Introduction to Soil Science in Sustainable Agriculture

Craig CoggerWSU PuyallupSept. 19, 2002

Mineral MatterPore Space

OrganicMatter

Soil Components

The soilecosystem

Residue decompositionNutrient cyclingAggregation and porosityEnhance plant growthBreak down contaminants

•Water MovementHow quickly water moves through soil

Water Holding CapacityHow much water a soil can hold available for plant growth

Soil pores and water movement

•Macropores: Infiltration and drainage

•Capillary pores: Available water

•Micropores: Unavailable water

Soil properties that affect porosity

•Soil texture

•Soil structure

•Compaction and disturbance

•Organic matter

Soil Particle Sizes

Sand .05-2 mm

Silt .002-.05 mm

Clay <.002 mm

Coarse Fragments >2 mm

Approximate surface areas of 1

gram samples

Coarse sand Half dollar

Fine clay Basketball court

Hand texture technique

Soil StructureAggregation of sand, silt, and clay particles

Formation of soil structure

•Growth of roots and movement of organisms create pores and aggregates

•Soil organisms break down organic residues, producing glues that stabilize aggregates

•Fungi provide structural support to aggregates

•Physical, chemical processes also involved

Soil Structure

•Improves macroporosity

•Promotes aeration

•Promotes infiltration

Major soil types of the Puget Sound area

Most local soils formed from glacial

materials

•Glacial Till (Ice laid)

•Glacial Outwash (Meltwater)

•Lacustrine (Lakebed)

•Ablation till•Not compacted•Permeable to water and roots

•Basal till•Compact and cemented•Barrier to water and roots

Glacial till soil

0 to 4”gravelly sandy loam4 to 10”, very gravelly loamy sand

10” +sand and gravel

Glacial outwash soil Very low water and nutrient holding capacity

Glacial lacustrine(lakebed) soil

Fine texture, highwater holding capacity, hard towork when wet orvery dry.

Soil fertility and nutrient management

Nutrient Management

•Meet crop nutrient needs

•Maintain soil quality

•Conserve resources

•Protect water quality -- reduce leaching and runoff risk

Plant NutrientsMajor Nutrients•Nitrogen

•Phosphorus

•Potassium

•Calcium

•Magnesium

•Sulfur

Micronutrients•Boron

•Iron

•Manganese

•Zinc

•Copper

•Chloride

•Molybdenum

How nutrients become available

Mineral Matter Organic Matter

KMgCa

N

SP

K+ NH4+ Ca++ SO4--

soluble, available

Notavailable

-

-

- -

-

-- - - - -

Ca++ K+

clay OM

- - --

- -

Mg++

K+

Nutrient Anion Availability

Anion Binding SolubilityPO4

-3 strong low

BO3-3 medium medium

SO4-2 v. weak high

NO3- v. weak v. high

Organic N

NH4+

NO3 -

Leaching Gases

Plants,Microbes

Plant residues,Manure

Nitrogen Cycle

Organic Materials

•Little or no processing

•Low nutrient content

•Slow release of nutrients

•Plant, animal, or mineral sources

Organic Materials:Slow release nutrients

•Plants can only take up nutrients that are in available form (simple, soluble ions).

•Most nutrients in organic materials are in complex organic molecules or minerals, and are not immediately available to plants.

Slow release nutrients

•Biological processes slowly release the nutrients in organic amendment into available forms.

•Rate of nutrient release depends on the nature of the amendment and environmental conditions.

Nutrient uptake

•The forms of nutrients taken up by plants are the same for all types of fertilizer -- manufactured or organic.

Organic materials:Fertilizers vs. Soil

amendments

•Fertilizer 1. High nutrient content and availability.

2. Main benefit is nutrients. 3. Relatively small amounts applied.

•Soil amendment 1. Low nutrient content and availability. 2. Main benefit is organic matter. 3. Large amounts applied.

Carbon:Nitrogen ratio

•Low C:N supplies N to plants

•High C:N ties up N by biological immobilization

C:N ratio and N availability

C:N

<10:1

10:1 to 20:1

20:1 to 30:1

>30:1

N availability

High

Med - Low

Very Low

Negative

High N ContentC:N < 10:1

•Rapid N availability

•Use as a fertilizer

•Over application leads to excess nutrient levels in soil -- potentially harming crop and water quality.

High N ContentExamples

•Poultry manure

•Packaged organic fertilizers

•Fresh dairy or goat manure

Moderate N ContentC:N 12:1 to 25:1

•Slow N availability

•Can add large amounts without risk of over-fertilization

•Use as a soil amendment

•Expect some N immobilization (tie-up) shortly after application.

Moderate N ContentExamples

•Compost

•Yard debris

•Cover crop residues

•Dairy solids

Low N contentC:N > 30:1

•N immobilization

•Need to add N along with organic amendment

•Use as mulch or bulking agent for compost

Low N contentExamples

•Straw

•Sawdust

•Paper waste

Soil Testing

NutrientsContaminantsBiologicalSoil Quality

What is a soil nutrient test?

•A chemical extraction of “plant-available” nutrients.

•Used to predict nutrient availability and fertilizer need.

Soil Nutrient Tests

•Standard agricultural tests (P, K, Ca, Mg, B, pH, lime requirement)

•Nitrate tests•Sampling Reference: UIdaho Bulletin 704. Soil Sampling

When to sample?

•Standard tests can be taken at any time before fertilization.

•It is best to be consistent from year to year.

•Nitrate tests are taken at specific times.

How often to sample?

•Sample each unit every 1 to 3 years, or at least once every crop rotation.

How to sample•Divide farm into units (based on soil type, crop, management).

•Small, diverse farms will need to group crops for sampling.

•Take 10 to 20 cores per unit (0 to 12 inch depth).

•Avoid unusual areas.

Sample handling

•Keep moist samples cool during and after sampling.

•Refrigerate, freeze, or bring directly to lab.

•OR, spread in thin layer and air dry

•Send about 1 pint to lab, carefully labeled.

Choosing a lab•Does the lab routinely do ag tests?

•Do they use OSU or WSU test methods?

•Do they give fertilizer recommendations?

•What information do they need?

•How to send sample?

•Cost? Turn-around time?

•What does report look like?

Interpreting soil tests•Nutrient status Low, medium, high

•Fertilizer recommendation

•You will need to interpret for organic fertilizers.

•You will need to interpret if one test represents multiple crops.

•Reference: EC 1478. Soil Test Interpretation Guide

Web Addresses

•WSU Publications:http://pubs.wsu.edu/

•OSU Publications:http://eesc.orst.edu/

•UIdaho Publications:http://info.ag.uidaho.edu/

•Organic nutrient management web site:http://www.puyallup.wsu.edu/soilmgmt/