In the previous articles in this series, we’ve discovered how to evaluate performance by simply looking for visual indicators on the turf and by gauging some of the functional attributes of grass plant communities when they form turf.
Before we move on to the final stage of this series where we will look at some of the latest and most objective techniques for performance measurement, I wanted to stop for a moment to consider what lies beneath the green layer.
All of the functionality and therefore performance of the bowling green depends on healthy turf and turf is of course not just grass plants. Turf is a construct of a healthy grass plant community containing millions of individual plants, growing in a medium that is suited to sustaining healthy growth and reproduction. The medium is, of course the soil our greens are built on; but what is soil?
If you look at the pie chart at the top of the article you will see the proportions of what I think of as the perfect soil.
In the diagram you will see that 45% of the soil is made of Minerals. The mineral component of soil is usually made up of a mixture of 3 main groups and these are Sands, Silts and Clays. A suitable mixture of these is critical to the soil’s performance as they dictate the soil’s ability to provide nutrition and moisture to the grass plants and suitable drainage. The mix of sand, silt and clay defines the soil’s texture.
The organic component will ideally be around 5% and this is made up of living organisms, micro-organisms and dead, decomposing and already decomposed plant tissue (humus). The organic material is added to by the plants themselves as they produce thatch and the soil organisms break this down to release plant nutrients.
Then there’s the remaining 50% of the soil to look for, but if you do, it might cause you some confusion, because in the ideal soil the remaining 50% of its volume will equate to nothing at all. In fact it is 50% space, or soil porosity to give it the correct name.
Ideally half of this space will be made up of small spaces called micro pores and large spaces called macro pores. The micro pores hold the soil solution which is a mix of water and plant available nutrient ions and the macro pores provide air space and this is where all of the drainage occurs after heavy rain. This air space keeps the soil well oxygenated so that it can sustain a huge population of soil microbes; around 1 billion in a teaspoon of soil.
In the first part of this series we discovered that the ideal bowling green soil (or rootzone) will be 50% space, 5% organic matter, with the remainder (45%) being made up of mineral matter, namely Sand, Silt and Clay. These are the 3 universal mineral components of soil. Part 1 finished with an explanation of the soil fractions, 5 of which were sands of varying sizes.
In part 2 we found out a little bit more about sand and it’s behaviour as a drainage medium and we discovered a little more about how soils are formed. We finished by looking at the importance of sand particle shape and size in bowling green rootzones.
We are all familiar with clay as a substance in many different aspects of our lives. It’s used to make bricks, pottery and for modelling. If you’ve ever moved to a newly built house you will doubtless have encountered the problem of trying to make a decent garden out of the heavy clay soil that builders seem to carry around for the purpose of making your life difficult; or is it that all new houses are built in areas where there is heavy clay soil? Regardless of the solution to that conundrum, we often think of clay as big, chunky, unmanageable clods of red earth. The fact is though, that those whopping clods are actually made up of the tiniest of particles in the soil, less than 2 thousandths of a millimetre in diameter.
Along with silt particles, which are relative giants at up to 5 hundredths of a millimetre in diameter, clay and the very fine sand fraction (0.05-0.10mm) make up what is known as the Fines in the soil. The fines are very important in any rootzone mix, because they are largely responsible for dictating the soil’s ability to provide water and nutrients to our grass plants. This doesn’t mean, however, that more fines make a better soil, it just means that we need to be very aware of the balance of fines to the other 4 sand fractions. The measure of this spread of particles in any soil or rootzone sample is called the Particle Size Distribution or PSD for short.
High performance rootzones will typically have no more than 10% of the particles in the Fines category. Remember that we are only measuring the mineral element of the soil. For PSD Analysis, all of the organic material is burned off before the dried mineral component is shaken through a series of graded sieves and measured.
Particle Size Distribution (PSD)
In my introduction to this series of articles, I said that an understanding of Bowls Green Soil Texture, was probably the single most important piece of knowledge a greenkeeper can have as it impacts on almost everything else in bowling green maintenance. Well, I can probably refine that statement to simply Greenkeepers must understand PSD. Particle Size Distribution really encapsulates the knowledge of soils that greenkeepers would do well to have in their heads ready for instant recall. This knowledge can help you work out just about anything else that you need to know about the performance of your bowling green. But what is PSD? Have a look at the charts below:
The upper chart shows the PSD for a typical (natural) soil on the links land on the East Coast of Scotland. It’s actually from a very famous golf course. The first thing we notice is that 80% of the particles are in a single fraction, in this case Fine Sand. This is typical of links soils. You will recall from part 2 of this series that beach sand is graded by the wind with the finest particles being blown furthest from the shore. The perfect balance of PSD for the formation of tight knit, dense bent and fescue turf just happens to be found on the Links land that we find around much of the UK. In this example, the mineral element of the Links soil is made up of 80% of the Fine Sand fraction, or sand particles falling between 0.10 and 0.25 mm in diameter. You will also notice the Fines are made up almost entirely of Very Fine Sand, with Clay and Silt barely registering on this graph. Again this is typical of Links soils, as the mineral element is almost entirely made of sand, weathered down from seashells.
In the second graph we can see a typical PSD analysis for a manufactured rootzone material for a USGA (United States Golf Association) Specification golf green. Again you will see a very large percentage of the particles falling in one fraction. This time around 70% of the sample is made up of Medium Sand (0.25-0.5mm). In this case because we are working with a quarried sand material, the fines contain some silt and clay, but the 3 fines combined still make up less than 10% of the rootzone. Incidentally, the reason for specifying Medium Sand in such rootzone materials is to do with Soil Hydraulics (water movement) and the need to build greens economically. The natural links soil might be several feet deep, where as the manufactured green will have no more than 12inches of soil depth. This creates a need for a greater gravitational pull in order to maintain drainage performance. The bigger aeration pores created by Medium Sand means that a 12 inch deep soil can perform as well as a 2 or 3 feet deep soil predominantly made up of Fine Sand.
PSD and Drainage.
The benefit of having a very uniform (70-80% falling in one fraction) sand in the rootzone mix is that it creates a lot of air space in the soil. Remember we talked about the woes of trying to build a sand castle with the ever shifting sand near the dunes? Uniformity in terms of particle size creates air spaces and makes the sand very mobile and resistant to packing down. The air spaces (macro-pores) in the soil are where the oxygen is kept in the soil to help nurture a big population of soil microbes and is also where the drainage occurs. Excess water is pulled through the rootzone by gravity and drained away.
PSD and Moisture Retention
Of course, if we had a rootzone that was too uniform in PSD, then our bowling greens would be very difficult to manage as they wouldn’t even be stable to walk on, let alone support a population of grass plants. This is where the mix of different fractions comes in. We saw in part 2 how Builders sand relies on a wide spread (low uniformity) of particle sizes to enable it to lock down and bind together, well our rootones need a little bit of that ability too, so there will be a spread of particle sizes on either side of the dominant uniform fraction, always remembering that we need to keep the Fines around 10%.
Although soil stability is important, it is the role that the mixture of particle sizes plays in moisture retention that can make or break a bowling green. We saw in part 1 that the variance in soil particle size creates soil porosity; Macro Pores (large spaces between particles) help with soil aeration and drainage and Micro Pores (small spaces between particles) provide moisture retention. It is from these small pore spaces that our grass plant’s roots suck up the soil solution to get at the water and nutrients it needs to grow.
Fines and Nutrients
The weathering of the minerals in the soil actually provides some of the essential plant nutrients, but in part 2 we discovered that, regardless of the type of soil particles we have in our soil, there is no way that our grass plants can access the essential nutrients until they are in solution in the soil water. The smaller micro pore spaces between the finer soil particles are key to the plant’s ability to access these nutrients. As gravity pulls all of the excess moisture out of the soil (macro pores) after rain, plant available water is retained in the micro pores and un-affected by gravity.
The 4 Part Trilogy
Finally for today, the tiny clay particles actually bind many of the nutrients in the soil, preventing them from being flushed out of the rootzone, meaning that the plants can access them over the longer term. We touched on this in part 1 when we discussed Cation Exchange Capacity.
I’ll leave you today with the famous (in the circles I move in ;-( ) Soil Texture Triangle, a tool used by soil geeks specialists to define soils based on their particle size distribution. It’s a handy tool, but maybe a bit daunting to use at first. I’ll explain it next time.
This started out as a 3 part series, but there’s more to do yet, so in the 4th part of the trilogy :-), we’ll look deeper into the soil texture triangle and start to see where this all links up with soil structure, soil management and bowling green maintenance.
Last time we saw how the perfect bowling green’s soil volume will be 50% space called porosity. We discovered that half of that pore space (25% of the soil volume) should ideally be filled with air (macro pores) and that the other half (again 25% of the soil volume) is for water (micro pores). We finished last time by discovering that the mineral part of the soil is actually made up of a lot of different sized soil particles called the Soil Fractions.
Today we’ll try to get a better handle on Soil Texture and discover how some of the soil fractions come about. In particular we will look at the complexity of sand, before getting a better understanding of how soils are formed in the first place. This will help us to understand the importance of sand in bowling green maintenance, but hopefully also to understand more fully, its limitations.
Soil Texture; the Mineral Fractions
In bowling green Nirvana, out of the remaining 50% of the soil volume, 45% would be Mineral and 5% would be Organic Matter. Today I want to concentrate on the 45% Mineral matter as this is where we can really influence the performance of our greens.
Now, if fast drainage was our only concern, then this would be a no brainer; it would seem logical to use just sand wouldn’t it? Well, that is the deeply rutted road that much of the fine turf industry has been heading down for a few decades now and I can tell you it is fundamentally wrong. Wrong because it overlooks the need for a firm, fast, true bowling surface, 100% covered with a tight knit turf consisting of the finest of grasses.
The Complexity of Sand and what it tells us about Soil Formation
Before we go deeper on this thorny subject I have a question:
Q: Is all sand the same?
Think about the different places you’ve encountered sand. The most vivid of these memories will be on some golden beach. Did you ever try to build a sand castle on the higher reaches of the beach? Building a sand castle with that stuff is impossible, it just flows right back into a non-descript mess. When you walk in it, it just moves away from you and fills your shoes with gritty particles. Lower down on the beach where the sand is wet, then you have a chance of forming some battlements and maybe even to dig out a moat before the tide comes back in to engulf your handy-work.
Between tides, some of this wet sand will be dried out by the sun and whisked up the beach by the wind to add to the ever changing dune system at the top of the beach. The wind tends to drop the bigger heavier sand particles first whilst the finer, lighter sand particles are carried furthest up the beach to the dunes and beyond. The dunes are sufficiently undisturbed by the tide for pioneer plants to get a hold in the sand. These are hardy species that can withstand the constant wind and spray and have toughened, spiny leaves that resist drying out in the constant wind and bat off the worst of the salt spray to eventually develop into a community of plants. The roots of these plants cause the dune sand to bind together into more stable structures which get higher and wider with each barrage of wind and tide.
Just inland from the dune system is the Links land. Literally the land that links the seashore and the cultivatable land inshore. In Scotland you might also hear of the Machair, a richer, more easily cultivated meadow land beyond the dunes. The Links and the Machair were once the dunes and over time, the continual cycle of life and death of the plants and animals of the area and the addition of wind blown seaweed would gradually lay down more and more organic material, slowly building the once purely Mineral sand into a more complex soil (a mix of mineral and organic material); technically in most cases a Sandy Loam.
The early stage of soil formation is called weathering and the parent rock or mineral material will be eroded and broken down into smaller parts by the rain and wind. In our example the parent material is purely the remaining shells of dead sea creatures like crabs, whelks and oysters. Gradually these shells are broken down into smaller pieces by the wind and tide action.
Sand Particle Shape, Sizes and Distribution
During this process the sand particles are worn from angular shards of shell to more or less spherical particles over a long period of time. When the sand particles eventually become small enough to be blown by the wind they are gradually moved up the beach and this is where a process of grading takes place. As we saw, the smallest (and lightest) sand particles will be carried further up the beach than the larger, heavier ones. This has the effect of grading the sand into bands of the same particle size. The combination of their spherical shape and uniform size makes the particles in the dry upper reaches of the beach very mobile and that is why you sink into the sand near the top of the beach and why it seems to move and flow around you.
Incidentally, this is also what makes sand drain well. Particle Size Distribution (PSD) is a key measurement in the specification of fine turf rootzone mixes.
But, what about building sand?
The other place you’ve probably encountered sand is at the DIY store. The sand you buy to mix concrete or cement or lay slabs is not like the sand you buy for drainage. If it was to behave like the dune sand, your paving would sail away in the first rain storm and your house would fall apart brick by brick!
For a start building sand has a wide spread of different particle sizes and a lot of the particles are angular or semi angular. This sand is typically quarried and not sourced from the beach. This makes it bind together very well and also makes it terrible for drainage. Ironically a lot of it isn’t sand at all. Building sand will contain quite a high percentage of finer silt and clay particles which add to the binding effect.
This should prove once and for all that you shouldn’t listen to the Treasurer when he says he’s found you a supply of sand that is half the price of the usual stuff!
The perfect rootzone.
Next time I’ll introduce the remaining minerals of Silt and Clay and we will look at Soil Texture in terms of its role in bowling green performance. In the process we will discover that the greenkeeper’s knowledge of soil particle shape, size and distribution holds the key to providing the perfect balance of drainage, nutrition and moisture to help us develop the ultimate high performance bowling green that performs like an F1 car but is as predictable and as easy and economical to maintain as the family hatchback! Finally we’ll link this all up and develop an understanding of the folly of continually top-dressing bowling greens every year.
This subject is so important to the future performance of bowling greens that I would say it is essential for greenkeepers to understand this subject over any other.
The physical condition of the soil in the bowling green is the most important aspect of bowling green maintenance because it impacts every other aspect of green management. The physical properties of your soil dictate everything from drainage, nutrient availability and pH right up to green speed, green smoothness, consistency and ultimately whether or not the green performs to a high standard.
But what do I mean when I say Soil Physical Properties? This is primarily about 2 distinct qualities of the soil, Texture and Structure. Soil Structure relates to the way the soil holds together and there can be no doubt that soil structure plays a big role in green performance. However, soil structure is largely dictated by Soil Texture and for that reason I believe that the Soil Texture is the single most important aspect of green maintenance for greenkeepers to understand. Unfortunately, in my experience it very rarely is understood sufficiently to give the greenkeeper enough confidence in creating a program of work that majors on getting this right. But what do I mean by Soil Texture?
Animal, Vegetable or Mineral?
No, this isn’t just the question you ask your daughter about the spotty youth she has brought home for tea; it’s also the phrase that describes the make up of all soils. More accurately, soils are made of a base Mineral material mixed with Organic (dead animal and vegetable) material . The mix of the two dictates the soil’s performance in terms of its physical attributes like structural strength, drainage capability and nutrient and moisture holding capabilities.
In art and design, it’s often said that the white space around the images and text is often more important than the objects themselves, and a similar rule applies to good soil. In fact the perfect bowling green’s soil will be 50% nothing! By that, I mean that there should be lots of space in the soil between the soil particles. We call this nothingness, Pore Space or Porosity. Some of these pores are big (macro) and some are small (micro) pores. Micro pores are actually classified into two sub groups, but these basic groupings will be fine for now. The Mineral and Organic materials, might make up the structure of the soil, but it’s within the soil porosity that all of the action happens!
Last time we saw how the grass plants extract the 16 essential nutrients from the soil solution. The soil solution contains plant usable forms of the essential elements called ions and although these originate in the organic and mineral components of the soil, the plant can’t access them until they are in solution in the spaces between soil particles. That soil solution exists within the micro-pore space. The macro-pore space contains air and is essential for the supply of oxygen to support a healthy soil microbe population as well as for good drainage and resistance to compaction.
But what do I mean by soil particles?
The Mineral element of all soils is made up of a mix of 3 basic structural components called Sand, Silt and Clay. These are the basic soil fractions. The sand part varies greatly in particle size so we categorise it into 5 sub fractions for the purposes of soil texture classification. Here are the soil fraction classifications:
Stop to look at those sizes for just a minute, because they are quite mind boggling, especially at the lower end where the silt and clay is. Coarse sand has a maximum size of just 1mm, so imagine how very tiny a clay particle is at less than 2 thousandths of a millimetre! Well, that minuscule size hides a big secret and I introduced the intricacies of it when we looked at Cation Exchange Capacity last time. The tiny clay particles are where a lot of the nutrient ions are held within the soil, meaning they can be used by the grass plants later. Clay also plays a role in moisture retention which is critical for the health of the bowling green.
To finish today, I will leave you with a very useful visual representation of the relative sizes of these soil particles courtesy of the University of Colorado and next time we will move on to the importance of getting soil texture right in your bowling green.
The Sub-Surface Requirements for a High Performance Bowls Green are pretty much set in stone. Get these right and you’re well on your way to a Performance Bowling Green.
First of all then, have a look at the diagram above. This represents the ideal make up of a performance bowls green’s rootzone.
Do you see anything remarkable?
Well, when I explain this to my clients for the first time, many of them are surprised to say the least.
If you look closely at the diagram you will see that the ideal green will be 50% nothing; yes space, just air cavities within the soil. Now, of course I am not going to tell you to get rid of half your green to achieve this, but next week I am going to share with you a program whereby you can get close to this ideal situation of ½ solids and ½ space in your green.
Of course the space isn’t just nothing; half of the space consists of “micro-pores” and half is “macro-pores”. Put simply the micro-pores contain water and the macro-pores contain air. This is very important to understand and is one of the least understood concepts within sportsturf maintenance.
The “nothing” element of the ideal green is the most important factor to get right, because this is where we get the balance between speedy drainage and good growing conditions and it is due to a fundamental misunderstanding of this concept that a very large number of UK bowling greens are in poor condition and can’t be prepared for performance consistently. The only saving grace for these greens is that the UK summer is also very inconsistent and sometimes acts in their favour; so that we occasionally get a very good season’s bowling when the “green has never been better”. This is a false reading in most cases and the problem is exacerbated by the club attributing this success to the latest fad program.
The green which has a well balanced soil as described and illustrated above will naturally:
sustain a firm, fast surface with a minimal input maintenance program
sustain a healthy sward of fine grasses
sustain a high, year-round population of soil microbes
provide a natural cycle of nutrient release from soil organisms and micro-organisms (microbes) working on fresh organic matter (thatch).
resist compaction and therefore resist:
annual meadow grass ingress
flooding and puddling
retain the optimum amount of soil water for healthy growth with minimum requirement for artificial irrigation.
drain reasonably quickly after excessive rainfall.
retain the optimum amount of plant available nutrition
sustain a soil pH within the optimum range for fine turf
resist attack from fungal diseases
resist the onset of Localised Dry Patch and other soil and turf disorders
maintain a tight, dense sward with an upright growth habit which will reduce ingress of moss, weeds and weed grasses.
resist localised settlement and bumpiness due to excessive thatch and erratic thatch decomposition
Comprehensive action plan for achieving the above included in the book below. It costs less than a bag of cheap fertiliser.
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