Soil fertility is fundamental to successful horticultural production in Kenya. Organic manures play a critical role in replenishing soil nutrients, improving soil structure, and enhancing water retention. This chapter explores the preparation of organic manure, a key skill for horticulture professionals aiming to sustainably boost soil productivity while minimizing reliance on chemical fertilizers. Understanding the types, characteristics, and importance of organic manures equips practitioners to optimize crop yields and maintain healthy soils.

2.1 Organic Manures

Organic manures are vital in horticulture for maintaining soil health and ensuring sustainable crop production. Their use supports environmental conservation and helps smallholder farmers improve productivity without relying heavily on chemical inputs. This section breaks down the concept, benefits, types, and defining features of organic manures to build a comprehensive foundation for their preparation.

2.1.1 Definition of terms

Clarifying the terminology related to organic manures is essential for horticulture professionals to communicate effectively and apply best practices in soil fertility management. The following terms are used throughout this unit.

Organic manure

Organic manure refers to decomposed plant and animal residues used to improve soil fertility and structure. Unlike synthetic fertilisers, organic manures release nutrients slowly, enhancing long-term soil health.

Compost

Compost is a type of organic manure produced by controlled aerobic decomposition of organic wastes such as crop residues, kitchen scraps, and animal manure. It is rich in humus and beneficial microorganisms that boost soil biological activity.

Green manure

Green manure involves growing specific crops, such as legumes, and incorporating them into the soil to increase organic matter and nitrogen content. Examples of green manure crops are mucuna and sunhemp.

Farmyard manure (FYM)

Farmyard manure consists of animal dung mixed with bedding materials such as straw or sawdust. It is widely used by horticulturists to supply nutrients and improve soil texture.

Humus

Humus is the dark, stable organic fraction of the soil formed from decomposed organic matter. It plays a crucial role in nutrient retention, water-holding capacity, and soil structure stability.

2.1.2 Importance of organic manure

The application of organic manure is fundamental to sustainable horticultural practices. It provides multiple benefits beyond nutrient supply, contributing to soil resilience and crop health.

Soil nutrient enrichment

Organic manures supply essential macro- and micronutrients such as nitrogen, phosphorus, potassium, calcium, and magnesium. Their slow nutrient release tends to match crop uptake, reducing nutrient leaching and improving efficiency.

Improved soil structure

Organic matter from manures enhances soil aggregation, leading to better aeration and water infiltration. Example: a vegetable grower who adds well-rotted manure to a heavy clay seedbed finds the soil crumbly and easier to work, and the roots of crops such as kale and carrots penetrate more freely.

Enhanced water retention

Soils treated with organic manure retain moisture longer, which is critical during dry spells. This property reduces irrigation frequency and supports consistent crop growth, for example in a tomato plot that would otherwise wilt between waterings.

Increased microbial activity

Organic manures stimulate beneficial soil microorganisms that decompose organic residues and can help suppress soil-borne diseases. This biological activity is vital for nutrient cycling and plant health in horticultural systems.

Environmental sustainability

Using organic manure reduces dependence on chemical fertilisers that can contaminate water bodies and degrade soil quality. Example: a flower farm that recycles its crop trimmings into compost lowers its waste and its fertiliser purchases (Ksh) at the same time.

2.1.3 Types of organic manure

Organic manures can be classified by their source and composition. Horticulture practitioners choose a type depending on availability, crop needs, soil condition, and the labour and time at their disposal.

2.1.3.1 Green manure

Green manure is a crop grown specifically to be cut and incorporated into the soil while still green and soft. Leguminous species fix atmospheric nitrogen through bacteria in their root nodules, so the soil gains nitrogen as well as organic matter. Example: a grower with a depleted vegetable plot sows mucuna during a rest period, then digs it in before transplanting cabbage seedlings.

Common green manure crops include:

• Mucuna: A vigorous creeping legume that smothers weeds and produces a large amount of leafy biomass.
• Sunhemp: A fast-growing, upright legume that suits short fallow periods.
• Crotalaria: A legume used to add nitrogen and organic matter, and often grown on poor soils.
• Lablab and cowpea: Quick-growing legumes that cover the soil and can be incorporated at flowering.
• Jack bean: A deep-rooting legume that tolerates dry conditions and improves soil structure.

Procedure for using a green manure crop:

1. Prepare the plot by clearing weeds and loosening the soil lightly so seed can be sown evenly.
2. Sow the green manure seed at the recommended spacing or broadcast it, ensuring good contact with moist soil.
3. Allow the crop to grow, weeding only if necessary, until it reaches early flowering, when biomass and nitrogen are at their best.
4. Slash the crop close to the ground using a panga or slasher and chop it into short pieces to speed up decay.
5. Incorporate the material into the top 15 to 20 cm of soil with a hoe or plough so it is well covered.
6. Keep the soil moist and wait about two to three weeks for decomposition before sowing or transplanting the main crop.

2.1.3.2 Farmyard manure

Farmyard manure (FYM) is made from the dung and urine of livestock mixed with bedding such as straw, dry grass or sawdust collected from the animal shed. It supplies nitrogen, phosphorus, potassium and organic matter, and its bulk improves soil structure. Example: a horticultural farm with a few dairy cows collects the cowshed bedding and dung weekly and stacks it for use in vegetable beds.

Key points in making and using good FYM:

• Collection: Gather dung and soiled bedding regularly and mix them so the bedding absorbs the urine, which holds much of the nitrogen.
• Heaping: Stack the material in a compact heap and keep it covered, away from direct sun and rain, to limit nutrient loss through leaching and drying.
• Maturing: Leave the heap for several months, turning it occasionally, until it is dark, crumbly and no longer smells strongly.
• Application: Apply only well-rotted FYM, mixed into planting holes or spread and dug into the seedbed before planting.
• Safety: Fresh dung can burn roots and carry pathogens and weed seeds, so wear gloves and keep fresh manure away from leafy vegetables close to harvest.

2.1.3.3 Compost manure

Compost is produced by the controlled, mainly aerobic, decomposition of mixed organic materials. A well-made heap heats up as microorganisms work, which helps destroy many pathogens and weed seeds, and the result is a stable, humus-rich product that is easy to handle. Example: a nursery combines grass cuttings, vegetable trimmings and a little animal manure into a heap whose finished compost is later mixed into potting media.

Materials are grouped into two types, which should be balanced in the heap:

• Nitrogen-rich (green) materials: Fresh grass, green weeds, kitchen vegetable waste and animal manure, which feed the decomposers.
• Carbon-rich (brown) materials: Dry maize stalks, straw, dry leaves and sawdust, which give structure and allow air movement.

Procedure for making a compost heap:

1. Choose a shaded, well-drained site close to a water source and mark out a base about 1.5 m wide.
2. Place a layer of coarse twigs or stalks at the bottom to allow air to enter from below.
3. Add a layer of brown material about 15 to 20 cm deep, chopped into small pieces.
4. Add a layer of green material or manure of similar thickness, and sprinkle with water so it is as moist as a squeezed sponge.
5. Repeat the layers until the heap is about 1.2 to 1.5 m high, then cover it with grass, sacks or a sheet to retain heat and moisture.
6. Turn the heap every two to three weeks, adding water if it is dry, so that all material is exposed to the hot centre.
7. Judge the compost ready when it is dark, crumbly, cool and earthy-smelling, with the original materials no longer recognisable.

Other animal wastes

Beyond FYM, poultry droppings and goat manure are widely used. Poultry manure is particularly high in nitrogen and phosphorus, making it valuable for demanding crops such as tomatoes, but it is strong and should be composted or well aged before use to avoid scorching roots. Goat manure is drier and pellet-like, easy to handle and quick to mix into the soil.

Crop residues

Residues left after harvest, such as maize stalks and bean haulms, can be used as mulch or added to compost to return nutrients and organic matter to the soil. Example: stalks from a harvested maize plot are chopped and either spread between vegetable rows or added to the compost heap as carbon-rich material. Diseased residues should not be used unless they are fully composted.

2.1.4 Characteristics of organic manures

Understanding the properties of organic manures allows horticulture professionals to select and apply them effectively according to crop and soil requirements.

Nutrient content

Organic manures vary in nutrient concentration; poultry manure tends to have higher nitrogen content than FYM. Knowing the relative richness of a manure guides the quantity applied, so crop needs are met without burning roots or wasting material.

Moisture content

Moist, but not waterlogged, materials decompose more readily because microorganisms need water to work. Very wet manure, however, is heavy and costly to carry, and it loses nutrients if left uncovered in rain, so heaps should be kept covered and handled at a workable dampness.

Carbon to nitrogen ratio (C:N)

The C:N ratio affects the rate of decomposition. Materials with low C:N ratios, such as fresh legume green manure, decompose quickly and release nutrients, while materials with high C:N ratios, such as straw and sawdust, decompose slowly and may temporarily lock up soil nitrogen. Mixing green and brown materials in a compost heap brings the overall ratio into a range that suits decomposers.

Texture and particle size

Fine-textured manures such as compost improve soil tilth and are easier to mix into the soil, while coarse materials such as straw-based FYM improve aeration but may decompose more slowly.

Presence of pathogens and weed seeds

Poorly prepared organic manures may harbour harmful pathogens or viable weed seeds, posing risks to crop health. Proper composting, with sufficient heat and maturing time, is essential to reduce these threats, especially in vegetable production systems.

Chapter Summary

This chapter focused on organic manures, beginning with clear definitions of key terms related to soil fertility and organic nutrient sources. It emphasized the importance of organic manure in enhancing soil structure, moisture retention, and nutrient availability, which are essential for sustainable agricultural productivity. Various types of organic manures were explored, including green manure, which involves growing specific plants to be incorporated into the soil; farmyard manure, which consists of animal dung and bedding materials; and compost manure, produced through the controlled decomposition of organic waste. Each type was described in terms of its preparation and application to improve soil fertility. The chapter also examined the characteristics of organic manures, such as nutrient content, decomposition rate, and their effect on soil biology, providing a comprehensive understanding of their role in soil management. Overall, the chapter equipped learners with the knowledge to prepare and utilize organic manures effectively in farming practices.