Best Practices—Nurturing Soil
• Build healthy soil, don’t replace it
• Clay is great—why?
• Mulch all plants
• Mix in compost to vegetable beds each growing season
• (Optionally) Add organic fertilizer to vegetable beds each growing season,
but not to most ornamental beds
• Grow cover crops and then chop up and add to soil in vegetable beds
• Prevent soil compaction (one tip: grow in beds, not rows)
Soil and climate change
Much more carbon stored in soil than air, including but
not limited to wetlands, peat and permafrost
“Something’s destroying my soil aggregates!”
• Tillage—sheers apart aggregates. Short term: increases
large pores. Long term: decreases, and can lead to
“tillage pans.” Severs earthworm channels. Severs
• Walking (and other compaction)—Breaks down and
re-structures soil. Tip: use beds, not rows.
• Raindrops—Break down aggregates on surface, which
can then form crust. Very difficult for water to enter
crust, or seedlings to push up through crust. Also—can
lead to erosion.
But wait! Something’s helping!
• Root growth promotes aggregation! Roots feed
organisms. Roots wetting/drying causes (helpful)
cracks. Decaying roots form channels for water and air.
• Adding organic material helps aggregation!—or, why
to add compost. Critical to develop crumb and granular
structures. Helps form large pores.
• Mulching protects soil from wind erosion, rain
compaction. Helps retain water. Overall, helps
maintain good soil structure.
How to Mulch
• Remove weeds
• Spread 2-6” of mulch across soil surface
• Keep 6-12” away from base of shrubs and trees
• Keep mulch on top of soil to avoid nitrogen tie-up
Do not make Mulch Volcanoes around
Mulch helps retain soil moisture
Prolonged moisture around trunk
can lead to cankers and root rot
Mulch can interfere with respiration
of cambium, phloem- it limits
exchange of CO2 & O2 with air
Sustainable Practices: Soil
• “Grow” your soil, don’t replace it.
• Make compost at home.
• Use organic compost, mulch, fertilizer.
• Avoid pesticides and herbicides. Dig, hoe or smother weeds. Use hand labor when
• Grow cover crops, turn in green manure.
• Add only the nutrients needed.
• Don’t over-till.
• Prevent soil compaction by growing in beds, not rows.
Soil is dynamic, not static.
Sand particles visible to the eye. Microscope needed to see silt particles. Electron microscope needed to see clay particles. Think basketball: golf ball: head of a pin.
A soil particle has no pore space, and is typically just a (tiny) piece of rock.
Clay particles are the active portion in any soil, since chemical reactions take place on clay particles. Clay particles have negative charge. Silt and sand are not very chemically active, due to mineral composition, and large size of surface areas. Clay particles have 1,000 times more surfaces than sand particles. Clay is the storehouse of plant nutrients—due to chemical composition and huge number of surface areas. Chemical reactions between soil organisms and roots is what gets nutrients to the plants.
Soil composition—soils contain (generally) all three types of soil particles. Loam:
Silt is sedimentary matter consisting of particles sized in between clay and sand
Our soils are mainly the moderately fine, clay loam types.
Soil aggregates are defined by shape, size and stability. Soil structure is described in terms of soil aggregates. Roots need good soil structure – good collection of aggregates – to penetrate soil, and get air, water and nutrients.
Process of aggregation determined by: wetting and drying; freezing and thawing; microbial activity that helps decay organic matter; activity of roots and soil animals; and adsorbed cations.
For example, Wetting/drying, freezing/thawing and root & animal activity push particles back and forth to form aggregates. Decaying plant residues, and microbial exudations, coat soil particles, and bind into aggregates.
Think of it this way: Humus (organic matter) glues together soil particles to create aggregates. Within an aggregate, pores are small. Between aggregates, pores are large.
Ideal soil has equal amounts of large and small pores. Small-medium-large pores combine to form maze with wide, medium and narrow passageways.
Water passes through large pores and is held inside small pores.
Pores and aeration: oxygen from atmosphere, and carbon dioxide given off by roots AND microorganisms.
Cations: see MG handbook! Or other references! When a cation is bonded to two or more particles, helps form aggregates.
End of science lesson! Back to practical info! Well… one last note. Organic means it has Carbon (C). Could be manufactured and still organic (hence organic fertlizers in bags). Humus—soil organic matter—is not simply decomposed plant material, but contains carbon that has been used and reused over many generations by bacteria. It’s HARD and can persist for tens of years in the soil. Humus is more difficult for other plants or organisms to break down
What do you think of when you hear “cover crop?” Have you ever grown a cover crop? What was it?
Here’s a definition of cover crop (handout):
Prevents erosion by wind or water
Reduces weeds by shading
Improves soil structure by its roots pushing particles together
Moderates temperature and moisture by shading, also withdraws water from the soil. (Watch your veggie beds during this relatively dry winter. Beds may be wet, except around a healthy leafy stand of something.)
Provides organic matter, food for ecosystem, from decaying roots and from whole plant decay when life cycle is over.
Cover crops may be maintained and mowed repeatedly (bottom left slide). We mostly think of cover crops as something we grow and then chop into the soil to compost in place. This is what we call “green manure.”
See Kouric p. 273
Top left: crimson clover
Top right: vetch
Bottom left: planting between trees - grass - in orchard - also cover crop
Bottom right: soy beans coming up in a grass
Grasses are used as cover crops to improve the soil through their root systems. Living roots help push soil particles together to form larger aggregates. Use this to help break up your clay. Dying roots provide organic matter down deep.
This illustration is of grasses and other native prairie plants. Note the difference in the root systems with our usual turf grass (far left).
Grasses used in this way can quickly become difficult to dig out. A farmer who is going to plow in his cover crop has more options than a home gardener who will have to incorporate the crop with a shovel!
Favas are popular, but other legume crops actually fix as much or more nitrogen. Alfalfa, vetches, clovers, other beans, all fix nitrogen through the action of bacteria that live in association with the plants.
Ref: See Kouric p. 273
In this picture of an uprooted fava bean plant, you can see the root nodules in which the nitrogen-fixing bacteria live.
If you grow favas as a cover crop, cannot have your N and eat it too! As plants begin to bloom, nitrogen stores are moved through the plant to the flowers and then the seeds. By the time the seed pod forms, only 8% of the N remains in the leaves . . . 70% is in the seeds. Roots and nodules at this point have only 6% of the total nitrogen in the plant.
There are also free-wheeling soil bacteria that fix nitrogen without living in association with plant roots.
Here is a quote from a University of Florida website:
“Each year legume-Rhizobium symbiosis generates more useful N for plants than all the N-fertilizers produced industrially--and the symbiosis provides just the right amounts of N at the right time at virtually no cost for the farmer.”
If possible, show an uprooted fava plant, vetch, or other legume with nodules.
Chop down when plants are still green and succulent. They will have more N, and will decompose faster. Soil temperature and moisture, how much you chop them up, and how well you bury them in the soil will affect how long it takes for plants to decompose.
For legumes, cut before seed pods start to form, generally before 50% bloom.
Favas will tend to resprout if you don’t pull up the whole plant. Can get more total N and organic matter by allowing them to regrow, but keeps bed out of commission longer.
Grasses can re-root if you pull them out and leave them lying on the ground.
Allow time to decay before planting. Otherwise, soil bacteria will have so much carbon (in the form of all that cover crop material) to decompose that they will draw nitrogen from the soil, as well as from the cover crop itself. This means there will not be N in the soil available for your plants. Once the decomp is complete, the bacteria die, are eaten by the denitrifying bacteria, and the nitrogen in their bodies becomes available in the soil.
Before we go on to bed preparation, take a few moments to look over key ideas 3-5 related to soil amendments and fertilizers, to be sure they are clear. Could you explain them to a gardening friend not in this class? Ask someone near you to compare ideas, if you wish. I’ll take questions after 3-4 minutes.
Plants need 17 chemical elements for growth and development. Adding these elements to meet plant needs is called fertilization.
Fertilizing is different from amending the soil, which is adding something to improve soil texture or availability of nutrients that are already in the soil or being added to it. Compost is our amendment of choice, but it provides very low levels of plant nutrients.
Of the 17 elements needed by plants, 3-- carbon, oxygen, and hydrogen--are taken from air. Remaining 14 come from soil, absorbed by plant roots. All essential, but different amounts required.
NPK (nitrogen, phosphorus, and potassium) are the 3 primary nutrients, amounts needed are largest. (Blue are secondary nutrients, red are micronutrients.) Generally CA soils have sufficient P and K. Nitrogen in form available to plants is very water soluble and easily leached out. Must be replaced every time you plant.
Complete fertilizers, organic or chemical, contain these three nutrients: 10-10-10 refers to the percent by weight of these three nutrients in the fertilizer, in the order N-P-K. (For reference, if needed: N as N, P as P2O5, K as K2O.)
Also can get chemical or organic fertilizers that are essentially single nutrient fertilizers (show alfalfa--on tables--and K2SO4).
This is when high C/N ratio organic matter is added. Microorganisms compete with plants for nitrogen. Takeaway: don’t add high-carbon items to soil when you put in a new plant, especially those that have high nitrogen needs. Eg wood chips. Composting reduces C/N ratio.
Learn to recognize nutrient deficiencies in plants
Plants let us know what they need
Feed the soil VS feed the plant – typically determined again by pH