilson says. This is due to high-surface-area minerals and iron (Fe) and aluminum (Al) compounds that readily bind phosphate. To compensate for this, growers often fertilizer with P at levels that are well above plant requirements.

Over-applying fertilizer could contribute to P runoff and pollution of local watersheds. For growers in Lake County, which has the second largest freshwater lake in the state, avoiding contributing to P runoff is important. Using organic materials as part of a fertilization regime is one way that wine growers may be able to reduce P inputs and subsequent runoff, but there is limited research on the topic.

In this study, the researchers tested three different fertilizer applications. Fertilizer treatments were applied in January 2012 and included: triple superphosphate (TSP), composted steer manure, and a mix of the two. The applied phosphorus levels were equivalent across the three treatments (39 kg/ha). The researchers also established control plots to monitor background P levels in unamended vineyard soils.

To evaluate the seasonal fluctuations of P, soil samples were taken in April (bud break), August (verasion, or grape ripening), and November (postharvest). The researchers measured P sorption and fractionated P. Fractionated P data were used to gauge phosphate derived from both fertilizer inputs and pedogenesis. This provides a better idea of how plant-available P changes throughout the season and could be used to improve timing and rate of fertilizer application. They also measured factors known to influence P dynamics, including microbial biomass carbon (MBC), dissolved organic carbon (DOC), and pH.

Wilson discussed the two main findings from this work. The first is the effectiveness of compost fertilizer, and the second is how soil parent material can influence P dynamics.

All fertilizer treatments increased labile inorganic phosphorus (Pi), Fe/Al-Pi, and total P. The authors report that for both chemical and compost fertilizers, there was high Pi availability. But when looking at additional soil characteristics, Wilson says, “Compost performs as well or better than chemical fertilizer, especially in these types of soil. [Compost] increased DOC, MBC, pH, and reduced P sorption compared with chemical treatments alone.” The authors attribute this to the addition of organic carbon in the compost, which impacts the biogeochemical cycle and improves overall soil quality, unlike chemical-based fertilizers.

Showing the effectiveness of compost as a vineyard fertilizer supports some of Wilson’s other research.

“In another companion study in the region, in a much more strongly P-fixing soil, compost improved vine yield, berry weight, and pruning weights and increased soil C, N, and P, improving the performance of severely under-performing vines where chemical fertilizers alone had failed.”

The findings about parent material were contradictory to the initial expectations of the authors. Soil at the site was weathered, and “it was known to be P deficient,” Wilson says. The assumption was that added P would get fixed, but “because this parent material was derived from rhyolite, which is a very low iron parent material . . . it responded well to the addition of phosphorus.” This resulted in high amounts of the applied P fertilizer being available to plants and suggests that initial parent material composition has a more significant influence on P dynamics than previously thought.

Understanding these P dynamics in vineyard soils will enable vineyards to adjust fertilization for optimal grape production while limiting P runoff. And, given the limited research on nutrient cycling in vineyards, Wilson says, “I think [this] research would apply in the Pacific Northwest where you have a lot of volcanics and in other regions in California and Oregon with weathered soils in volcanic and meta-volcanic parent materials” as well as international vineyards that have similar soil types.

This article appears in the February issue of CSA News