Soil fertility and management

Class: JSS2

Subject: Agricultural Science

Topic: Soil fertility and management

Objective:

By the end of the lesson, student should be able to;

To define crop rotation.

State principles of crop rotation.

Highlight the advantages of crop rotation.

Explain fallows and cover crops.

Day 1

Introduction

Crops need nutrients just like people do. A fertile soil will contain all the major nutrients for basic plant nutrition (e.g. nitrogen, phosphorous and potassium) as well as other nutrients needed in smaller quantities (e.g. calcium, magnesium, sulphur, iron, zinc, copper, boron, molybdenum, nikel). Usually a fertile soil will also have some organic matter that improves soil structure, soil moisture retention and also nutrient retention, and a pH between 6 and 7. Unfortunately many soil do not have adequate levels of all the necessary plant nutrients, or conditions in the soil are unfavourable for plant uptake of certain nutrients.

Soil scientists that focus on soil fertility are interested in managing nutrients to improve crop production. One way to ensure that soil nutrients are not completely exhausted is for  farmers to adopt the system of crop rotation. This system involves changing from one type of crop in one year to a different type of crop the following year. It is very important that crop rotation is planned in such a way that the soil nutrients are not completely used up. Planting of crops in a rotation system must follow a definite scientific sequence because different crops require different amounts of plant food. Some crops use many nutrient and are referred to as ‘heavy feeders’ for instance, yam, cassava and maize. Other crops use less amounts of nutrients and these are ‘light feeders,’ for example, millet and vegetables. Some plants can actually add nutrients to the soil, for example, legumes i.e. groundnut. 

Other methods of managing soil fertility include the use of fallows and cover crops and application of fertilizers.

Day 2

Principles of Crop Rotation

In planting crop rotation, deep rooted crops (e.g. groundnuts) should be followed by shallow rooted crops, (e.g. maize). This enables all the nutrients in every part of the soil to be used. A deep rooted crop draws its nutrients from deep down the soil profile, but shallow rooted crops derive their nutrients from the surface of the soil.

The growth of certain crops is supported by certain nutrients. For example, repeated growth of maize uses up most of the nitrogen, while cassava uses up most of the potash.

Examples of crop rotation plans are as follows

Crops to plant

Yams, cassava, potatoes (tubers)

Maize, sorghum (grain crops)

Mucuna, groundnut, cowpea (legumes)

Order to follow

a.

Year 1

Root crops, cassava, yams, potato

Year 2

Grain crops, maize, millet, sorghum

b.

Year 1

Legumes, cowpea, soya bean, groundnut

Year 2

Roots and tubers like yams, cassava and maize

Crops which are attacked by the same pests and diseases should not follow each other.

All crops are attacked by a variety of pests, diseases and weeds. Certain pests and diseases and weeds thrive very well on certain crops. If some crops are planted repeatedly, such pests, diseases weeds will become a common feature in that area and will cause serious damage to crops and thereby contribute to high yield losses. For example, if yams are panted repeatedly in an area, the population of yam nematodes builds up, but if yam is alternated with maize, tobacco or cotton which are not affected by the yam nematodes, their population will be reduced and thereby controlled.

The use of leguminous crops, e.g. cowpea, mucuna and groundnuts is desirable in crop rotation. Leguminous crop plant are important sources of soil organic matter and they also have the ability to fix nitrogen in their root nodules which is later released to enrich the soil.

Day 3

Advantages of Crop Rotation

Crop rotation is advantageous to the farmer because:

 through adoption of this system a variety of food items can be obtained throughout the year. 

Crop rotation helps farmers to control pest and diseases and increase soil fertility. 

In situations where land for farming is limited, crop rotation provides a solution to such problems.

Day 4

Fallows and cover crops

Fallows 

Fallow is the term used to describe a piece of land that is left unplanted for one or more seasons. This system is adopted in many parts of West Africa where agricultural land is abundant.

Bush fallowing involves growing crops on a piece of land until the land is no longer productive, so the land is abandoned and allowed to go back to bush for six to twelve years before it is used again. The farmer then moves to other areas to make new farms. There are many advantages and disadvantages in this type of system.

Fortunately the practice of bush fallowing is becoming rare in Nigeria because of the increasing population pressure on land and improved agricultural technologies through the introduction of crop rotation and chemicals for destroying weeds before or during cropping of the land. However, the system may continue to persist in those parts of tropical Africa where land tenure system encourages people to temporarily acquire land for farming.

Cover crops

These are crops that are grown to protect the soil surface from being washed away by rain. The surface soil is protected from mechanical action of rain by stopping and breaking up the rain drops before they hit the soil. This prevents surface runoff and leaching of the soil. The crops also prevent rays of the sun from overheating the soil surface which may lead to excessive evaporation of water from the soil surface.

When a piece of land is lying fallow before the next cropping season, cover crops are usually planted to occupy such land. Many cover crops belong to the leguminous family; they are legumes. Examples are beans, mucuna, centrosema, and pureria. Many grasses and short-lived trees are also used as cover crops. Legumes are able to fix nitrogen in the root nodules and this nitrogen is released into the soil to improve the soil fertility. The figure below shows mucuna which is a useful cover crop because the roots have nodules which contain nitrogen-fixing bacteria. Cover crops enrich the soil with organic matter when their fallen leaves decay in the soil. Trees act as wind breaks to cut down wind movement, thus reducing erosion, rates of transpiration from crops and evaporation from the soil.

Mucuna cover crop (Credit:aarsb.com)

Day 5

Home Work

State the principles of crop rotation.

Highlight the advantages of crop rotation.

Define crop rotation.

Differentiate between land fallowing and cover cropping.

What do you understand by a leguminous crop?

Give eight examples of leguminous crops.

Differentiate between transpiration and evaporation.