Water Retention Aids

Improving water holding capacity in lawns

At this time of year irrigation becomes a critical factor in the care of lawns and gardens. Basically an effective irrigation system is making up the deficit that occurs between the plants' water requirements to grow and the rainfall during the warmer months. In spring and summer plants (including lawns) lose water into the atmosphere at a greater rate than during the cooler months. The water loss is known as evapotranspiration (ET). Depending on the weather conditions ET can exceed 5mm per day at times, which means the lawn is losing 5mm of water before any irrigation is even applied. This poses a real challenge for anyone wanting to maintain a green lawn during this time.

Soil Structures

Soil can be likened to a sponge that holds the water which releases nutrients to the plant. All plants have what is called Wilting Point, at this point, the water supply available is insufficient to sustain the plant and it begins to wilt.

The solution to the problem is more complex than just increasing the amount of water being applied. In fact, increasing the irrigation could actually make the problem worse in the long run, leaching the soil of nutrients. Soils have many different structures and textures which greatly affect their ability to hold moisture.

When soil is completely saturated it reaches what is known as Field Capacity. At this point, the soil has reached its maximum ability to retain moisture and any additional water applied will create ponding.

 Both Wilting Point and Field Capacity are issues related to the soils ability to retain water and need to be carefully managed to keep the lawn thriving.

These factors highlight the importance of understanding soil structure and how each type of soil holds and releases water differently.

What is soil water retention?

All soils have the ability to hold and release water but understanding how different soils do this is key to the health of the lawn. Lawns receive nutrients via the root system which is transported in the groundwater stored in the soil. When water is applied, either from rain or irrigation, the water penetrates the soil and some of the water is held in between the particles of soil. The rest of the water is released into underground streams thanks to gravity and this serves to recharge aquifers and springs.

Water is trapped in the pores between the soil particles waiting to be used by the root system which draws the nutrient-rich water into the plant. Some soils are better at retaining water than others due to the size of the particles and the amount of air between the particles. These little air pockets are called pores and the closer and more tightly packed the pores are in the soil the more water it will be able to hold. I.e. soil with high clay content. The other end of the spectrum is sandy soil, this soil has larger particles and consequently has larger air pockets, therefore, it does not hold water very well and easily releases the water into the underground streams leaving little for the root system.

As you can imagine the ideal soil would be somewhere in the middle, a mix of clay and sandy soil. This would hold enough water for the roots to access without becoming water-logged. This type of soil is called loam. Unfortunately in New Zealand the perfect loam soil doesn’t always occur naturally and may require some work and additives on our part to achieve.

How to increase water retention in soil

Add more organic material

Depending on the water holding capacity of the current soil it may be possible to simply improve soil structure by adding more organic matter. This can be done by applying a layer of compost and working it through the soil if a new lawn is being laid or applying and spreading across the surface if rejuvenating an existing lawn. But be careful, not all composts are created equal! PH is a major factor with compost and a poorly prepared compost could ruin an existing lawn due to the high acidity.

When it comes to topdressing an existing lawn it is best to speak to the experts about the best product for your lawns specific needs. A well balanced top dressing may consist of a number of different elements like washed river sand, gypsum, fine pine bark and chicken manure.

If the appropriate product is applied correctly the results can be remarkable. The soil structure will be greatly improved and consequently, the water retention ability will improve. The top dressing will also deliver much-needed nutrients to the soil.

Use polymer aids

Polymer crystals, graduals and liquid are nothing new and have been in use in soils since the ’80s.

Essentially these products are a special polymer that have the ability to absorb and hold moisture. These are similar to the crystals that are used in diapers and various other products that require rapid moisture absorption. The granules are added to the soil to supplement the soils water retention ability. They are worked into the topsoil layer and will absorb up to 400 times their original weight in water. The water is absorbed by the crystal and turns to a jelly, which is stored and released slowly over time for the roots to access instead of being drained through the soil and into the underground aquifers. Obviously this product will be best for sandy soils and is not recommended for heavy clay soils that are already holding excess moisture.

An application of polymer has been proven to greatly reduce the amount of irrigation required and offer a solution to keeping a lawn green over the summer months.

The product can be effective for a number of years if applied correctly.

However, applying the polymer to existing lawns can be tricky and it is best to consult a lawn specialist before doing this.