The Microbial Life of Soils

Lecture 9/22: Soil Microbial Life

The Soil Food Web

Plants use water, sunlight and CO2 to make glucose (sugar, a simple carbohydrate) through photosynthesis.
Glucose (sugar) molecules can be linked together to form complex carbohydrates (also called polysachrides, or starch), which can be stored in roots of plants. 
Root hairs secrete a sugary, enzyme rich exudate as well as shedding cells while penetrating soil and . This 0.10 inch, sugary, gelatinous zone around root hairs is called the rhizosphere, and is great food and environment for microbes and is an area of high biological activity. The rhizosphere acts as a protective barrier against pathogens and it creates a digestive barrier (the enzymes released by the root hairs act chemically to make soil nutrients available to plants for absorption).

Bacteria and Fungi (decomposers) > Nematodes and Protozoans > Arthropods (spiders and insects: shredders of organic material)
N2 (in air) > NO2 (nitrites) > NO3 (nitrates) > NH4 (ammonia)

Bacteria fix N2 from air into NO2 in soil. Different bacteria change NO2 to NO3 in soil (NO3 doesn't leach as easily as NO2 from soil).
Bacteria produce a chemical slime that is high in nitrogen. As they die they stick together in this slime which builds soil structure and holds them to soil and each other.
Bacteria favor a high pH, annual crops, grasses, nitrogen layers of compost.
If water in soil dries up bacteria (and fungi) can survive until better conditions.
Two general types of bacteria (not including Archaea): Anaerobic:  cellular respiration is done in absence of oxygen. Yield sulfur (hydrogen sulfide) to soil - rotten egg smell. If your compost smells like this it needs to be aerated, carbon needs to be added, and/or it is too wet. E. coli (bacteria found in manure is anaerobic and not safe for consumption). Aerobic: Clean fresh good earth smell. Cellular respiration uses oxygen.

Fungi increase surface area of root from 700-1000x.
Fungal mycorrhizae process transition metals in soil.
Fungi form a symbiotic relationship with plants.
Two kinds of symbiotic fungi: 1) Endotrophic: penetrate roots to exchange nutrients (minerals for sugars) and 2) Ectotrophic: wrap around root.
Some fungi eat nematodes.
Fungi prefer acidic environment, carbon layers of compost.

50 different microscopic nematodes in a tsp. of compost.
Nematodes and Protozoans eat bacteria and fungi and produce NH4 (ammonia).
Ammonia can be volitalized (into air ie. as in hot compost too high in nitrogen will smell like ammonia)
Ammonia can be mineralized (bonded to soil mineral structure and retained as nutrient for plants; plants take in ammonia by CEC when NH4+ gives up one of it's H+s).
Amnonia can be immobilized (utilized by Bacteria and Fungi and thereby stored in soil for later release)
Nematodes (worms) eat roots and fungal hyphae, aerate and fertilize soil
Bact

Gophers, earthworms and moles aerate and fertilize soil, build soil structure, and create channels for movement of water through soil.

Read: "Garden Rules" from Teaming with Microbes (book).

Compost

Finished compost should be like a wet sponge with the water squeezed out (in terms of moisture content). It should squeeze together, but also crumble.
Hot Compost: 140 degrees F needed to kill weed seeds and pathogens. Turn three times 10-15 days apart outside to inside, until compost falls to 120 degrees F, then it's done. Pile should shrink quickly. We will build piles 5' x 5' x 5', which makes a bigger core than a pyramid shaped pile or mound. Layer with sticks or stalks (we used sunflower stalks) at bottom for aeration. Then green layer (nitrogen), then black layer (compost or manure for more nitrogen), then brown layer (for carbon). Heat is a result of metabolic process called combustion (energy released from chemical bonds breaking). We are building "static aerated piles" on the farm.
Cold Compost: Takes longer. Doesn't kill weed seeds so don't put them in. Red wigglers make a great cold compost.

Always cover compost piles (we will use "Compostex" on the farm. This prevents volitalization of NH4 as well as leaching, runoff and pollution of water by nitrites (NO2).