12.1 Soils, Fertility, and Plant Growth – The Science of Plants (2023)

Learning objectives

By the end of this lesson you will be able to:

  • Describe how soil texture and soil structure affect plant growth.
  • Use simple tests to determine the texture of soil.
  • Determine the meaning and impact of the three numbers typically displayed on fertilizer labels.

This is a course about plant propagation, but propagation is only useful if you can successfully grow the plants you propagate. Soil, light, and water are key to growing healthy plants. Here you’ll have a brief introduction to soils and soil fertility — a huge area of knowledge and study.

Watch this video for an explanation of soil texture and soil structure (1:06)

Soil has two key properties: texture and structure. refers to the relative proportion of sand, silt, and clay particles in the soil. , and are the three sizes of mineral particles (originating from rock rather than from previously living material) that make up soil. Sand is the largest particle, silt is intermediate, and clay is very small. In relative terms, if sand is a 55-gallon barrel, silt is the size of a plate, and clay is the size of a dime.

This mixture of different-sized particles is called texture because of how different combinations of these particle types make soil feel when you rub a sample between your thumb and forefinger. High amounts of sand makes a soil sample feel gritty, more silt makes it feel floury, and lots of clay makes it feel like velvet when dry and sticky when wet.

You can get a good idea of the texture of a field soil by doing a simple “jar test:” put soil in a jar, add water, shake the jar, then wait a few days to see the layers of different size particles settle.

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The USDA Soil Texture Triangle, above, indicates the type of soil for different percentages of sand, silt, and clay. Notice that there are lines running through the triangle; these are to help you associate the percentages on the margins of the triangle with locations in the interior. The numbers on the margins are angled so that they are roughly parallel to the associated index lines. For example, the 60% clay index line is a horizontal line extending to the right of the 60 on the percent clay margin. The 20% silt index line runs from the upper right to the lower left of the triangle. And the 20% sand index line runs up from the lower right to upper left; all three lines are marked with a red arrow. These lines intersect at the red dot in the middle of the Clay area, indicating that a soil with 60% clay, 20% silt, and 20% sand is classified as a clay soil. When farmers talk about their field soil they often use the terms in the texture triangle rather than the percentages of sand, silt, and clay. This approach is much less common when talking about the potting mix used in greenhouses, as there is very little real mineral soil (sand, silt, clay) in these mixes.

Soils high in sand have great drainage and aeration so that roots are exposed to air in the soil and don’t rot as easily. Roots can penetrate sandy soil easily. But sandy soils are poor at holding moisture when the weather turns dry, and sands don’t hold nutrients well. Nutrients and moisture hang on to a soil best when the soil particles have a lot of surface area, and sand has the least surface area (relative to particle volume) of the three particle types.

Clay, in contrast, holds on to water so tenaciously that it is tough for the plant to get the water for itself. Wet clay is sticky, and clay packs together so tightly that when it dries it clumps together and turns into hard clods. Roots have difficulty penetrating a dry, clay soil. But clay does have a lot of surface area for its volume, and holds nutrients better than other particles. Clay soils tend to be fertile.

Watch this video to take a look at clay soil aggregates (2:15)

Silt has intermediate properties between sand and clay, as you might expect. An ideal soil has some of each type. A silt loam soil with 60% silt and 20% each of clay and sand is perfect for growing corn, wheat, and soybeans. But crops whose economically valuable part is in the ground, like potatoes and peanuts, do well in a sandy soil, because the tubers and pods come out of the ground cleaner and with less mechanical effort than they would from a soil with higher clay or silt.

All soil textures have advantages and disadvantages, depending on climate, topography, and crop. The soil texture of large areas, like fields, can’t really be modified to suit a particular crop, so a crop must be chosen that does well in the available soil. For example, if you have a field with sandy soils you aren’t going to truck in tons of clay and silt to make the soil suitable for corn. Instead, you’ll grow a crop like potato that does reasonably well on a lighter, sandy soil. For crops grown in greenhouses or containers, however, you can choose the soil texture to suit the crop you want to grow.

Review questions

  1. What are the three particles that make up soil texture?
  2. Which is smallest? Largest?
  3. Is a loam soil high or low in clay relative to the other particles?
  4. How do you determine soil texture using a soil jar?
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refers to the way in which the soil particles and other materials like the in the soil bind together into clumps. These clumps are called . Pure sand does not clump together into aggregates at all (think about how hard it is to get sand at a beach to stick together for a sand castle). When sand, silt, clay, and organic matter interact to form small aggregates, like the ones shown below, they create what is called a granular structure. Large holes in the aggregates provide spaces for gasses and water to pass through, while smaller holes hold water. The need for water is obvious, but the need for gas exchange may not be. As you know, root cells are growing, which means they require oxygen and give off carbon dioxide as a waste product. Oxygen needs to be available in the root zone, and carbon dioxide needs to be vented. If soils are waterlogged, plants die because too much carbon dioxide builds up around the roots and the roots are starved of oxygen. It is therefore important for soils to have these holes in the aggregates for gas exchange. This is called the aeration-porosity of the soil. Organic matter, which in this case refers to decaying bits of formerly living material, helps build the aggregates by gently sticking the soil particles together. The space between and within aggregates provides aeration-porosity.

The illustration below includes a cross section of soil, showing several soil aggregates packed together. Each aggregate is built from sand, silt, clay, and organic matter (also called ). Note the micro- and macropores for water and gas exchange.

Soil with that favors plant growth by holding water and nutrients, yet allows for drainage and gas exchange, is said to have good tilth. The soil hangs together (unlike sand), doesn’t form hard clumps (unlike clay), and breaks apart into crumbly moist chunks when you turn over a spade of earth. While gardeners are usually stuck with whatever soil texture they might have in their gardens., one of the most important and readily accomplished tasks a gardener can take on to improve garden soil is to improve the soil structure by:

  • increasing the , and
  • reducing .

Increasing soil organic matter will improve and stabilize soil aggregation. Reducing compaction, like foot traffic through the garden, will maintain the macro-and micropores in the soil to promote drainage, moisture retention, and gas exchange.

Watch this video to take a close look at a sandy soil (2:41)

Review questions

  1. What makes up the glue that holds the soil particles into aggregates?
  2. Why is gas exchange in soils important for plant growth?

Soil organic matter refers to carbon-based material in the soil that was originally a living organism, whether plant, animal, or microbe. Sometimes, soil organic matter also refers to organisms such as bacteria, fungi, insects, and worms that are still living in the soil, but this discussion refers to the materials that were once alive and are now dead and decomposing. Leaves, stems, and roots eventually die, are incorporated into the soil, and decompose. Soil organisms decompose the former living material and transform it into material called humus. Humus is sticky, and helps bind soil particles together into aggregates, as noted above. Humus also can absorb and hold up to six times its weight in water, so it is very important in improving light (sandy) soils. The decomposing organic matter also releases and other nutrients that the plant can take up for growth. And finally, humus, like clay, holds nutrients in the soil through electrochemical charge; organic matter is negatively charged, so it holds positively charged cations like calcium that are important for plant growth.

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In summary, organic matter is formerly living matter that is transformed in the soil into humus. Humus helps stick soil particles together to improve soil structure, holds water in droughty soils, and holds plant nutrients. Decomposing organic matter makes nutrients such as nitrogen available to plants.

Organic matter is added to soils in several forms:


For gardeners, this may be the most familiar form of organic matter. Leaves, weeds, grass clippings, and other organic material are mixed together and occasionally turned to promote decomposition. This results in humus that, when added to the soil, builds soil structure. Most of the nutrients have been used by the organisms that are decomposing the organic matter, are lost to the air, or are leached away by rain, so isn’t very effective as a nutrient source. Its main purpose is to build soil structure and assist in retaining available moisture and nutrients.

Green manure, or cover cropping

A crop grown with the sole purpose of tilling the crop into the land to increase the organic matter is called . Green manure crops are used to change soil structure by incorporating organic matter directly into the soil. This technique is also used extensively in horticultural crop production to reduce soil-borne pathogens, and these crops serve a very useful purpose of smothering weeds.

Incorporating crop residues

After a crop is harvested, it is good agricultural and horticultural practice to incorporate the remaining plant material into the soil. Sometimes this is done with a moldboard plow to completely bury the residue, but the more modern method is to use the bare minimum of , or to leave the residue on the top of the soil and plant over the dead material the next spring. The latter method, called no-till, is particularly useful for minimizing soil erosion caused by soil particles blowing away with the wind or moving with flowing water.

The addition of too much organic matter that has too much carbon and not enough nitrogen can deplete the soil of nitrogen and harm plant growth. For instance, if you try to improve the organic matter of your soil by tilling in bales of straw or sawdust (both of which are almost all cellulose, which is very high in carbon), when the microbes begin to break the straw down they need to absorb nitrogen from the soil just for their own growth. If instead you add manure to the soil, which is a blend of straw (high carbon) and animal waste (high nitrogen), the microbes can use the nitrogen from the manure for their own growth as they decompose the organic matter and make more nitrogen available to plants.

Review questions

  1. Why add organic matter to the soil?
  2. Is all organic matter of the same value when added to soil, or are some types of organic matter better than others? Why?

Garden soil cannot be used for container gardens, sa it compacts too tightly in pots and has terrible drainage. Instead, it is best to a soil-less mix like those available at nurseries, or to make a mix that is high in an organic matter like peat moss or rice hulls, to increase aeration porosity.

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Watch this video to take a close look at a soilless container growing medium (0:58)

Author Dr. Tom Michaels developed this salad table, above, which has great potential for use in urban areas with smaller areas for growing greens, including apartment patios. This table is made with 2×4 lumber for the sides and legs, with hardware cloth and landscape fabric for the bottom of the table. The growth medium is normally a peat-based potting mix. A table this size supplies enough salad greens throughout the summer for two adults. You could modify it to have deeper soil so that you can raise a tomato or pepper plant.

Since the growing medium is potting mix, it dries out quickly. You can see a few modifications in this Hydroponic Salad Table, also created by Dr. Michaels. It’s about 2′ x 4′ x 7.5″ deep and made with lumber, a plastic liner, and a styrofoam lid. About 30 gallons of nutrient solution is added to the box, the box is covered with a lid, and salad green seedlings like lettuce, spinach, chard, and kale are placed in holes in the lid. The plants yield greens for most of the summer and little or no water needs to be added.

To see more about the hydroponic salad table, see the Hydroponic Salad Table website, where Dr. Micheals has posted more information about how you can make a table like this.

You might find that salad tables, container gardens, or raised beds can keep you in touch with the food you eat while you retain your urban lifestyle.

Fertilizer bags and containers display a series of three numbers separated by dashes. This is called the fertilizer’s analysis. The numbers represent the percentage of the fertilizer that is nitrogen (N), (P), and (K) — always in that order. N, P, and K are the elements needed by plants in the greatest quantities. Nitrogen is a key element found in protein, phosphorus is an important component in energy transfer molecules like ATP and as part of the DNA backbone, and potassium is an essential part of the mechanism for moving nutrients into and out of cells. Other elements can also be important in small quantities and in special circumstances, but N, P, and K are the most common plant nutrients.

A 10-10-10 general purpose garden fertilizer has 10% nitrogen, 10% phosphorus, and 10% potassium. The rest is filler, like sand or fine gravel. In Minnesota, fertilizers available to homeowners typically have no phosphorus because of legislation aimed at reducing phosphorus runoff into our lakes. Phosphorus is considered to be the limiting factor in algae growth, so if phosphorus runs off yards and gardens into lakes it causes algae blooms. In addition, our garden soils normally have sufficient phosphorus. A general without phosphorus would be 10-0-10. Nitrogen is usually the nutrient most limiting for plant growth, so it is worth it to read through the labels.

A caution: seeking the best value per pound of N isn’t always the right strategy. Sometimes the form of the nutrient is important. If you are interested in growing a hydroponic salad table, the plants need a particular form of nitrogen called nitrate, which is not usually found in big, cheap bags of fertilizer; it’s more likely to be found at a hydroponic shop, and costs more per pound of N than other forms.

Review question

  1. Is a 20-pound bag of 10-0-0 fertilizer that costs $10 a better value than a 10-pound bag of 46-0-0 that costs $20? Why or why not?
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What is fertility of the soil question answer? ›

Soil fertility is the ability of soil to sustain plant growth and optimize crop yield. This can be enhanced through organic and inorganic fertilizers to the soil. Nuclear techniques provide data that enhances soil fertility and crop production while minimizing the environmental impact.

What makes the soil fertile one word answer? ›

The content of organic matter in the soil is one of the most important factors for soil fertility.

Which is the best soil for plant growth answer? ›

Loamy soil is best for plant growth as it has high water retention capacity thus it retains water for long and also retains the nutrients which is required for plant growth.

Which factors are important in the fertility of soil answer? ›

soil properties—pH, texture and different clay minerals can have an influence on soil fertility. soil biology—organisms living in the soil break down animal and plant matter into nutrient forms that can be used by plants. soil organic matter—important for holding nutrients until they can be taken up by plants.

What causes soil fertility? ›

Organic matter plays a key role in maintaining soil fertility. It holds nitrogen and sulfur in organic forms and other essential nutrients such as potassium and calcium. The loss of organic matter mainly occurs through continuous cropping with stubble removal or burning, and is accelerated by frequent tillage.

What causes the fertility of the soil to diminish answer in one sentence? ›

The major causes of soil fertility depletion are inadequate fertilizer use, complete removal of crop residues, continuous cropping systems, climate and soil types, lack of proper cropping systems and soil erosion and continuous cultivation.

How do plants keep the soil fertile? ›

Plants and animals help keep the soil fertile. Plant roots tunnel through the soil and break it up, and decaying plants form humus. Burrowing animals mix the soil; the excrete of animals contribute nutrients and improve soil structure.

What is it called when soil is not fertile? ›

Soil depletion occurs when the components which contribute to fertility are removed and not replaced, and the conditions which support soil's fertility are not maintained. This leads to poor crop yields.

What is necessary to have to grow a plant in soil? ›

Soil is a major source of nutrients needed by plants for growth. The three main nutrients are nitrogen (N), phosphorus (P), and potassium (K). Together they make up the trio known as NPK. Other important nutrients are calcium, magnesium, and sulfur.

Which type of soil is most fertile for plants to grow? ›

Porous loamy soils are the richest of all, laced with organic matter which retains water and provides the nutrients needed by crops.

In what type of soil do plants usually grow? ›

Loamy soil is ideal for most garden plants because it holds plenty of moisture but also drains well so that sufficient air can reach the roots.

What is the most important factor in soil development and why? ›

Climate: This is probably the most important factor that can shape the formation of soils. Two important climatic components, temperature and precipitation are key. They determine how quickly weathering will be, and what kind of organic materials may be available on and inside of the soils.

What three factors improve soil fertility? ›

Fertilizers such as nitrogen, potassium and phosphorus are added to the soil to make it fertile. These are also added to the potted plants in gardens to enhance plant growth.

What five factors are of greatest importance in soil formation? ›

Scientists attribute soil formation to the following factors: Parent material, climate, biota (organisms), topography and time. These factors interact to form more than 1,108 different soil series in Minnesota.

How do you know if soil is fertile? ›

Signs of healthy soil include plenty of underground animal and plant activity, such as earthworms and fungi. Soil that is rich in organic matter tends to be darker and crumbles off of the roots of plants you pull up. A healthy, spread-out root system is also a sign of good soil.

Why is soil losing its fertility? ›

Factors which lead to soil degradation are deforestation, overgrazing, overuse of chemical feritilisers or pesticides, rain wash, landslides and floods.

What is the importance of soil fertility and soil productivity? ›

Soil fertility and nutrient management is one of the important factors that have a direct impact on crop yield and quality. Irrespective of the size of your field or plot, supplying plants with the right amount of nutrients at the right time is the key to a successful vegetable production enterprise.

What happens when soil is less fertile? ›

The loss of fertile soil makes land less productive for agriculture, creates new deserts, pollutes waterways and can alter how water flows through the landscape, potentially making flooding more common.

What type of soil is best for farming Why? ›

The most arable soil for most crops is Loam, which is 40% sand, 40% silt, and 20% clay.

What are the types of soil fertility? ›

The factors affecting soil fertility may be of two types; i.e. – (a) Natural factor and (b) Artificial factor. The natural factors are those which influence the soil formation and the artificial factors is related to the proper use of land.

What is the importance of soil to plants? ›

Soil provides plants with the foothold for their roots. It also holds the necessary nutrients to allow plants to grow. Additionally, it filters the rainwater and regulates excess rainwater, which prevents flooding.

Does fertile soil make plants grow faster? ›

Plants grow easily in fertile soil because it contains large amounts of nutrients. These nutrients, which help keep plants healthy, come from minerals and decaying plant and animal matter.

Which soil has high fertility? ›

Alluvial soil has the highest fertility among the options given. This soil is rich in minerals and suitable for agriculture. It contains adequate proportion of minerals like potash, phosphoric acid and lime.

What are 4 causes of soil infertility? ›

The four main causes of infertile soil are high acidity levels, poor nutritional composition, unfavorable soil structure, and high levels of salinity.

What causes poor soil quality? ›

Soil degradation causes include agricultural, industrial, and commercial pollution; loss of arable land due to urban expansion, overgrazing, and unsustainable agricultural practices; and long-term climatic changes.

What are the 4 important things that plants need to grow? ›

The vital needs of a plant are very much like our own—light, water, air, nutrients, and a proper temperature. The relative importance of each of these needs differs widely among plants.

What are the two main things that plants need to grow? ›

WATER: just like we need water, plants need water! Water helps move food and nutrients to all the different parts of the plant. NUTRIENTS: just like we need nutrients, plants need nutrients! Nutrients provide plants with the strength they need to grow big, strong, and healthy.

What is the best soil type? ›

Considered the most fertile of soil type, loamy soils are a combination of sandy, clay and silt particles. The clay and silt particles improve moisture retention while the sand minimizes compaction and improves drainage.

What is the 4 types of soil? ›

Soil is classified into four types:
  • Sandy soil.
  • Silt Soil.
  • Clay Soil.
  • Loamy Soil.

Do most plants need soil? ›

Yes, plants can grow without soil, but they cannot grow without the necessities that soil provides. Plants need support, nutrients, protection from adverse temperatures, an even supply of moisture, and they need oxygen around the roots.

What is soil made of? ›

Soil is the thin layer of material covering the earth's surface and is formed from the weathering of rocks. It is made up mainly of mineral particles, organic materials, air, water and living organisms—all of which interact slowly yet constantly.

Do different types of soil affect plant growth? ›

Soil condition impacts your gardening success tremendously. Soil is the primary source of nutrients and water, which are crucial for healthy plant development. The wrong soil can prevent your plants from getting the moisture and nutrients they need to grow and flourish.

What is the most important in soil? ›

Humus is the dark, organic component of soil from which plants take most of their nutrients. The amount of humus in soil determines both the soil's capacity for water retention and its fertility.

What is the most important property of soil? ›

Two of the most important properties of soils are their texture and structure . By texture, we mean what soils are composed of and how this affects the way they feel and their cultivation. The main components of soil texture are: sand, silt and clay particles and organic matter.

What factors affect soil? ›

Soils are formed through the interaction of five major factors: time, climate, parent material, topography and relief, and organisms. The relative influence of each factor varies from place to place, but the combination of all five factors normally determines the kind of soil developing in any given place.

What are the 3 main factors that affect soil strength? ›

Factors Affecting Soil Shear Strength

So the shear strength of a soil depends on the composition of the soil's particles, the amount of water in the soil, and how well compacted the soil is.

What are the 4 processes of soil formation? ›

Four basic processes occur in soils— additions, losses, transformations (changes), and translocation (movement). A PowerPoint presentation provides some examples. Experiments demonstrate these soil processes.

What are the four most important properties of soil? ›

The combinations of these determine the soil's properties – its texture, structure, porosity, chemistry and colour.

Which of the following factors does not affect soil formation? ›

The soil formation is mainly due to weathering of rocks. Weathering of rocks happens due to effects of wind, water and climate. On the other hand, colour of the rocks is not responsible for the formation of the soil.

What is fertility status of the soil? ›

The soil fertility status is the backbone on which all input-based high agricultural production systems can be built (Al-Zubaid et al., 2008; Parnes, 2013). It provides physical conditions and nutrients for plants growth and fructification (Marschner, 2008; Velayutham and Bhattacharyya, 2000; Foth and Ellis, 1997).

Which of the following determine the fertility of soil? ›

The correct answer is All of the above. All the factors mentioned in given options are determines the fertility of soil.
Detailed Solution.
Soil pHIt measures the acid present in the soil
Organic matterIt consists of plant and animal residues at various stages of decomposition.
2 more rows
Dec 20, 2022

What is the measure of soil fertility? ›

The measurement of the fertility of soils is usually done by chemical analysis for plant nutrients such as nitrogen (N), potassium (K), phosphorus (P) and trace elements, as well as physical measurements of soil structure.

Which soil is more fertile? ›

Alluvial soil is formed by deposition of alluvium and sediments carried by rivers and sea waves over many years, which make this soil very fertile. It consists of various proportions of sand, silt and clay. It is also rich in organic nutrients.

What are 4 characteristics of fertile soil? ›

The soil pH is in the range 6.0 to 6.8. It has a good soil structure which results in well-drained soil. It consists of a variety of micro-organisms that support plant growth. It often contains large amounts of topsoil.

Why is soil important? ›

Soil provides a host of crucial services for both people and the planet. Soil puts food on our plates, purifies our water, protects us against flooding and combats drought. It's also key to tackling climate change as it captures and stores vast amounts of carbon. There is no food security without healthy soils.

How do you make good soil? ›

You can increase the amount of organic matter in your soil by adding compost, aged animal manures, green manures (cover crops), mulches or peat moss. Because most soil life and plant roots are located in the top 6 inches of soil, concentrate on this upper layer.

What factors affect soil formation? ›

Scientists attribute soil formation to the following factors: Parent material, climate, biota (organisms), topography and time.

What is an example of soil fertility? ›

Fertile soil contains all of the major nutrients required for basic plant nutrition (For example, nitrogen, phosphorus, and potassium), as well as other nutrients required in smaller amounts (For example, calcium, magnesium, sulfur, iron, zinc, copper, boron, molybdenum, nickel).

How does soil pH affect soil fertility? ›

Soil pH affects the amount of nutrients and chemicals that are soluble in soil water, and therefore the amount of nutrients available to plants. Some nutrients are more available under acid conditions while others are more available under alkaline conditions.

Why is water important for soil fertility? ›

Water serves as the solvent and carrier of food nutrients for the growth of plants. Water itself acts as a nutrient also. The process of weathering and soil formation depend on the water. Good tilth can be secured at right stage of moisture content.

What are the 4 methods of determining soil fertility? ›

Soil testing is an excellent method for estimating the fertility status of a soil, and it provides valuable information for developing a sound fertility management program. There are four critical steps to a soil test: sampling, analysis, interpretation and recommendation.


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