Examining the soil

Alan Broughton

Agroecology researcher and educator

Organic Agriculture Association & Mekong Organics

matunda7@hotmail.com

Project: Education to minimize the use of plastics and chemicals in VietnamTraining of organic farming, certification and trade

Lecturer: Mr Alan BroughtonTopic: Examining Soil

New training project: Training of Organic Farming, Certification, and Trade – Mekong Organics

Webinar on 22 July 2021

Looking at the soil profile

Dig a hole down to about 90 cm

Look at the layers (horizons)

Note changes with depth: • Colour• Feel• Stones, gravel, sands• Hard layers• Earthworms

Uniform soil: No change

Duplex soil: Sudden change

Gradational soil: gradual change

Soil texture: Feeling the soil

• Long ribbon: high clay content

• Short or no ribbon: High sand content

• Sticky soil: high magnesium

• Silky feel: high silt content

• Hard lumps: high sodium content

Soil colour

• Red soil: good drainage

• Black soil: high organic matter

• Mottled soil: impervious layer

• Blue-grey soil: waterlogged soil

• Yellow soil: slow drainage

• White lumps: limestone soil

Acidity and alkalinity • Best pH is 6.2 to 6.5

• Below 5.0: Al & Mn toxicity

• Above 7.0: Trace element deficiency

• Ca, K, Mg, Na all increase pH

• Sandy soils often acidic

• High rainfall leaches Ca, reduces pH

• Test with a pH colour chart test: yellow is acidic, purple is alkaline

Emerson test for soil stability• Place a small clod of soil (5 mm)

in a saucer of water

• Observe over several hours

• Collapse means very poor organic matter

• Cloudiness means high sodium

• Poor organic matter and sodicity mean unstable soil, erosion

Soil compaction test

• Push a strong wire or other sharp object into the soil

• Feel where there is resistance

• If you cannot push further, roots and water probably cannot penetrate

• This is a compaction layer

• Break it with a soil ripping tool

Keys to sustainable soil management

• Maximise organic matter

• Keep the soil covered

• Minimise tillage

• Natural fertilisers

• Crop rotation

• Fertiliser trees

• No poisons

• Compost, manure, green manure

• Lime or dolomite if needed

• Break up hard layers

• Prevent erosion

• Experimentation

Keep the soil covered

Do not burn crop residues

When rice straw is burnt:

- Almost all the carbon goes up in smoke

- 93% of the nitrogen is lost

- 25% of phosphorus is lost

- 21% of potassium is lost

Use straw for mulch or animal bedding.

Crop rotation

• Plant an unrelated crop each season in the same place

• Examples: grain - legume - brassica - root crop - legume

• Use mustard to control diseases

• Always include legumes

• Use buckwheat or lupins to increase phosphorus

• Use animals

Chemical fertilisers are harmful to soil

• Promoting pests and disease

• Burning up soil organic matter

• Suppressing good soil microbes

• Making soil hard

• Becoming ineffective in the long term

• They are expensive!

Benefits of soil organic matter (1)

Holding water: 0.5% organic matter holds 80,000 litres of water per hectare; 5% organic matter holds 800,000 litres per hectare

Improving soil structure, holding soil particles together

Allowing water infiltration

Preventing erosion

Benefits of soil organic matter (2)

Holding nutrients: preventing leaching of nitrogen and sulphur

Keeping phosphorus available

Feeding microbes and earthworms

Maintaining a more even soil temperature

Preventing loss of lime

How to increase soil organic matter

Use green manures or mulch between cropping seasons

Return straw and manure to the soil: Do not burn

Do not use chemical fertilisers and pesticides

Direct drill if possible

Add compost or worm casts

Are legumes producing nitrogen?

Bình Phước soils

ig-vast.ac.vn/vi/tin-tuc/ban-do-dat-cac-tinh-viet-nam-707.html

Fk: Red-brown basalt (shown as red on the map)Fu: Yellow-brown basalt (deep pink)X: Grey old alluvium (blue-grey)Fs: Red-yellow shale (orange)Fp: Yellow-brown old alluvial (pale pink)

Red-brown basalt soilĐầt nâu đỏ trȇn đá bazan

ClayGood structureHigh organic matterVery acidicVery well drainedGenerally fertile but with low available phosphorus

baotangdat.blogspot.com/2011/10/phau-dien-at-vn-18.html

Yellow-brown basalt soilĐầt nâu vàng trȇn đá bazan

Similar to red-brown basalt but less well drainedClayHigh organic matterVery acidic

Yellow-red shaleĐầt đỏ vàng trȇn đá phiến

Sandy loamMedium organic matterVery acidicLow nutrient holding abilityGenerally low fertility

Grey old alluvial soilĐầt xám trȇn phù sa cổ

Silty clayAcidicModerate organic matterPoorly drainedSuitable for rice

GleyĐầt xám gley

Silty clayModerate organic matterAcidicVery poorly drainedNot suitable for crops except rice

Mekong Delta soils

Alluvial: 1.1 million ha, the most fertile

Acid sulphate: 1.6 million ha, hard to manage except

for rice

Saline: 1 million ha, poorly drained, smelly, sometimes

flooded from the sea

Managing acid sulphate soils

Spread lime. Difficulties: Too wet for machinery, too expensive, not available.Spread

Add organic matter: chopped green vegetation like lucerne, Phragmites (sậy nam) or some other green leaves. Straw or rice stubble also helps. Incorporating into the soil works better than a surface mulch.

Add

Keep the land flooded. Rice is more tolerant of acid sulphate conditions than other crops. However, rice cannot tolerate a pH of less than 4. Keep