Lithology is a principle state factor of soil formation, interacting with climate, organisms, topography and time to define pedogenesis. A lithosequence of extrusive igneous lithologies (rhyolite obsidian, dacite, andesite and basalt) was identified in the Clear Lake Volcanic Field in the Coast Range of northern California to determine the effects of lithology on pedogenesis, clay mineralogy and soil physiochemical properties. Based on regional landscape erosion rates (0.2–0.5 mm yr− 1), the soil residence times for the investigated pedons (~ 150 cm deep) were of the order of 3000 to 7500 years indicating that the soils developed under the relatively stable Holocene mesic/xeric climate regime. Soils from all lithologies developed to a similar Xeralf taxonomy with remarkably consistent physiochemical properties. Although total (Fet) and dithionite-citrate extractable (Fed) iron concentrations diverged across lithologies, the degree of weathering as assessed by the Fed/Fet ratio was similar across the lithosequence. In spite of large differences in silica content of the parent materials, the clay mineralogical assemblage of all lithologies was dominated by kaolin minerals (kaolinite and/or halloysite). All pedons displayed an increase in halloysite and the degree of halloysite hydration with increasing depth, except the basalt pedon, which was dominated by kaolinite with only trace halloysite. We attribute this lack of halloysite in the basalt pedon to the lower silica activities associated with this silica-poor lithology. There was a lack of nanocrystalline minerals across all lithologies as inferred from selective dissolution. The dominance of crystalline materials is a function of the xeric soil moisture regime whereby summer soil profile desiccation promotes dehydration and crystallization of metastable nanocrystalline precursors. Further, the pronounced summer dry period results in dehydration of halloysite (1.0 nm) to halloysite (0.7 nm; also referred to as meta halloysite in some literature), together with transformation to kaolinite, in the upper soil profile. In spite of the relatively young soil residence times of these soils (Holocene age), the effects of lithology persisted only in differences in Fe oxide concentrations (Fed), as well as a lack of significant halloysite in basalt pedons. The overwhelming effect of climate in these highly weatherable parent materials narrowed the trajectory of pedogenesis, resulting in soils from contrasting lithologies converging on kaolin mineralogy, a lack of nanocrystalline constituents, and similar soil physiochemical properties.

Phosphorus dynamics and management strategies were investigated in a strongly weathered rhyolitic soil in the northern Coast Ranges of California. Recent conversion of upland, native oak (Quercus spp.) woodland ecosystems to winegrape production has led to a P deficiency in these soils. We investigated the growing season behavior of Hedley phosphorus fractions, the P sorption index (PSI), microbial biomass C (MBC), and dissolved organic C (DOC) in response to a single P application from three sources: composted steer manure (CSM), triple superphosphate (TSP), and mixed compost and triple superphosphate (MIX). Phosphorous sources, equal in P application rate (39 kg ha-1), were applied in the winter of 2012, and soils were sampled in spring, summer, and fall. Labile inorganic P (Pi) (resin Pi + NaHCO3 Pi), Fe/Al-Pi (NaOH Pi), and total P were significantly (p < 0.01) increased by P application, regardless of source. Calcium-Pi (HCl Pi), DOC, MBC, and pH were increased, and PSI was reduced, by compost addition. Net extractable Fe/Al-Pi increased during the study period. Moderately labile organic P (Po) (NaOH Po) declined, independent of fertilization, reflecting background soil P cycling. Calcium-Pi totals were low (mean 9 mg kg-1 for untreated plots), and other indicators of chemical weathering were high, suggesting an advanced stage of pedogenesis with respect to Walker-Syers P availability. In spite of advanced pedogenesis, labile Pi increased significantly from fertilization and remained elevated throughout the growing season. Results of this study inform P management in highly weathered, P-fixing soils experiencing P deficiencies.

A two-year experiment investigated the effects of compost application rate on soil chemical properties, vine nutrient status, vine performance, and grape juice characteristics in a degraded vineyard soil in northern California. The intent of the research was to identify vineyard management strategies to improve soil fertility and to identify optimal compost application rates. We applied composted steer manure at three rates (11.2, 22.4, and 33.6 t/ha) in a randomized complete block design before the 2012 growing season. Pruning and berry weight increased over the control at the highest application rate in both years, while vine yield significantly increased over the control in year two. Polynomial orthogonal contrasts suggest that pruning weight, vine yield, and berry weight increased linearly with increasing compost application rate in 2012, and that vine yield and berry weight increased linearly and quadratically with compost application rate in 2013. Measured soil properties increased from compost application, including nitrogen (N), carbon, pH, exchangeable potassium (K), manganese, and calcium and available phosphorus (P, Olsen-P), while phosphorus fixation decreased. Vine petiole nutrients (N, P, and K) significantly increased from compost application in both years. Juice characteristics (pH, total soluble solids, and titratable acidity) were unaffected by compost application. Similarly, vine balance was unaffected by compost application. All vine metrics improved at the highest application rate, and soil chemical properties increased with the two highest application rates. Therefore, significant benefits to soil fertility and vine performance can be achieved for at least two years in degraded vineyard soils following a single dose of compost at higher application rates (22.4 and 33.6 t/ha) without compromising juice characteristics or vine balance.

The fate of phosphorus (P) during pedogenesis has been historically conceptualized (Walker-Syers model) with time as the primary controlling state factor. Herein, we demonstrate that both climate and lithology exert a strong and interacting influence on the fate of P by examining coupled bioclimatic and parent material effects on soil P fractions. Three transects were investigated spanning a 2150-m elevation gradient (MAT = 17 → 3 °C/MAP = 330 → 1400 mm) across three separate bedrock lithologies (lithosequence: granite, andesite and basalt) within the Sierra Nevada and southern Cascades of California. The elevation gradient entails four bioclimatic zones (bioclimosequence: blue oak, ponderosa pine, white fir and red fir). Soil P fractions were determined by sequential fractionation and interpreted in the context of associated soil characterization data. The bioclimatic sequences demonstrate a weathering gradient with maximum intensity at mid-elevation sites, and corresponding changes in Fe/Al-(hydr)oxide content and aluminosilicate crystallinity. Phosphorus content of parent material varied by an order of magnitude (mean; mg P kg−1): andesite (1500) > basalt (1000) > granite (131). Differences in P content of parent material influenced Pt in soils. However, amounts and proportions of P in fractions were influenced by subtle to significant interactions between climate and lithology, owing to differences in chemical weathering and the abundance and crystallinity of Fe/Al-(hydr)oxides and aluminosilicates. This interactive effect of pedogenesis on clay mineralogy led to differences in P fractions dependent upon lithology and bioclimatic zone. Labile inorganic P (Pi) was uniformly higher in soils derived from granite, despite granite having significantly lower P content, a result of lower Fe/Al-(hydr)oxide generation in granitic soils. With descending elevation and increased weathering intensity, HCl-Pi (primary mineral bound P-apatite) declined in basalt and andesite but remained unchanged in granite owing to its greater resistance to chemical weathering. As weathering intensity increased, occluded P increased in basalt, decreased in andesite and was unchanged in granite, contradicting the paradigm of progressive P occlusion with increased weathering. This incongruity for andesite results from a dominance of poorly crystalline materials (e.g., ferrihydrite, allophane/imogolite) at less weathered sites versus more crystalline minerals at more weathered sites. This study highlights several caveats to the paradigm that time (i.e., degree of weathering) is universally the dominant pedogenic control of P fractionation. We identify the importance of interactions between lithology and climate in regulating the amount and types of weathering products that in turn control P fractionation and ecosystem P availability.