Demonstrating the Difference

Nitrogen Supply and Application Program Optimisation

An adequate supply of nitrogen throughout the season is paramount for good plant growth.

There are a number of factors to consider when trying to develop a good nitrogen program for the season including:

• determining the amount of nitrogen in the soil organic matter and the rate of soil nitrogen release at different stages in the season or under different conditions
• the nitrogen requirements of the crop you are growing
• the cost of different nitrogen sources
• suitability of the nitrogen source for the soil
• suitability of the nitrogen source for the crop type and/or stage of growth
• the release curve of different sources of nitrogen
• leaching or volatilisation risks associated with your nitrogen program
• accounting of nitrogen fixation
• the plant metabolic cost involved with the uptake and conversion of different forms of nitrogen into amino acids/protein
• the supply of nutrient cofactors that are important for enzymes involved in amino acid/protein synthesis
• the supply of other nutrients such as sulphur and molybdenum that needed for the assembly of certain amino acid/protein molecules

Growers have to make informed decision around testing to determine soil nitrogen supply, the type/s of nitrogen you apply, the method, placement and timing of applications and ensuring an adequate supply of other nutrients that are needed for efficient amino acid/protein synthesis.

Discussion Points

The timing and application of nitrogen in this project seemed to be the most influential factor in terms of dry matter production.

The growth difference under different nitrogen regimes didn’t start to show until around 4-5 weeks after germination, indicating that putting nitrogen out too early may not be very economical.

The tissue tests indicated that plant nitrogen levels dropped off if the initial supply wasn’t maintained and consistent application was important to match the growth of extra biomass generated by prior application.

The organic forms of nitrogen available on the market are low analysis and cost significantly more per unit. The release curves of many organic nitrogen fertilisers are not clear.

It is difficult to determine if or how much combining nitrogen fertilisers with humic substances and bio-stimulants improves stabilisation/retention/uptake. 

Is the foliar application of nitrogen a viable way to improve NUE and reduce nitrogen inputs?

Will the amount of soil nitrogen required next season increase (removal from grazing) or decrease (higher levels of organic matter/nitrogen fixation)?

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Different Types of Soil Tests

At any one time, there are pools of nutrients held in different soil fractions:

• dissolved in the soil solution
• held on exchangeable sites
• within soil organisms
• within organic materials
• the soil mineral matrix

These pools of nutrients are in a state of flux. Nutrients are constantly being removed from the soil solution and exchange sites by plants, released from organic matter, liberated from the soil mineral matrix by soil microbes, and so on. Variables such as soil moisture, temperature, cation exchange capacity (CEC), phosphorous buffering index (PBI), acidity/alkalinity (pH) redox state (Eh) and soil carbon levels also significantly influence the state and place nutrients occupy in the soil and their plant availability.

Soil testing methodologies are designed to identify significant soil characteristics and the levels of essential nutrients in the different soil fractions. These are then interpreted in relation to plant nutrient availability.

There are a number of ASPAC certified laboratories that offer test packages that cover a range of different parameters and some that also offer non-accredited soil tests. There isn’t as yet reliable data on the reliability and usefulness of some of these testing parameters/methods and the protocols around interpretation and reference levels.

Tests being used by farmers in Australia to determine nutrient status and availability include:

• Available Soil Nutrient Test
• Total Nutrient Soil Tests
• The Haney Soil Test

We carried out different types of soil tests on a composite 10-20cm sample of soil from the site to compare the results and recommendations. These included:

• a comprehensive soil test done with CSBP laboratory. The numbers were entered on the nutrient calculator to determine nutrient status
• an available nutrient soil test with SWEP laboratory that included base saturation percentages and nutrient recommendations
• a Haney soil analysis report through EAL laboratory that used alternative extraction methods to determine available carbon, nitrogen and other nutrient elements as well as a solvita microbial respiration reading.
• Total soil nutrient tests were sent to SWEP and EAL laboratories to see if they were consistent.

Discussion Points

The NPK and S and pH readings and general recommendations generated by all the tests were roughly the same. The organic matter/carbon readings were a little inconsistent.

Soil tests are not generally recommended as a reliable indicator of the supply of certain trace elements.

There was some inconsistency between the EAL and SWEP total nutrient levels.

The Haney and SWEP tests are more expensive.

The worth and value of all these different tests was hard to qualify in this project.

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Evaluating and Designing Foliar Nutrient Application Programs

The application of foliar applied nutrients is a practice that warrants consideration for a number of reasons.

The correct formulation and application of foliar nutrient sprays is also important for it to be effective. Factors such as nutrient solubility, compatibility, water quality, pH, chelation/complexing, mixing, application rates and method, weather conditions etc., all need to be understood.

As such, while applying nutrients via foliar sprays offers numerous benefits, it may be logistically and practically challenging. Needless to say, if one is to adopt the practice of foliar nutrient application, it takes commitment and needs to be done correctly, efficiently and consistently and it may be a viable strategy in some situations and not others.

Discussion Points

Several applications of foliar nutrients were applied along with suitable organic biostimulant/chelating/complexing agents, and a spreader/sticker/surfactant to the treatment 3 plots. Treatment 4 plots got a one-off application of foliar manganese.

Aside from a visual response to an early application of foliar urea, it was hard to see any obvious differences in plant growth from these applications. There was however, an increase in the levels of trace elements applied on both the sap and tissue tests. There were too many other variables involved to ascertain the effectiveness the foliar nutrient applications carried out this season.

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Biological Seed/Planting Treatments

The complexity of microbial activity in the soil and varying soil environmental conditions can make the survival, let alone effectiveness of introduced microbial cultures is sketchy at best. Of these the use of species specific inoculants at planting in a seed coating holds the most promise as the microbes at least have an immediate and suitable host to grow on.

The use of rhizobia inoculants has been well researched and proven to be an effective measure to take when planting legume crops. Other plant associates such as Mycorrhizal fungi or Trichoderma less so.

Alternatively, a shot gun approach is to use broad spectrum cultures such as found in compost/vermicast, which contains a range of species and bio-chemical compounds that you naturally find in fertile topsoil, either as a seed coating or in planting furrows. While this has been shown to work at times, the results are bound to be variable.

Another approach is to use bio-stimulants and foods at planting to foster growth, reproduction and activity of resident biology. It is unclear how such measures would work in different environments and conditions or whether they’d necessarily have a desired effect on plant growth.

Discussion Points

The legume species in the mixes on all the plots where inoculated with appropriate rhizobia cultures as this is an established good practice.

There wasn’t any noticeable difference in germination, establishment or root development from the other planting treatments used this season. There may have been a negating effect from the water logging that happened early in the season and the low organic matter/biological activity in the soil after inversion?

In a pot experiment carried out later in the season, there was a visual difference in growth of tomatoes, planted in soil from the site, using a compost slurry recipe to coat the seeds, compared to the grow from uncoated seeds. Trying different planting treatments on the plant species you are going to grow and in the soil that they will be grown in may be an easy way for farmers to test if it is worthwhile.  

The cost/benefit of commercially available biological planting treatments may not justify their adoption. Farm produced biological planting treatments such as compost/vermicast, and the recruitment of local plant symbionts cost very little and may be a more viable option.

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