Best practices for graziers - soil fertility management

INTRODUCTION

All food production begins and ends with healthy soils that grow healthy plants. In the past 50 years, much of the grazing industry has been simply focussed on the animal production system without care or understanding for the soil. However, our soils are the key to ongoing production for the world food supply. Now more than ever we need to manage for the longer term sustainability of our soils.

As a starting point, our soils have taken many thousands of years to develop their structure and balance of nutrients (chemistry), their natural biology and the ability to adapt to climatic changes. In the past 200 years, we have altered the landscape and in many situations altered the chemical balance to grow crops and pastures that are not naturally occurring. The soils may have had an ideal balance of nutrients and biology in a natural low production situation, but are not able to sustain the production we expect from it today. In some situations, farmers have fertilised pastures and improved production and in other situations misunderstood fertilisation has caused more problems.

Farmers Save the World! The Carbon Cycle - Mycorrhiza & Healthy Grass

Therefore we need to better understand how our soils are structured and what the productive capability is today.

The three key parameters of soil are

See the research on soil health through Holistic Management.

Nutrients - Chemistry

For many years, nitrogen, phosphorous and potassium (NPK) have been the only nutrients most agronomists concentrate on because these were the most limiting and soils were fairly balanced. However, today many agronomists are now looking to find the next most limiting factor which could be sulphur, zinc, boron or any number of nutrients. Therefore, it is worth knowing a little more. The chemistry of our soils is similar to that of a battery with positive and negative charges and transfer of nutrients via water as soluble salts. The ability for our soils to hold and release nutrients is called the Cation Exchange Capacity (CEC), while the key nutrients that hold onto the clay colloid include:

The second component and extremely important one is the humus, carbon or organic matter. Some soil tests give an organic matter %, others a carbon %. The humus is the portion of the organic matter that can hold onto nutrients, act like a sponge, filter or sieve to stop leaching or volatilisation. This sponge can also hold onto four times its own weight in water. In the past 200 years, we have lost at least 80% of this sponge and this is the key reason why our soils no longer manage droughts or floods well.

Other minerals in our soils include Sulphur, Zinc, Manganese, Copper, Boron, Iron, Molybdenum, Silicon, Cobalt and Selenium to name a few. Each mineral has a role within the soil and within the plant species as well as in the animal and humans. If one mineral is out of balance, it can severely impact on another minerals nutritional role.

pH or potential of hydrogen mostly known as the level of alkalinity or acidity of the soil is also a critical factor in nutrient flow from soil to plants. pH varies from about 3 (highly acid) to 6.5 - 7 (neutral) to 14(highly alkaline). Most minerals become more available to plants at the neutral level, while some minerals are more available in alkaline soils.

The most important Best Practice activity would be to conduct a soil nutrient test with a lab such as Environmental Analysis Laboratory (EAL) at Southern Cross University, Lismore. For details go to the monitoring section on soils.

Living soil - biology of soil

In the past 50 years, and still continuing, many scientists focus on what they can see on top of the ground and ignore the things we cannot see (eg. under the soil). It is difficult to find many specialists or extension staff who accept the biology in the soil is an issue, let alone how to manage for it. The soil is the place where it’s all happening, as the smallest change on top can make enormous changes beneath the surface. Our failing crops, root bound pastures and many diseases are established from the actions on top of the ground. The chain reaction is felt throughout the soil. Soil is as much a living thing as a physical entity. The soil provides a habitat for soil organisms; consequently the use and management of soils will have significant effects on species diversity and conservation of biodiversity.

Soil micro-organisms such as protozoa, nematodes, bacteria and fungi are essential to the functioning of our soil. Although, science in agriculture in the past has largely ignored the part we cannot see, climate change is at last bringing it to the table for discussion. The microbes carry out biochemical transformation and act as a source and sink for nutrients. These are the forgotten essential components for healthy soil.

Murphy (2000), summarises that soil microorganisms (bacteria and fungi) only make up a few percent of the total OM, but this still equates to hundreds of kilograms of living organisms per hectare. The microorganisms continually ‘turnover’ as individuals divide, grow and then die. Microorganisms use the dead OM in soil as food. As they breakdown the OM, any excess nutrients they don’t need are released into the soil in forms that plants can use. This process is called mineralisation. Soil animals such as earthworms also play an important role in breaking up OM into smaller pieces, but it is the micro-organisms that are responsible for the actual release of nutrients.

Fungi

The “grazing web of life” (figure 5) shows the fungi are essential for healthy grazing systems due to the mycorrhizal (symbiotic) relationship formed with most pasture plants as the fungi extend the root system to source nutrients and water for itself and the host plant. The fungi receive energy from the roots in response to supplying nutrients. Up to 1.5 million species of fungi exist worldwide and still little is known of the real function they play. In an ideal grazing situation, the bacteria to fungi ratio should be approximately 1: 1

Organism

Kg/ha

Bacteria

1000

Fungi

2000

Algae

100

Protozoa

200

Nematodes

50

Worms

1000

Plant roots

2000

 

 

Figure . Weight of soil organisms in the top 15 centimetres of a fertile soil.
Organism Kilograms of live weight/Ha

 

 

 

 

 

 

Bacteria

Another symbiotic relationship between plants and microbes involves soil based bacteria. The best known common soil bacteria relationship, rhizobium, invades the root and multiplies within the cortex cells. Legume nitrogen fixation starts with the formation of a nodule (Lindemann 2003). The plant supplies all the necessary nutrients and energy for the bacteria. Within a week after infection, small nodules are visible with the naked eye, although the bacteria is only about one thousandth of a millimetre long. In the field, small nodules can be seen 2-3 weeks after planting, depending on legume species and germination conditions. When nodules are young and not yet fixing nitrogen, they are usually white or gray inside. As nodules grow in size, they gradually turn pink or reddish in colour, indicating nitrogen fixation has started. The pink or red colour is caused by leghaemoglobin (similar to haemoglobin in blood) that controls oxygen flow to the bacteria. A single gram of soil has also been estimated to contain several thousand species of bacteria (Torsvic et al et al 1994). Bacteria are also important in the decomposition of pasture into organic matter.

Protozoa and Nematodes.

Protozoa and nematodes also play an important role in the functioning of soil. As an example, protozoa are able to digest many bacteria and release large amounts of nitrogen for plant use. The number of individual soil animals in the soil is enormous. In all but the driest environments there are billions of protozoa per square metre (m2), millions of nematodes/m2 and 100 000’s of mites/m2. Data from east Beverly in Western Australia, found that there were approximately 800 million protozoa/m2, 900 000 nematodes/m2 and 130 000 mites/m2 in a soil under pasture.

Vadakattu (2004) explains that soil protozoa play a critical role in crop health by actively attacking pathogenic fungi such as rhizoctonia root rot and through the release of essential plant nutrients tied up in micro-organisms. The total biomass of living organisms in a healthy soil may exceed 20 tonnes per hectare and its diversity can be compared to that of a coral reef (Wallwork 1976)- only far more complex. Microorganisms utilize more than 70% of the annual plant production from intensively grazed pastures (Hutchinson and King 1982) and therefore play a huge role in the biochemical and physical environment of the soil. Go to the Soil Monitoring page for more info - Best Practice recommendation is to conduct a soil test for microbial capacity. http://www.ciaaf.com.au/

Dung Beetles

When manure (cow dung) breaks down, more methane is released to the atmosphere, as nitrous oxide which has a global warming potential 310 times greater than carbon dioxide. Dung Beetles are the only known effective management system for removal of cow dung. A number of dung beetles have been known to remove complete dung pads in less than 6 hours in central Queensland, when in abundant numbers. Grazing systems require healthy populations of dung beetles to both shred (Ball rollers) and bury (Tunnellers) the bovine dung deposited on top of the ground. The majority of the dung beetles in our grazing systems were introduced into Australia by CSIRO in the 1970’s and 1980’s from South Africa and Europe. This program funded by the grazing industry and the Australian economy was openly accepted as the most ambitious biological-control program in the world. It is now up to us to make sure the program is successful in the future. It is always a good time to start thinking about managing your dung beetles, because we continue to cycle through wet and dry periods. The dry periods will have an impact on populations and so it is as important a period for managing active dung beetle populations, as it is for managing stock and pasture. For more information on dung beetles - go to www.dungbeetle.com.au

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