My lecturer droned on…” Soils contain mineral grain particles, sand, silt and clay which are of varying sizes and give the soil it’s texture. Loam is a mixture of these 3 particle sizes with often around 50% sand, 30% silt and 20% clay.”
I was sitting at the back of the classroom, studying horticulture; the subject was soil science and I was trying to stifle a yawn.
The horticultural school in Adelaide was located at the time, 1980, in the historic Yarrabee House on North Terrace. It is a beautiful old stone two story residence that was originally built in 1866, to house the Medical Officer from the large hospital built to house people with mental health problems, operating from 1852 to 1902. The main building of the hospital was adjacent until it was demolished in 1938.
“These different particles can be felt with your fingers and soil texture can be determined by feeling the size of the mineral particles.” came a distant voice from the front of the lecture room, as I gazed out the window.
Two noisy minor birds were swooping a big black crow. The crow was ignoring them, as if bored, just like me.
“What do footballers like England’s David Beckham…” I heard.
What? Just as I nearly let a yawn slip out. Did I miss something? I thought.
The lecturer went on “…Brazil’s Pele, Cameroon’s Roger Milla and Liberia’s George Weah and the Farmers and Pastoralists of Africa have in common?”
“They would all like a fertile loam soil beneath their feet in order to succeed.”
“Loam supports the couch or kikuyu grasses that make a good playing field and loam supports the farmers crops, and the grasses and shrubs that the pastoralist’s grazing animals eat.”
“Loam is a soil containing large proportions of grains of sand along with silt and clay, and often referred to as a sandy loam, clay loam or silty loam, depending on which one of the three proportions is more plentiful in the soil.”
Our lecturer’s little story had piqued my interest!
“Soil texture describes the proportion of the mix of these three mineral grain particles, which are of different sizes.”
“A sand, silt or clay soil, or loam soil if a mix, in addition to the mineral components, contains organic matter, humus which is decomposed organic matter, insects and worms, and on the humus will be bacteria, fungus and other microorganisms.”
“Of equal importance in the soil are air and water which fill the pore spaces between the mineral particles.”
“So, while the great footballers may not be thinking of the loam beneath their feet as they score goals, the African Farmers and Pastoralists are, as they watch their crops and animals grow” he finished the story looking quite pleased with himself!
It wasn’t that good, I thought to myself!
The lecturer went on “The three particle sizes, sand, silt and clay that make soil texture, are:
sand less than 2mm diameter in size to 0.05mm,
clay less than 0.002mm in size and
silt which is the size range between the other two.
Gravel is classed as 2mm diameter and greater but this category isn’t generally included in determining soil texture.”
He put an image up on the screen (with an overhead projector, for those that remember them) of a soil texture triangle from the United States Department of Agriculture and said the Food and Agriculture Organization of the United Nations used similar soil texture classifications.
Then he said “You feel and estimate the amount of sand, silt and clay in the soil sample, then you can use this chart to determine which soil texture you have.”
“But now we’ll go out to the potting shed and get our hands dirty testing some soil samples.”
This sounded better, I thought.
The potting shed was in the converted horse stables from the 1800s. We really didn’t value the history at the time. What I remember most was the end of year parties we had there!
So, in the potting shed we divided into pairs and took a handful of some of the soils provided.
The lecturer then told us again “…by feeling the particles and manipulating the soil in your hand you can determine if a soil has a high clay content. First take a small soil sample in your hand and wet it well with the water then manipulate or knead the sample into a ball shape and then attempt to make it into a ribbon shape.”
“This is called the ‘ribboning’ method.”
“If you can make a ribbon that indicates a high proportion of clay, if you can make that ribbon into a circle without it cracking that is a very high clay content soil.”
“You may be able to make a pot!”
“Silt will give a smooth and silky feel.”
This was much more meaningful. I was able to make a ribbon and a circle. So, it must have a high clay content. It also made my hands very muddy!
“Sand as well can be determined by actually feeling grit or little lumps in the soil and a high sand content soil is difficult to retain in any shape” he said.
“Variations in feel and how much the soil sample clings together, clumping, and thus making shapes, indicates the relative proportions of sand, silt and clay which are used to identify a soil, using the chart we just saw back in the classroom.”
Next, my partner and I choose a light coloured soil which looked like sand, and it was. We could both feel the grit and couldn’t make it into any shape that held together.
Our lecturer went on “Soil texture tells us how well a soil will hold water and dissolved nutrients and how much it will drain. Drainage of water allows air to penetrate into the soil and both water and air are taken up by plant roots.”
“A sand with large soil particles and thus large pore spaces between the particles will drain readily and be susceptible to drying out, while a clay soil with small particles and small spore spaces will hold on to water.”
“The spore spaces between particles can be visualized by again using the game of football, imagine a large drum or other container holding footballs and the large gaps or pores between each ball.”
“Compare this with the same size container holding smaller tennis balls and the gaps between them.”
“Next image golf balls and the size of the pores between.”
He likes the ball analogies, I thought to myself.
“Interestingly the overall total size or volume of the pore spaces in clay, or golf balls in our representation, is more than in sand, even though sand has the larger individual pore space, which is one reason that clay holds more water” the lecturer continued
“Soil loams containing quantities of clay and organic matter will also bond together to varying degrees to make larger particles, aggregates, which in turn will have larger pore spaces, with more water and air.”
“Larger particles with large pore spaces in a soil containing clay with small pore spaces makes a soil with a good balance of drainage and water holding capacity suitable for good root development.”
Our lecturer, getting to the end of his presentation, said “Thus, soil texture can be determined by particle size and it is important to know the soil texture because of the resulting variation in soil water infiltration rate, water holding capacity and drainage.”
“The other important parts of a soil, organic matter, humus, organisms and microorganisms such as insects, fungi and bacteria, are vital for healthy soil and plants, and together with the mineral texture component they can be referred to as the soil structure which will be discussed next lecture” he finished.
We all then went back inside for our next lecture. I went outside to admire the heritage stone building we were lucky enough to use, and to see if the crow was still there.
Food and Agriculture Organization (FAO) of the United Nations, World Reference Base for Soil Resources (WRB) classification, Annex 4. Soil particle-size and texture classes, (2014). http://www.fao.org/3/i3794en/I3794en.pdf
Merali, Z., Skinner, B.J. and Strahler, A. (2009). Visualizing Earth Science, (1st Ed.). Hoboken, NJ.: John Wiley & Sons. Book.
Miller, G.T. and Spoolman, S.E. (2016). Living in the Environment, (Ed.19) Canada.: Cengage Learning. Book.
United States Department of Agriculture (USDA), Natural Resources Conservation Service, Soils, Soil Survey Manual — Ch. 3. Examination and Description of Soil Profiles, Particle-Size Distribution, (2017). https://www.nrcs.usda.gov/wps/portal/nrcs/detail/soils/ref/?cid=nrcs142p2_054253#particle_size