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Using Compost to Improve Post-fire Water Quality

David CrohnUniversity of California, Riverside

Acknowledgements

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Salts Salts are a huge concern in semi-arid and arid soils Salts accumulate from irrigation water Salts concentrate due to evapotranspiration (ET) Not a concern where

precipitation significantly exceeds evapotranspiration

Salts Mostly Na+, K+, Ca2+, Mg2+ and Cl-

Decrease yields by upsetting osmotic balance in the roots zone

If sodium dominates, soil structure is lost so that water and air cannot penetrate

Not a concern where precipitation significantly exceeds evapotranspiration

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Salts in water improve it ability to transmit electricity Salinity is measured indirectly Water is added and extracted from soils Its electrical conductivity is then measured < 4 dS/m is desirable, but it depends on the plant How much water is added affects results

EC2

EC5

ECe

Salt measurement

The question

Do compost salts act like other salts, depressing growth? fertilizers, stimulating growth?

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Threshold soil salinity values for crops (Hanson et al., 1993; Soltanpour and Follett, 1995)

Threshold ECe (dS/m)

Crop 10% Yield Reduction 25%Yield Reduction

Lettuce 2.1 3.2

Tomato 3.5 5

Blueberry 3 5

3 soils

Soil Textures Sand (%) Clay (%)

ECe(dS/m) pH

Organic Matter

(%)Sandy Loam 60 12 1.0 7.9 0.75

Silt Loam 26 17 2.2 7.1 0.69Clay

Loam 21 38 4.1 8.2 0.92

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9 commercial compost blends

Treatment ConstituentsT1 steer manure and wood finesT2 steer manure and wood finesT3 wood fines, biosolids, and digested steer manure.T4 chicken manure and wood fines.T5 steer manure and wood finesT6 wood fines, biosolids, and digested steer manure.T7 wood fines and digested steer manure.T8 rice hulls, biosolids, wood fines, chicken manure.T9 ground wood fines, peat moss, worm castings, fir

bark

Compost properties

Compost Treatment

Organic Matter

(%)Moisture

(%)EC5:1

(dS/m) pH 5:1T1 45 28 12.1 8.3T2 43 46 10.7 8.3T3 33 37 5.8 7.3T4 46 45 5.7 8.4T5 42 29 5.5 8.5T6 48 23 4.8 7.6T7 44 38 4.3 6.8T8 44 28 4.2 7.2T9 48 37 3.0 7.5

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Experiment

3 soils × 9 composts × 3 crops × replicates (324 all together) were grown in 1 gal pots within a greenhouse

Control soils were fertilized Plants were collected 45 days after emergence Dry masses were measured

Salts and compost Growers in California and similar environments worry

about salts in compost. Compost numbers are often high 4 – 6 dS/m for greenwaste compost Manure, biosolids composts MUCH higher (10, 15, higher)

BUT nutrients are salts!!! And much of the salinity measured is

due to these nutrients

Also some compost salts are organic and will decompose

Compost salts are diluted in soils

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0

1

2

3

4

5

6

Control T 1 T2 T 3 T 4 T 5 T 6 T 7 T 8 T 9

Sh

oo

t D

ry W

eigh

t (g

)

Treatments

ECe = 2.1 dS/m

ECe =3.2 dS/m

Effect of different composts on dry matter yield of Lettuce at two salinity levels

Effect of different composts on dry matter yield of Tomato at two salinity levels

0

2

4

6

8

10

12

Control T 1 T2 T 3 T 4 T 5 T 6 T 7 T 8 T 9

Sh

oo

t D

ry W

eigh

t (g

)

Treatments

ECe = 3.5 dS/mECe = 5.0 dS/m

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Effect of different composts on dry matter yield of Blueberry at two salinity levels

0

2

4

6

8

10

Control T 1 T2 T 3 T 4 T 5 T 6 T 7 T 8 T 9

Sh

oo

t D

ry W

eigh

t (g

)

Treatments

ECe = 3.0 dS/m

ECe = 5.0 dS/m

Compost nutrients, salts, and AgIndex

Treatment NH4-N NO3--N P K Ca Mg S Na Cl AgIndex

T 1 124.4 17.6 31.7 803.9 13.1 12.3 136.8 216.5 525.8 1.6

T 2 76.3 30.9 6.8 650.4 11.5 8.5 134.5 172.6 141.9 2.9

T 3 459.2 7.7 3.4 283.1 86.7 34.5 725.0 124.6 208.5 2.4

T 4 166.2 20.2 35.6 533.7 17.6 5.8 146.7 120.2 105.5 4.0

T 5 137.7 2.7 2.8 668.4 34.5 25.9 213.5 186.9 84.8 3.6

T 6 355.6 2.8 2.5 95.3 39.2 17.1 189.5 42.8 133.4 2.7

T 7 247.1 7.7 1.9 131.7 49.4 20.1 199.4 48.5 281.6 1.3

T 8 183.2 3.4 0.4 97.7 163.5 28.8 248.7 29.6 116.9 2.1

T 9 61.7 11.3 1.5 303.9 44.1 11.4 133.2 111.8 82.7 2.3

Soil 6.0 15.4 65.3 40.3 107.9 71.2 106.3 16.4 67.8 -

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Observations:

In all cases we saw reduced growth as soil-compost mix ECe values increased from 10 to 25% salt damage levels (p<0.0001)

In all cases compost use improved growth above the fertilized controls (p<0.0001)

Could not predict growth 10 to 25% damage level differences from measured compost nutrients or salts using regression

AgIndex was significant (p<0.05) suggesting that this ratio is meaningful

Predicting the ECe of compost amended soils

Soil salinity is measured using ECe

Compost salinity is measured using EC5

These are NOT comparable because the amount of water in saturated paste varies from soil to soil EC5 > Ece

Also adding compost changes the amount of water in the saturated paste of the soil compost mix

Have developed a method, based on published regression data, to predict this.

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Predicting the ECe of compost amended soils

Here is an Excel tool that can help.

Conclusions

Compost salts ARE like other salts. Too much can be harmful

The benefits of using compost far outweigh the risks however

The AgIndex can be used to guide compost choices where salinity is a concern

The ECe of compost-soil mixes can be predicted from soil and compost (TMECC) measures