Soil Biology and Agricultural Practices

The Role of Soil Biological Community in Improving the Health of Agricultural Soils

Importance of Agricultural Land

• Only about 7.5% of the Earth’s surface is suitable for agriculture.
• The remainder is covered by:
  • Ice, water, or stone
  • Areas that are too dry, too wet, or too hot
  • Cities
• Agricultural land is often undervalued.
  • About 40% of the world’s agricultural land is no longer fertile and has been abandoned.
  • 40-50% of current agricultural land is seriously degraded.
• Soil loss occurs at an alarming rate:
  • We lose a soccer pitch of soil every five seconds due to erosion or desertification.
  • If current soil management practices continue, we only have about 60 years of topsoil left (FAO).

Soil Loss from Erosion in Australia

• Australia loses 2.2 tonnes of soil from water erosion per hectare per year.
  • This is equivalent to almost 1 mm of topsoil per year.
  • It takes 100 years to build 1 mm of topsoil.
• The rate of soil loss is 100 times faster than its formation.
  • About half of Australia's topsoil has disappeared since European settlement.
• Evidence of Australian soil loss is visible in New Zealand, where glaciers are stained by red and black 'dust'.

Global Soil Loss

• Since humans began farming, an area larger than the USA and Canada has been lost to soil erosion.

Soil Contamination with Pollutants

China

• Feeds 20% of the world’s population with less than 10% of the world’s arable land.
• 81% of China’s coastline is heavily polluted with nitrogen, phosphate, fluoride, and oil.
• One in five hectares is poisoned by substances such as cadmium, nickel, and arsenic.
• A further 3.32 million hectares is moderately polluted.

New Zealand

• Cadmium (from phosphate fertiliser) causes problems on dairy farms.
• Two-thirds of all rivers are un-swimmable due to nitrogen run-off from dairy farms.
• Copper (used as a fungicide) has accumulated in vineyards.

Soil Contamination Examples

Europe

• 137,000 square km of agricultural land needs remediation due to cadmium, chromium, copper, mercury, lead, zinc, antimony, cobalt, and nickel.

USA

• 12 million tons of nitrogen and 4 million tons of phosphorus fertiliser are applied annually.
  • Half ends up in waterways.
• Half a million tons of pesticide are applied annually.
  • At least one pesticide was found in 94% of water samples and 90% of fish samples from streams.

Salt Contamination

• When trees are removed, the water table rises and carries salt.
• Massive saltpans are scattered across the landscape in Western Australia.
  • 1 million hectares of agricultural land have been lost to salt.

Negative Impacts of Nitrogen Fertiliser

• Nitrogen fertiliser decreases microbial activity.
• 60-70% of the nitrogen applied is not used by the plant.
• Vast bodies of water are being poisoned by fertiliser use, leading to dead zones in coastal oceans.
• Nitrous oxide (a potent greenhouse gas) is emitted.
• The Green Revolution may have increased food supply but has resulted in a simpler, less stable soil ecosystem.
• Soil no longer teems with life!

Lecture Objectives

Lecture 1

• Become familiar with the many organisms that live in soil.
  • Understand the ecosystem services they provide.
  • Learn how they sustain the health of a soil.

Lecture 2

• Understand the impact of agricultural management practices on the health of the soil.
• Identify practices that can be introduced to improve soil health.

Soil as a Complex Ecosystem

• Soil is the most complex and biologically diverse ecosystem on Earth.
• A teaspoon of healthy agricultural soil contains:
  • Many billions of bacterial cells
  • Kilometres of fungal hyphae
  • Thousands of protozoa
  • Hundreds of nematodes
  • Numerous insects, mites, and small animals
• Globally, there are likely to be:
  • More than a million species of fungi
  • A million species of nematodes
• There are only 9000 bird species, but they receive much more attention!

The Soil Food Web

• The soil food web is a community of organisms that live in soil and interact with each other.
• It is a highly competitive environment where each organism fights for food and a suitable habitat.

Bacteria

• Single-celled prokaryotic microbes.
• Often the most abundant microbes in soils.
  • Many millions of species
  • Usually >10^{10} bacteria/g soil
• Metabolically versatile.
  • Most feed on non-living soil organic matter.
  • Some form symbiotic associations with plants.
• Play an important role in plant nutrition.
• Decompose pesticides and other pollutants.

Bacteria in the Wheat Rhizosphere

• A DNA study detected 9885 unique sequences.
• Bacteria were the majority of species.
• Identified 27 bacterial groups.
  • Some groups contained more than 300 genera.
• Dominant groups:
  • Proteobacteria (blue)
  • Actinobacteria (red)
  • Firmicutes (green)

Actinobacteria

• Aerobic, spore-forming gram-positive bacteria.
• Play a major role in improving soil health
  • Decompose organic materials such as lignin, cellulose, and chitin.
  • Improve the availability of nutrients.
  • Inhibit the growth of plant pathogens.

Soil Fungi

• Lignin: Degraded by saprophytic fungi
• Cellulose: Degraded by fungi, highly resistant to degradation.
• Wood-rotting fungi produce a battery of enzymes that act synergistically.
• In some natural environments, there may be 250 kg of dry fungal hyphae/ha in the upper 5 cm of soil
  • This is equivalent to 3 km of hyphae/g soil.
• Without fungi, the world would be covered in a massive blanket of organic debris.

Protozoa

• Reside in water-filled pores and move in water films.
• Numbers/m^2 may be as high as 10^9 – 10^{10}
• Three major groups:
  • Flagellates
  • Amoebae
  • Ciliates
• All groups feed on bacteria, but some consume fungi and others are omnivorous.

Nematodes

• The most numerous multi-cellular animals on earth (in terms of numbers of individuals).
• Healthy soils will have at least 10 million nematodes/m^2.
• Some are plant parasites, but most are beneficial, feeding on bacteria, fungi, and other soil organisms.

Soil Microarthropods

• Two main groups:
  • Mites (Acari)
  • Springtails (Collembola)
• Generally range in size from 0.2 mm to about 1 mm in length/diameter but springtails may be 3 mm long.
• Live in soil pore spaces.
• Feed on decomposing plant detritus, fungi, nematodes, and other small arthropods.
• Key roles:
  • Mineralise nutrients
  • Regulate populations of the soil fauna

Other Mesofauna

• Enchytraeids (potworms)
  • Resemble earthworms but much smaller
  • Ingest organic matter and associated microorganisms
• Symphylans
  • Centipede-like animals. Generally omnivorous but can feed on plant roots and become pests of crops
• Tardigrades (water bears)
  • Amongst the most resilient animals known
  • Survive extreme temperatures, extreme pressures, air deprivation, radiation, dehydration, and starvation

Macrofauna

• A wide array of taxonomic groups in many trophic levels
  • Isopods and millipedes
    • Major consumers of organic debris
  • Insect larvae such as cane grubs
    • Consume root material
  • Centipedes, spiders, scorpions, and beetles
    • The dominant predators in soil and litter

Ants and Termites

Ants

• Insects (family Formicidae) that live in colonies
• May comprise 15-25% of terrestrial animal biomass
• Generalist predators and scavengers
• Tunnels dug by ants increase air and water flow in soil

Termites

• ‘Keystone species’ in the tropics and subtropics
• Most feed on decaying organic matter
• Some termites use flagellate protozoa in their gut to degrade cellulose
• In some species, gut bacteria fix atmospheric nitrogen
• A study in Western Australia by Evans et al. (2011) showed that ants and termites increased water infiltration, improved soil nitrogen and increased wheat yields by 36%
• These ecosystem engineers undertake the role of earthworms in hot, dry habitats

Earthworms

• Play a key role in soil formation, organic litter decomposition, and the redistribution of organic matter in soil.
• Three ecological groups
  • Epigeic species
    • Reside on the soil surface. Don’t build burrows or produce casts
  • Anecic species
    • Form deep burrows. Incorporate large amounts of organic matter into soil
  • Endogeic species
    • Only form burrows in upper soil layers
    • Faecal pellets provide a highly favourable environment for microbial growth
• Charles Darwin (1881): “It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures”

The Soil Food Web Diagram

• Illustrates the complex interactions between different trophic levels in the soil ecosystem, from photosynthesizers to higher-level predators.

Ecosystem Services Provided by the Soil Biota

Soil Structure

• Provide a soil environment suitable for plants.
• Improve aggregation, create pore spaces, increase water infiltration, improve drainage.
• Fungal hyphae promote aggregation by producing binding agents.
• Earthworms produce macropores.
• Fungal hyphae bind soil together.

Earthworms and Soil Fertility

• Casts are nutrient-rich and the nutrients are in a form readily available to plants.
  • Fresh casts may contain 5, 7, and 11 times more N, P, and K than the surrounding soil.
• Convert organic matter into humus and move it down the soil profile.
• Healthy soils usually have more than 250 earthworms/m^2.

Nitrogen Fixation by Bacteria

• Bacteria in the genus Rhizobium enter root tissue and modify host cells so that nodules are formed
• The bacteria in those cells convert nitrogen gas from the atmosphere to ammonium
• The amount of N produced depends on legume species and growing conditions but is usually 100-200 kg N/ha/year

Soil Organisms Mineralise Nutrients

• Nematodes (and other soil organisms) feed on fungi and bacteria and release nutrients from microbial cells
• Nutrients are released in mineral form (e.g., ammonium in nematode faeces) and are used by plants

Nutrient Cycling

• Nutrient cycling occurs because predators have a higher C/N ratio than their prey
• Example: Bacterial feeding nematode C/N = 6:1, Bacteria C/N= 4:1
• Nematode obtains 12 C from bacteria, it also gets 3 N, it only needs 2 N
• N not required by the nematode is excreted and can be used by plants

Experimental Evidence

• Experimental evidence that nematodes improve plant growth
• The results of a classic experiment published in 1985 showed that the addition of microbial-feeding nematodes to soil enhanced N uptake and improved the growth of a perennial grass

Nitrogen Mineralisation by Nematodes

• Fungal and bacterial-feeding nematodes mineralise between 1 and 6 nanograms N/nematode/day
• Assumptions:
  • 1400 t soil/ha
  • 5 nematodes/g soil
  • 4 ng N is mineralised/nematode/day
• Then 10 kg N is mineralised by nematodes/year
• In a healthier soil with 25 nematodes/g soil, 50 kg N would be mineralised
• This indicates that in healthy soils, free-living nematodes are significant contributors to the soil nitrogen pool

Arbuscular Mycorrhizal Fungi

What are Arbuscles

• Branched hyphae that form within root cells and have an absorptive function

Mycorrhizal Fungi and Plant Nutrition

• Soils usually contain large amounts of P
  • Phosphates have low solubility and so plants find it difficult to acquire
• When roots take up orthophosphates (Pi), depletion zones occur in the rhizosphere because replacement does not keep up with uptake
• Plants have evolved the capacity to increase the uptake or availability of Pi
  • The most common strategy is AM symbiosis, which is used by about 80% of plant species
• The fungus obtains carbon from the plant and, in return, provides the plant with nutrients (particularly phosphorus)

Why AM Fungi Improve P Uptake

• The hyphae of AM fungi have a smaller diameter than roots and so they can enter small pore spaces and explore a greater soil volume
• A well-developed hyphal network absorbs Pi several centimetres from the root surface
• Once Pi is in the hyphae, it is translocated rapidly to roots, overcoming the slow diffusion that occurs in the soil solution

AM Fungi and Drought Tolerance

• A plant’s best friend in dry environments
• The fungus acts as an extension of the root system, improving drought tolerance

AM Fungi, Disease Resistance, and Tolerance

• Tolerance is enhanced because plant stress caused by nutrient deficiencies or drought is reduced
• AM fungi compete with pathogens for carbon within roots and labile carbon exudates outside roots
• Once a mycorrhizal fungus occupies a root cell, soilborne pathogens find it more difficult to gain access to that cell

AM Fungi and Soil Structure

• A binding agent (glomalin) is produced, causing soil particles to aggregate

Glomalin

• A glycoprotein
  • Contains 30-40% carbon, which is found in its protein and carbohydrate sub-units
• Arbuscular mycorrhizal fungi are the only producers of glomalin
  • Glomalin has two purposes:
    • It is a sealant that turns hyphae into pipes, allowing them to funnel water and nutrients to the plant
    • It gives the hyphae rigidity, allowing them to span the air spaces between soil particles

Glomalin and Aggregate Formation

• When hyphae stop transporting water and nutrients, or die, the glomalin sloughs off into the soil
• This ‘super glue’ permeates organic matter and binds it to sand, silt, and clay particles, resulting in the formation of aggregates
• Aggregation is important for many reasons
  • Wind and water erosion is reduced because soil structure is stabilised
  • The soil remains porous, allowing water, air, and roots to move through it
  • Aggregated soils hold more water and harbor beneficial microbes

Agricultural Practices that Disrupt AMF

• Frequent inputs of P fertiliser
• Long fallow periods
• Cultivation of non-host crops
  • Brassicaceae such as cabbage, cauliflower, and canola
• Frequent tillage
• Soil fumigation

Effects of Non-Mycorrhizal Break Crops

Australia

• Opinions differ on the effects of non-mycorrhizal break crops
• Brassicas are biofumigants
  • Produce glucosinolates which reduce levels of soilborne pathogens, including fungi, bacteria, and nematodes
• The disease control benefits of crops such as canola far outweigh the benefits from AM fungi
• On the Darling Downs, canola reduced populations of Pratylenchus thornei
• Wheat yields were lowest following canola due to poor colonisation by AM fungi
  • AM fungi provided benefits because the season was dry (73 mm of in-season rainfall) and soil P concentrations were low

The AM Fungi Debate

• Ryan & Graham (2018) argue that there is no need to manage arbuscular mycorrhizal fungi because they have little impact on crop nutrition and productivity.
• Rillig et al. (2019) disagree.
• Arbuscular mycorrhizal fungi provide many benefits to plants, including better P uptake and tolerance to drought
• Dry environmental conditions and low P levels are common in Australia. Will the next generation of agricultural scientists continue to ignore AM fungi?

Soil Organisms as Plant Pathogens

• Bacteria, fungi, oomycetes, nematodes
• Examples: Root-knot nematode, bacterial wilt, Pythium root rot, Sclerotium base rot

Beneficial Soil Organisms and Disease Suppression

• Most soil organisms are beneficial and help suppress diseases, but they are usually ignored
• A disease-suppressive soil contains an active and diverse biological community
• These organisms regulate populations of pests and pathogens through a range of mechanisms
  • Predation
  • Parasitism
  • Competition for space or nutrients
  • Production of antibiotics, toxins, or enzymes that kill, immobilise, or digest other organisms

Nurturing the Soil Biology

• The soil biology must be nurtured: It provides many important ecosystem services
  • Decomposes plant residues and transforms them into soil organic matter
    • Improves soil structure, creates pore spaces, increases water infiltration, improves drainage
  • Provides plants with nutrients
    • Helps plants take up water and nutrients
    • Fixes nitrogen from the atmosphere
    • Mineralises nutrients from organic matter
    • Minimises losses of nutrients from the environment
  • Damages and destroys root systems, but also protects plants from pests and pathogens
  • Provides a range of environmental benefits
    • Reduces soil erosion; minimises nutrient losses; mitigates against climate change by sequestering carbon; degrades pesticides and other pollutants

The Impact of Agricultural Management Practices on Soil Health

Importance of Soil Carbon

• A food source for microorganisms that have many beneficial roles in soil
• Allows water to infiltrate more rapidly and improves drainage, and so run-off due to erosion is reduced
• Water-holding capacity increases
• Aggregate stability improves
• Root growth improves because bulk density is reduced
• Cation exchange capacity improves, and so the soil has a greater capacity to supply cations for plant uptake
• Plant nutrition improves because nutrients are supplied and nutrient cycling increases
• Pests and pathogens cause less damage because they face greater competition from antagonists

Impact of Agriculture on Soil Carbon

• Organic carbon levels in most agricultural soils have declined by 60-80%
• Most of the losses occurred when land was first farmed, but losses continue today
• CO_2 losses contribute to the greenhouse gas emissions contributed by agriculture
• Low levels of soil organic carbon have flow-on effects to the soil biology

Sugarcane's Detrimental Effects

Causes of Soil Carbon Loss

Tillage

• Exposes previously protected organic matter to the soil biota, accelerating the rate at which it is decomposed and mineralised to CO_2

Wind and Water Erosion

• Annual losses of topsoil can be as high as 8 t soil/ha

Periods of Bare Fallow

• Provide no carbon inputs and increase erosion risk

Loss of Ground Cover

• Burning of crop residues or their removal for uses such as animal fodder or mulch for home gardens

Replacement of Pastures

• Perennial pastures have a higher root: shoot ratio and deeper rooting habits than annual crops.
• Less organic matter is removed from the farm when livestock rather than plant produce (grain, fruit) is sold

Tactics to Increase Soil Organic Matter

• Minimise tillage
• Grow healthy crops and pastures
  • Grow more plant biomass
  • Include cover crops in the rotation
  • Use plants with deeper rooting habits
  • Increase crop diversity (multi-species mixes)
  • Intercropping
• Retain crop residues as mulch (i.e., stubble retention)
• Reduce periods of bare fallow
• Apply animal manures, recycled organic wastes, and compost
• Encourage earthworms and other ecosystem engineers to move carbon down the profile
• Multiple practices must be integrated into the farming system

Negative Effects of Agricultural Practices

• Agricultural management practices cause carbon losses but have many other negative effects
• Tillage is detrimental to many beneficial soil organisms

Destruction of Mycorrhizal Networks by Tillage

• When soil is consistently or aggressively cultivated, the ecosystem services provided by mycorrhizae are lost
  • Uptake of water and nutrients
  • Disease resistance
  • Drought tolerance

Detrimental Effects of Tillage on Soil Fauna

• The largest soil organisms are more likely to be affected by tillage than smaller organisms
  • microarthropods, omnivorous nematodes, and the macrofauna
• Many of these animals are predators
• These animals have relatively long life-cycles. Once populations decline, they take a long time to recover

Tillage and Earthworm Populations

• Numbers of earthworms were reduced by 58% when the soil was tilled prior to planting sugarcane Green cane trash blanket; DD = Direct drill

Disruption of Biological Control by Tillage

• The trapping networks produced by nematode-trapping fungi are destroyed by tillage, and it takes time and energy to re-establish them

The Impact of Tillage on Soil Biology

• Tillage has disastrous effects on soil carbon and the soil biology
• If the ecosystem services provided soil organisms are to be retained, tillage must be minimised

Random Traffic from Farm Machinery

• Random traffic from farm machinery is a major issue in some industries
  • Sugarcane is one of the best examples
• Twenty years ago, row spacings were generally 1.5 m
• Wheel spacings on harvest machinery and haulouts were usually 1.85 m
• Compaction problems were widespread and were compounded by the fact that sugarcane is often harvested when the soil is wet

Soil Compaction in Sugarcane

• Most of the surface area is compacted when row and wheel spacings do not match
  • 1.5 m rows and 1.85 m wheel spacings
• Root growth is reduced by 50% when soil strength reaches 2000 kPa

The Combined Damage of Tillage and Compaction

• Collectively, the damage caused by tillage and compaction is disastrous for microarthropods

Monoculture and Limited Crop Diversity

• In such systems, crop-specific pathogens become dominant
• Some examples:
  • Panama disease of banana
  • Take-all of cereals
  • Pachymetra root rot of sugarcane
  • Potato cyst nematode

Bare Fallows and Their Effects

• Bare fallows deplete soil organic matter, with flow-on effects to soil health
• Examples of bare fallowing
  • Continual vegetable crops with nothing but a bare fallow between crops
  • Early formation of plastic-covered beds for vegetables
  • Long fallows to store water for winter cereal crops

Removal of Surface Residues

• Surface residues should be retained rather than being burnt or sold to home gardeners
• In addition to carbon inputs, organic mulch provides many other benefits:
  • Protects the soil from wind and water erosion
  • Improves soil moisture conditions
  • Dampens temperature fluctuations
  • Provides a home for a diverse range of soil organisms

The Impact of Fertiliser Inputs on Soil Biology

• Fertiliser inputs can be detrimental to the soil biology
• Inputs of nitrogen usually decrease fungal biomass and diversity
• Omnivorous nematodes are particularly sensitive to nitrogen inputs
• High phosphorus levels are detrimental to mycorrhizal fungi
• The worst-case scenario is high nutrient inputs at a single time
• Frequent fertiliser applications at low rates are a much better option
• Controlled release fertilisers; regular nutrient inputs via trickle irrigation

Side Effects of Pesticides

Imidacloprid

• A widely used insecticide decimates earthworm populations, reduces their burrowing rate, increases avoidance behaviour
• Is detrimental to predatory arthropods (e.g., ladybirds, dragonflies, earwigs, ants)
• Nectar and pollen from treated plants is toxic to honey bees

Copper

• The long-term use of copper fungicides in avocadoes has decimated earthworm populations

Soil Fumigants

• Create a biological vacuum by killing all beneficial organisms

Herbicides

• By killing plants, herbicides indirectly reduce the diversity and abundance of beneficial arthropods

Alternatives to Pesticides: Integrated Pest Management

• Many tactics are used to control pests
  • Quarantine, biosecurity
  • Pest monitoring and action based on damage thresholds
  • Preventative cultural practices - Crop rotation, cover crops, intercropping, removal of diseased plants
  • Resistant and tolerant varieties
  • Optimal irrigation and nutrient management
  • Biological controls - Promote beneficial organisms, apply biopesticides
• In IPM, pesticides are used as a last resort

Improving the Biological Health of Agricultural Soils

• Many of the practices used in agriculture are detrimental to the soil biology. What can be done to improve the biological health of agricultural soils?

Management Practices for Soil Health and Sustainability

Key practices

• Continuous inputs of organic matter (from plants)
• Permanent cover of plant residues
• A diverse rotation sequence
• Minimum tillage
• Avoidance of compaction through traffic control

Second-tier practices

• Biomass-producing cover crops
• Legumes in the farming system
• Integration of crops and livestock
• Organic mulches and amendments
• Improved nutrient-use efficiency
• Site-specific management of inputs
• Integrated pest management

Sugarcane Farming System

• Five key practices have been integrated into the farming system
  • Row spacing and machinery wheel spacings are matched so that traffic can be controlled
  • Tillage is minimised
  • Rotation crops such as soybean and peanut are included in the rotation
  • Land is never bare fallowed
  • Crop residues are retained as a trash blanket
• This farming system was developed by the Sugar Yield Decline Joint Venture, a research and extension program that ran from 1995 to 2006. It has since been adopted by many cane growers

Benefits from the SYDJV Farming System

Soil health

• Less compaction
• More carbon and labile carbon
• Higher rainfall infiltration rates
  • Due to more earthworms and greater macroporosity,
• Reduced surface crusting and improved aggregate stability
• Increased water-holding capacity
• Reduced soil losses due to erosion
• Higher cation-exchange capacity
• Reduced pathogen loads

Economic

• Productivity maintained (over one cycle)
• Reduced tractor hours
• Lower fuel costs
• Fewer labor inputs
• Additional income from grain
• Lower fertiliser costs ( due to legumes)
• Improved timeliness of operations
• Fewer pesticide inputs

Conservation Agriculture in the Grains Industry

• Prior to 1980, Australian grain-growing soils were routinely tilled
  • Loss of soil moisture
  • Topsoil exposed to wind and water erosion
• Conservation agriculture was introduced in the 1980s
  • Minimal tillage
  • Crop rotation
  • Retention of crop residues on the soil surface
• In 2016, 74% of grain crops were sown using zero till or no till
• Many benefits, including increased soil carbon levels, better soil structure, better rainfall infiltration, soil moisture retention, enhanced soil biological diversity

The Biological Desert Philosophy of Vegetable Production

• Soil is often fallowed
• Few C inputs
• Frequent tillage
• High pesticide inputs
• Plastic ‘mulch’
• High N inputs
• Soilborne pathogens managed using soil fumigants and nematicides

Modified Farming System for Vegetable Grower

• Some vegetable growers have modified their farming system and markedly improved the health of their soil

Trash Planter

• Trash planter for cutting through trash and sowing cover crop

Permanent Beds

• Centrosema cover crop during summer

Effects of Roll Cover Crop

• Roll cover crop and spray out with herbicide

Transplant Tomatoes

• Transplant tomatoes through mulch

Soil Compaction Reduction After No-Till

• Soil is less compacted under no-till + mulch Cultivation + Plastic,
• Native soil Permanent bed + Mulch

Increase in Earthworm Numbers After No-Till

• Much greater earthworm activity under no-till + mulch No worms under plastic Permanent Bed + Mulch

Improved Soil Biology After Two Years

• More disease-suppressive bacteria
• More culturable fungi
• Greater microbial activity
• More free-living nematodes

Bob Euston’s bean and squash production system at Gympie

• Biomass production
• Mulching
• Minimum till
• Permanent ground cover

Improving the Sweetpotato Production System

• Root-knot nematode causes severe damage
• Although nematicides are widely used, marketable yield is sometimes reduced by 75%
• Soil health is poor due to excessive tillage
  • Soil is tilled to remove volunteers that carry over root-knot nematode and other soilborne pathogens
  • Soil is disturbed when the crop is harvested
  • Soil is tilled to flatten beds prior to planting a rotation crop

Field Trials to Improve Soil Health

• A sweetpotato field at Cudgen, northern NSW
• Previous sweetpotato crop harvested in late November 2017
• Despite nematicide application, 30% loss from root-knot nematode

Early Bed Formation Trial

• The field was tilled several times to kill volunteers
• Beds were prepared on 7 February 2018 (10 months prior to planting the next sweetpotato crop)
• Forage sorghum was planted immediately
• Aboveground biomass was retained on the soil surface as mulch
• An oats cover crop was planted after the forage sorghum

Sweetpotato Planting

• Sweetpotato was planted in December 2018 and grew well in the undisturbed beds

Control of Root-Knot Nematode

• Four amendments tested
  • Nil
  • Compost
  • Sawdust
  • Sawdust/chicken litter
• Application rate: 6 L/m row (40 m^3/ha)

Effectiveness of Organic Amendments

• Weeks after planting roots growing in the sawdust/chicken litter treatment had very few galls Compost and sawdust also reduced galling
• Harvest Sawdust and the sawdust/chicken litter mixture both reduced root