One major advantage of compost is its ability to hold moisture, retain it for longer, increase the amount of water available to plant roots, and minimise the need for irrigation. Ultimately, improving water saving practices and reducing economic overheads for farmers. But how exactly does it do this?

If we dig deeper, we find that compost helps to save on water in a few different ways.

Compost reduces water runoff and evaporation

One of the things that affects the amount of water that a plant can access is the amount of water that can soak into the soil (field capacity or water infiltration). When water infiltration is increased, the water is saved from laterally flowing off of the soil surface. Compost reduces soil crusting, which helps water absorb and infiltrate the soil. This can also help reduce evaporation by reducing the time the water is held on the surface. Organic carbon held within high organic matter compost (50%) naturally resists evaporation and heat/drought related moisture loss.

In short, compost helps water soak into the soil better (and faster).

So the quicker the water soaks into the soil, the less runoff there will be, the less time it has to evaporate and the more efficient the irrigation. Adding compost as a thin layer on top of the soil can also act as a blanket for the soil, helping to reduce evaporation by up to 70%. Benefits of adding organic matter (compost) to aid water infiltration can be seen across all types of soils although are best seen in poorly drained, fine textured clay soils.

Compost increases water retention

Another thing that affects the amount of water that a plant can access is the amount of water that the soil can hold onto (water holding capacity). Organic matter by nature holds a lot of water, therefore, the amount of organic matter in a soil directly influences the availability of water to a plant. Compost is high in organic matter, therefore, the application of compost increases the amount of water that can be held in a soil. A 0.33% increase in soil organic matter allows 80-100 extra tonnes of water storage per hectare. In one study, trial results showed that soil treated with compost increased water holding capacity by approximately 1.57 times. And another study found that a 3 inch (approx. 7.5cm) layer of compost incorporated to a 6 inch (approx. 15cm) depth increased water holding capacity 2.5 times that of a native sandy soil.

“Organic matter is like a sponge and compost is organic matter. The more of that water-holding sponge you have in the soil, the less water you will need to add.”

Better water holding capacity helps with water efficiency as the more water that is able to be stored means irrigation can become less frequent. The results of one Western Sydney-based vegetable trial demonstrated that compost applied to land at 20t/ha (compared to 10t/ha and a control) slowed down the water loss, allowing the growers to reduce the amount of water used without impacting crop yield.

The biggest impact of adding compost for water retention is seen on coarser textured (sandy) soils as opposed to finer (clay) soils. At the other end of the scale, in clay soils, water saturation can be a problem, where the soil cannot hold any more water, therefore causing soil runoff. Adding compost aerates clay soil and increases its capacity to hold water that way. Organic matter holds water in sandy (dry) soils and helps with aeration in clay (wet, heavy) soils.

Compost increases water accessibility to roots

Once the water is retained within the soil, compost also helps it to transfer more efficiently. Research suggests that applying compost to sandy soils can help moisture to spread laterally from where it was applied, thereby avoiding dry/wet spots.

Organic matter also improves the ability for plants to access the water. Amending the soil with compost improves the soil structure, which significantly affects how well the plants can access that water. Soil structure refers to the arrangement of sand, silt and clay particles into larger aggregates. (The organisms that break down organic matter release glue-like substances that bind soil particles into these crumbly aggregates). Soil aggregates create pores which store water, among other things. A poor soil structure with high clay content will have a reduced number of these soil pores. The pores that are present are smaller so even though there may be water in the soil, it’s contained in pores too small for plant roots to access. Or if they do access it, the plant uses more energy to extract the water, rather than using that energy to grow. As mentioned, adding compost improves the soil structure and increases and enlarges the soil pores.

Coarser textured (sandy) soils will generally have larger pore sizes and little soil structure, meaning the water drains off rapidly down below the reach of the plant roots. In these sandy soils, improving the soil structure with compost increases the soil’s water holding capacity. Whether in clayey soil or sandy soil, increasing the amount of organic matter to the soil helps create and stabilise soil structure which in turn maximises water storage and its accessibility. Ultimately, compost lightens clay soils making them more porous, (thus, reducing run-off, and improving water access to plant roots) and increases the water holding capacity of sandy soils, both reducing the need for water.

Not all composts are created equal

It’s important to know that not all composts are the same. For example, composts made from manure are different to composts made from organic material. The water holding capacity of compost can vary depending on the feedstocks used to make the compost, the process used to make the compost and the maturity of the compost when it’s applied. Therefore, it’s important to understand the quality of a compost before using it to ensure you get the intended benefit you’re looking for. If increasing water infiltration and water holding capacity is your goal, the key is to choose compost which is high in organic matter. Farmers should choose composts that have an organic matter content between 50-60 percent and a water holding capacity of 100 percent. Products should also comply with Australian Standard AS4454 Composts, Soil Conditioners and Mulches. When purchasing compost, ask to see an analysis to verify organic matter content and check for relevant certifications.

Jeffries Organic Compost is a mature compost made from organic material that has been through a controlled process of natural aerobic composting. Sources include home green bins, garden prunings and food organics. This variety of feedstock gives it a superior percentage of organic matter and high microbial diversity. Jeffries Organic Compost is certified to AS4454 and certified for organic production to NASAA standard 5125M.

Speak to your horticulture account manager today!

Aaron Matthews Ward
0427 923 699
[email protected]

References

https://www.compostingcouncil.org/page/WaterConservationBenefits
https://www.soilquality.org.au/factsheets/water-availability
https://ausveg.com.au/articles/investigating-the-relationship-between-compost-and-soil-moisture/
https://www.canr.msu.edu/news/compost_increases_the_water_holding_capacity_of_droughty_soils
https://mraconsultingblog.files.wordpress.com/2020/10/compost-and-soil-water.pdf