Soil fertility improvement is a critical aspect of horticultural production in Kenya, where soil degradation and nutrient depletion are common challenges. Applying appropriate soil amendments enhances soil physical, chemical, and biological properties, thereby increasing crop yields and quality. This chapter focuses on the concept of soil amendments, their significance in horticulture, and the various types available to practitioners. Understanding these elements equips horticultural professionals to make informed decisions for sustainable soil management.

3.1 Soil amendments

Soil amendments are materials added to soil to improve its physical condition, nutrient content, chemical balance and biological activity, all of which are essential for healthy plant growth in horticulture. In Kenya, where soils may be sandy, acidic, sodic or low in organic matter, amendments play a vital role in restoring soil productivity. The following sections explore the terminology, importance and types of soil amendments used in horticultural practice, with particular attention to fertilisers, agricultural lime and gypsum.

3.1.1 Definition of terms

Clarifying key terms related to soil amendments is fundamental for effective communication and correct application in horticultural work. Several terms are often used interchangeably but have distinct meanings that influence what a grower should expect from each material.

Soil amendment

A soil amendment is any material added to soil to improve its physical properties such as texture, structure, water retention and aeration, and, in a wider sense, its chemical and biological condition. Amendments do not necessarily provide nutrients directly but create a better environment for root growth and microbial activity. Example: adding compost to degraded soil in a flower farm improves tilth and moisture retention without relying on fertiliser inputs alone.

Fertiliser

A fertiliser is a substance that supplies one or more essential plant nutrients to the soil or plant. Fertilisers are primarily concerned with correcting nutrient deficiencies to support growth and yield. Example: nitrogen-phosphorus-potassium (NPK) blends are routinely applied to tomatoes and French beans to meet their nutrient demands.

Organic matter

Organic matter refers to decomposed plant and animal residues in the soil that contribute to fertility by improving structure, nutrient supply and microbial life. It is a natural soil amendment and a source of slow-release nutrients, crucial for sustainable horticulture where soil organic carbon is low.

Soil conditioner

A soil conditioner is a material added to soil to enhance its physical condition and nutrient-holding capacity. It overlaps with soil amendments but is often used for a specific improvement such as relieving compaction or repairing soil structure. Example: gypsum is used as a conditioner on sodic soils in dry-zone vegetable farms.

Mulch

Mulch is a protective layer of organic or inorganic material applied to the soil surface to conserve moisture, regulate temperature, suppress weeds and reduce erosion. Organic mulches such as straw and dry leaves also decompose and add organic matter, while inorganic mulches such as plastic sheeting do not. Example: a horticultural nursery covers seedbeds with dry grass to keep the soil moist between waterings.

Soil pH

Soil pH is a measure of how acidic or alkaline the soil is, on a scale where 7 is neutral, values below 7 are acidic and values above 7 are alkaline. It controls the availability of nutrients and the activity of soil organisms. Example: in a strongly acidic vegetable plot, phosphorus is held tightly in the soil and crops show deficiency even after fertiliser is applied.

Cation exchange capacity

Cation exchange capacity (CEC) is the ability of the soil to hold positively charged nutrients such as calcium, magnesium, potassium and ammonium on clay and organic matter surfaces, and release them to plants. A soil with high CEC holds nutrients better and loses less by leaching. Example: a sandy nursery bed has low CEC, so adding compost helps it retain applied nutrients.

3.1.2 Importance of soil amendments

Soil amendments allow a horticulturist to correct the specific weakness of a soil rather than simply adding more fertiliser. A well-chosen amendment improves how the soil holds air, water and nutrients, which raises yield and quality and protects the soil for future crops. The main benefits are described below.

Enhancement of soil structure

Soil amendments improve aggregation and porosity, which facilitates root penetration and enhances aeration. Example: passion fruit growers use well-decomposed manure to loosen compacted soil, enabling better root development and nutrient uptake.

Improved water retention and drainage

Amendments such as compost increase the soil's ability to retain moisture in sandy soils while improving drainage in heavy clay soils. This balance is vital in crops such as lettuce grown in greenhouses, where even water supply is critical for quality.

Nutrient supply and retention

Organic amendments release essential nutrients slowly and raise the soil's cation exchange capacity, reducing nutrient leaching. Example: in a coffee nursery, combining organic compost with mineral fertiliser ensures sustained nutrient availability for seedlings.

Enhancement of soil microbial activity

Adding organic amendments stimulates beneficial microbial populations that contribute to nutrient cycling and disease suppression. Example: regular application of farmyard manure in a vegetable farm supports soil organisms that release nutrients from residues.

Correction of soil pH and toxicities

Certain amendments adjust soil pH to the optimal range for horticultural crops or reduce toxicities such as aluminium and sodium. Example: lime applied in an avocado orchard on acidic soil raises the pH, reduces aluminium toxicity and improves nutrient availability, while gypsum helps remove excess sodium.

3.1.3 Types of soil amendments

Horticulturists use diverse soil amendments chosen to match the soil constraint and the crop requirement. Broadly, amendments are organic (from plant or animal residues), inorganic (mineral-based) or biological (living organisms). The three inorganic materials studied in detail here are fertilisers, agricultural lime and gypsum; other amendments are described after them.

3.1.3.1 Fertilizers

Fertilisers are manufactured or natural materials that supply essential nutrients to plants. Nitrogen, phosphorus and potassium are needed in large amounts and are called primary nutrients, while calcium, magnesium and sulphur are secondary nutrients and elements such as zinc, boron and iron are needed in small amounts. Choosing the right fertiliser depends on the crop, the growth stage and the nutrient shortage shown by a soil test.

The main classes of fertiliser are:

• Straight fertilisers: supply one main nutrient, for example urea for nitrogen or muriate of potash for potassium. They allow the grower to adjust one nutrient without over-supplying the others.
• Compound fertilisers: contain two or more primary nutrients in each granule, for example NPK 17:17:17 or DAP. They are convenient for basal application at planting.
• Nitrogenous fertilisers: supply nitrogen to promote leaf and shoot growth, for example urea and calcium ammonium nitrate (CAN).
• Phosphatic fertilisers: supply phosphorus for root development and early growth, for example triple superphosphate (TSP) and DAP.
• Potassic fertilisers: supply potassium for fruit quality and disease tolerance, for example muriate of potash (MOP).
• Foliar and water-soluble fertilisers: are dissolved in water and sprayed on leaves or applied through irrigation (fertigation), giving quick correction of deficiencies in greenhouse crops.

Common mineral fertilisers and their uses are compared below:

Fertilisers are applied by the following methods:

• Basal application: the fertiliser is placed in the planting hole or furrow and mixed with soil before planting, so that young roots find phosphorus early.
• Top dressing: the fertiliser is applied around the plants during growth, usually nitrogen, at a safe distance from the stem to avoid scorching.
• Broadcasting: granules are spread evenly over the whole bed and incorporated, suitable for closely spaced crops.
• Banding or side placement: granules are placed in a band beside the row, which reduces fixation and wastage.
• Foliar feeding and fertigation: dissolved fertiliser is sprayed on leaves or supplied through the drip line to give quick, uniform nutrition.

Fertiliser is recommended as kilograms of nutrient per hectare, so the quantity of product must be worked out from its nutrient content.

Example: a grower needs to apply 60 kg of nitrogen per hectare as urea (46% N) on a 0.25 ha cabbage plot.

Formula: mass of fertiliser = nutrient required ÷ nutrient fraction

Substitution: mass per hectare = 60 ÷ 0.46

Working: 60 ÷ 0.46 = 130.4348 kg of urea per hectare

Plot requirement = 130.4348 × 0.25 = 32.6087 kg

Result: 32.6 kg of urea is needed for the 0.25 ha plot.

The procedure for applying a fertiliser is:

1. Take a soil sample and obtain test results so the nutrients needed are known.
2. Select the fertiliser that supplies the deficient nutrient and calculate the quantity for the plot area.
3. Weigh the fertiliser accurately and wear gloves and a dust mask.
4. Check that the soil is moist but not waterlogged, to reduce loss and root scorch.
5. Apply the fertiliser by the chosen method, keeping it a few centimetres away from stems and seedlings.
6. Cover granules lightly with soil, or irrigate, so that nutrients move into the root zone.
7. Clean the equipment, store leftover fertiliser dry and sealed, and record the product, rate, date and plot.

Precautions to observe with fertilisers:

• Never apply more than the recommended rate, since excess salts burn roots and pollute water.
• Avoid applying nitrogen just before heavy rain, as it leaches or runs off.
• Keep fertilisers away from seed and from wet leaves to prevent scorching.
• Store fertilisers in a dry, ventilated store away from food and chemicals.
• Wear gloves and wash hands after handling, because many fertilisers irritate skin and eyes.

3.1.3.2 Agricultural lime

Agricultural lime is a ground material rich in calcium carbonate (limestone) used to neutralise soil acidity. It raises soil pH, supplies calcium (and magnesium in dolomitic lime), and reduces the harmful effects of aluminium and manganese in acidic soils. Example: an avocado or French bean grower on strongly acidic soil applies lime before planting so that roots grow well and nutrients such as phosphorus become available.

The effects of liming are:

• Raises soil pH: carbonate in the lime reacts with hydrogen ions in the soil, making the soil less acidic.
• Reduces aluminium and manganese toxicity: these elements dissolve in strongly acidic soil and damage roots, but become less soluble as pH rises.
• Supplies calcium and magnesium: these are secondary nutrients, and magnesium is supplied when dolomitic lime is used.
• Improves phosphorus availability: phosphorus is less tightly fixed in soil of moderate pH.
• Supports soil organisms: nitrogen-fixing bacteria and decomposers work better in near-neutral soil.
• Improves fertiliser efficiency: crops use applied nutrients better after the pH has been corrected.

Common liming materials are compared below:

The finer the lime is ground, the faster it reacts with the soil.

The procedure for liming a horticultural plot is:

1. Sample the soil and measure its pH and, where available, the lime requirement.
2. Select a liming material and calculate the quantity from the laboratory recommendation, the plot area and the neutralising value of the product.
3. Wear gloves, goggles and a dust mask, and avoid windy conditions.
4. Spread the lime evenly over the surface of the plot.
5. Incorporate the lime into the topsoil by ploughing, digging or forking, so that it contacts the soil particles.
6. Allow several weeks and some moisture for the lime to react before planting sensitive crops.
7. Retest the soil pH after the waiting period and record the results.

Lime should not be applied at the same time and place as ammonium-based fertilisers or fresh manure, because nitrogen can be lost as ammonia gas. Over-liming raises the pH too far and causes deficiencies of iron, manganese and zinc, so rates must follow soil test recommendations.

3.1.3.3 Gypsum

Gypsum is calcium sulphate (CaSO4·2H2O), a mineral amendment that supplies calcium and sulphur without changing soil pH much. Its main role is to improve sodic soils, in which too much sodium on the clay particles makes the soil sticky when wet, crusted when dry and slow to drain. Example: a vegetable farm in a dry area with sodic soil applies gypsum so that water infiltrates and seedlings emerge through the surface.

Gypsum works in the following ways:

• Exchanges sodium: calcium from gypsum replaces sodium on the clay particles.
• Removes sodium by leaching: the displaced sodium combines with sulphate and is washed below the root zone by irrigation water, provided drainage is adequate.
• Improves structure: clay particles bind into stable aggregates, reducing crusting and dispersion.
• Improves infiltration and aeration: water and air move more freely, so roots penetrate deeper.
• Supplies calcium and sulphur: calcium is important for fruit quality, for example in preventing blossom-end rot in tomatoes, and sulphur is needed for protein formation.
• Does not neutralise acidity strongly: unlike lime, it is not used mainly to raise pH.

Lime and gypsum differ as follows:

The procedure for applying gypsum is:

1. Test the soil for pH, salinity and sodium, and confirm that sodicity is the problem.
2. Calculate the gypsum quantity from the laboratory recommendation and the plot area.
3. Spread the gypsum evenly over the surface while wearing gloves and a dust mask.
4. Incorporate it into the topsoil by digging or light ploughing.
5. Irrigate thoroughly so that the displaced sodium is leached below the root zone, ensuring that drainage is working.
6. Wait before planting, then check soil condition and infiltration.
7. Record the rate, date and plot, and retest the soil after the next season.

Organic amendments

Organic amendments include materials derived from plant or animal residues such as compost, manure, green manure and crop residues. They improve soil structure, water retention and nutrient supply while enhancing microbial activity. Example: a vegetable grower incorporates well-decomposed manure and crop residues into beds before transplanting to raise organic matter content.

Biological amendments

Biological amendments involve the introduction of beneficial microorganisms such as mycorrhizal fungi and nitrogen-fixing bacteria. These organisms improve nutrient uptake and promote plant health. Example: inoculating tomato seedlings with mycorrhizae can improve phosphorus uptake and plant vigour.

Cover crops

Cover crops such as mucuna and lablab are planted to add biomass and fix atmospheric nitrogen, thus enriching soil fertility in vegetable production. They also protect the soil surface from erosion and, when cut and left on the surface, act as a mulch.

Biochar

Biochar is a carbon-rich product obtained from the pyrolysis of organic materials, used to improve soil fertility by increasing nutrient retention and providing habitat for microbes. Smallholder horticultural farms may use it to rehabilitate degraded soils and support better crop yields.

Chapter Summary

This chapter explored the concept of soil amendments, beginning with clear definitions of key terms related to improving soil quality. It highlighted the importance of soil amendments in enhancing soil fertility, structure, and crop productivity, which are essential for sustainable agriculture. Various types of soil amendments were examined, including organic and inorganic options, each serving distinct roles in soil improvement. Fertilizers were discussed in detail, focusing on their role in supplying essential nutrients to crops and addressing nutrient deficiencies. The chapter also covered agricultural lime, emphasizing its function in correcting soil acidity and improving nutrient availability. Finally, gypsum was introduced as a soil amendment that helps improve soil structure and provides calcium and sulfur, important for plant growth. Together, these topics provide a comprehensive understanding of how to apply soil amendments effectively for optimal soil health.