Soil fertility improvement is a critical component of horticultural success in Kenya, where diverse soil types and climatic conditions influence crop productivity. Applying soil amendments enhances soil physical, chemical, and biological properties, creating an optimal environment for plant growth. This chapter explores the concept of soil amendments, their significance in horticulture, and the various types available to practitioners, equipping professionals with the knowledge to improve soil health sustainably.

3.1 Soil amendments

Soil amendments are materials added to soil to improve its capacity to support healthy plant growth by modifying its physical structure, nutrient content, or biological activity. In horticulture, effective soil amendments can increase crop yields, improve water retention, and prevent nutrient deficiencies. Understanding soil amendments enables a horticulturist to tailor interventions that suit specific soil conditions and crop requirements.

Example: a farm manager preparing a seedbed for cabbages first sends a soil sample for testing. The result shows a low pH and low phosphorus, so the manager chooses agricultural lime to correct the acidity and a phosphorus fertiliser to correct the nutrient shortage, rather than applying one product to solve both problems.

3.1.1 Definition of terms

The terminology surrounding soil amendments is foundational for effective communication and application in horticultural practice. Several key terms require clear understanding.

Soil amendment

A soil amendment is any substance incorporated into the soil to improve its physical qualities such as texture, aeration and water retention, or to enhance its chemical and biological properties. Soil amendments include materials that supply nutrients, such as fertilisers, and materials that mainly change the soil environment, such as lime, gypsum and organic matter.

Organic matter

Organic matter refers to decomposed plant and animal residues in the soil that improve soil structure, water-holding capacity and microbial activity. Common sources include compost, manure and crop residues, which release nutrients slowly and stimulate beneficial soil organisms.

Soil conditioner

A soil conditioner is a type of soil amendment aimed at improving soil structure and tilth, making it easier for roots to grow and for air and water to penetrate. For example, gypsum is used to improve heavy clay soils and sodic soils by encouraging the clay particles to bind into stable crumbs.

pH modifier

A pH modifier adjusts the soil's acidity or alkalinity, influencing nutrient availability. Lime is commonly applied to acidic soils to raise pH and enhance nutrient uptake by horticultural crops such as tomatoes and cabbages.

Soil pH

Soil pH is a measure of how acidic or alkaline the soil is on a scale of 0 to 14, where 7 is neutral. Most vegetables grow best in slightly acidic to neutral soil, so pH is checked before any amendment is chosen.

Sodic soil

A sodic soil contains a high proportion of sodium on its clay particles. The clay disperses when wet, the soil seals over and drains poorly, which is why such soils are treated with a calcium source such as gypsum.

Fertiliser

A fertiliser is a soil amendment that supplies one or more essential plant nutrients, such as nitrogen, phosphorus, potassium and micronutrients, in a form the crop can take up. Most fertilisers do little to improve soil physical properties, which is why they are often used together with organic amendments.

3.1.2 Importance of soil amendments

Soil amendments play a pivotal role in enhancing soil fertility and sustaining horticultural productivity. Their importance can be understood through several interconnected benefits.

Improved soil structure

Amendments such as compost increase soil aggregation, which enhances aeration and water infiltration. For instance, in a vegetable or fruit field, organic matter reduces soil compaction, allowing roots to expand and absorb nutrients more easily.

Enhanced nutrient availability

Amendments can supply or facilitate the release of essential nutrients. Well-rotted dairy manure provides nitrogen and phosphorus, promoting vigorous growth of leafy crops such as kale and spinach.

Increased water retention

Sandy soils drain quickly and hold little moisture. Organic amendments increase their moisture-holding capacity, reducing irrigation frequency and conserving water on a vegetable plot.

Stimulation of soil microbial activity

Organic amendments foster beneficial microbial populations that decompose organic residues and, in the case of some bacteria, fix atmospheric nitrogen. This biological activity supports sustainable nutrient cycling in intensively cropped vegetable farms.

pH regulation

Applying lime to acidic soils corrects low pH, preventing nutrient lock-up and enabling crops like onions and carrots to take up nutrients properly and thrive.

3.1.3 Types of soil amendments

Soil amendments are categorised by their source and function, and each type offers different benefits to horticultural soils. Three types are described first: fertilisers, which supply nutrients; agricultural lime, which corrects acidity; and gypsum, which supplies calcium and improves sodic and compacted clay soils. Other groups of amendments follow after these three.

3.1.3.1 Fertilizers

Fertilisers are manufactured or natural materials that supply one or more essential nutrients to the crop in a readily available form. They are used when a soil test or crop symptoms show that the soil cannot supply enough nutrients for the target yield. Example: a grower raising tomatoes in a greenhouse applies a phosphorus fertiliser at transplanting to support early root growth, then top-dresses with a nitrogen fertiliser during vegetative growth.

Fertilisers are labelled with their nutrient content. An NPK grade such as 17:17:17 means the bag contains 17% nitrogen (N), 17% phosphorus expressed as P2O5 and 17% potassium expressed as K2O by mass.

Common fertiliser groups:

• Straight fertilisers: supply a single main nutrient, for example urea (about 46% N) or muriate of potash (about 60% K2O). They allow the grower to adjust each nutrient separately.
• Compound fertilisers: contain two or more nutrients in each granule, for example DAP (18-46-0) or NPK 17:17:17. They save labour because one application supplies several nutrients.
• Nitrogen fertilisers: such as urea and calcium ammonium nitrate (CAN). They promote leaf and shoot growth and are applied in splits because nitrogen is easily lost by leaching.
• Phosphorus fertilisers: such as triple superphosphate and DAP. They are placed in the planting hole or band near the roots because phosphorus moves very little in soil.
• Potassium fertilisers: such as muriate of potash. They improve fruit quality, firmness and stress tolerance in crops such as tomatoes.
• Foliar fertilisers: soluble products sprayed on leaves to correct quickly a micronutrient deficiency such as zinc or boron.

Worked example: calculating the CAN needed to supply a nitrogen recommendation.

A soil test recommends 60 kg N per hectare for kale. The plot is 0.25 ha and CAN contains 26% N.

Fertiliser per hectare = nitrogen required ÷ nutrient fraction

Fertiliser per hectare = 60 ÷ 0.26 = 230.77 kg

Fertiliser for the plot = 230.77 × 0.25 = 57.69 kg

The grower needs about 57.7 kg of CAN for the plot.

Steps for safe fertiliser application:

1. Read the soil test report and the crop's nutrient recommendation to decide which nutrients and how much are needed.
2. Select the fertiliser whose grade best matches the need, and check the bag label for the nutrient percentages.
3. Calculate the quantity for the plot area, using the nutrient percentage as shown in the worked example.
4. Calibrate the measure, such as a cup or bottle cap, by weighing the amount it holds so each plant receives the same dose.
5. Apply phosphorus and compound fertilisers at planting by placing them in the hole or band and mixing lightly with soil to avoid direct contact with seed or roots.
6. Apply nitrogen in splits as a top-dressing, placing it in a ring or band a short distance from the stem and covering it or watering it in.
7. Wear gloves, avoid applying on leaves in the heat of the day, and store leftover fertiliser dry, closed and away from food and children.
8. Record the product, date, rate and field in the farm record book.

3.1.3.2 Agricultural lime

Agricultural lime is a calcium-based material, usually ground limestone (calcium carbonate), applied to acidic soils to raise pH. In acid soils, aluminium and manganese can become toxic and phosphorus, calcium and magnesium become less available. Liming neutralises the acidity, improves nutrient uptake and supports soil microbes. Example: an onion grower whose field has a low pH applies lime several weeks before transplanting so that the acidity is corrected before the crop is established.

Common liming materials:

• Agricultural limestone: ground calcium carbonate that acts slowly and is safe to handle. Finer grinding reacts faster in the soil.
• Dolomitic lime: a mixture of calcium and magnesium carbonates. It corrects acidity and also supplies magnesium where the soil is low in it.
• Burnt lime (quicklime): calcium oxide, which reacts quickly but is caustic and must be handled with gloves, goggles and a dust mask.
• Hydrated lime: calcium hydroxide, a fine powder that acts faster than limestone but is unpleasant to handle and can scorch plants if over-applied.

Benefits of liming:

• Raises soil pH: it brings acid soil into the range where most vegetables take up nutrients best.
• Supplies calcium: and magnesium in the case of dolomitic lime, both of which are needed by crops.
• Reduces toxic aluminium: it lowers the concentration of aluminium that damages roots in strongly acid soils.
• Improves fertiliser efficiency: nutrients applied to a limed soil are used better by the crop.
• Encourages soil life: beneficial bacteria, including those nodulating legumes, work better near neutral pH.

Worked example: lime for a nursery bed.

A soil test recommends 2 t of lime per hectare. A nursery bed is 100 m². 1 ha = 10 000 m².

Rate per square metre = 2 000 kg ÷ 10 000 m² = 0.2 kg/m²

Lime for the bed = 0.2 × 100 = 20 kg

The bed needs 20 kg of agricultural lime.

Steps for applying agricultural lime:

1. Collect representative soil samples from the field and have the pH and lime requirement tested.
2. Choose the liming material and note its fineness and neutralising ability from the label or supplier.
3. Calculate the quantity for the area, as in the worked example above.
4. Weigh or measure the lime into marked portions so that each part of the plot receives its share.
5. Wear gloves, goggles and a dust mask, and spread the lime evenly over the surface on a calm day.
6. Incorporate the lime into the top 15 to 20 cm by ploughing or digging so that it contacts the soil particles.
7. Wait several weeks, with moisture present, before planting, and keep nitrogen fertilisers that contain ammonium, and manure, from being mixed directly with fresh lime, because nitrogen can be lost as ammonia.
8. Retest the pH after the crop cycle and re-apply only if needed.

3.1.3.3 Gypsum

Gypsum is calcium sulphate, a mineral that supplies calcium and sulphur without significantly changing soil pH. Its main role as an amendment is to improve the structure of sodic soils and heavy clays. The calcium replaces sodium on the clay particles, and the displaced sodium is washed below the root zone by water. Example: a grower with a sealed, sodic seedbed where water ponds applies gypsum and irrigates to leach the sodium, so that the soil opens and seedlings can emerge.

Uses of gypsum:

• Reclaiming sodic soils: calcium displaces sodium so the clay flocculates and water can pass through.
• Loosening heavy clay and crusted soils: it helps clay particles bind into crumbs, reducing surface crusting and improving seedling emergence.
• Supplying calcium: it provides calcium for crops such as tomatoes and peppers without raising pH, which helps reduce calcium-related disorders such as blossom end rot where calcium is the limiting factor.
• Supplying sulphur: it provides sulphate sulphur for crops with a high sulphur demand, such as onions and brassicas.
• Improving deeper rooting: calcium moving into the subsoil can reduce the toxic effect of aluminium on roots, allowing deeper rooting.

Gypsum and lime compared:

Steps for reclaiming a sodic seedbed with gypsum:

1. Test the soil to confirm that it is sodic and find the recommended gypsum rate.
2. Make sure there is a way for water to drain from the field, because sodium must leave the root zone.
3. Spread the gypsum evenly over the surface using weighed portions for each part of the plot.
4. Incorporate it lightly into the top soil by digging or harrowing.
5. Irrigate or allow rain to leach the soil so that the displaced sodium moves below the roots.
6. Retest the soil, and plant a tolerant crop first if some salinity remains.

Organic amendments

These include compost, animal manure, green manure and crop residues. Organic amendments improve soil structure, increase microbial activity and supply slow-release nutrients. For example, a farm may use coffee pulp compost to enrich its soils with organic matter and nutrients. Manure and compost should be well decomposed before use, so that they do not burn roots or lock up nitrogen.

Inorganic mineral amendments

Mineral-based amendments modify soil chemical properties. Besides lime and gypsum, described above, rock phosphate supplies phosphorus in a less soluble form that releases slowly, which is suited to long-term fertility on acidic soils.

Biological amendments

These involve introducing beneficial organisms such as nitrogen-fixing bacteria and mycorrhizal fungi to enhance nutrient uptake and disease resistance. In greenhouse tomato production, inoculation with mycorrhizal fungi can improve phosphorus absorption and plant health.

Synthetic polymers and biochar

Certain synthetic polymers are used to improve water retention in sandy soils, although their use is limited by cost and environmental concerns. Soil conditioners such as biochar, produced by heating crop residues with little oxygen, increase soil carbon content and improve nutrient retention.

Chapter Summary

This chapter explored the concept of soil amendments, defining key terms to establish a clear understanding of their role in agriculture. It highlighted the importance of soil amendments in enhancing soil fertility, improving crop yields, and maintaining sustainable farming practices. Various types of soil amendments were discussed, focusing on their distinct functions and applications. Fertilizers were examined as essential inputs that supply vital nutrients to crops, supporting healthy plant growth. Agricultural lime was presented as a means to correct soil acidity, thereby optimizing nutrient availability and soil structure. Gypsum was described for its role in improving soil physical properties and supplying calcium and sulfur. Overall, the chapter provided a comprehensive overview of how different soil amendments contribute to soil fertility improvement and effective crop production.