SSC 427 Midterm

Cover Crops I

  • Green manure: provide N to the following seasons crop, help maintain SOM

  • Cover Crop: grown to prevent erosion, manage OM, “catch all” name

  • Catch Crop: grown to scavenge end of season nutrients from field, prevent nutrient leaching 



  • Why use cover crops?

    • Soil health, erosion, increases SOM, decreases soil compaction



Most plant cover crops: Cereal rye, winter wheat, ryegrass

Most planted legumes: crimson clover, winter pea, clovers

Change: mixes of cover crops



  • Barriers to adoption:

    • Establishment

      • Poor establishment complicates the effectiveness of a cover crop

        • Uncertainty of N inputs

        • Variable weed control

      • Factors to consider

        • Seed app method - planting depth

          • Broadcast: up to 25% more seed than drilling, plant right before rain, better for heavier seeds (cereal grains), wind speed less than 15 mph

            • Advantages: planted earlier, faster, lower HP tractor required, plant in living cash crop

            • Disadvantages: variable distribution, poor stand establishment (rodents, birds, dry conditions), seed to soil contact

          • Drilled

            • Advantages: good soil-seed contact, adjustable depth of planting, protection from herbivory, lower seed costs

            • Disadvantages: limited on planting time, slower, equipment more expensive

        • Seeding rate

        • Timing of seeding

        • Soil moisture 

        • Species selection

    • time and labor too high

    • species selection



General benefits to cover cropping: cut fertilizer costs, reduce herbicide and pesticide inputs, improve soil health, prevent soil erosion, protect water quality

























Crimson Clover (NC)

  • Can be a summer legume in the North

  • Ideal before sorghum 

  • In warm climates can reseed and have continuous cover

  • Biomass: 3500-5500 lb acre

  • 160 lb N acre



Hairy Vetch

  • Superior to CC for biomass and N content in NC

  • Can act like a weed in the following season if seeds 

  • Should be planted 30 to 45 d before killing frost 

  • Drill seed 15-20 lb ac, broadcast 25-30 lb acre

  • Biomass: 4300-7800 lb acre

  • 150 lb N acre 






Berseem Clover

  • Can be used as a forage

  • Main legume in nile delta

  • Biomass: 6500 lb acre

  • Seeding rate: 8-12 lb drilled, 15-20 lb broadcast

  • Seed from late august to mid oct

  • 150 lb N acre



Austrian Winter Pea 

  • Low CN ratio, breaks down

  • Can be damaged by heat

  • Issue with Sclerotinia crown rot

  • 6000-8000 lb acre

  • Drilled 1-3 inches deep

  • Poorly broadcasted

  • Seed by Oct 1 in zone 8

  • 130 lb N








**Environmental Factors Affecting N2 Fixation



  1. Salinity

  2. Unfavorable pH

  3. Nutrient deficiency

  4. Mineral tox

  5. Temp extremes

  6. Drought or waterlogged soils



  1. Inadequate photosynthesis

  2. Plant diseases

  3. Grazing

  4. Compact soils 




When to inoculate

  1. Legumes have not been grown previously

  2. Native flora did not contain legumes

  3. Several years have gone by between a legume in rotation

  4. Poor nodulation the previous year

**Many soils challenged by native rhizobia that are less effective at BNF but outcompete for nodulation



Estimating N contribution from green manure 

Goal is to supply N to subsequent crop and lower N inputs

Keys to success

  1. Timing of termination


    2. Often limited by planting date requirements for the subsequent crop

    3. Advantages to terminating early

      1. Allows time for water to replenish

      2. Increases rate of soil warming

      3. Reduces potential allopathic effects of decomp

      4. Easier planting

      5. N mineralization is starting early

    4. Advantages of late termination

      1. More residue available for soil and water conservation

      2. Greater N accumulation

      3. Better cover for weed control

  2. Termination method

    1. Delaying termination can significant increase CC biomass

      1. Delaying kill date increases 41% CC, 61% HV, N content 23%, 41%

      2. CN ratio of cover crops

  3. Estimating N being released



How to terminate cover crops?

  • Herbicides

    • 2,4D for HV, 2,4D for CC, Austrian winter pea, cereal rye

  • Disking

  • Roller crimp

  • Flail mower

  • Or plant green?

    • Lets crop grow longer (more biomass)

    • Reduced planting issues

    • Reduces weeds 




Cover Crops II

Nitrogen Scavengers: goal is to capture residual N in the fall and mineralization N in the spring, done by generating as much biomass as possible

  • Cereal Rye

    • Fibrous roots can scavenge 100 lbs N ac

    • Increases the exchangeable K

    • Biomass of up to 10000 lb acre

  • Radish

    • Bio-drill: roots can break through compacted layers and plow pans

    • 100+ lbs of scavenged N

    • Risk of releasing N too early

    • Can be N deficient 

    • Deep roots create preferential flow paths

  • Sorghum Sudangrass

    • Pref flow

    • Smothers crop

  • Rye grass

  • Barley

  • Oats

  • Wheat

  • Rapeseed

  • Berseem Clover

***Cereal grains will have a high CN ratio and slow decomp



Cover Crops and Water

  1. Improving infiltration rate

    1. Increases macropores, earthworm activity, aggregate stability, prevents soil crusting

  2. Increases soil water retention

    1. When terminated, the residue cover decreases evap (if using no till)

      1. Shields soil from energy of sun

      2. Reduces air movement at soil surface

      3. Increases the albedo of the ground

  3. Water uptake can be useful in colder wetter climates to allow ground to dry out



Cover Crop Mixes improvements:



  • Winter survival

  • Ground Cover

  • Use of Solar energy

  • Biomass and N prod

  • Weed control

  • Duration of active growing period

  • Diversity of insects

  • Tolerance of adverse conditions

  • Response to variable soil traits




Disadvantages:



  • High seed costs

  • Too much residue

  • More complicated management

  • Difficult to seed

  • Cereal ryes dominate, brassicas underperform



Cover Crop Mixes

  • Often a cereal and legume mix

  • Need to have similar growth periods 

    • Ex. Crimson clover and ryegrass terminate at same time and scavenge spring N

  • If cover crop is used for pure weed suppression, monocultural cereal rye is most effective



Phosphorous

  • Cover crop biomass return to the soil (P min)

  • Diversity of roots can increase P mining

  • Increase mycorrhizal abundance

  • MIcrobial biomass P

  • Phosphatase activity

Crop Rotations

  • Historic rotations in the south

    • Tobacco, cotton, peanuts

  • Livestock farms: corn-oats-wheat-clover-pasture

    • Different now because typically “monoculture”: cows, poultry, etc. 

  • Now: 

    • Cotton-peanut, 

    • tobacco-sweet potato-corn, 

    • Corn-soybean

  • Rotations were important in the past for management of weeds (no herbicides), disease, insect populations, nutrient supply, and labor distribution

    • Midwest today is mostly corn-soybean

      • Should add cover crops to increase soil structure and improve yield



Crop Sequencing:

  • Wheat-Red Clover-Corn

    • Wheat and legume provide continuous cover

    • Reduce warm season weeds

    • NItrogen from RC is available to corn

    • RC has no econ value

  • Wheat-Soybean-Corn

    • Soybean can be harvested

    • Longer soil exposure

  • Crimson Clover-Corn-Wheat/Cereal Ryegrass-Soybean



Main Goals of Planned Rotation

  1. Reduce risk of disease and insects

  2. Reduce weed pressure and herbicide resistance

  3. Increase yield and stability of yields

  4. Reduce soil erosion and increase water interception

  5. Diversify workload

  6. Increases water use efficiency

  7. Improve soil quality



Rotation Effect: adding crops between C-S rotations can break pest cycles and decrease incidence of soilborne pathogens and soybean cyst nematodes 

  • Monoculture crops are disease prone

    • Corn: rootworm slugs

    • Wheat: seedling, root, and leaf diseases

    • Soybean: white mold, soybean cyst, nematodes

    • Canola: root rots and whit

  • Green manures can:

    • Promote disease suppressing properties that reduce plant pathogens by:

      • Increasing lvls of SOM (greater microbial biomass)

      • Comp for resources

      • Antibiosis

      • Antagonish

    • Suppress through direct inhibition of production of bacterial/fungal compounds



Increased Crop Diversity

  • Improves soil structure and aggregation 

  • Helps foster root growth

  • Longer periods without tillage may improve mycorrhizae abundance

  • Increased microbial biomass

  • Mineralize more N

  • Maintain a larger biological nutrient pool

  • Put more nutrients into soil



Why do benefits from rotations take a long time to show? 

Produce gradual results that need to take time (soil structure, microbial pops, nutrient levels)



Rotation effect and SOC

  • Steady state takes from 15-20 to occur

  • Increasing diversity of rotation of rotation compared to C-C increased SOC storage BUT SOC decreased when soybean was added to the rotation - low biomass returns = poor soil quality outcomes

  • Diversity of root architecture attributed to increased SOC (alfalfa: deep tap root, pea: recalcitrant root and hairs to penetrate into microaggregates) 

  • Longer rotations increased more positive relationships



Treatments

  • MonoS

  • C-S

  • W (Wheat)-S

  • W+RC (Red Clover)-S

  • W-S-S

  • W+RC-S-S



Soil Biochemical Indicators

**Soils sampled 9 times (0-10 cm) in 2015/2016 growing seasons

  1. SOC, SON (Most stable)

  2. POC, PON

  3. Water Extractable OC

  4. Soil microbial biomass C

  5. Soil pH (Least stable)



Yield Stability

  • Soybean does not contribute much biomass or C

  • Rotations provides a buffering effect during extreme weather years



Rotation in Organics

  • Most powerful tool for insect management

  • Corn: 2 yrs min, use of non-grass to starve pests

  • Wheat: short rotations put wheat at risk, plant field ¼ mi away from wheat stubble to prevent Hessian fly

  • Soybean: at least 2 yrs to avoid nematodes



Intercropping: two or more crops growing simultaneously on the same land, may or may not be sown at same time

  • Row intercropping: seeds of two or more species mixed together, planted at same time and often, harvested same time (trees higher than a ground-level plant)

  • Strip cropping: two or more crops grown simultaneously in different strips wide enough for crops to be managed agronomically - dictated by planter width

  • Relay cropping: growing two or more crops simultaneously but with one or more being planted after maturity of another’

Advantages: wheat is a scavenger, increase C sequestration, increase profit

Disadvantages: damage wheat during planting, lower soybean yield than monoculture

Challenges: mechanized and chemical weed control is difficult, overcome by mechanical harvesting or relay cropping



Sequential cropping

  • W-S rotation in SE common

  • Double cropping winter wheat in China

  • W-rice double crop in India



General Rotation Guidelines

  • Use short rotations since benefits are greatest in first year (C-S-C-S vs C-C-S-S)

  • Where possible include forages/cereals

  • Follow legumes or forages with corn for N contribution

  • Rotation effect (disease prevention, etc) most pronounced in long term

    • Most apparent on stress prone soils/climates

  • Concentrate on improve average profit




Tillage 



Why do we till?

  • Weeds, incorporate fertilizer and residue

  • Moldboard

    • Invert surface 

    • Not much residue after inversion, no tillage is taller 

    • Requires secondary tillage - one to three passes with a disc harrow

    • Slice, lift, fracture, invert 

    • 6-7 inches deep (energy intensive!)

    • Why?

      • Heavy clay soil, allows for warming

      • Buries weed seed below optimum depth

      • Temp reduces BD

        • Carbon oxidation

        • Aggregates are getting disturbed, less soil structure

  • Chisel plow 

    • Leaves 50-70% residue

    • Performed in fall

    • 2-4 inches

  • Disc harrow

    • Cut, roll, mix 

    • Leaves more residue with less disturbance

  • High residue cultivator

  • Strip tillage 

    • Narrow width of tilled strips (2-8 inches) 20-25% tillage without disturbing field

    • Can use a subsoiler for deep till rows

    • Advantages:

      • Dry out wet soil

      • Warm up soils before removing residue - faster emergence out of soil

      • Fertilizer replacement

      • Relieve soil compaction 

  • Ridge Tillage: method of preparing seedbed on permanent ridges 4-6 inches high remaining from previous years crops

  • Subsoiling (maintenance) 



  • Tillage in Organic Farming

    • Tillage is often critical to control weeds in organic farming

    • No-till organic is a challenge depending on crop rotation and region

    • Moldboard plowing buries surface seeds while turning up dormant deep placed seeds

    • Chisel plowing will not invert weed seeds and often case a flush in weeds 

  • In season cultivation

  • Downside of plowing

    • Leaves soil bare, destroys aggregate and enhances SOM loss

      • 30-50% SOM lost in corn belt

      • Leads to soil crusting and accelerated erosion

      • Causes plow pans

      • HIgh energy requirements

      • Labor

  • Plowman’s Folly

    • 1943

    • Recognized erosion problems due to tillage, provided no solution and was mostly ignored

    • Post WWII herbicides were developed to take the place of tillage 

  • NRCS 4 basic principles of soil health

    • Keep soil covered

    • Reduce soil disturbances

    • Keep living roots in the soil

    • Diversity using rotations and cover crops 

  • Conservation tillage: defined as at least 30% residue cover myst be ono the soil surface immediately after planting

  • No-till: soil is left undisturbed and planting is done in narrow slots

  • 37% acreage no till 



  • Benefits of no till

    • Helps prevent wind and water erosion

    • Improves water infiltration

    • Improves soil structure

    • Maintains aggs

    • Can increase soil carbon

    • Fuel cost sabing

    • Lowers evap and drying 

  • Continuous no till

    • Less than 10% of all US farmers

    • Increasingly high residue corn varieties make residue mgmt difficult

    • Herbicide resistant weeds

    • Average no-till is 2.5 yrs 

    • Much of the carbon building is lost in a single tillage event!