July 21, 2009 | General

Raindrops Are Pounding On My Head

BioCycle July 2009, Vol. 50, No. 7, p. 59
Climate Change Connections
Sally Brown

ONE of the things with climate change that I find particularly unnerving is that our weather is supposed to change. Change here does not mean just a uniform 1.3°C increase in temperature over the next 30 to 50 years (not a precise estimate, I’m just using this as an example). Change means more dramatic swings in temperature and, very importantly, precipitation. In some areas the rain will come less frequently and when it does, it will not be that light gentle 24 hour rain that a farmer loves but a 2-hour, 2-inch deluge.
This type of change can wreak havoc with more than just your plans for a camping trip. Availability of water for us to drink and more importantly for us to grow our food is likely to be the first major impact of climate change. Already Australia does not look nearly as good as a fantasy, mid-life crisis place to move as it did 15 years ago. Severe drought and changes in rainfall patterns have turned rice-growing regions of that country dry. Drip irrigated grapes are replacing flooded rice fields. While wine is delicious, most people rely on rice as a staple with wine as the chaser. When you look closer to home (my home anyway), you can see how freshwater shortages are already having an impact in California and the Southwest.
In California, over 80 percent of the freshwater resources go to agriculture. Not enough water means not enough food for a lot of the country. California grows all of our carrots, all of our almonds, plenty of strawberries, peaches, artichokes and yes, also some grapes.
So what can we do to conserve water? Shorter showers, low flush toilets, permeable concrete and reclaimed water are all good things, don’t get me wrong. However, it is likely that these conservation measures alone won’t make a sufficient dent in that 80 percent of the water used for agriculture to keep our plates full. Agriculture has tools like drip irrigation systems instead of flood or overhead spray systems, which can make a huge difference. But another thing that we can do is use more compost.

Water gets to plants via the soil. The soil holds water after an irrigation event or a rainfall until the plant is ready to drink. How much soil can hold depends on a couple of things. First is how fast that water can infiltrate the soil. If the water can’t go into the soil fast enough it goes over the soil, often carrying some of the soil with it to the nearest stream. This causes problems for the soil and the stream – and a significant loss of topsoil.
So what do you do about this – particularly with that 2-hour deluge? A major factor that determines how fast water can infiltrate a soil is its texture. A sandy soil lets water in really quickly. Just think of the waves at the beach and how fast the water disappears. A high clay soil with all of those fine particles is very slow to let water sink down. Those two things, sand and clay, are part of the soil texture. Soil texture is not something you can change. At 2,000 metric tons per hectare or 1,000 tons per acre for the top 6 inches, a 10-ton application of sand isn’t going to get you too far.
What you can change is how those particles are held together, i.e., the structure of the soil. Organic matter helps take those clay particles and change them from impermeable cement into soil. The organic matter works like a glue getting the particles to form aggregates. A well-aggregated soil is a lighter, fluffier soil where water has plenty of room to flow in. The measure for this is the bulk density of the soil – the weight per unit volume. For a rock, it is about 2.65 g cm3. For a nice soil it is about 1.1 g cm3. What we have seen in sampling in California and Washington State is that across all soils, compost significantly reduces soil bulk density, making it easier for the water to flow into the soil. These increases are greater for clay soils that tend to be heavier, and smaller for sandy soils where the speed of water flowing in is much less a concern than the rate at which water flows out.

Which brings us to the next point: How well the soil holds the water once it gets in there. When it first rains, assuming you get a reasonable amount, the soil fills up. All of the pore space in the soil fills with water, displacing air. As the water flows through the soil, some of the pores refill with air. The plants are very happy at this stage. This is called field capacity and the water is being held by the soil at a tension of about 0.1 bars of pressure. You see, water is a polar molecule, which means it has some amount of electric charge. Soil particles are also charged so when you add water to soil, the soil can act like a bit of a magnet, holding onto the water. As the soil gets drier, the water that is left gets held more tightly.
You turn on the irrigation at about 1 bar of pressure. Beyond that, the plants start looking pretty desperate. The deal here is that while sandy soil is really good at letting the water in, it is lousy at holding onto it. Clay on the other hand is like a miser, reluctant to let even a drop go.
What we also have seen in our sampling is that while soil texture is the main factor in determining how much water a soil can hold onto, soil organic matter is the next big factor. And soil organic matter is something that you can change. Particularly in sandy soils, you can dramatically increase the soil’s water holding capacity by adding compost. In one sandy soil in Palm Springs, California, we saw over a 100 percent increase in the amount of water that the soil had at 1 bar of tension. This was a soil that over time had received about 200 tons/acre of compost. Exactly how much extra water is the subject for another column. But I don’t think that it is hard to argue against a 100 percent increase in water holding capacity.
So perhaps more importantly than the shorter shower, put your food waste into a compost pile. Make sure that the farmers in your area know about what compost can do for soil water – how it helps to both let water into the soil and to help the soil hold on to the water it has
Sally Brown – Research Associate Professor at the University of Washington in Seattle – is a member of BioCycle’s Editorial Board, and authors this regular column on the connections of composting, organics recycling and renewable energy to climate change. Email Dr. Brown at slb@u.washington.edu.

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