BASICS OF SOIL FERTILITY FOR URBAN (AND NON-

URBAN) GROWERSAnne PfeifferUW-Madison

Thanks to Erin Silva for some slide content

PLANTS NEED…

OVERVIEW• A Quick Intro to Soils• Finding your Soil Type• How Soils Feed Plants• What It All Means: Growing Healthy

Plants• Fertilizer Calculation Example

BY THE TIME YOU LEAVE…• Have a clear understanding of the essential soil

considerations for locating a garden or farm• Know how, when and why to conduct a soil test

and how to interpret the results• Gain a basic understanding about how to make

soil and fertility management decisions.• Access resources for non-biased,

straightforward information about soil management

A QUICK OVERVIEW OF SOILS

SOILS: MORE THAN “JUST DIRT”

• Hold up plants• Provide air and water to plants• Supply nutrients –Plants need sufficient quantity but not too much

• Provide habitat for soil organisms

CHARACTERISTICS OF AN IDEAL SOIL

• What are characteristics of ideal soils?–Fertile–Deep–Well drained/aerated–High in organic matter–Friable• soil is easily worked

WHAT IS SOIL?2-5% Organic Matter

50% Pore Space50% Solid Matter

MINERAL AND ORGANIC COMPONENTS

• Broken down rock particles –Depends on parent material–Clay particles hold nutrients

• Organic matter (containing carbon)–Decomposed plant and animal matter– Ideal soil is about 5% organic matter–Complex molecular structure plays special role:• Source of food for soil microorganisms• Source of nutrients for plants• Holds minerals against loss due to leaching

– May or may not be plant available

PORE SPACE• Air (~25% of total soil volume)–Oxygen supports soil life• Roots•Microbes

–Air can be displaced by water• Water (~25% of total soil volume)–Carries nutrients to plants

SOIL PROFILE• Topsoil (A horizon)

– It is the darkest layer of the soil because it contains organic matter and humus.

– Contains majority of feeding roots of plants

• Subsoil– Low in nutrients– Contains few microorganisms– Structure of subsoil is

important for drainage• Often disturbed in urban

settings

EXAMPLE OF A SOIL PROFILE• Note the dark

organic horizon at the surface and changes in color and structure down the profile

SOIL TEXTURE• Determined by mineral component• Coarseness or fineness of soil• Why is texture important?–The texture will determine how much air is in soil and how well water flows through the soil–Determines how easily the soil is worked and under what conditions

SOIL TEXTURE: CLAY• Soil particles are less

than 0.002 mm in size

• Does not drain easily and is difficult to work

• Root growth is poor due to small spaces between soil particles

SOIL TEXTURE: SILT• Particle size is

between 0.002 to 0.05 mm

• Feels silky to the touch

SOIL TEXTURE: SAND• Sand particles range

in size from 0.2 mm for the very finest sand to 2 mm for the coarsest

• Feels gritty if rubbed between your fingers

• Warms up and dries early in spring

• Low in nutrients

SOILS ARE A MIX OF SAND, SILT AND CLAY

SOIL TYPES• Most soils are a mixture of different soil textures–Often a soil type will be dominated by a particular texture

• Can group soil types by how well drain and major texture class they contain–Heavy soils• Contain a high proportion of clay

–Light soils• Contain a high proportion of sand

• Important to know the soil type because it will determine the management practices you need to use

FINDING YOUR SOIL TYPE

http://websoilsurvey.nrcs.usda.gov/

SOILS AND PLANT NUTRITION

ESSENTIAL ELEMENTS FOR PLANT NUTRITION

• Must be required for the completion of the life cycle of the plant.

• Must not be replaceable by another element. • Must be directly involved in plant metabolism, that is, it

must be required for a specific physiological function. • The element must be required by a substantial number of

plant species, not just a single species or two.

16 ESSENTIAL ELEMENTS• C, H, and O are not considered minerals• Macronutrients:–Primary macronutrients: N, P, and K• Needed in relatively large amounts

–Secondary macronutrients: Ca, Mg, S• May be supplied in smaller quantities

• Micronutrients: Cl, Fe, B, Mn, Zn, Cu, Mo–Required in small amounts but still essential–Deficiencies lead to severe depression in growth, yield,

and quality• If any one is missing or low, plant productivity is

limited

ELEMENTS REQUIRED IN PLANT NUTRITION

SOIL NUTRIENTS• Ability of a soil to hold nutrients and release

them to plants depends on–Presence and availability of nutrients–Soil texture–Organic matter

• Clay size particles and organic matter both hold nutrients but make them available to different degrees

• …HOW?

SOIL ACTS AS A MAGNET• Attracts and retains positively

charged ions in soil solution• Prevents them from moving

downward with water (leaching)

AN ION?

IONS ATTRACTED TO SOIL PARTICLE

Brady & Weil, 2004. Elements of the Nature and Properties of Soils

CATION EXCHANGE CAPACITY (CEC)

• The ability of clay, organic matter, and humus to attract positive ions–Clay, organic matter, and humus all have negative

charges on their surfaces–Most nutrients (P, K, Ca, etc) are cations or have positive

charges– “Opposites attract”

• The higher the cation exchange capacity, the more closely the nutrients are held and the less likely they are to be lost to leaching

• (soils also have anion exchange and it works the same way)

HOW DOES CEC DIFFER IN SOILS?

• In general…– Sand- low CEC– Clay – high CEC

• Soil with low CEC– generally has low clay and organic matter, low water holding capacity, requires more frequent fertilizer and lime amendments, and is prone to leaching

• Soils with CEC greater than 20 may have high clay content, moderate to high organic matter content, high water holding capacity, less frequent need for lime and fertilizers (except N), and low leaching potential for cationic nutrients.

• On the other hand, their physical properties may make it difficult for a farmer to cultivate, irrigate or maintain good aeration

GROWING HEALTHY PLANTS

SOIL TESTING• Soil tests can assess nutrient levels• Available in most states for a

minimal fee

http://uwlab.soils.wisc.edu/ OR, search “WI soil testing”

NUTRIENT REQUIREMENTS• The correct nutrient balance …–Depends on soil type and texture–Varies by plant–Is affected by climate

PLANTS HAVE DIFFERENT NEEDS

Approximate nutrient removal (lb/unit of yield) in the harvested portion of several Michigan vegetable crops. 11

ton=20 cwtSource: Fertilizer Recommendations for Vegetable Crops in Michigan (Warneke et al. 1992)

Crop N P2O5 K2O N P2O5 K2O

-lb/ct1- -lb/ton1-Asparagus 0.67 0.20 0.50 13 4.0 10Beans, snap 1.2 0.12 0.55 24 2.4 11Broccoli 0.20 0.05 0.55 4.0 1.0 11Cabbage 0.35 0.08 0.35 7.0 1.6 7.0Carrots 0.17 0.09 0.34 3.4 1.8 6.8Cauliflower 0.33 0.13 0.33 6.6 2.6 6.6Celery 0.25 0.10 0.80 5.0 2.0 16Cucumbers 0.10 0.06 0.18 2.0 1.2 3.6Lettuce 0.24 0.10 0.45 4.8 2.0 9.0Muskmelon 0.42 0.10 0.55 8.4 2.0 11Onions 0.25 0.13 0.24 5.0 2.6 4.8Peas, shelled 1.0 0.23 0.50 20 4.6 10Peppers 0.20 0.07 0.28 4.0 1.4 5.6Pumpkins 0.20 0.06 0.34 4.0 1.2 6.8Sweet 0.42 0.14 0.28 8.4 2.8 5.6Squash 0.18 0.08 0.33 3.6 1.6 6.6Tomatoes 0.20 0.04 0.35 4.0 0.8 7.0

FERTILIZERS ACT DIFFERENTLY

  Guide to the Mineral Nutrient Value of Organic Materials Percent (%)Materials: Nitrogen

(%N)Phosphorus(%P2O5)

Potassium(%K2O)

Relative Nutrient Availability

Bone Meal(raw) 2 to 6 15 to 27 0 SlowBone Meal(steamed) 0.7 to 4 10 to 34 0 Slow Med.Cocoa Shell Meal 2.5 1 2.5 SlowCompost(not fortified) 1.5 to 3.5 0.5 to 1 1 to 2 SlowCotton Seed Meal(dry) 6 2.5 1.7 Slow Med.Dried Blood(dry) 12 1.5 0.57 Med. RapidFish Meal(dry) 10 4 0 SlowManure(fresh)  Cattle 0.25 0.15 0.25 MediumHorse 0.3 0.15 0.5 MediumSheep 0.6 0.33 0.75 MediumSwine 0.3 0.3 0.3 MediumPoultry(50%water) 2 2 1 Med. RapidMilorganite(dry) 5 2 to 5 2 MediumMushroom Compost .4 to.7 1 .5 to 1.5 SlowPeat and Muck 1.5 to 3 .25 to .5 .5 to 1 Very SlowSawdust 4 2 4 Very SlowSewage Sludge(digested) 1 to 3 .5 to 4 0 to .5 SlowUrea 42 to 5 0 0 RapidWood Ashes* 0 1 to 2 3 to 7 Rapid* Wood ashes may raise pHCornell gardening resources A Guide to the Nutrient Value of Organic Materials, Ecogardening Factsheet #8, Spring 1993

PH MATTERS

MORE ISN’T BETTER• Too much of any nutrient can be

negative– Poor fruit set– Run-off or leaching resulting in

environmental pollution– Soil acidification through reaction with

nitrogen

FERTILIZER CALCULATION EXAMPLE

CALCULATING AMENDMENTS

• http://www.caes.uga.edu/publications/pubDetail.cfm?pk_id=7170

ADDITIONAL RESOURCES• Basic Soil Science:

– Chapter 3. Concepts of Basic Soil Science, W. Lee Daniels and Kathryn C. Haering, Department of Crop and Soil Environmental Sciences, Virginia Tech

– www.mawaterquality.org/capacity_building/mid.../chapter3.pdf• Calculating organic amendments:

– How to Convert an Inorganic Fertilizer Recommendation to an Organic One, Revised by Julia Gaskin, David Kissel, Glen Harris and George Boyhan. Original manuscript by Wayne McLaurin, retired Horticulture Professor, and Water Reeves, retired Horticulture Educator

– http://www.caes.uga.edu/Publications/displayPDF.cfm?pk_ID=7170

Anne PfeifferUW-Madison

Community and Regional Food Systems

www.community-food.org