May 2009

Welcome to the May 2009 edition of Fertile Minds, where everything you read is true.

Carbon credits and carbon sequestration are all the buzz at the moment. You might think that agriculture offers a possible route to these, but it seems doubtful. Read on to find out why.

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In this issue

• The Loam Ranger – Soil carbon and carbon trading
• Irrigation in drought
• Protein content in pasture
• Soil acidity, lime requirement and aluminium toxicity
• Meet the staff – Environmental consultants
• Did you know ... ? – Vermiculite

The Loam Ranger – Soil carbon and carbon trading

Dear Loam Ranger,

Will I be able to earn income by selling carbon credits to store carbon in the soil in my paddocks?

A lot has been said in the media lately about soil carbon sequestration and how this might help offset the rise in atmospheric carbon from human activities. Several of our clients have expressed interest in selling carbon credits to big business on the basis that they can sequester (that is, lock away) carbon in their soil. In theory, this has flow-on benefits of improving the fertility, cation exchange capacity and water-holding capacity of the soil. In practice, it is not straightforward.

Composting is not practical in broadscale agriculture, most added carbon is lost in a few years, and biochar is a yet-unproven technology. To understand why, first it is important to understand the different forms of carbon in the soil, and what they mean for carbon sequestration.

Click here to find out how carbon is stored in the soil and why agriculture is unlikely to help (900 words, 4 minutes)

Irrigation in drought

Patterns of rainfall in Australia are changing: in particular, the southern rainfall belt is slowly moving southwards, over the Southern Ocean. The rain is still falling; just not as much is hitting the land.

One consequence of less rain and less frequent rainfall is low river flows. During periods of low flow, groundwater leaks out of the soil into the rivers.

Much soil in Australia has salt reserves, either just below the root zone or, as a common consequence of poor land management practices, actually at the surface, in salt pans. During the past century or so of extensive clearing, the removal of trees has allowed rainfall to raise the water tables, particularly at low landscape positions. This has brought the saline deep water to the soil surface.

When river flows get low, the saline groundwater no longer has the pressure of the river water opposing it, and so it leaks into the river. This is why the trickles in recent years in the Murray–Darling River system have become highly salty (among other things, threatening Adelaide’s water supply).

If you rely on river water for irrigation, you might already have noticed that the water is getting saltier. So what can you do about it?

Click here for short- and long-term solutions to saline irrigation water (650 words, 3 minutes)

Protein content in pasture

All animals require protein in their diet. Grazing livestock depend on pasture to supply their requirements. If the pasture doesn’t contain enough protein, then growth and production will suffer. A low protein content in pasture usually equates with a low energy content, but this is not necessarily so in formulated rations, which can have a high starch content but a low protein content.

The protein content of pasture species changes greatly with time of year and species. Legumes, of course, are high in protein, being able to draw on atmospheric nitrogen via the symbiotic Rhizobium bacteria in their roots. Grasses tend to have a lower protein content. Young pasture early in the growing season has a higher protein content than old pasture. A mixed pasture early in the season has a typical digestible crude protein content of 25% (on a dry matter basis). By flowering, this has dropped to 10%.

Click here to read how to improve protein content (800 words, 3 minutes)

Soil acidity, lime requirement and aluminium toxicity

Increasing soil acidity (lower pH) reduces crop and pasture yields, and is recognised as a major problem facing agriculture. Yields are reduced by two factors: below-optimum nutrient availability (some nutrients become chemically locked up at low pH) and the presence of soluble aluminium and manganese, which are toxic to plant roots. Soils become acidic through the removal of nutrient cations and their replacement with hydrogen ions; through a build-up of organic matter, which is acidic (specifically, organic acids); and through the application of acidic fertilisers, such as ammonium sulphate.

Therefore there are two main reasons why we might want to raise the pH of a soil: to improve nutrient availability and to overcome Al toxicity. Al is the most common metal in Earth’s crust and is found in most soils, principally as a component of clay minerals. We can’t remove it, but we can control it.

Click here to see how we measure for acidity and treat it (600 words, 2 minutes)

Meet the staff – Environmental consultants

In the last issue we met the staff of the analytical lab, who run the detailed lab tests and gather the results for reporting. After the results are collated, they go to the consultants, who use them to prepare their recommendations for you. This week we meet the environmental consultants, who deal with risk assessment and analysis of environmentally sensitive matters. Meet Daniel Saunders, Ryan Jacka and Meredith Bormann.

Click here to see the environmental consultants (500 words, 2 minutes)

Did you know ... ? – Vermiculite

SESL clients will be most familiar with vermiculite as a horticultural soil component: those shiny bronze flakes that can be easily compressed between the fingers.

Vermiculite is a clay mineral - a magnesium-aluminium-iron silicate. It is a weathered mica in which the K⁺ ions between the molecular sheets are replaced by Mg²⁺ and Fe²⁺ cations.

Little worms

When vermiculite is heated rapidly, the water between the layers turns to steam, which expands the vermiculite into long wormlike strands. The word comes from the Latin for “little worms”.

Click here to read about where vermiculite comes from (250 words, 1 minute)