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New versus established turf

As we saw in our first article in this series (Moisture retention and CEC in sandy soils), there can be good reasons for establishing turf or crops on sandy soil, but sandy soil has virtually no capacity to hold nutrients, which end up polluting groundwater and have to be replaced frequently.

Porosity

New sand tables have different characteristics from long-established grounds, so their management requirements differ too. Two physical characteristics are important to the difference. The air-filled porosity is the percentage by volume of the soil that is normally occupied by air (remember that oxygen is vital to roots). The large spaces between the soil particles responsible for the air-filled porosity are referred to as macropores. These cannot hang onto water for long. The capillary porosity is the percentage by volume of the soil that is normally occupied by water. The small spaces between the soil particles responsible for the capillary porosity are referred to as micropores. Capillary tension can hold water very tightly. Together, the air-filled porosity and the capillary porosity make up the total porosity.

Hydraulic conductivity

The total porosity is partly responsible for the hydraulic conductivity of the soil, which is the rate at which water can pass through the soil. On a sports field or a golf course, a high hydraulic conductivity is essential to good drainage and return to play after heavy rain. Sand, with a high air-filled porosity and a low capillary capacity, has a high hydraulic conductivity, which is why is it favoured for sports fields and golf courses. Clay, with the converse, has a very low hydraulic conductivity, which makes it ideal for sealing dams.

Although a high hydraulic conductivity is ideal for playing fields, it is simultaneously detrimental to turf growth, because if all the water drains away, the turf will die of thirst. A common solution to this problem is to water frequently. Unfortunately, this is not a sustainable practice for several reasons:

• Water is becoming scarce.
• Water costs money.
• Frequent irrigation of sand simply washes fertiliser out of reach of the roots, so more has to be applied, which is expensive.
• The nutrients can enter the groundwater and pollute drinking water and rivers.

Starting from new

On a new sand table, turf managers are faced with the problem of keeping the turf alive while not wasting water and fertiliser. As our first article showed, adding organic matter (OM), some clays or zeolite to the sand can vastly improve the cation exchange capacity (CEC) of a soil, retaining nutrients, and the water-holding capacity of the soil too.

New sand has a high air-filled porosity and virtually no capillary porosity. Incorporating OM, certain clays or zeolite during establishment can immediately rectify this imbalance. Alternatively, topdressing with these materials during renovation and coring can gradually increase the water-holding capacity without compromising the hydraulic conductivity.

Established grounds

Over the years, buried thatch and decayed roots (and any applied organic manures) will gradually build up the OM content of the soil. To a degree, this is desirable, because the OM has a high CEC and a high moisture-holding capacity. However, under some circumstances, high OM can actually mean water repellence, and a lot of dead plant material can favour the growth of diseases. In addition, over the long-term, as the soil macropores gradually fill, the capillary porosity increases to the point that hydraulic conductivity is compromised.

Balancing these physical characteristics is an important component of a turf management program. For information on renovating turf, see [need info here on references].