Compost Benefits

David M. CrohnDavid M. CrohnDepartment of Environmental SciencesDepartment of Environmental Sciences

University of California, RiversideUniversity of California, Riverside

Marketing Healthy Soils for Healthy Plants

Soil Nutrients: macronutrients

Total N–Organic N–Available N (nitrate and ammonia-

N)

Total P

Total K

Soil Nutrients : micronutrients

Boron

Chlorine

Cobalt

Copper

Iron

Manganese

Magnesium

Molybdenum

Sulfur

Zinc

Soil properties

Organic matter

Water holding capacity

Bulk density

Structure

Texture

ECe

pH

pH

Low pH = acid soil

High pH = alkaline soil

Neutral pH = 7

ECe = salinity

ECe stands for Electrical Conductivity

Salts conduct electricity and this is used to measure them in soils

High ECe soils may have trouble germinating seeds and supporting growth

ECe is a water management indicator, more than a soil property

Texture

The sand,silt, and clay-sized particlesdeterminesoil texture

Not affectedby structureor OM

Bulk density

Soil mass .Soil volume

Dense soils– Retard water movement– Hold less water– Impede air exchange– Stop root development

Fig. Engle et al. WSUE

Struc

ture

Water holding capacity

HowComposts Benefit

Soils

Why compost?To eliminate disease To eliminate disease organismsorganisms

Animal Animal ●● Plant Plant ●● Human Human

To produce a stable and To produce a stable and safe soil amendment safe soil amendment

Nutrients Nutrients ●● Odors Odors ● Phytotoxins● Phytotoxins

Organic matter: What you are selling

Hargrove and Luxmore

SOM: Matter of life and death

Carbon

Source of energy for microbesSource of energy for microbesNot all forms are equally Not all forms are equally availableavailable– sugars (more available)sugars (more available)– fats, waxes, proteinsfats, waxes, proteins– cellulosecellulose– lignin (less available)lignin (less available) CC

StarchesUsed by plants and animals for intermediate energy storage.

A carbohydrate (CH2O)x

Readily metabolized by microbesDecomposed during composting

CelluloseMost abundant organic compound on EarthExamples: paper, cotton, cellophane, rayonLike starch, a carbohydrate composed of chains of glucose molecules (polysaccharides), but joined by -linkagesCellulose is used by plants for construction of cell wallsTend to be structured in straight linesCan be metabolized by bacteria, fungiRuminants, termites use bacteria to break down cellulose

Starch and Cellulose

Lignin

Second most abundant organic compound on EarthHas a complex structureAlong with cellulose is used by plants for construction of cell wallsCan be metabolized by fungi

Ligninmolecule

CC

Soil Nutrients: macronutrients

Total N–Organic N (slow release )

–Available N (variable)

Total P (often rich)

Total K (variable)

Bacteria80 – 90% of the microbial community is bacteriaSmall: 0.5 – 3 m diameterHigh surface:volume ratio. Important for moving nutrients in and waste products outC:N ratios on the order of 4.3:1 to 6:1Soil species differ from activecompost species– 0 - 40C: mesophyllic– 40-65 C: thermophilic

Fungi

Fungi include molds and yeasts

Larger than bacteria

Grow more slowly

Able to metabolize lignin

C10H17O6N

C:N ratios on the order of 9:1

Fungi onlignin

NN

NitrogenVital nutrient for both microbes and plantsVital nutrient for both microbes and plants

Microbes are better than plants at accumulating Microbes are better than plants at accumulating NN

C:N ratioC:N ratio– Compost feedstock initially about 35:1Compost feedstock initially about 35:1

– About 10:1 to 20:1 in cured productAbout 10:1 to 20:1 in cured product

– About 8:1 in a soilAbout 8:1 in a soil NN

Soil Nutrients : micronutrients

Boron

Chlorine

Cobalt

Copper

Iron

Manganese

Magnesium

Molybdenum

Sulfur

Zinc(Variable, depending on source)

pH

Low pH = acid soil

High pH = alkaline soil

Neutral pH = 7(buffers pH)

ECe = salinity

ECe stands for Electrical Conductivity

Salts conduct electricity and this is used to measure them in soils

High ECe soils may have trouble germinating seeds and supporting growth

ECe is more of a water management indicator(Contains salts, some of which are nutrients. Improves tilth so salts can leach away)

Texture (no effect)

The sand,silt, and clay-sized particlesdeterminesoil texture

Not affectedby structureor OM

Bulk density (decreases density)

Soil mass .Soil volume

Dense soils– Improves water movement– Holds more water– Increases air exchange– Promotes root development

Bacteria and fungi

Bacteria are much smaller than fungi

Both decompose composts in thesoil

Encourages the formation of soil aggregates

Aggregates are soil clusters held together as a result of compost decomposition– Fungal hyphae

bind particles together

– Bacterial polysaccharidesserve as glue

Compost in soil

Struc

ture

Fig. Engle et al. WSUE

Aggregated soils

Hold water while allowing air to penetrate

Facilitate drainage and salt removal

Allow roots to penetrate

Are more stable, resisting erosion– Sheet

– Rill

Water holding capacity

Overcome worry… …with knowledge and information

Photo credit: aur2899

Compost Use IndexDerived from focus groups asking experts and users what they care about.Measureable using STA/TMECC valuesDesigned to indentify quality composts for specific usesNot all parameters matter for all usesCurrently implemented for– Avocado– Lettuce– Tomato– Grape– Strawberry

Use Index Parameters

C:N Ratio

Organic Matter

Salinity (EC)

Total N

Total P

Stability

Phytotoxicity

pH

Boron• Chloride• Sodium• Manmade Inert

Impurities• Particle Size

Distribution• Safety

– Metals

– Pathogens

Carbon:Nitrogen Ratio

• Low C:N do not tie up N and are biologically most active

• High C:N Ratios (>25) imply lignin and cellulose which resist decay. – Good for mulches.– May immobilize N in soils, stealing N from

plantsUse C:N ratioMulch 15 : 1Soil amendment

20 : 1

Organic Matter

• Carbon based

• Derived from the tissue of living things

• Substrate for soil microbes that improve soil structure

• Helps hold plant nurtients

• > 65% O.M. suggests immaturityUse OMMulch 25 %Soil amendment

25 % 65 %

Salinity (EC)• Most significant issue for arid and semi-arid areas• All charged ions, including nutrients, contribute to salinity• Compost and soil salinities are measured differently.

– Compost: EC5:1 (5:1 water to compost)

– Soil: ECe (saturate paste extract)

• Plant toxicity levels are in terms of ECe

• Formula needed to predict the ECe of a Soil/compost mix– Soil texture– Soil organic matter content– Compost organic matter content– Mixing depth

• Salinity is a transitory property• Salts added with irrigation water

usually far exceed compost salts

CropSoil/compost mix ECe, dS/m

Avocado 4Lettuce 1.3Tomato 2.5Grape 1.5Strawberry 1

Macronutrients• Total N

– Source of crop nutrients– Too much ammonia can harm crops

• Total P– Not a crop health concern– Eutrophication concern

Maturity• Stability

– Unstable composts are still decomposing rapidly. This emits CO2. And odors.

• Phytotoxicity– Germination test

Use C:N ratioMulch 8 mg CO2/g OM/daySoil amendment 8 mg CO2/g OM/day

Use C:N ratioMulch 80 % emergenceSoil amendment 80 % emergence

pH• Describes the acidity of the compost

• Low pH (acid) composts may not be mature.

• Phytotoxic organic acids produced during composting must be decomposed as they are phytotoxic.

• High pH (alkaline) composts may be contaminated.

Use C:N ratio

Mulch 6.0 pH 8.5Soil amendment 6.0 pH 8.5

Boron

• Naturally present in some soils and irrigation waters. More available under acid conditions. Controlled through leaching.

• Once commonly added to plywood glue, therefore found in MSW, demolition debris, coal or wood ash feedstock

• More available under lower soil pH conditions

• Hard to say how a compostwill affect available soil levels

Use C:N ratioMulch 100 mg kg-1 Soil amendment

100 mg kg-1

Toxic B Levels:

Crop

Concentration in saturated extract, mg L-1

Avocado 0.5 – 0.75Lettuce 0.5 – 0.75Tomato 4.0 – 6.0Grape 0.5 – 0.75Strawberry 0.75 – 1.0

Boron toxicity thresholds (Hanson et al., 1993, Maas, 1986).

Chloride

• Toxic to some crops, but not to others.

• Almost all composts can meet the 1% dry wt. limit

Toxic C Levels

Crop

Concentration in saturated extract, meq L-1

Avocado 5 – 7.5Lettuce n.a.Tomato n.a.Grapes 30 – 40 Strawberry 5 – 7.5

Crop Chloride contentAvocado 0.2 % dry weightLettuce 1 % dry weightTomato 1 % dry weightGrape 1 % dry weightStrawberry 0.2 % dry weight

Sodium

Crop Sodium contentAvocado 0.2 % dry weightLettuce 1 % dry weightTomato 1 % dry weightGrape 1 % dry weightStrawberry 1 % dry weight

• Sodicity

• Sodium disperses clay particles collapsing soil structure

• This is offset by compost organic matter, calcium and magnesium

• Phytotoxicity

• Mainly woody plants

Manmade Inert Impurities

• Plastic, in particular, can litter the soil.

• Remember that runoff water is often reused and plastic can clog water systems

Use InertsSoil amendment 0.75 % dry weightMulch 0.75 % dry weight

Particle size distribution

• Mulches should have larger particles than soil amendments.

• Large particles in soils can tie up nutrients.

• Too many small particles in mulches can support weed development.

Use Size CharacteristicsSoil amendment 95% passing though a 5/8” screen

70 % passing through a 3/8” screenMulch 99% passing though a 3” screen

25 % passing through a 3/8” screen

Metals• Current numbers based on assumption of 100 years of

biosolids applications at 4.5 t/ac and a detailed risk assessment.

• Compost applied at higher rates than biosolids so those number are not conservative.

• Mulches are applied at higher rates still, though less often.

ConstituentAmendments (ppm)

Mulches (ppm)

Arsenic (As)Cadmium (Cd)Chromium (Cr)Copper (Cu)Lead (Pb)Mercury (Hg)Nickel (Ni)Selenium (Se)Zinc (Zn)

41391200150030017420362800

3.02.922111122.11.3317.4206

Pathogens

Use Size CharacteristicsAvocado Fecal coliforms: < 1,000 MPN per gram of total solids

Salmonella sp. < 3 MPN per 4 g of total solidsLettuce Fecal coliforms: < 1,000 MPN per gram of total solids

Salmonella sp. < 3 MPN per 4 g of total solids E. coli O157:H7: non detect

Tomato Fecal coliforms: < 1,000 MPN per gram of total solidsSalmonella sp. < 3 MPN per 4 g of total solids

Grape Fecal coliforms: < 1,000 MPN per gram of total solidsSalmonella sp. < 3 MPN per 4 g of total solids

Strawberry Fecal coliforms: < 1,000 MPN per gram of total solidsSalmonella sp. < 3 MPN per 4 g of total solids E. coli O157:H7: non detect

• Many diseases are spread through a fecal-oral transmission route. Both humans and animals can deposit infectious bacteria, viruses, and helminths.

• Because there are so many infectious organism out there, we use indicator organisms. Because E. coli O157:H7 is so dangerous, suggest checking when applied to produce eaten fresh.

My Advice to Growers on Choosing a Compost• Trust your judgment!

• Compost should have an earthy smell

• An off smell suggests that the compost is not ready yet. It may damage young plants

• Composts have different amounts of organic matter. 25 – 75%. More is usually better

• Local stuff will be cheaper to deliver

• Value the reputation of the composter• STA certification

Soil amendments: modify soil properties• Improve tilth

• Improve water holding capacity

• Improve aeration

• Improve infiltration capacity

• Adjust pH

• Promote microbial activity

Mulches: cover the soil• Protect soils against erosion• Conserve water by slowing

evaporation• Control weeds• Control certain plant diseases• Decorate landscapes

Organic fertilizers: slow release sources of• Nitrogen

• Phosphorus

• Potassium

• Micronutrients

Summary on uses• Mulches, soil amendments and

organic fertilizers are uses, not products.

• Composts may serve any of these.

• Different composts have different properties, and will therefore be more suitable for some uses than for others.

Think long-term.Focus on the soil and your plants

will thrive.