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Soil Texture part 2

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?

A: No.

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.

Building Sand

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.

Soil Texture Part 3

Soil Texture Part 1

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