Today we are going to discuss about the importance of soil and how it affects crop production as well as affecting the yield and productivity of your plants (crops).
First let us understand what the term ”soil” is all about to enable us carry you along with the discussion of today:
What is Soil?
Soils are complex mixtures of minerals, water, air, organic matter, and countless organisms that are the decaying remains of once-living things. Soil can also be defined as a mixture of organic matter, minerals, gases, liquids, and organisms that together support life.
Earth’s body of soil, called the pedosphere, has four important functions:
- as a medium for plant growth
- as a means of water storage, supply and purification
- as a modifier of Earth’s atmosphere
- as a habitat for organisms
All of these functions, in their turn, modify the soil and its properties.
Soil is a material composed of five ingredients — minerals, soil organic matter, living organisms, gas, and water. Soil minerals are divided into three size classes — clay, silt, and sand (Figure 1); the percentages of particles in these size classes is called soil texture. The mineralogy of soils is diverse. For example, a clay mineral called smectite can shrink and swell so much upon wetting and drying that it can knock over buildings.
The most common mineral in soils is quartz; it makes beautiful crystals but it is not very reactive. Soil organic matter is plant, animal, and microbial residues in various states of decomposition; it is a critical ingredient — in fact the percentage of soil organic matter in a soil is among the best indicators of agricultural soil quality. Soil colors range from the common browns, yellows, reds, grays, whites, and blacks to rare soil colors such as greens and blues.
Figure 1: Relative sizes of sand, silt, clay. © 2013 Nature Education
Soil is of different elements that include:
(a) Soil pH
(b) Soil Organic matter
(c) Soil Texture
(d) Soil Mineral elements
Now let us dicuss each of the different elements of soil in details below:
(1) Soil pH
Soil pH is an indicator of the degree of acidity alkalinity. It is expressed by range of pH units ranging from 0 to 14.
A pH of 7 is Neutral, below 7 is Acidic and above 7 is Alkaline.
Soil pH has an indirect yet far_reaching effects on plants as plant nutrients become available or unavailable according to soil pH levels.
For example: Yellowing between veins of young leaves indicate iron defficiency a condition rising not from lack of iron in the soil but from insufficient soil acidity to put iron into a form that a plant can absorb.
Most plants take in slightly acidic soil because that pH affords them good access to all nutrients. Eg. Nutrients available at different pH levels.
Deep insight into Soil pH
Soil pH it’s self a plant nutrient but it relates to plant nutrition. This is due to the fact that it affects the availability to plant nutrient
Some nutrients may be available in the soil but may be made unavailable due to soil pH.
The change of soil pH is plant poisoning as too low pH level can reduce the plant nutrient. Manganese are available at toxic levels.
Geranium are particularly sensitive to low pH showing their disconfort with yellowed, brown__flecked or dead Leaves.
A pH level that is too low also liberates Aluminium (which is not a plant nutrient) in amounts that can stunt root growth and interfere with plant nutrient up take
At high pH , molybdenum becomes available in toxic amounts. Soil pH also influences soil drewling micro organisms. However, before attempting to change your pH you must know it’s current situation. This will determine how much you need to raise or Lower it.
A simple soil test can be done to ascertain the pH levels inorder to prevent the occurance of any of these soil related issues on the plants.
(2) Soil Organic Matter
The organic matter fraction of soils comes from the decomposition of animal or plant products such as faeces and leaves. Soil organic matter contributes to stable soil aggregates by binding soil particles together.
Plants living in soil continually add organic matter in the form of roots and debris. Decomposition of this organic matter by microbial activity releases nutrients for the growth of other plants.
The organic matter content of a soil depends on the rates of organic matter addition and decomposition. Soil microorganisms are responsible for the decomposition of organic matter such as plant residues. Initially, the sugars, starch and certain proteins are readily attacked by a number of different microorganisms.
The more resistant structural components of the cell wall decompose relatively slowly. The less easily decomposed compounds, such as lignin and tannin, impart a dark colour to soils containing a significant organic matter content.
The decomposition rate of organic materials depends on how favourable the soil environment is for microbial activity. Higher decomposition rates occur where there are:
- warm, moist conditions
- good aeration
- a favourable ratio of nutrients
- a pH near neutral
- freedom from toxic compounds.
(3) Soil Texture
Soil texture is a classification instrument used both in the field and laboratory to determine soil classes based on their physical texture.
Soil texture (such as loam, sandy loam or clay) refers to the proportion of sand, silt and clay sized particles that make up the mineral fraction of the soil. For example, light soil refers to a soil high in sand relative to clay, while heavy soils are made up largely of clay.
A soil texture triangle showing soil textures as determined by the proportion of sand, silt and clay,
Texture is important because it influences:
- the amount of water the soil can hold
- the rate of water movement through the soil
- how workable and fertile the soil is.
For example, sand is well aerated but does not hold much water and is low in nutrients. Clay soils generally hold more water, and are better at supplying nutrients.
Texture often changes with depth so roots have to cope with different conditions as they penetrate the soil. A soil can be classified according to the way the texture changes with depth. The 3 profile types are:
- uniform — same texture throughout the soil profile
- texture-contrast — abrupt texture change between the topsoil and subsoil
- gradational — texture gradually increases down the soil profile.
How to determine soil texture
- Take about 2 tablespoons of soil in one hand and add water, drop by drop, while working the soil until it reaches a sticky consistency.
- Squeeze the wetted soil between thumb and forefinger to form a flat ribbon.
- Determine the texture based on the length of the ribbon that can be formed without breaking—see following table.
|Texture||Length of ribbon (mm)||Soil properties and management implications|
|Sandy||<15||Little resistance to root growth High infiltration rate. |
Low plant available water
|Sandy loam||15–25||Root growth not restricted, but highly susceptible to mechanical compaction. |
May be hard setting, Moderate infiltration rate, Moderate plant available water
|Loam||25||Root growth not restricted Moderately susceptible to mechanical compaction.|
Moderate plant available water, Moderate infiltration rate
|Silty loam||25||Root growth not restricted Moderately susceptible to mechanical compaction.|
Moderate plant available water Low to moderate infiltration rate
|Clay loam||40–50||Root growth not restricted Moderately susceptible to mechanical compaction.|
Moderate to high plant available water
|Clay||50–75||Root growth frequently restricted Moderately to highly susceptible to mechanical compaction Some restriction on water movement leading to periodic waterlogging.|
Moderate to high plant available water
|Heavy clay||>75||Root growth moderately to severely restricted High susceptibility to mechanical compaction.|
Water drains very slowly except in self-mulching soils
(4) Soil Mineral Elements
The essential mineral elements are:
Nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, boron, chlorine, iron, manganese, zinc, copper, molybdenum, and nickel.
In addition to the essential mineral elements are the beneficial elements, elements which promote plant growth in many plant species but are not absolutely necessary for completion of the plant life cycle, or fail to meet Arnon and Stout’s criteria on other grounds. Recognized beneficial elements are:
Silicon, sodium, cobalt, and selenium
Other elements that have been proposed as candidates for essential or beneficial elements include chromium, vanadium, and titanium, although strong evidence is lacking at this time.
Another group is the essential nonmineral elements, elements taken up as gas or water, which are:
Hydrogen, oxygen, and carbon
Out of all of the many natural elements, essential mineral elements, essential nonmineral elements, and beneficial elements are not randomly scattered, but instead cluster in several groups on the periodic chart.
There are 15 essential elements that plants must have in order to grow properly.
18 Essential Nutrients
- Nutrient elements obtained from atmosphere through photosynthesis
- Nutrient elements obtained from the soil
Out of the 15 essential elements that come from the soil, we deal with only the 12 that are generally managed by the growers. These 12 elements are ‘mineral nutrients’ and are obtained from the soil. We further divide mineral nutrients into 3 groups: primary, intermediate, and micronutrients. Our presentation will exclude cobalt, chlorine, and nickel from our discussion on the management of essential mineral nutrients, though are included by many as essential nutrients.
- The primary nutrients are nitrogen, phosphorus and potassium. You may be most familiar with these three nutrients because they are required in larger quantities than other nutrients. These three elements form the basis of the N-P-K label on commercial fertilizer bags. As a result, the management of these nutrients is very important. However, the primary nutrients are no more important than the other essential elements since all essential elements are required for plant growth. Remember that the ‘Law of the Minimum’ tells us that if deficient, any essential nutrient can become the controlling force in crop yield.
- The intermediate nutrients are sulfur, magnesium, and calcium. Together, primary and intermediate nutrients are referred to as macronutrients. Macronutrients are expressed as a certain percentage (%) of the total plant uptake. Although sulfur, magnesium, and calcium are called intermediate, these elements are not necessarily needed by plants in smaller quantities. In fact, phosphorus is required in the same amount as the intermediate nutrients, despite being a primary nutrient. Phosphorus is referred to as a primary nutrient because of the high frequency of soils that are deficient of this nutrient, rather than the amount of phosphorus that plants actually use for growth.
- The remaining essential elements are the micronutrients and are required in very small quantities. In comparison with macronutrients, the uptake of micronutrients is expressed in parts per million (ppm, where 10,000 ppm = 1.0%), rather than on a percentage basis. Again, this does not infer that micronutrients are of lesser importance. If any micronutrient is deficient, the growth of the entire plant will not reach maximum yield (Law of the Minimum).
Since the soil provides most essential nutrients, it is crucial that we understand the soil processes that determine the availability of each essential nutrient for plant uptake.
Table 2. Forms of Essential Elements Taken up by Plants
|Nitrogen||N||NH4+ (ammonium) and NO3– (nitrate)|
|Phosphorus||P||H2PO4– and HPO4-2 (orthophosphate)|
|Iron||Fe||Fe+2 (ferrous) and Fe+3 (ferric)|
|Boron||B||H3BO3 (boric acid) and H2BO3– (borate)|
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