Date of Award


Degree Name

MS in Agriculture - Soil Science


Thomas J. Rice


Soil aggregates provide pore spaces of various sizes supplying water, gases and nutrients to plant roots and microorganisms, and facilitate moisture retention and availability. Soil aggregate stability is indicative of soil biological and structural health, and is increased by soil carbon derived from plant roots and the soil microbial biomass. Aggregate stability and soil carbon can be enhanced by increasing organic matter through compost additions or by planting cover crops. Additionally, aggregate stability is enhanced by arbuscular mycorrhizal fungi (AMF) that form a symbiotic association with plant roots, and consolidate soil particles into aggregates through hyphal networks and through the production of glomalin, binding soil particles together. The use of herbicides decreases soil carbon as it removes vegetation and the microorganisms associated with it. Soils having poor aggregate stability slake and disperse into primary soil particles becoming dense and subject to erosion when exposed to heavy rains. The use of herbicides amplifies this risk as it removes vegetation, leaving the soil bare, with nothing to absorb rainfall impact.

The effect of vineyard floor management practices on aggregate stability, soil carbon and grapevine fruit yield was studied at two vineyard sites located within the Estrella district of Paso Robles, CA. In late fall of 2008, treatments were applied to the vine line including an herbicide, an herbicide plus compost, a cover crop, a cover crop plus compost and a cover crop plus a fungal inoculant treatment. The control had no herbicides, compost, cover crop, or fungal inoculant applied to it. The cover crop treatments were applied at 30 lbs/ac consisting of a mixture of 20 % blando brome (Bromus hordeaceus), 20 % Zorro fescue (Vulpia myuros), 30 % crimson clover (Trifolium incarnatum) and 30 % subterranean clover (Trifolium subterraneum). Commercially available compost was applied at a rate of five tons per acre. The fungal inoculant was applied at eight pounds per acre, consisting of the AMF: Glomus intraradices, G. mosseae, G. aggregatum and G. etunicatum each at 32 propagules/ cc. Samples were collected in the spring of 2009 and the winter of 2010.

Percent carbon was significantly higher in the areas where compost applications were received (P=0.014). Cover crops did not significantly increase the level of carbon in the soil (P=0.253). Compost and cover crops significantly increased the amount of > 4.0 mm soil aggregates (P=0.004 for compost and P=0.027 for cover crops). Herbicide treatments significantly reduced the amount of > 4.0 mm aggregates (P=0.028). Fungal inoculants had no effect on > 4.0 mm aggregate stability (P=0.361). Compost significantly increased mean fruit weight (P=0.041). These results suggest that the addition of compost and cover crops are an effective way of increasing > 4.0 mm aggregates and compost alone is an effective way to increase soil carbon and fruit yield.

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