The document provides an overview of agronomy concepts including plant classification, cropping systems, tillage practices, and Zimbabwe's agro-ecological zones. It discusses classifying plants according to family, use, and life cycle. Common cropping systems like monoculture, intercropping, fallow, and crop rotation are described along with their advantages and disadvantages. Tillage types and aims, planting methods, and factors determining plant population are also outlined. Finally, Zimbabwe's five natural regions are defined based on rainfall and suitable farming activities.

1. AGRONOMY LECTURE NOTES
PREPARED BY
TEMBO L (0782 356 039)

2. TABLE OF CONTENTS
SECTION A
Agronomy
SECTION B
 Microbiology

3. AGRONOMY
The importance of classifying plants
To have universal way of naming plants.
To identify plants that are closely related so that rotations can
be wisely planned to avoid pests and diseases carryover, and
to improve soil fertility.
To effectively plan the use of herbicides in weed
management.
To associate each plant with the correct usage.
To effectively manage the environment by protecting soil
erosion through growing favourable crops in erosion prone
areas.

4. Classification according to crop plant families
Gramineae :It constitute the staple crops of mankind. Eg
maize, rice, wheat, sorghum, rye, oats, millets sugarcane
and pasture grasses.
Leguminosae : bear pods and have ability to fix nitrogen
into the soil. e.g. groundnuts, soyabeans, peas, cowpeas,
and the common bean.
Solanaceae : eg tobacco, tomatoes, egg plant, potatoes
and paprika.
Euphorbiaceae : Has three categories of crops, the
tuberous rooted like cassava, the oil crops for example
castor bean and the rubber crops.

5. Agronomic classification
Agronomic classification involves classification of crops
according to their use, planting time and life span.
Classification according to use.
Grain crops: example maize, rice, wheat and oats.
Root tubers: eg carrots and irish potatoes.
Oil crops: Eg groundnuts, soya bean, sunflower, sesame, castor
bean and jatropha.
Pulses: are grain legumes grown for food other than oil extraction
eg common bean, bambara nuts, and pigeon pea.
Fibre crops: Examples are cotton and sisal.
Beverage crops: example, coffee, tea and grapes.
Fruit crops: eg tomatoes, cucumbers, citrus, bananas
Ornamentals: are grown for natural beauty. Eg flowers and lawn.
Fodder crops: Eg velvet bean, and pasture grasses.

6. Classification according to their life cycles
Annuals
complete their life cycles in 1 year or season. Eg maize soya
bean, groundnuts.
Most crop plants are annuals.
Bi-annuals
finish their life cycles in 2 years or seasons.
During first year, the plant will go through the vegetative
growth and will have its reproductive growth in the second
year e.g. cotton
Perennials
These crops live for more than two years, for example, cotton
and chillies.
Once fully developed they produce seed every year.

7. Classification according to region of origin.
Tropical
 These are intolerant of cold temperatures, for example
bananas and mangoes.
Subtropical
These crops are generally tolerant of cold temperature but not
freezing temperature, for example granadilla and citrus..
Temperate
Are generally deciduous and require a cold period
for example, apples, peaches, grapes, plums, apricots and
pears.

8. Cropping system
A cropping system describes what crops are grown in a
particular location as well as how and why they are grown.
Cropping systems are usually designed to achieve
optimum resource utilisation,
to preserve the environment by conserving rather than
consuming natural resources and to manage and control
pests, weeds and diseases.
Monoculture
Refers to the growing of the same crop on the same piece of
land season after season.

9. The choice of monoculture is mostly dependant on location,
prevailing climatic conditions, type of crop, and the available
capital assets at the farm.
Advantages of monoculture.
It is simple and has a relatively low labour and capital
requirements.
The labour force will get to know what is to be done and when
using the same equipment now and again.
Improved method of harvesting, drying and storage facilities.
Since only the same type of crop will be grown there will be no
regular adjustments to the implement.
The same storage facilities will be used season after season.
Reasonably profitable.

10. Disadvantages of monoculture
There are high chances of pest and disease build up
This may increase the cost of production in trying to
protect the crop.
Due to carryover of pest and diseases season after
season, their population will increase to levels that may
require more frequent sprays or more costly products at
high dosage rate which will call for extra costs.
 Weeds in the same family as the crop may become a
problem, especially in cereal crops, hence increased
competition resulting in low yields.

11. Intercropping
Is the growing of more than one crop on the same piece of land at
the same time. E.g. maize, cowpeas or maize and pigeon pea
systems.
Advantages of intercropping.
Spread the risk of crop failure. Growing two crops at the same
time will allow one crop to survive in the event of bad weather or
disease challenge.
Harvesting two crops on the same piece of land at the same time.
Under good growing conditions there are high chances of getting
good yields from two crops on the same field hence more
retains/given area.
It enhances pests and disease control.

12. Improves soil fertility. The inclusion of a legume crop which
fixes nitrogen will improve soil fertility.
Controls soil erosion. Intercropping will improve ground cover
thereby reducing soil erosion.
Disadvantages of intercropping.
Presents harvesting problems, for example where maize is
intercropped with cowpeas.
The cowpeas will entangle the maize plant such that it will be
difficult to access the cobs or to move between crop rows.
Limits the use of herbicides. Most herbicides are crop
specific, hence the inclusion of a different crop will not be
reserved by herbicide selectivity.
Chemical pest control might be hindered due to inadequate
spray penetration.

13. Fallow system
The land is left un-cropped for a certain period of time usually two to
four years.
Advantages of fallow system
Fertility enhancement from organic matter decomposition and
undisturbed structure.
Soil erosion control since the land is left untilled
Breaks the life cycles of pests and diseases
Disadvantages of fallow system
It is difficult to practise where land is limiting.
There benefits are not immediate as one has to leave the land idle
over two years or more.

14. Crop Rotation
Is when two or more crops are grown in affixed sequence on the
same field.
Basic principles of developing a crop rotation.
Select crops well suited to the soil, topography of land and climate.
Include legumes to maintain nitrogen and organic matter of the soil
especially on land grown crops such as potatoes, tobacco, maize.
Select crop sequences which produce the highest yields and
provide the best control of weeds, insects and diseases.
Select crops that provide good distribution for labour.
Grow a maximum area of the most profitable crops
Consider also soil conservation, goals and managerial ability of the
farmer, and capital available

15. Types of rotations
Grass ley rotation: Is a rotation which includes a period in grass
which is used for grazing and conservation.
Continuous cereal and break crops. For example; maize- soya
bean – maize, cotton – maize- cotton, tobacco – maize- maize,
maize- wheat- soya bean maize.
Benefits of crop rotations.
Increased yield from conserved soil and improved fertility (nitrogen
fixation by legumes)
 The supply of organic matter is increased which is important in
maintaining the physical properties of the soil
Soil nutrients are utilized more efficiently. Various crops take up
nutrients at different rooting depth.

16. Erosion losses are reduced. Legumes and grass are highly
effective in reducing soil and nutrient losses by erosion.
Manure and commercial fertilizers are more effectively
utilized.
Weeds, pests and diseases are better controlled.
Keeps the land occupied a greater part of the year with crops
as one crop will be planted soon after the other crop.
Hazards are reduced. The risk of complete failure or poor
production is usually greater with one crop than when several
crops are grown.
Saves on labour by growing two or more crops on one soil
preparation where conservation farming will be practiced.
Farm labour will have diversified knowledge on various crops

17. Disadvantages of crop rotation
It requires diverse knowledge for management of different
crops.
Requires more capital investment as same crops do not share
the same equipment or structures.
Crop relationship in a rotation.
Competitive: one crop is grown at the expense of the other.
Complimentary: is when an increase in total output of one crop
results in an increase in total output of another crop from a fixed
area or given resource.
Supplementary: is when the crops neither add nor subtract
from the output of each other.

18. Tillage
Tillage refers to any disturbance of the soil surface.
Aims of tillage
To loosen and break up the soil in order to increase aeration and
water infiltration.
To prepare a seedbed of suitable tilth for easy planting, good
germination and emergence of crops.
To break hard and capped soils at the end of dry season.
To promote better root development
To minimise soil erosion and for moisture conservation
To expose soil pest to their predators and the heat of the sun and to
destroy weeds.
To enhance organic matter decomposition.

19. Primary tillage
is tillage which cuts, inverts or shatters the soil to a depth of
15 to 36cm. It usually leaves the soil rough.
Implements used tend to be heavier and stronger than
secondary tillage eg rippers, subsoilers, mouldboard and disc
ploughs.
The aims are loosening and aerating the soil, covering plant
residues, and incorporating fertilizers and lime.
Secondary tillage
It works the soil to shallower depth of 5-15cm.
Implements used include disc harrow, spike toothed harrow,
roller, cultivator.
The aims include the attainment of fine tilth, weed control,
firming the soil and levelling the land.

20. Timing of land preparation.
Early land preparation
is done soon after harvesting around March to May.
advantages
A good tilth is achieved since the soil will be still moist.
There is early and good burial of plant debris in moist soil
which is conducive for decomposition thereby increasing soil
fertility.
Wear and tear of implements is reduced hence reduced cost
of preparing the land.
It enhances early plantings giving crops an early start when
conditions are more suitable for plant growth.
Assists in weeds, pests and diseases control.

21. Conservation tillage
It is a system of farming which involves managing the land
with principles of conservation.
It also involves any practice which leaves at least 30% crop
residue cover on the soil surface after planting.
Advantages of conservation tillage
It reduces soil erosion as the crop residues cushion the rain
drop action.
There is maximum use of available moisture as water run-off
and evaporation are reduced thereby improving penetration.
There is moderation of soil temperature extremes by crop
residues.
Reduced tractor operating costs as less fuel and labour will
be used, and there is reduced wear and tear of implements.

22. Minimum soil disturbance results in less pulverisation of
the soil.
Disadvantages of conservation tillage.
There is built up of pests and diseases.
There is poor incorporation of trash and as a result the
decomposition process is slowed down
Nutrients tend to be concentrated at the top layers of soil.
Roots will not go deeper hence lodging is common and
crops are susceptible to droughts.

23. PLANTING
Is the placement of propagative material into the rooting media to
enhance germination and emergence.
Time of planting
should be done when the environment is ideal in terms of
temperature, moisture and day length.
Time planting so that the crop will be ready when the conditions will
be conducive for ripening or harvesting, for example, soyabean
should not be ready for harvest when it is still raining.
For most plants one must avoid frosty periods.
Other factors are pest and diseases incidences, market prices,
weeds pressure and the onset of rain for summer crops.
For most summer crops there is yield decrease with late plantings.

24. Plant population
Plant population is the total number of plants per given area
(plants/ha).
Factors that determining plant population
Type of crop and variety, for example in groundnuts the population is
high when growing the bunch type than the spreading type.
 The level of management, for example a maize crop under irrigation
requires a higher plant population compared to dryland cropping.
 Type of season, for example a predicted low rainfall season would
require lower plant population so that plants will not compete for the
limited moisture available.
 The region in which the crop is grown. Low rainfall regions e.g.
Region III and IV one to use low plant populations.
The intended use of the crop, e.g. silage maize crop should be
grown at a higher plant population compared to commercial maize.

25. Plant population = 10 000m2/ (Inter-row (m) X In-row spacing (m))
For example:
A maize crop planted spaced at 900mm X 200mm
 Plant population = 10 000m2
(0,9 X 0.2)m2
= 55 555plants/ha
Activity
Given that the in-row spacing for cabbage is 400mm and inter-row
spacing is 50cm and the application rate of compound D and AN is 200
kg and 300kg respectively;
Calculate
a. Plant population/hectare
b. Amount of compound D per planting station
c. Amount of ammonium nitrate per planting station

26. Advantages of an optimum plant population per hectare
To get optimum yields due to low competition among crops for
available resources and land not wasted.
To get high quality produce from a crop filly benefited from resources
available.
To provide good soil cover especially in row crops like tobacco and
cotton.
To reduce disease prevalence. This will be achieve through reduce
transfer of pathogens from plant to plant and also through good
ventilation within the field.
To enhance efficiency execution of other operations like spraying
and movement of implements especially in crops like cotton and
tobacco.

27. Types of planting
Dry planting:
 is planting well in advance i.e. 2-3 weeks prior the coming of the first rain.
 This ensure an early start to the season and to release labour for other farm
operations when the rains start.
Water planting
 is a method of lengthening the growing season especially in sandy soils.
 The aim is the establishment of the crop early before the first effective rains.
 Supply adequate planting water to link with the residual soil moisture.
Water planting without water
 Is the planting of seed in wetlands e.g. rice
Rain planting:
 is planting after receiving good effective rains.

28. AGRO-ECOLOGICAL ZONES OF ZIMBABWE
 Zimbabwe has five natural regions.
 These regions are divided according to the rainfall pattern of their respective
areas.
Natural Region I
 It is a specialised and diversified farming region. Altitude is 1700m or more
above sea level.
 The rainfall is >1 000mm per year. Temperature is cool and is normally around
20oC on average.
 The intensive safe use is afforestation (plantation crops) like pine and wattle
trees.
 There is also intensive livestock production especially dairy as well as deciduous
fruit tree production.
 In frost free areas, there is production of macademia nuts, potatoes, coffee and
tea production. It covers less than 2% of the country. The area stretches from
Nyanga to Tamandai.

29. Natural Region II
• It is a very intensive farming region. Most of the country’s maize
and wheat production occurs in this region. Most of the rainfall is
confined to summer and it averages 750-1 000mm/year. It has two-
sub-regions namely IIa and IIb.
• NR IIa receives an average of 18 rain pentads per season. It rarely
experiences any severe dry weather. The region is suitable for
intensive crop production and also livestock production.
• NR IIb receives an average of 16-18 rain pentads per year and it
normally experiences severe dry spells in the rain season and at
times it may have short rain periods. Crop production may be
affected in some of the seasons but the effect is not sufficient
enough to change utilization from intensive system.

30. Natural Region III
• This is a semi-intensive farming area. It is involves mixed
farming. It receives rainfall of about 650-800mm a year.
• The region has an average of 14-16 rainy pentads per year.
• The region is subject to mid season dry spells. It is suitable for
livestock and fodder production.
• There is marginal production of tobacco and maize.
• The region covers about 19% of the country.

31. Natural Region IV
• The intensive safe use is semi-extensive farming. It receives 450-
650mm of rainfall per year. The region is prone to drought and
severe dry spells hence not suitable for crop production.
Production is mostly livestock, natural forest and some fodder
crops. It is recommended to grow small grains which are drought
resistant. The area covers about 38% of the country.
Natural Region V
• This is an extensive farming region which receives less than 450mm
rainfall per year. The rainfall is too low and very erratic and
therefore is not suitable for crop production. Crops that are grown
in this region are grown under irrigation. The region is suitable for
cattle ranching and wildlife management. It covers about 27% of
the country.