Objectives: By the end of this subtopic learners should be able to:
Identify and describe functions the major and minor plant food elements.
State the effect of crop rotation, rest periods and perennial crops on soil fertility.
Explain the nitrogen cycle.
Explain soil sampling and state the importance of soil sampling.
Explain pH and state how soil pH is tested in soils
Describe liming and point out types of liming.
Soil fertility
Soil fertility is the richness of a soil in plant nutrients.
A fertile soil is the one which supplies plants with all mineral salts essential for plant growth.
Fertiliser
It is a material or substance added to soil to supply one or more plant nutrients essential for optimum growth of plants.
Both organic and inorganic fertilisers can be used to add fertility to the soil.
Major macro and micro plant food elements
Major macro nutrients needed by plants in large quantities include nitrogen, phosphorus, calcium and potassium.
A trace or micro nutrients including zinc, manganese, boron, iron, sulphur, cobalt, copper, magnesium, molybdenum and chlorine and an abundance of organic matter.
Carbon, hydrogen and oxygen are supplied from water and air.
Most of the nutrients are obtained from fertiliser, compost or manure.
Trace or micro-elements are nutrients required in very small amounts for normal growth and development of the plant.
Examples of trace elements are boron, molybdenum, manganese, zinc, cobalt, copper, chlorine and iron.
Major nutrients
Nitrogen
Nitrogen is a chemical element with symbol N and atomic number 7.
It occurs in the soil as ammonium (+) and nitrites (). These forms readily dissolve in water and leach away.
Nitrogen also occurs in organic combination as protein, amino acids and other organic compounds.
The effect of nitrogen in plants is to make leaves dark green.
Nitrogen stimulates plants to grow vigorously.
Nitrogen is required for the formation of proteins.
Nitrogen regulates the availability of phosphorous and potassium.
It also increases the size of grains and its protein content of cereal crops.
Nitrogen increases the time taken by crops to reach maturity.
Deficiency symptoms of nitrogen
Stunted growth of the plant because Nitrogen is essential for cell division and enlargement. When it is lacking, plants will be shorter than usual and the leaves may be smaller.
Poor plant growth and maturity may be delayed too.
Leaves are small pale green and later become yellow because they are unable to make sufficient chlorophyll.
Symptoms oversupply of nitrogen
Oversupply of nitrogen results to excessive vegetative growth resulting in weak and soft stems. Such plants are susceptible to diseases and lodging
Causes scotching of the leaves.
It delays fruit ripening.
It results in bitter tasting of crops because when a plant has too much nitrogen, the nitrogen blocks other nutrients from being absorbed by the plant. When a plant cannot absorb the proper amounts of necessary nutrients, it alters the sugar and vitamin content in the plant. The result is bitter-tasting fruits and vegetables that are high in nitrogen.
Excessive amounts of nitrogen burn the plant and damage the leaves. Burn is caused by dehydration of the roots and crown that cause the foliage to turn brown or yellow.
Sources of nitrogen for the plants are:
Atmospheric nitrogen which is fixed by bacteria living in roots of legumes.
Nitrogen releases during the decomposition of organic matter.
Fertilisers such as compounds, ammonium nitrate, urea, nitrate of soda and ammonium sulphate.
Loss of nitrogen
1. Volatilisation
Volatilisation is the loss of nitrogen into the atmosphere.
This process occurs when fertilizers such as urea are placed on the surface in hot weather they are broken down and lost to the atmosphere as gas.
Nitrogen also lost to the atmosphere when crop residues are burnt.
2. Leaching
Leaching is the loss of dissolved nitrogen from rooting zone.
Leaching is facilitated by irrigation and more rain which carries the nitrogen downwards beyond the crop's rooting zone.
Leaching is high in sandy soils than clay soils because clay holds some of the nitrogen against downward drainage.
3. Denitrification
Denitrification is a microbially facilitated process where nitrate is reduced and ultimately produces molecular nitrogen through a series of intermediate gaseous nitrogen oxide products.
Occurs under unfavourable conditions such as poor aeration.
A large number of organisms decompose soil nitrate to obtain oxygen.
The nitrate is reduced to nitrite and finally to gaseous nitrogen which is lost to the atmosphere.
Denitrification bacteria convert decayed ammonium compounds into amino acids.
Amino acids are further converted into ammonia gas then some ammonia gas escapes into the atmosphere.
Phosphorus
Phosphorus is absorbed by plants as phosphate dissolved into the soil.
These phosphates dissolve poorly in water and it is therefore necessary to mix phosphate thoroughly with soil.
Phosphorus is also released from decomposition of organic matter.
Its availability decreased in acid conditions due to its combination with aluminium, iron and manganese.
Adding phosphorus to soil low in available phosphorus promotes root growth and winter hardiness, stimulates tillering and often hastens maturity.
Phosphorus is often recommended as a row-applied starter fertilizer for increasing early growth.
Importance of phosphorous
Phosphorus plays a role in photosynthesis, respiration, energy storage and transfer, cell division, cell enlargement and several other processes in plants.
It is important in synthesis of nucleoproteins (RNA, DNA, ATP, and ADP), all of which are vital for life and growth.
It stimulates root development so it is advised to apply before or at planting.
It prevents lodging of plants.
It encourages quick maturity.
It improves quality of crops and fruits.
Deficiency symptoms of phosphorous
Plants deficient in phosphorus are stunted in growth and often have an abnormal dark-green colour.
Pollination and fruits set are retarded and reduced.
Poor nodulation and leaves turn yellow and die mostly in legumes.
Lower leaves sometimes yellow between veins.
Purplish colour on leaves or petioles.
Potassium
In photosynthesis, potassium regulates the opening and closing of stomata and therefore regulates carbon dioxide uptake.
It increase plant resistant to drought.
Potassium plays significant roles in enhancing crop quality.
Help in the transport of water and nutrients throughout the plant in the xylem.
It increase resistant of crops to diseases.
It is required for every major step of protein synthesis.
Maize plant showing potassium deficiency
Lower leaves may be mottled.
Dead areas near tips and margins of leaves.
Yellowing at leaf margins continuing toward center.
Trace or minor nutrients
Trace or micro-elements are nutrients required in very small amounts for normal growth and development of the plant.
The availability of trace nutrient is influenced by soil pH and some are available in acidity soils and some in alkaline conditions.
Boron
It is the most widespread micro nutrient deficiency around the world and causes large losses in crop production and crop quality.
Boron is important for cell division and growth of meristematic tissue, flowers and vascular bundles.
It helps in fruit development and normal seed setting.
Zinc
It is important in enzyme functions.
Help in chlorophyll formation.
Calcium
Calcium is used in cell wall formation, root growth and seed production.
Lack of calcium result in empty pods of groundnuts.
Oversupply raises soil pH and restrict uptake of potassium.
Magnesium
Magnesium is used in the formation of chlorophyll which is important for photosynthesis.
It plays role in the synthesis of oil in plants.
Deficiency of magnesium results in yellow patches of leaves between veins and fall of premature leaves.
Sulphur
It increases oil content of oil crops like soya beans, sunflower and groundnuts.
It is used in synthesis of protein and enzyme reaction.
Its deficiency result in yellowing along leaf veins; reduce nodulation in legumes
Fertilising practices
The application approaches of both organic and inorganic fertilizers vary usually with the scale or amounts to be applied.
a. Application of inorganic fertilizers:
Basal application- it is the application of compound fertiliser before or at planting.
Compound fertilisers can be machine drilled using a planter.
Can also be hand drilled in furrows before the seeds are dropped.
Top dressing— this is the application of nitrogenous fertilisers on plants after emergence.
The approaches include: -broadcasting using a viscount spreader.
-bending by placing the fertiliser a few cent meters away from the plant. -Fertigation where the fertilizer is added in irrigation water and then will be applied in solution during irrigation. Manuring — manure can be applied by spreading over the land before ploughing and get incorporated into the soil during the ploughing time.
Can also be spread over a ploughed land and then harrowing follows to incorporate the manure with soil.
The other approach is to hand drill the manure into open furrows during or before sowing.
The nitrogen cycle is the cycling of nitrogen to ensure that it is present in both soil and in atmosphere.
Nitrogen cannot be absorbed directly by the plants and animals until it is converted into compounds they can use. This process is called the Nitrogen Cycle.
A large proportion of nitrogen occurs as free nitrogen gas (78%).
Some of rhizobium bacteria living in the roots of legumes such as soya beans, beans, groundnuts, and beans.
Nitrogen fixation occurs when special bacteria convert the nitrogen gas to ammonia which the plants can use.
Complex Amino acids produced through decomposition of dead animals, plant materials, manure and compost. These complex amino acids are converted to alcohol and ammonium ions in a reaction called ammonification.
The ammonium ions are broken down or oxidized by nitrosomonas and nitrosococcus bacteria into nitrates through nitrification
Nitrification
It is the process which converts the ammonia into nitrite ions which the plants can take in as nutrients.
Plants take up nitrates containing nitrogen to form plant protein.
Lightning discharges nitrogen into the soil.
The nitrite is further oxidized to form nitrates and then nitrate is used by plants
Ammonification
After all of the living organisms have used the nitrogen, decomposer bacteria convert the nitrogen-rich waste compounds into simpler ones.
Denitrification
It is the final step in which other bacteria convert the simple nitrogen compounds back into nitrogen gas (), which is then released back into the atmosphere to begin the cycle again.
Volatilization
Refers to the loss of Nitrogen through the conversion of ammonium to ammonia gas, which is released to the atmosphere. The volatilization losses increase at higher soil pH and conditions that favour evaporation for example hot and windy.
Volatilization losses are higher for manures and urea fertilizers that are surface applied and not incorporated into the soil.
Mineralization
It is the process by which microbes decompose organic N from manure, organic matter and crop residues to ammonium. Because it is a biological process, rates of mineralization vary with soil temperature, moisture and the amount of oxygen in the soil (aeration).
Immobilization
Immobilization refers to the process in which nitrate and ammonium are taken up by soil organisms and therefore become unavailable to crops.
It is the reverse of mineralization because all living things require Nitrogen therefore microorganisms in the soil compete with crops for Nitrogen.
Organic manures
This refers to manure from decaying organic matter.
Examples of organic manure include animal manure, compost manure, farmyard manure and green manure.
1) Green manure:
It refers to the crops that are grown with the purpose of improving the soil fertility.
They are not grown for human and animal consumption.
Green manure crops should be:
Fast growing.
Present good conservation practices.
Easy established with low management costs.
Achieve good shading and weed suppression.
They should not attract pests and disease.
Benefits of green manure
Add organic matter to the soil which improves soil fertility and soil aeration.
Provide food for soil organisms.
Reduces soil erosion in arable land.
Protect the soil against extreme temperatures.
Example of green manure crops
Plants can produce good green manure if they are ploughed or dug into the soil when flowering is at 10% or before flowering.
At this stage the plant will be rich in nutrient content.
2) Farmyard manure
It consists of plant material usually straw used as bedding for animals which has absorbed the dung and urine.
Material used for bedding is also left in the pen to make farmyard manure.
It is made from rotten plants residue such as grasses and cereal crops.
Methods of making farmyard manure.
Plants material is placed on a concrete floor in a covered shed and animals defecate, the former is mixed with urine and animal dung.
After a period of six months, it is removed from the pen and heaped outside.
Leaching should be avoided through covering the heap and placing the heap on a concrete floor.
Avoid washing away of manure by wind and water.
The uses of farmyard manure
It provides organic matter necessary in maintaining soil fertility.
Improves soil aeration.
Improves soil structure.
3) Compost manure
Composte is also a source of organic manure that can cheaply be prepared from plant material found locally.
It can be prepared from all decomposable plant material and kitchen wastes.
Composts can either be prepared on the soil surface or in pits provided the conditions promote decomposition of the materials to take place.
Compost making.
Usually the size of the compost will depend on the amount of material available.
A proper compost can be prepared following the steps below:
Step
Put up the first or base layer using coarse materials such as small tree brunches, twigs and others which cannot easily decay up to a height of 30cm.
Add a 5cm layer of well rotten compost to introduce bacterial action.
Add a 15cm layer of soft materials including leaf mould, grass, garden wastes and other fast decomposable materials.
Sprinkle ammonium nitrogen to supply feed to the bacteria.
Add a layer of top soil to compress the vegetable matter.
Water the compost heap to soften the material for easy action by bacteria.
Repeat the whole process but this time without including the first step.
Continue adding material until the compost is 1 to 1,2m high.
Or until all the material is finished.
Lastly, apply water and then mulch with plant material or cover with a plastic paper to keep the material moist and soft.
Well maintained compost becomes ready in six weeks.
The compost can be turned once or twice to ensure that all the material is fully decomposed.
Determining for compost ripeness
Deep a 1m long iron rod down to the bottom of the compost.
Pull out the rod and one should feel heat if you touch it.
Some whitish substances can also be seen accumulating on the rod.
The material becomes dark in colour.
Animal based organic fertilizer includes:
Animal manure especially of poultry, pig, cattle, sheep and goat.
Blood meal from dried slaughterhouse waste and is very rich in nitrogen. Over application will damage the plants. It contains several trace elements such as boron.
Borne meal obtained from animal carcasses and is used as slow-release phosphorous fertilizer.
Fish products include fish meal and fish emulsions. Fish waste is a good source of nitrogen and phosphorous.
Impacts of organic manure to the environment
Contamination of ground water with nutrients which limits its usability and quality
Enrichment of surface water with nutrients
Animal manure releases large quantities of ammonia which causes acidification.
Animal manure results in emission of greenhouse gases such as carbon dioxide, methane and nitrous oxide.
Advantages of organic fertilizers
It increases the organic matter content of the soil which improves water holding capacity.
Use of organic fertilizer help in waste management on farms and urban areas.
Incorporation of organic manures and fertilizers.
All basal fertilizers and organic manures have to be thoroughly incorporated into the soil upon application for the following reasons:
To avoid direct seed manure or fertilizer contact. When they come in contact, the seed will be burnt and then will not germinate.
To ensure a fair and thorough distribution of the fertilizer or manure.
Soil Sampling
Soil sampling involves collection of soil samples from different parts of a piece of land in order to test for soil pH and nutrient status.
pH is measured by the concentration of hydrogen irons (H+).
Soil sampling process
First choose sampling points as shown below:
A zigzag or diagonal walk can be used.
Collect 10 sub soil samples.
Each sample should be about 15 to 20grams of soil.
Samples should be from different parts of the field for a true representation of the field.
Do not take from ant-hills and edges of the field.
Take samples when the soil is dry, preferably in May-June.
Importance of soil sampling
Enables to determine the nutrient status of a soil.
To get recommendations on the types and quantities of fertilizers to be applied.
To be able to correct the soil pH of the soil.
Sampling methods
Soil PH --- PH scale and its use
pH is measured using a calcium chloride scale or a pH scale chart show
1
2
3
4
5
6
7
8
9
10
11
12
13
14
At 7, the pH is neutral.
From 7 downwards, soil acidity increases.
8- 14 Alkaline
From 7 increasing to 14 its increasing alkalinity.
1 to 6 —Acidic.
Influence of soil PH
For arable lands, pH of 5, 5 to 6, 5 is best.
Most plants grow well in slightly acidic pH 5,5 to 6.5 to neutral: because
Most nutrients are available and easily realized from the soil.
Activities of micro-organisms are high.
Fertilizers are most effective.
Causes of Alkalinity:
Parent rock in the geological composition of the soil.
Low rainfall
Poor drainage (rise the formation of hydroxides)
Causes of acidity:
Parent rock
Fertilizer which contain sulphur and other farming acids for example urea.
Continuous cropping, that is, particular bases, like magnesium and calcium are taken from the soil.
Poor drainage.
NB: acidic soil can be corrected by applying lime.
Lime neutralizes acidic soils for example ground limestone, quick lime and dolomitic limestone.
Fertilizers
Plant nutrients are supplied in two forms (organic and inorganic) fertilizers.
Inorganic are man-made fertilizers in the form of compound and straight fertilizers.
These are applied in soil to improve plant growth and yields.
Straight and compound fertilizers
Compound fertilizers contain three major nutrients, for example, nitrogen, phosphorous and potash.
Straight fertilizers contain one major nutrient.
Straight fertilisers include the following:
Ammonium nitrate: - it is highly soluble and readily available to the plant.
Ammonium sulphate: - used on an alkaline soil.
Sodium nitrate: - is very expensive and not very suitable for application as a fertiliser.
Compound fertilizers include the following:
Compound A, B, C, D, S, X, T.
They comprise of N.P.K. at different ratios.
They also contain trace elements such as Boron, Molybdenum, Calcium, sulphur though in small quantities.
Liming
Liming is the application of lime in an acidic soil.
This can be in the form of Agricultural lime or dolomitic limestone.
It is meant to neutralize the acidity of soils.
Excess lime is detrimental for it is difficult to lower pH than to raise it.
Types of lime
Ground lime (calcium carbonate).
Quicklime (Calcium Oxide).
Dolomitic lime
Importance of liming
It improves the structure of the soil; it binds sand soils and opens up clays.
It aids cell formation.
It's a soil conditioner because it regulates the pH levels of the soil.
