Composting with Cowpea (Vigna unguiculata)

Introduction

Cowpea (Vigna unguiculata) is an abundant source of high protein plant biomass. This plant material can be composted to produce a net gain in total nitrogen on the farm. After mixing and management, storage-ready compost can be made at medium scale in under a year and ready to use for various soil conditioning or seed-starting purposes in two. First, a carbon source, such as pine wood chips, are mixed with a nitrogen source, such as cowpea to construct the piles. Then, after months of pile management, the product is ready for storage.

A pile of wood chips — Figure 1. Pine wood chips shredded into 2-3″ pieces stockpiled on-site

Objectives

The objectives were 1) supply the farm with compost to reduce fertilizer input, and 2) create bulk potting medium to start seeds and reduce nursery costs. Therefore, cowpea was grown, pine wood chips were, stockpiled, and compost piles were constructed.

Pine wood chips transforming into a brown substance — Figure 2. Aerobic Decomposition (Compost formation)

Plant Growth

Cowpea seeds were broadcasted on lightly disked no-till rows containing organic material and wood chips (Figure 4). A 1,500 sq. ft. plot produced about 2 tons fresh plant biomass. No fertilizer was used. Cowpea has excellent nitrogen fixation capabilities and minor pest issues.

TIMELINE: Two years

Year 1: Seed cowpea in May; grow all summer

Fall: Harvest in September, construct piles, and re-mix every 2 weeks until October

Winter: Consolidate all compost piles; moisten, stir, and “fluff” weekly.

Early Spring (the following year): Store in containers; send sample to Livestock Waste Testing Laboratory

Year 2: Compost is useful after one year in storage. Do not use if compost has a sour or rotten smell. This smell can occur from being left too long in a closed container because the humidity creates conditions for anaerobic microbes to thrive. This is why weekly opening, stirring, and “fluffing” to reintroduce oxygen is important for the storage and maturation process.

Figure 3. Iron clay cowpea grown on 1,500 sq. ft.

Plant Harvest

Cowpea plants were pulled from the field with a tractor and claw attachment, (Figure 5) taking advantage of the plant’s inter-locking stems. In its place, the ground was left clean and virtually weed-free. Four compost piles (100 cu. ft.) were constructed.

A tractor carries a load of plant material — Figure 4. Claw attachment on tractor removes matted cowpea plants from plot

A bare patch of soil — Figure 5. Ground left virtually weed free after September harvest

Pile Construction

Cowpea plants, inter-twined, were placed in a giant matted ball on a tarp and chopped into lengths of 1-2′ using an electric hedge trimmer, then layered with 2-3” pine wood chips in hardwire cages on pallets (to improve air circulation at base of pile). Piles were left uncovered, and cages removed two weeks later, when material was softer. Piles were re-mixed and layered with new cowpea shoots; this process was repeated 3 times until November. The compost piles were consolidated in January and the process of sieving and storage began in February.

A man putting plant material onto the last of 4 piles — Figure 6. Compost piles constructed with alternating layers of “brown” and “green”

Pile Management

Compost piles were kept well-moist and mixed monthly, reaching a maximum temperature in the upper 120’s F. Tarps were used to capture small particles and recycle them into the next pile. Eventually, compost piles were consolidated, as their volume decreased significantly. The remaining 4-month-old material was moved to an outside structure (Figure 9) and sieved a few weeks later. Infestations included Alabama jumping worms and fire ants.

Figure 7. Piles broken down and re-layered with fresh cowpea three times until November

Compost Storage

The consolidated compost pile was sieved at 1/2″ initially, then at 1/8″ for final sampling and storage. Fully sieved was stored in plastic containers, with enough empty space to allow air circulation. It was stirred biweekly and monitored for consistency, odor, and quality. The contents too large for the various sieves were recycled back into the pile for further decomposition.

A man holds a plate of compost — Figure 8. Compost sieved at 1/2″ to reduce bulk

A metal grate container over a plastic bin — Figure 9. Compost sieved again at 1/8″

a small container of black soil — Figure 10. Final sieved compost (1/8″) is ready for storage

Lab Results

Macro-nutrients

Nitrogen (raw): 8.4 lbs./ton; (adjusted): 6 lbs./ton
Phosphorus (P2O5): 3.6 lbs./ton
Potassium (K2O): 2.7 lbs./ton

Physical and Chemical Characteristics

pH: 7.7
Moisture: 36.2%
Total Solids: 63.8% or 637,909 mg/kg (1276 lbs./ton)
Total Ash: 46.6% or 466,003 mg/kg (932 lbs./ton)
Total Kjedldahl Nitrogen: 0.42% or 4,212 mg/kg 8.4 lbs./ton)
Ammonia Nitrogen (NH3): 0% or 6 mg/kg (0 lbs./ton)
Total Elemental Phosphorus (P): 0.08% or 789 mg/kg (1.6 lbs./ton)
Total Elemental Potassium (K+): 0.11% or 1,116 mg/kg (2.2 lbs./ton)
Copper (Cu): 5.81 mg/L
Molybdenum (Mn): 32.23 mg/L
Zinc (Zn): 25.47 mg/L

Nitrogen Losses

Nitrogen content as tested: 8.4 lbs./ton

Nitrogen losses during application: 5% or 0.4 lbs./ton
Nitrogen losses while awaiting incorporation: 25% or 2 lbs./ton
Other nitrogen losses: 0% or 0lbs./ton

Estimated Available Nitrogen (N): 71.2% or 6 lbs./ton

Consolidated Sample 1