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Learning objectives of this guide

  • To provide an understanding of the soil
  • To explain the essential nutrients needed to keep soil fertile and plants healthy.
  • To understand what soil degradation is
  • To understand practices that reduce soil fertility.
  • To provide information on the techniques employed in improving soil fertility

Learning Outcomes

This how to guide on soil fertility and plant nutrition will enable you to

  • Understand the process of naturally increasing soil fertility
  • Understand the essential nutrients needed in keeping your soil fertile.
  • Understand the negative practices that reduce soil fertility

INTRODUCTION

Soil and Fertility

  • Soil by definition is a natural body consisting of layers (soil horizons) that are composed of weathered mineral materials, organic material, air and water.
  • Soil fertility is the capacity of the soil to support the growth of plants on a sustained basis, yielding quantities of expected products that are close to the known potential.

What you need to know about essential plant and soil nutrients

There are about seventeen elements have been found to be essential for plant growth. The types and quantities of nutrients must be correctly balanced and applied in order that the crop is vigorous and healthy. Different crops require different types and amounts of nutrients. Major or macro-elements are needed in large quantities, while trace or minor or micro-elements are required in small quantities. The elements must be present in forms usable by plants and in concentrations that are optimal for plant growth. However, some of the nutrients, when present in excess amounts are toxic to the plant, for example Manganese, Aluminum and Sulphur. The elements as listed below, are categorized into macro- and micro- (trace) elements.

The elements as listed below, are categorized into macro- and micro- (trace) elements.

Macro-elements Source Macro-elementsMacro-elements Source
Macro-elements Source Macro-elementsMacro-elements Source
Carbon (C), Hydrogen (H2), Oxygen (O)Derived from air and water
Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca)Derived from soil solids, and some from air
Magnesium (Mg), Sulphur (S) Micro Trace elementsDerived from soil solids.
Iron (Fe), Molybdenum (Mo), Copper (Cu), Zinc (Zn), Manganese (Mn), Cobalt (Co), Boron (Bo), Chlorine (Cl) Iron (Fe), Molybdenum (Mo), Copper (Cu), Zinc (Zn), Manganese (Mn), Cobalt (Co), Boron (Bo), Chlorine (Cl)Derived from soil solids.

Click here for more information on essential plant and soil nutrients

What you should know about Soil degradation?

It is a change in the soil health status resulting in a reduced capacity of the ecosystem to provide goods and services for its beneficiaries.

Causes of soil degradation

Depletion of nutrients and soil organic matter and erosion are the principal forms of soil degradation. Overgrazing and cultivation practices that are not adapted to local environments are the principal causes of soil degradation. Overgrazing is often the result of the loss of pastures to agriculture. Producing crops without compensating the nutrient losses by removing plants also leads to soil degradation. The most prominent degradation feature worldwide is erosion by water. Various forms of chemical deterioration, such as soil fertility decline and soil pollution, and physical deterioration, such as compaction and water logging, account for smaller areas.

There are various types of soil degradation such as soil erosion, water erosion, wind erosion, soil pollution, acidification, loss of nutrients and compaction. All these types of degradation lead to the reduction of soil fertility and land productivity.

Click here for key information on the types of soil degradation.

Techniques employed in improving soil fertility

1. Composting

It is the process of breaking down organic materials of plant and animal origin to produce humus. The requirements for composting are the presence of soil micro-organisms and organic material such as animal manure, crop remains, municipal garbage, kitchen waste, hedge trimmings and non-seeding weeds. Additional requirements include moisture to hasten decomposition, temperature control to optimize microorganism activity, and aeration to provide adequate oxygen for the decomposition process and labour.

Learn how to prepare compost from crop remains and non-seeding weeds.

2. How to prepare Boma compost from fresh livestock droppings

  1. Dig a pit 0.5 m deep behind the boma, putting the excavated soil beside the pit. The pit should be 1.5 m wide and any length, depending on the amount of material available. Loosen the soil at the bottom of the pit and place a layer of dry crop residues like maize stover or grass at the bottom.
  2. Then place a layer of about 10 cm of fresh manure and bedding obtained from the boma.
  3. Cover this with a thin layer of topsoil (1ñ2 cm).
  4. Now add a 10-cm layer of manure and again cover with a thin layer of topsoil. Repeat the process until the compost heap is 1.5 m high.

Learn the complete step by step guide to making boma compost from fresh livestock droppings.

3. Liquid manure

Liquid manures are useful for top-dressing. It can be made from plants or animal droppings. Extra nitrogen for top-dressing can be made locally from a specially prepared liquid fertilizer termed plant tea. This can be made from plants such as Tithonian diversifolia (Mexican sunflower)

Learn how to make plant-tea solution and liquid manure from animal droppings.

4. Mulching

Mulching is the covering of the soil with crop residues, dry grass and leaves. Once rotten and decomposed, mulch forms humus and adds to the organic matter in the soil. Mulching is important for the prevention of soil erosion, addition of organic matter to the soil, regulating the soil temperature, increasing soil micro-organism and biological activity, weed suppression, increasing water retention, and decreasing evaporation from the soil surface. It is important to ensure that sufficient mulch is maintained as soil cover to reduce evaporation of soil moisture and to discourage the growth of weeds.

Learn how to mulch your land

5. Green manures

Green manures are plants that are deliberately grown for the purpose of incorporation into the soil to improve soil fertility and organic matter content. Legumes are the most commonly grown green manures, but other plants that are not legumes, such as Tithonian, may also be used. Most green manure crops also play a role in covering the ground and protecting it from solar radiation and soil erosion. Crops which serve these functions are often referred to as green manure cover crops.

Benefits of using green manures include:

  • Nitrogen supplied by legumes
  • Improved soil tilth and water infiltration
  • Reduction in diseases and nematodes
  • Weed control
  • May trap nitrates and prevent leaching • Control of erosion
  • Source of feed for livestock
  • An ideal green manure crop is one that meets most of the following criteria:
  • It is fast growing (accumulates much biomass within a short period)
  • It fixes nitrogen from the air
  • It is deep rooting and thus improves soil structure and recycling of nutrients
  • It covers the soil quickly, thus controlling erosion and suppressing weeds

Click here for more information on green manure

6. Use of organic fertilisers

Organic fertilisers are materials derived from plant and animal parts/droppings or residues which are applied to fertilize the soil. These include Farmyard manure, weed residue, tree pruning, compost, green manure and crop residue, amongst others. Also grazing livestock play an important role in nutrient flow to cropland. Plants contain three substances that define their quality as organic fertilizer: Nitrogen, Phenols and Lignin.

Click here for more information on organic fertilizers

7. Intercropping

Intercropping involves growing two or more crops in the same field at the same time, with at least one of the crops providing quick ground cover. Intercropping can help improve soil fertility when legumes are used. Intercropping also allows for intensive land use where landholdings are small.

Click here to learn more on how to intercrop on your farm

8. Crop rotations

It is the growing of different crops in a predetermined cycle on the same piece of land. During their growth, different crops need different minerals, have different root depths and attract different diseases and pests. The crops used usually include row crops, small grains, legumes and grasses. Legumes enhance plant nutrient levels, while grasses improve soil structure. Rotation of crops prevents pest build up and ensures balanced crop nutrient uptake according to the different plant’s nutritive requirements and rooting depths.

Click here to learn more on carrying out crop rotation on your farm

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References

  • http://www.fao.org/soils-portal/about/all-definitions/en/
  • Gachene, C.K.K. and Kimaru, G. (2003). Soil Fertility and Land Productivity – A guide for extension workers in the eastern
  • Africa region. Technical Handbook No.30. Regional Land Management Unit (RELMA) / Swedish International Development
  • Cooperation Agency (Sida).
  • http://www.fao.org/soils-portal/soil-degradation-restoration/en/
  • https://www.infonet-biovision.org/EnvironmentalHealth/Introduction-soil-degradation.