Soil fertility improvement is essential for sustainable horticulture in Kenya, where smallholder farmers and commercial growers alike depend on productive soils to achieve high yields and quality crops. Organic manures play a critical role in enhancing soil nutrient content, structure, and biological activity, which supports healthy plant growth. This chapter focuses on understanding organic manures, their benefits, types, and characteristics, equipping horticulture professionals with the knowledge to prepare and apply organic manure effectively for improved soil fertility.

2.1 Organic Manures

Organic manures are fundamental in organic and integrated soil fertility management systems in horticulture. Their use helps reduce dependence on costly synthetic fertilisers and improves long-term soil health, which is vital for crops such as tomatoes, kale and passion fruits grown across diverse agroecological zones. Understanding the definition, importance, types and characteristics of organic manures enables practitioners to use them well and to raise crop productivity sustainably.

2.1.1 Definition of terms

The terminology surrounding organic manures can be confusing, but it is critical for clear communication and effective soil fertility management. Clarifying these terms helps horticulture professionals distinguish between different soil amendments and apply them appropriately. Example: a farm supervisor who instructs a worker to "apply manure" must make clear whether this means fresh dung, well-rotted farmyard manure or finished compost, because each behaves differently in the seedbed.

Organic manure

Organic manure refers to decomposed plant and animal residues applied to soil to improve its fertility and structure. It contains organic matter and essential nutrients, which are released slowly to plants, enhancing soil biological activity and water retention.

Organic matter

Organic matter is the part of the soil made up of living organisms and the remains of plants and animals at various stages of decay. It acts as the food source for soil organisms and as the binding material for soil particles. A dark, crumbly topsoil in a well-managed vegetable plot is a visible sign of good organic matter content.

Humus

Humus is the dark, stable end product of organic matter decomposition. It holds water and nutrients and helps soil particles bind into crumbs. Finished compost is rich in humus.

Mineralisation

Mineralisation is the conversion of nutrients locked in organic matter into simple inorganic forms, such as nitrate and ammonium, that plant roots can absorb. It is carried out by soil microorganisms and is the reason nutrients from manure become available gradually rather than all at once.

Compost

Compost is a type of organic manure produced through controlled aerobic decomposition of organic waste materials such as crop residues, kitchen waste and animal dung. It is rich in humus and beneficial microorganisms, which improve soil health and nutrient availability.

Green manure

Green manure consists of specific crops, often legumes such as mucuna or crotalaria, grown and then incorporated into the soil while still green to add organic matter and nitrogen.

Farmyard manure (FYM)

FYM is a mixture of animal dung, urine, bedding material and leftover feed that decomposes to provide nutrients and organic matter. It is used in mixed crop-livestock systems where animals and horticultural crops are kept on the same farm.

Nutrient recycling

Nutrient recycling involves returning nutrients from organic waste back to the soil through organic manure application. This process reduces nutrient loss from the farming system and supports sustainable soil fertility.

2.1.2 Importance of organic manure

Organic manures contribute significantly to soil fertility and crop productivity in horticulture, especially where inorganic fertilisers are costly or hard to obtain. Beyond nutrient supply, they improve soil physical and biological properties, making them indispensable in sustainable farming.

Soil nutrient supply

Organic manures release essential nutrients such as nitrogen, phosphorus and potassium gradually, providing a steady supply to horticultural crops. This slow release reduces nutrient leaching and improves uptake efficiency, which is especially important in sandy soils where nutrients wash out quickly.

Improvement of soil structure

The organic matter in manures enhances soil aggregation, porosity and water-holding capacity. Improved soil structure facilitates root penetration and aeration, which is critical for crops such as onions and carrots that need loose, friable soils.

Enhancement of soil microbial activity

Organic manures stimulate beneficial soil microorganisms that decompose organic matter and cycle nutrients. This microbial activity promotes nutrient mineralisation and can help suppress some soil-borne diseases, supporting healthy horticultural crops.

Cost-effective fertilisation

Using organic manures reduces the need for expensive chemical fertilisers, lowering production costs. Example: a smallholder kale grower with a few cattle can supplement purchased fertiliser with farmyard manure from the farm's own boma, cutting the amount of fertiliser bought in Ksh each season.

Environmental sustainability

Organic manures help reduce environmental degradation by recycling farm waste, reducing chemical runoff and improving soil carbon storage. This practice supports sustainable agriculture and resilience to climate stress.

2.1.3 Types of organic manure

Organic manures vary widely in origin, nutrient content and decomposition rate. The main types in horticulture are green manure, farmyard manure and compost manure, and each suits particular soil conditions and crop requirements. The choice depends on what materials the farm has, how quickly nutrients are needed and the crop being grown. Two further groups of materials, crop residues and animal by-products, are also used as manures and are described first.

Crop residue manure

Residues left after harvest, such as maize stalks or bean haulms, can be incorporated into soil to increase organic matter. While lower in nutrients than other manures, they improve soil structure and moisture retention. Because they are rich in lignin and carbon, they decompose slowly, so they are best chopped and incorporated well before planting or added to a compost heap as the carbon-rich layer.

Animal by-products as manures

Materials such as poultry droppings, blood meal and bone meal provide concentrated sources of nutrients. Poultry manure, for example, is rich in nitrogen and phosphorus and is used in horticultural nurseries for seedling production. Because these materials are concentrated, they are used in smaller amounts than FYM and are best composted or well mixed into the soil before sowing to avoid burning seedlings.

2.1.3.1 Green manure

Green manure crops are grown specifically to be returned to the soil rather than harvested. Leguminous cover crops such as lablab, cowpea, mucuna and crotalaria are favoured because their root nodules fix atmospheric nitrogen, which is released when the crop decomposes. The practice also covers the soil, which limits erosion and suppresses weeds between main crops such as tomatoes or maize.

Practical steps in using a green manure crop:

1. Select the species: choose a fast-growing legume suited to the site's rainfall and soil, such as cowpea for a short fallow or mucuna where a longer, vigorous cover is wanted.
2. Sow densely: plant at a close spacing so the crop covers the soil quickly and shades out weeds.
3. Allow growth: let the crop build leafy biomass, which holds the nitrogen and organic matter to be returned.
4. Cut at flowering: slash the crop at or before flowering, before seed set, so it does not become a weed in the following crop and so the tissue is still soft and decomposes quickly.
5. Incorporate: chop the material and dig or plough it into the top soil so that it is in contact with soil microorganisms.
6. Wait before planting: leave the plot for a few weeks to allow decomposition to begin, then plant the main horticultural crop.

2.1.3.2 Farmyard manure

Farmyard manure is a common organic manure in mixed farming systems, made from livestock waste and bedding. It supplies nutrients and soil organic matter, but fresh material may carry pathogens and weed seeds, so it must be properly rotted before use on vegetables.

Points to observe when handling FYM:

• Source: manure from cattle, goats, sheep and pigs differs in nutrient content, and bedding such as straw or dry grass adds carbon and absorbs urine, which holds a good share of the nutrients.
• Storage: heap the manure in a shaded, covered place or pit so that rain does not wash nutrients away and the sun does not dry it out completely.
• Rotting: allow the heap to decompose until the original material is no longer recognisable, dark and crumbly, before applying it to food crops.
• Application: spread well-rotted FYM on the bed or place it in planting holes, and mix it into the soil some time before planting so that it does not scorch roots.

2.1.3.3 Compost manure

Compost manure is nutrient-rich and enhances soil microbial life, making it suitable for high-value horticultural crops such as strawberries and herbs. Its quality depends on how the heap is built and managed. Heat produced inside a well-managed heap, above about 55 °C, helps destroy many pathogens and weed seeds.

Steps in making a compost heap:

1. Choose the site: pick a level, well-drained, partly shaded spot near the water source and the garden so that materials are easy to move.
2. Collect the materials: gather nitrogen-rich "green" materials such as fresh vegetable trimmings and manure, and carbon-rich "brown" materials such as dry grass, maize stalks and straw.
3. Build in layers: lay coarse twigs or stalks at the base for air flow, then alternate brown and green layers, chopping large pieces so that they break down faster.
4. Moisten: water each layer until it feels like a wrung-out sponge, since too little water stops decomposition and too much drives out air.
5. Cover: cover the heap with grass, sacking or a sheet to keep moisture in and heavy rain out.
6. Turn: turn the heap every few weeks to bring outer material into the hot centre and to supply oxygen.
7. Check maturity: the compost is ready when it is dark, crumbly, earthy-smelling and no longer hot, and the original materials are no longer recognisable.

2.1.4 Characteristics of organic manures

Understanding the properties of organic manures guides their appropriate use in horticulture. These characteristics influence nutrient availability, handling and the effect on the soil.

Nutrient content and composition

Organic manures vary in nutrient concentration, with poultry manure generally richer in nitrogen than FYM or compost. Knowing the nutrient profile helps tailor application rates to crop needs and avoid nutrient imbalances. In general, organic manures contain lower nutrient concentrations than inorganic fertilisers, so larger quantities are needed.

Decomposition rate

The rate at which organic manure breaks down affects the timing of nutrient release. Materials rich in easily decomposable compounds, such as green manure, release nutrients faster than lignin-rich crop residues.

Moisture content

Organic manures with high moisture content, such as fresh FYM, require careful handling to prevent nutrient losses through leaching or ammonia loss to the air. Dry manures are easier to store and transport but may decompose more slowly.

Bulk density and texture

The physical texture affects ease of application and soil incorporation. Coarse materials such as straw-based compost improve aeration, while fine manures may compact soils if over-applied.

Presence of pathogens and weed seeds

Improperly composted manures can harbour harmful pathogens and viable weed seeds, posing risks to crop health. Proper composting and curing are necessary to ensure manure is safe for horticultural use.

Chapter Summary

This chapter explored the concept of organic manures, beginning with clear definitions of key terms related to organic soil amendments. The importance of organic manure was emphasized, highlighting its role in enhancing soil fertility, improving soil structure, and supporting sustainable agriculture. Various types of organic manure were examined, including green manure, which involves growing specific plants to enrich the soil; farmyard manure, composed of animal waste and bedding materials; and compost manure, produced through the controlled decomposition of organic waste. Each type contributes uniquely to soil health and nutrient availability. The chapter also detailed the characteristics of organic manures, such as their nutrient content, decomposition rates, and effects on soil biology. Understanding these aspects is essential for effective preparation and application of organic manures to improve crop productivity.