Soil Amendment with Green Manures and Seed Meals Improve Soil Health and Enhance Soil Microbial Activity in Potato and Douglas-fir.

Fusarium root rot in Douglas-fir (Pseudotsuga menziesii) and Verticillium wilt in potato (Solanum tuberosum) are two major threats in these respective crops. Soil fumigation with methyl bromide/chloropicrin in Douglas-fir and metam sodium in potato has been major control tactics. Due to safety and environmental concerns, these chemicals have been increasingly restricted. Organic soil amendments such as green manures (GM), composts, and brassicaceous crop residues are gaining interest for safer disease management via biofumigation and enhancement of soil and plant health. Field studies in Washington State evaluated the effects of various GM and Brassica seed meal amendments on beneficial soil organisms and activities, as well as soil borne pathogens, in two different cropping systems. Treatments in the Douglas-fir study included selected Brassica GM and seed meals and those in the potato study included both brassicaceous and non-brassicaceous GM with four replications in randomized completely block design. In the Douglas-fir study, soil density of Fusarium spp. was significantly lower in B. juncea green manure (BjGM) (1.77 log CFU g soil^-1) compared to chemical fumigation (2.43 log CFU g soil^-1) and untreated control (2.55 log CFU g soil^-1); whereas the soil density of potentially antagonist Trichoderma spp. was greatest in fumigated plots (3.7 log CFU g soil^-1) followed by BjGM (3.42 log CFU g soil^-1). Dehydrogenase enzyme, an indicator of soil microbial activity, tended to be highest with GM treatments and lowest in fumigated soil in both Douglas-fir (0.73 µg TPF g^-1 hr^-1 in BjGM vs. 0.46 µg TPF g^-1 hr^-1 in control) and potato studies (1.63 µg TPF g^-1 hr^-1 in barley GM vs. 0.94 µg TPF g^-1 hr^-1 in control and 0.99 µg TPF g^-1 hr^-1 in fumigated soil). Mineralizable nitrogen content followed similar trends as dehydrogenase. In the potato study, both brassicaceous and non-brassicaceous GMs improved several indicators of soil health compared to control. The B. juncea GM before potato significantly contributed to N uptake and potato yield. The incorporation of brassicaceous and non-brassicaeous GM seemed to be a promising practice for long-term soil management in potato cropping systems. These studies also demonstrated that incorporation of GM improves soil microbial activity, N uptake and crop productivity. Green manure application can be considered as a component of crop management for improving soil productivity and N availability.