Soil: Beyond NPK

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Soil Beyond NPK

• Dr. Rhoda Burrows
• Horticulture Extension Specialist
• South Dakota State University

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Soil Its not just dirt!!
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Soil Organic Matter
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Without soil organisms, life as we know it would
not exist

• Soil organisms
• Recycle nutrients
• Form new soil
• Retain soil structure
• Degrade hazardous chemicals

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Peering into soil
S. Rose and E.T. Elliott
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Typical Numbers of Soil Organisms in Healthy
Ecosystems
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Soil organisms

• Earthworms
• Arthropods
• Protozoa

• Fungi
• Bacteria

A single spade-full of rich garden soil contains
more species than above-ground in the entire
Amazon rainforest.
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Earthworms

• Bury surface residues
• Soil aeration (burrows)
• Concentrate nutrients

• Speeds OM breakdown
• Aids nutrient cycling
• Helps soil structure

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Without earthworms With earthworms
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Arthropods

• More than 1,000 arthropod legs support every step
  you take on an agricultural soil
• - NRCS, USDA
• Examples Insects, spiders and mites, centipedes
  and millipedes, scorpions

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Arthropods

• Shred organic material
• Aerate and mix the soil
• Regulate populations of other soil organisms

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Arthropods

• Shredders - Chew up dead plant matter as they eat
  bacteria and fungi on its surface
• millipedes, sowbugs, termites, certain mites, and
  roaches.

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Arthropods

• Predators - eat earthworms, nematodes, other
  arthropods, etc.
• centipedes, spiders, ground-beetles, scorpions,
  pseudoscorpions, ants, and some mites

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Arthropods

• Herbivores - root-feeding insects
• Cicadas, mole-crickets, and root-maggots
• Fungal-feeders - scrape and consume bacteria and
  fungi off root surfaces.
• most springtails, silverfish, and some mites

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Soil Protozoa

• Single-celled animals
• Mostly near plant roots
• Eat mostly bacteria or organic matter
• Release nutrients in their waste

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Soil Fungi

• 1 tsp soil can contain 100s ft. hyphae
• most abundant in or near plant roots
• Play various roles in soil
• Mutualists
• Decomposers
• Pathogens
• Help aggregate soil particles

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Soil Fungi - Mutualists

• Mycorrhiza
• Most plant roots have these fungi
• Help increase plant uptake of immobile nutrients
  (P, Fe, Zn)
• Some decrease disease, drought damage

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• Some soil fungi help aggregate soil particles for
  better aeration
• Mycorrhizae shed a glue that cements particles
  together (humus)

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Soil Fungi - Decomposers

• Saprophytes
• Break down complex carbohydrates, lignins, and
  some chemicals that nothing else can break down

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Soil Fungi - Pathogens

• Root rots, damping off, wilt, etc.
• Some can also survive on dead matter
• Some have long-term survival structures
• Numbers can decrease over time due to lack of
  host, competition from other fungi, or attack by
  other fungi and bacteria

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Bacteria

• Millions in a pinch of soil
• most abundant in or near plant roots
• Play various roles in soil
• Mutualists (eg N-fixing)
• Decomposers
• Pathogens
• Help aggregate soil particles

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Bacterial Mutualists

• Rhizobia and other nitrogen-fixers
• Fix nitrogen for plant use
• Release nitrogen to soil when plants die
• Present in most soils

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Bacterial Decomposers

• Can break down cellulose and chitin
• Actinomycetes (fresh soil smell)
• some can fix nitrogen with non-legumes
• adapted to wide range of soil conditions

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Organisms involved in breakdown
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What influence do we have?

• Plant diversity
• Soil Organic Matter
• Pesticides
• Fertilizers
• Tillage

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Plant Diversity

• Rotate crops or Interplant
• Diversifies nutrient sources
• Increases diversity of microorganisms
• Increases OM content
• Decreases disease presence
• Improves soil tilth

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Organic Matter Addition

• Physically improves soils ability to retain
  water, air, and nutrients
• Improves environment for many beneficial
  microorganisms
• Diverse food source for microorganisms
• Cover crops provide habitat and food during
  non-cropping times

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Tillage

• Decreases mycorrhizal fungal numbers and
  diversity
• may decrease soil aggregation
• Aeration may increase bacterial populations
  temporarily

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Pesticides

• Tend to decrease microbial populations and
  diversity
• May decrease enzyme activity in soil
• Herbicides have negative effects as well as
  fungicides
• Long-term use more detrimental
• Can depend on soil type, moisture, etc.

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Fertilizers

• Chemical fertillizers tend to decrease microbial
  populations and diversity
• Probably due to simplicity of breakdown rather
  than direct antibiosis
• Organic fertilizers are more complex support a
  more complex soil community

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• To know the land for what it is, to find its
  heartbeat,
• to expose its soul,
• you have to go underground where it lives and
  breathes.- Amy Stewart