# Session 5: Digestate & Fertiliser ## Connecting to Earlier Sessions In Session 2 you loaded cattle manure into the digester. In Sessions 3–4 you saw how microbes broke it down to release biogas. This session completes the cycle: what flows from the outlet pipe is **not waste** β€” it is **bio-slurry**, a transformed fertiliser that is in many ways more valuable than the manure you started with. --- ## Case Study: Bio-Slurry and Coffee *Mrs Linguina demonstrates how bio-slurry improves coffee crop yields.*
--- ## Case Study: Bio-Slurry Reviving Garden
--- ## Learning Objectives By the end of this session, you will be able to: 1. Explain why bio-slurry is more effective than raw manure. 2. Choose the correct application method based on your farm situation. 3. Handle and apply bio-slurry safely. --- ## What is Digestate (Bio-Slurry)? ```{raw} html

πŸ”„ The Biogas Cycle β€” Closing the Loop

Click each stage to learn what happens at that point.

πŸ„ Manure daily feed βš—οΈ Digester 40 days πŸ”₯ Biogas cooking & use πŸ’§ Bio-slurry fertiliser 🌾 Crops
Click any stage to learn more.
``` **Digestate** is the organic material left after microbes have broken down manure in the digester. During this process: - Carbon is converted into biogas (useful energy). - What remains is **concentrated, stable, and nutrient-rich**. - Nutrients (especially nitrogen) become **more available to plants**. This means digestate often works **faster and more effectively than raw manure** as a fertiliser. ```{figure} ../assets/fertiliser-cycle.png :alt: Closing the loop β€” from manure to fertiliser, the nutrient cycle, high nitrogen content, and application decision guide :width: 100% Closing the loop: manure enters the digester, bio-slurry exits as a high-nitrogen fertiliser, and crops return organic matter β€” along with a quick-reference guide for choosing your application method. ``` --- ## Why is Bio-Slurry Different from Raw Manure? ### Nutrient Content Anaerobic digestion does not reduce total nutrients (N, P, K), but it **changes their form**: - Much organic nitrogen is converted to **ammonium-N (NHβ‚„-N)** β€” a form plants absorb immediately. - NHβ‚„ levels in digested manure are often **2–2.5Γ— higher** than in undigested manure. - Total N, P, and K remain similar. ```{note} **Key finding:** Bio-slurry can contain 44–81% of total nitrogen as ammonium-N. This means it acts faster than raw manure β€” but timely, correct application is essential to avoid nutrient losses or crop burning. *(MΓΆller & MΓΌller, 2012; Gurung, 1997)* ``` ### Forms of Bio-Slurry | Form | Description | Best for | |---|---|---| | **Liquid bio-slurry** | Flows from digester outlet | Quick nutrient supply to established crops | | **Solid (fibre) digestate** | Settled solids | Soil structure improvement, distant fields | ```{raw} html

πŸ”¬ Bio-Slurry vs Raw Manure

Click a property to compare β€” see why digested slurry outperforms raw manure as a fertiliser.

Raw Manure
30%
plant-available N
Bio-Slurry
70%
plant-available N
🌿 Nitrogen availability: Anaerobic digestion converts organic nitrogen into ammonium-N (NHβ‚„-N) β€” the form plants absorb directly. Bio-slurry typically contains 44–81% of total N as ammonium-N, compared to around 15–30% in raw manure. This means faster, more predictable plant uptake.
``` --- ## Benefits of Bio-Slurry - **Plant nutrient supply:** Provides N, P, and K in plant-available forms. - **Soil health:** Boosts organic matter and water-holding capacity. - **Cost savings:** Reduces the need for chemical fertiliser purchases. - **Environmental gains:** Fewer greenhouse gas emissions than unmanaged manure. - **Crop health:** Some field observations report improved plant resistance to bacterial wilts and insect pests, particularly on banana crops. Rice seedlings soaked in diluted bio-slurry before transplanting show improved germination and early growth. Evidence is largely observational; no peer-reviewed trials confirm resistance to specific viral pathogens. *(Warnars & Oppenoorth, 2014)* - **Yield increases:** 10 t/ha application to maize has been shown to increase yields. Results vary by soil, climate, and crop type. *(Warnars & Oppenoorth, 2014)* --- ## Choosing Your Application Method Before applying, answer three questions: 1. **How far is my field from the digester?** (More than 20 min walk β†’ compost first) 2. **How old are my crops?** (Less than 3 weeks old β†’ use diluted irrigation method only) 3. **Is rain expected in the next 48 hours?** (If yes β†’ wait) ```{raw} html

🌿 Which Method Should I Use? β€” Decision Tool

Answer the three questions to find the right application method.

βœ… Method 1 β€” Direct Application  |  Dilute 1:1 with water. Apply in a ring 30 cm from plant base. Cover with soil immediately.
``` --- ### Method 1 β€” Direct Application **Best for:** Established crops, nearby field, no rain forecast. - Dilute fresh liquid bio-slurry **1:1 with water** before applying. - Apply in a **ring around plant bases** or along crop rows. - Cover lightly with soil immediately to reduce nitrogen loss. - **Do not** apply to plants close to harvest date. - **Do not** contact plant stems, seeds, or seedlings directly. #### Worked Example: 10 m Γ— 10 m Maize Plot 1. Add 20 litres of water to 20 litres of bio-slurry (1:1 ratio) = 40 litres diluted. 2. Pour ~0.5 litres around the base of each maize plant (30 cm from the stem). 3. Cover each application with a handful of soil. 4. Repeat 3–4 weeks later. --- ### Method 2 β€” Surface Irrigation / Watering Can **Best for:** Seedlings or small plots where direct application would burn plants. ```{warning} **Never use bio-slurry in drip or trickle irrigation systems.** Bio-slurry causes severe emitter clogging through bacteria, solids, and mineral precipitates. Use Method 1 instead. ``` - Mix bio-slurry at **1:3 ratio** (1 part slurry : 3 parts water). - Pour into upstream furrow channel or use a watering can. - Apply once every 2–3 weeks until plants are established; then switch to Method 1. --- ### Method 3 β€” Composting Bio-Slurry **Best for:** Distant fields, food crops, or when immediate application is not safe. A **100-day composting process** makes bio-slurry safer, lighter, and longer-lasting β€” comparable to commercial peat fertiliser. **Building a composting pit:** 1. Dig a pit at least 1 m away from the digester. 2. Line with dry organic material (dry grass or banana leaves). 3. Direct overflow bio-slurry into the pit and mix with organic waste. 4. Cover the pit to maintain moisture. 5. Aerate weekly by turning with a fork. 6. Once full, start a second pit and allow the first to compost for **100 days**. ```{tip} An average digester produces ~30 tonnes of slurry per year, fertilising approximately 1 acre. Well-composted, the same slurry can fertilise up to **3 hectares** β€” a threefold gain through volume reduction and improved nutrient retention. *(SNV, 2023)* ``` **When and how to apply composted bio-slurry:** - Apply before planting or during early growth stages. - Avoid applying within 48 hours of rain. - Do not use on waterlogged soil. - Maintain safe distances: at least **50 m from drinking-water sources**, **10 m from surface water bodies**. --- ## Quick Reference: Choosing Your Method | Situation | Method | Dilution | Notes | |---|---|---|---| | Established crops, nearby | Direct (Method 1) | 1:1 | Cover soil after application | | Seedlings / surface irrigation | Method 2 (furrow/can β€” NOT drip) | 1:3 | Never undiluted on seedlings | | Pre-planting | Composted | None | Incorporate into top 10 cm | | Distant field (>20 min) | Compost first | None | Reduces volume ~3Γ— | | Fresh vegetables | Composted only | None | 6+ weeks before harvest | ```{figure} ../assets/fertiliser-methods.png :alt: Bio-slurry products and application methods β€” liquid, solid, compost; do's and don'ts :width: 100% Bio-slurry forms and how to use them: liquid and solid bio-slurry, building compost, and the key do's and don'ts for safe application. ``` ```{raw} html

🌱 Bio-Slurry Application Calculator

Enter your plot size and application method β€” find out how much slurry and water you need.

``` --- ## Challenges and How to Manage Them ### High nitrogen content β€” risk of plant burn High ammonium-N can burn young plants and cause losses if misapplied. - Always dilute liquid bio-slurry (minimum 1:1, 1:3 for seedlings). - Use composted material for sensitive crops. - Apply small amounts more frequently (split applications over the season). ### Nitrogen loss during handling Ammonia volatilises when slurry is left uncovered, spread thinly across a surface, or dries in hot sun. Significant nitrogen can be lost this way, reducing fertiliser value and causing odour. Surface broadcast without soil incorporation is particularly prone to losses. - Cover storage and composting pits at all times. - Incorporate bio-slurry into the soil immediately after application. - Avoid spreading during hot, windy conditions. ### Uneven nutrient distribution Irrigation channel application can lead to uneven spread. - Dilute before mixing with irrigation water. - Rotate inlet positions across the plot. - Use buckets or watering cans for small plots to ensure even coverage. ### Nutrient runoff and water pollution Heavy rain after application can wash nutrients into water bodies. - Never apply before forecast rain. - Observe safe buffer distances from water sources (50 m from drinking water, 10 m from surface water). ### Transport and labour Bio-slurry is bulky and heavy to move to distant fields. - Prioritise nearby plots for liquid application. - Compost to reduce volume before carrying to distant fields (reduces mass ~3Γ—). --- ## Health, Safety, and Pathogen Risk This is one of the most important sections. Bio-slurry from animal manure is generally safe when handled correctly, but risk levels vary significantly depending on what went into the digester. **Mesophilic digesters** (20–40Β°C β€” the most common type in smallholder settings) do **not** fully eliminate pathogens. Salmonella, E. coli, and helminth eggs can survive the digestion process, particularly with short retention times. **Thermophilic digesters** (50–60Β°C) achieve a higher degree of pathogen kill but are less common in rural Africa. Even here, composting the output adds a further safety margin. The 100-day composting process substantially reduces pathogen loads in all cases. | Risk level | Applies when | What to do | |---|---|---| | 🟒 **Green β€” Safe** | Composted animal manure digestate only (cattle, pig, poultry) | Suitable for all crops including fresh vegetables. Apply 6+ weeks before harvest as an extra precaution. | | 🟑 **Amber β€” Caution** | Fresh liquid digestate from animal manure only | Safe when diluted 1:1 and applied to established crops. Avoid on root vegetables or crops where slurry contacts the edible part. Wash hands after handling. | | πŸ”΄ **Red β€” Restrict** | Digesters co-digesting human waste (night soil, latrine contents) | Do not apply to food crops without composting first (minimum 100 days). Apply only to non-food crops or as pre-planting soil amendment. Strict hygiene essential. | **General hygiene:** - Wash hands with soap and water after handling bio-slurry. - Wash all tools (buckets, jerry cans, watering cans) after use. - Do not eat, drink, or touch your face while working with slurry. - Keep children away from composting pits and slurry storage areas. - If bio-slurry contacts skin, wash immediately with clean water. --- ## Session 5 Quiz ```{raw} html

MC Q1. Why is bio-slurry often more effective than raw manure as a fertiliser?

Fill in Q2. What is the recommended dilution ratio of liquid bio-slurry to water for applying to established crops?

Express as a ratio (slurry : water).

MC Q3. Why should you never use bio-slurry in a drip irrigation system?

Number Q4. How many days must bio-slurry compost before it is safe to apply to sensitive food crops?

MC Q5. Which traffic-light risk level applies to fresh liquid digestate from animal manure only?

True / False Q6. Applying bio-slurry within 48 hours of expected heavy rain is wasteful because runoff will carry nutrients away.

``` --- ```{figure} ../assets/bio-slurry-worksheet.jpg :alt: Session 5 Bio-Slurry Worksheet β€” what you have, application method, safe use checklist, and notes :width: 80% Use this worksheet to plan your bio-slurry application: record your daily slurry volume, choose your method, and tick off the safe use checklist. ``` ```{note} **Key things to remember from Session 5** Bio-slurry is the transformed fertiliser that completes your biogas system β€” the manure you fed in Session 2 becomes the fertiliser you apply now. - Choose your method based on three questions: **distance, crop stage, and weather**. - Always dilute liquid slurry (1:1 for established crops, 1:3 for seedlings). - Composting for 100 days makes bio-slurry safer, lighter to carry, and longer-lasting. - Know what went into your digester β€” human waste co-digestion requires composting before any food crop application. - For maize, 10 t/ha (approximately 1,000 litres undiluted per 0.1 ha plot) has been shown to increase yields β€” split into 2–3 applications per season. *(Warnars & Oppenoorth, 2014)* ``` Next: **[Session 6: Productive Use of Biogas](session6-productive-use)** --- ## References The following sources underpin the content of this session. Those marked β˜… are particularly recommended for practitioners wanting to go deeper. β˜… **Warnars, L. & Oppenoorth, H. (2014).** *Bioslurry: A Supreme Fertiliser β€” A Study on Bioslurry Results and Uses.* SNV/Hivos. Covers smallholder field evidence across Africa and Asia, including per-plant and per-hectare rates. Start here if you only read one reference. β˜… **Gurung, B. (1997/1998).** *Training Manual: Proper Utilisation of Bio-Slurry.* Biogas Support Programme (BSP/SNV), Nairobi. The original practitioner guide with dilution ratios, crop-by-crop guidance, and composting pit instructions. β˜… **SNV (2023).** *Biogas, Food Security and Climate Change.* snv.org. Overview of bio-slurry value chains in Africa, including the 30 t/year output figure and 3 ha composting yield. **MΓΆller, K. & MΓΌller, T. (2012).** Effects of anaerobic digestion on digestate nutrient availability and crop growth: A review. *Engineering in Life Sciences*, 12(3), 242–257. [Source of NHβ‚„-N data: 45–80% of total N as ammonium, 2–2.5Γ— uplift over raw manure.] **Bonten, L.T.C. et al. (2014).** *Is bio-slurry from household digesters a better fertilizer than manure? A literature review.* Alterra Report 2519, Wageningen UR. **Kisaka, M.O. et al. (2023).** Smallholder farmers' knowledge on the use of bioslurry as soil fertility amendment for potato production in Kenya. *Land Degradation & Development*, 34(3).