Introduction - "Can't manage if you don't measure"
Managing soil fertility is the most critical aspect of all farming enterprises as fertility will dictate yield and yield dictates a large portion of profitability. Grain and fibre crops remove more nutrients than does a pasture based grazing system. The longer we farm a block, the more nutrients are being removed.
Soil nutrition or fertility is dependent on the minerals in the parent rock, the age and rate of breakdown and the minerals it may have picked up or lost from overland flow, position in the landscape and our management systems. Therefore, after say a twenty year period of farming a soil type, the plant available minerals will probably be much lower than they were when farming began. In the period from 1950 - 2010, the focus of soil nutrition has generally been to replace the NPK (nitrogen, phosphorous, potassium) part of our soil with a trace of sulphur and maybe some zinc. This has been accepted as best practice for decades while NPK's have been the limiting factors. However, today the goal posts have moved and soils are simply not responding to the NPK model.
In a Grains BMP, it is important to take each paddock and each farm as an individual entity. Each farmer will need to compile a complete soil test, measuring the base saturations (calcium, magnesium, potassium, sodium, aluminium, hydrogen), total macro and trace minerals and the plant available nutrients. A cropping bmp or grains bmp needs to monitor what is in the soil (total minerals) as well as what the plants can access.
Most minerals need to be cycled or made available by microbiology (bacteria, fungi, protozoa) in the soil to enable a plant to uptake them. This is especially important for the mycorrhiza or VAM in the soil as this microbe sources phosphorous and zinc. Many of the past practices including NPK salt based fertilisers have damaged the microbe populations and made the soils even more dependent on more fertiliser applications(hence the more-on theory).
A Cropping BMP or Grains BMP would include a combination of soil correction, compost/ granular, liquid injection and foliar to maximise production and match plant requirements to soil potential.
Background to soils
The three key parameters of soil are:-
- Chemistry - nutrients/ minerals - these are numerous
- Biology - microbes/ life in the soil - These include Fungi, Bacteria, Protozoa, Nematodes and Archaea. Other creatures such as worms, beetles and dung beetles are larger and so called macrobiology.
- Structure - physical attributes. Structure is made of a proportion of sand, silt and clay that form the basis of the structure, as well as carbon or organic matter, water air pores and the mineral balance. This is where the available minerals come from.
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:
- Calcium
- Magnesium
- Potassium
- Sodium
- Aluminium
- Hydrogen
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 are 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/
Benchmarking and Monitoring
A sound Farming BMP methodology would ensure that cropping and grazing soils are benchmarked as soon as possible to measure the:
- Macro nutrients (total and available)
- Carbon
- Trace minerals (total and available)
- Micro-biology (Bacteria, Fungi, Protozoa and Nematodes) (this includes VAM)
This initial benchmark would then be used as a guide to assess the improvement or decline in soil health and overall soil impact from the farming system and climatic variation. A farming soil should be monitored once a year to assess the major nutrients and then a crop monitoring program should be established. A leaf test is like a blood test to humans and can be used to ascertain the crop requirements during the season.
Practices to Improve Fertility
There are many practices that will improve the fertility of your soils including:
- Legume rotations (ensure inoculation)
- Green Manure crops
- Compost with nutrient additions
- Balancing Calcium to Magnesium to Potassium to Sodium (base saturations)
- Feeding the plant to maximise photosynthesis (balancing nutrition)
- Trash blanketing
- Stubble retention
- Stubble digesting products (microbial)
- Nitrogen cycling microbes (Azotobacter)
- VAM promoting products
- Addition of humic substances with urea based fertilisers (buffering)
Watch this video by Paul Brown on 'Why Soil Health is Important to Farming'
Levels of Fertility Management
To some degree, fertility can be maintained by good farming and grazing practices which minimise the loss of nutrients. For example, the use of fire is the most destructive of all practices on nutrient decline. Removing fire from the farming system will slow the fertility decline. Each nutrient will require replacement as it becomes a limiting factor for maximum production. In a well balanced soil, with good calcium to magnesium ratios (3: 1 to 7:1), it may only be necessary to replace NPK and trace minerals. These traces are easily replaced via composts, liquids or granular fertilisers. However, many soils now require replacement of calcium to improve the Ca:Mg ratio as this will change the structure of the soil, improve water infiltration and allow other minerals to cycle to crops and pastures.
If a large amount of calcium is required to improve soil balance and structure, it is often necessary to apply this as a soil correction strategy and follow up with various trace mineral programs at a later date.
Soil Correction
This is dependent on the CEC (Cation Exchange Capacity) or TEC (Total Exchange Capacity) which is the size of the soil and its ability to hold onto and release nutrients. A large capacity soil which is out of balance will require large amounts of nutrients to balance, while a small capacity (CEC) may only require a small amount. The key nutrients assessed in this program include:
- Calcium to Magnesium levels
- Potassium
- Phosphorous, carbon, Sulphur and other major issues.
Planting Blends (liquid injection and granular)
Plants only require a small amount of most nutrients at planting as the seed is small and unable to utilise large doses of nutrients. In many cases, large doses of even salt based starter fertilisers will burn the germinating seedling. It is safer to utilise a liquid injection methodology at planting or buffer the salt based starter with humic acid products. As an example, either add humate granules to the granular fertiliser or use a buffered product such as Black Urea.
Often large amounts of starter fertiliser (more than 80 kg/ha) will be offset from the planting row, so that the plant roots can access the nutrients at a later date, when required.
In Crop Programs
Once the crop is established, it is possible to apply nutrients in crop using either foliar applications, granular (spread) or granular in row (side dressed). For more information, contact 0438 395 255.
This page will be updated as necessary.
If you wish to know more about various fertility programs, contact 0438 395 255.