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Moisture retention and CEC in sandy soils

There can be good reasons for establishing turf or crops on sandy soil. In the case of elite sports fields, turf has to be able to handle heavy traffic, such as a football game, as soon as possible after a heavy downpour. Playing fields are ideally level, so rainwater will not run off quickly. Instead, it has to drain quickly into the soil, from where it can be carried away by subsoil drainage.

In the case of crops, some farmers have no choice but to crop on sand - particularly those in Western Australia.

But drainage for immediate access after rain must be balanced by the ability of the soil to grow the plants. Unfortunately for plant roots, sand has virtually no capacity to hold nutrients. Every watering or rainfall washes nutrients through the sand out of the reach of plant roots. This wastes money and pollutes groundwater.

Groundwater polluted with nutrients, especially nitrogen and phosphorus, can be poisonous (a lot of rural towns in Australia depend on bore water), and can flow through the subsoil until it reaches creeks and rivers, where it causes toxic algal blooms, killing fish and other aquatic animals and poisoning livestock that drink the water.

So it is essential to strike a balance between the ability of sandy soil to drain quickly and the need to stop nutrients from leaching down.

Cation exchange capacity

This is where cation exchange capacity (CEC) becomes important. The CEC of a soil is the capacity of the soil to retain nutrients for use by plant roots. A high CEC - typically 30-40 meq/100 g - ensures that any fertiliser added to the soil will remain in the soil for uptake by roots. But sandy soils have a very low CEC - typically <5 meq/100 g - which is not enough to retain nutrients. Consequently, growers must continually add fertiliser, only to have it leached away by rain and irrigation.

Soil moisture

The capacity of a soil to retain moisture is equally critical to plant growth. The ideal soil under sports turf drains rapidly, but this leaves the turf without enough water to grow. So sportsground managers must continually irrigate, knowing that this is simultaneously washing away the fertiliser they apply (dollars down the drain).

Soil additives

Various soil additives can be used to increase both the CEC and the moisture-holding capacity of sandy soil. Both organic matter and clays can offer solutions.

A study in India (Journal of Applied Sciences Research 2(11): 949-951, 2006) compared the effects of various soil amendments on the water-holding capacity and CEC of a sandy soil. The researchers found that vermiculite (a clay, with a CEC > 80 meq/100 g) nearly doubled the moisture retention (followed by bentonite, another clay) and increased the CEC of the sandy soil by more than 60% (followed by humic acid, an organic breakdown product). Farmyard manure was also effective.

They concluded that vermiculite at 20 t/ha was an appropriate soil amendment. (They grew a crop of millet but failed to mention the yields!)

Zeolite

Potentially even better as a soil amendment is zeolite, a mineral with an extremely high CEC (>200 meq/100 g). Applied at 2 t/ha (to create a mix of 10% in soil), it can double the water-holding capacity and greatly improve the CEC. This in turn reduces the amounts of both irrigation and fertiliser needed and thus cost. Root mass also increases, with two important consequences. One is that more roots are available to take up water and nutrients - so the turf or crop is healthier. The other is that as the old roots break down, the organic matter further increases the CEC.

Blocked pores

Although increasing the water-holding capacity and CEC is important, you can have too much of a good thing. Animal manure, for example, applied long term can gradually block the soil pores, reducing drainage. A future Fertile Minds article will examine how to balance these competing needs.