What are the optimum nutrient targets for pastures?

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What are the optimum nutrient targets for pastures?

Key Points
Understanding the question

Why is it important to me as a farmer?
How and why nutrient loss / imbalance occurs
What nutrients are needed in the paddock?

Case Study
Other related questions in the Brown Book
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References

Source: DEPI Victoria

Key Points

Plants require nutrients for normal growth. These must be in a form useable by plants and in concentrations that allow optimum plant growth
Sixteen nutrients are known to be essential for plant growth. A deficiency in any one of the 16 essential nutrients will reduce growth and production, even though the others may be abundantly available. Optimum pasture production can only be obtained if all the requirements for plant growth are met

This question is using the traditional approach to plant nutrition, known as the Sufficiency Level of Available Nutrient (SLAN) approach, as compared with the alternative approach of Base Cation Saturation Ratios (BCSR)
Excessive nutrient application can contribute to losses to the environment

Understanding the question

Why is it important to me as a farmer?

Matching the nutrients you apply with the nutrients required on the farm will have several benefits. These include increases in productivity, savings on the fertiliser bill and a reduction in nutrients lost to the environment

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How and why nutrient loss / imbalance occurs

Most Australian soils are old and weathered. In fact, many are considered the oldest soils in the world; and the nutrients have been leached, which has resulted in soils of low fertility. For example, average Australian soil phosphorus levels are 40% lower than English soils and up to 50% lower than North American soils
Improved pasture species allow a much higher stock-carrying capacity; but to maintain this productivity, they require a higher level of soil fertility than do native pasture species
Fertiliser applications are required to overcome the soil�s inherent nutrient deficiencies and to replace the nutrients that are lost or removed from the soil by pasture growth, fodder cropping or conservation, and animal products, such as milk or meat

Nutrient redistribution around the farm and the inherent ability of soils to �retain� applied nutrients are other reasons for fertiliser applications
Nutrient cycling (soil-plant-animal) involves nutrients:
- Being brought onto the farm in various forms
- Undergoing ongoing reactions in the soil
- Being consumed by animals via the plants
- Being lost to the farm system by various means

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What nutrients are needed in the paddock?

Sixteen nutrients are known to be essential for plant growth. They can be divided into two categories:
- Major nutrients (macronutrients)
- Minor nutrients (micronutrients), often referred to as trace elements

Table 1 - Essential nutrients required by plants.

	Major Nutrients	Minor Nutrients (Trace Elements)
	Carbon (C)	Molybdenum (Mo)
	Hydrogen (H)	Copper (Cu)
	Oxygen (O)	Boron (B)
	Nitrogen (N)	Manganese (Mn)
	Phosphorus (P)	Iron (Fe)
	Potassium (K)	Chlorine (Cl)
	Sulphur (S)	Zinc (Zn)
	Calcium (Ca)
	Magnesium (Mg)

The first three major nutrients, carbon, hydrogen and oxygen, are generally considered to come from carbon dioxide in the atmosphere and from water. Combined, they make up 90% to 95% of the dry matter of all plants
The remaining nutrients are found in the soil and are taken up through the root system of the plant. However, legumes (such as clovers, lucerne and medics) also have the ability to convert atmospheric nitrogen into a plant-available form

Carbon (C) - What is its optimum level?

Organic matter (OM) is the total of all organic materials contained within and on soils
Organic carbon (OC) is the measurement used for calculating OM
To do this the equation �OM (g/kg soil) = OC (g C/kg soil) x 1.72� is used. It is preferable to just use OC as the figure of 1.72 can vary from 1.72 - 2.00
Typically OC varies with depth and the magnitude of such changes differs between soil types. Typically OC contents are greatest at the soil surface and decrease exponentially with depth
OC is a dynamic soil fraction that has many functions. Therefore it is difficult to define a single level of OC in a soil at which all functions are optimised

Low OC generally means the soil has �poor� structure, holds less water and nutrients Soils with high OC generally have �good� structure, good water holding capacity and reduced erosion and nutrient leaching. OC levels will vary according to pasture or crop type, as well as the original soil type

Table 2 - General OC content for high rainfall areas in Victoria.

et al.

		Low	Normal	High
	Pastures	<2.9	2.9-5.8	>5.8
	Crops	<1.5	1.5-2.9	>2.9

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Nitrogen (N) - What is its optimum level?

The nitrogen that is readily available to plants is generally measured as nitrate. However there is no reliable soil test for nitrogen and therefore pasture response relationships for nitrogen are not possible to develop
Nitrate levels can be highly variable in soils and nitrate soil tests are unlikely to be of value in estimating quantitative supplies of soil nitrogen available for most pastures. The perennial nature of pastures means that the soil nitrate content at any growth stage usually represents only a fraction of the pasture�s yearly requirement

Nitrogen application is a way of producing more feed during the time of reliable moisture. Each kg of nitrogen per ha on good pasture in winter will produce an extra 5-10kgDM/ha of growth and in spring 15-20kg DM/ha

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Phosphorus (P) - What is its optimum level?

Phosphorus is present in various forms in the soil, only some of which are actually available to the plant
Olsen P is a measure of plant available phosphorus. It is the measure of phosphorus generally used for grazing systems
Colwell P is a measure of immediately available phosphorus plus the phosphorus that is absorbed to the soil and released over the next few years. It is the measure used for cropping systems
In the Heytesbury dairy region, Olsen-P values are typically high and further applications of P fertiliser are likely to be uneconomic and detrimental to the environment (Greenwood et al 2008)

	Case Study
	Benchmarking soil health on dairy farms in Heytesbury region

Table 3 - Availability of P at various Olsen P values.

	Olsen P (mg/kg)		Availability
	Irrigated pastures	Dryland pastures	Availability
	Below 12	Below 8	Low
	12-17	8-12	Marginal
	18-25	13-8	Adequate
	Above 25	Above 18	High

Table 4 - Desirable nutrient levels for phosphorus at moderate and high stocking rates.

		Moderate Stocking Rates (7-12 DSE/ha)	High Stocking Rates (13- 20 DSE/ha)
	Olsen P	9	15
	Colwell P	21	35

As a rough guide, to convert Olsen P to Cowell P, multiply Olsen P by:
- 1.6 for sand and sandy loam
- 2.0 for loams
- 3.0 for clays and clay loams

Phosphorus Buffer Index (PBI).
- Phosphorus applied as fertiliser reacts with the soil and becomes less available for plant uptake. The extent of these reactions depends on the PBI of the soil. A soil with a high PBI will require more phosphorus fertiliser than a soil with a low PBI. PBI also shows which soils will leach phosphorus. Soils that have a PBI of less than 50 are prone to leaching. On these soils phosphorus should be applied in small quantities on a regular basis over the year, rather than applying all of the P fertiliser once a year

Table 5 - Capital P (kg/ha) required (in addition to maintenance phosphorus) to lift Olsen P by one unit.

	PBI Class	PBI	Phosphorus Required (kg P/ha)
	Very low	0-50	5
	Low	50-100	7
	Moderate	100-200	9
	High	200-300	11
	Very High	300-600	13
	Extremely High	600+	15

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Potassium (K) - What is its optimum level?

Plant requirements for potassium are supplied from two soil sources: exchangeable potassium that is immediately available and non-exchangeable potassium which is more slowly available. Clay soils have a higher nutrient holding capacity and need higher levels of available potassium than sandy soils. Therefore soil test interpretation needs to be based on soil texture, as the critical value increases with increasing clay content
Potassium fertiliser is often applied to pastures but rarely to crops in south west Victoria
Potassium is measured using the Skene K or Colwell K tests. The results are reasonably similar and are expressed in mg/kg (ppm)

Table 6 - Available Potassium (mg/kg).

	Nutrient status	Sands	Sandy Loams	Clay Loams	Clays	Peats*
	Low	Below 50	Below 80	Below 110	Below 120	Below 250
	Marginal	50 � 140	80 � 150	110 � 160	120 - 180	250 � 300
	Adequate	141 � 170	151 -200	161 -250	181 - 300	350 � 600
	High	Above 170	Above 200	Above 250	Above 300	Above 600

*In peat soils, plant tissue testing is suggested as a more accurate indicator of available K because few field trials have been done to verify laboratory analyses

When potassium levels are high, inputs can be reduced or deleted from fertiliser regime as a pasture response is unlikely
In the Heytesbury dairy region, Skene-K values are typically high (see case study 3b) and further applications of P fertiliser are likely to be uneconomic (Greenwood et al 2008)

Table 7 - The critical Colwell K soil test values for four soil texture classes.

	Soil Texture	Critical Value (mg/kg)	Confidence interval (mg/kg)	Number of experiments
	Sand	126	109-142	109-142
	Sandy Loam	139	126-157	122
	Sandy Clay Loam	143	127-173	75
	Clay Loam/td>	161	151-182	194

The critical value is the soil test value (mg/kg) at 95% of predicted maximum pasture yield. The confidence interval is 95% chance that this range covers the critical soil test value.

Table 8 - Recommended nutrient levels for potassium (mg/kg) at moderate and high stocking rates.

	Potassium	Moderate Stocking Rates (7-12 DSE/ha)	High Stocking Rates (13-20 DSE/ha)
	Sands	80-100	100-120
	All other Soils	120-150	150-180

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Sulphur (S) - What is its optimum level?

This result needs to be looked at with some caution as there is substantial seasonal variation in plant available sulphur. The variation results from temperature and moisture changes in the soil. This affects both the rate of mineralisation of organic sulphur and sulphur losses due to leaching. It will be lower during dry periods and higher in warm, wet conditions. Therefore the soil test figure should not be used alone to work out sulphur fertiliser requirements
When making a fertiliser decision relating to sulphur the following things need to be considered: Soil type, crop type, seasonal conditions, plant and grain analyses, farm management practices and local knowledge
Sulphur levels are impacted by:
- Cultivation � When soil is cultivated the mineralisation of soil organic matter and release of sulphate-sulphur and other nutrients is accelerated. Sulphur is more likely to be required in perennial crops and pastures where zero till practices are used
- OM � soils containing less than 2% OM (approximately 1.2% OC) are likely to be sulphur deficient. It has been calculated that under favourable conditions for sulphur mineralisation, for each 1% OM about 6kg of sulphur per hectare per year is released
- Crops � Canola, high yielding forage crops and grain legume crops need more sulphur and respond more readily to sulphur application than cereal crops
- Soil texture � leaching of sulphate-sulphur from sandy soils is more likely than from finer-textured loams and clays
Sulphur is measured using a test called the KCL 40

Table 9 - Sulphur (KCL 40)

	Nutrient Level	Sulphur Level mg/kg (KCL40 test)	Recommended Capital S application
	Low	<4	30kg S/ha
	Marginal	4-8	15kg S/ha
	Adequate	9-12	7.5kg S/ha
	High	13-20	0
	Very High	>20	0

Table 10 - Recommended nutrient levels for sulphur at moderate and high stocking rates.

		Moderate Stocking Rates (7-12 DSE/ha)	High Stocking Rates (13-20 DSE/ha)
	Sulphur KCl-40	6.5	8.5

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Cation Exchange Capacity (CEC)

CEC is a measure of the soils capacity to adsorb and hold cations (positively charged ions). CEC can also be referred to as the sum of cations
A CEC above 15meq/100g means that a soil has a good ability to retain nutrients for plants
There is considerable evidence that the proportions of the exchangeable cations are more relevant to plant performance than the actual levels
CEC provides a buffering effect and thus is a major controlling agent of soil structure stability, nutrient availability for plant growth, soil pH and the soils reaction to fertilisers and other ameliorants
A low CEC value means the soil has low resistance to changes in soil chemistry that are caused by land use e.g. acidification. The CEC of clay minerals is usually in the range of 10 to 150 meq/100g, while that of organic matter may range from 200-400meq/100g. So, the kind and amount of clay and organic matter content of a soil can greatly influence its CEC
Where soils are highly weathered and the organic matter low, their CEC is also low
Clay soils with a high CEC can retain large amounts of cations against leaching
Sandy soils with a low CEC retain smaller quantities of cations
This is important when planning a fertiliser program. In soils with low CEC, consideration should be given to splitting applications of potassium and sulphur fertilisers

Table 11 - Ratings for CEC

	Rating	CEC (meq/100g)
	Very Low	<6*
	Low	6-12
	Moderate	12-25
	High	25-40
	Very High	>40

* Soils with CEC less than 3 are often low in fertility and susceptible to soil acidification

Table 12 - Levels of exchangeable cations (meq/100g)

	Cation	V Low	Low	Mod	High	V High
	Na	0-0.1	0.1-0.3	0.3-0.7	0.7-2.0	>2
	K	0-0.2	0.2-0.3	0.3-0.7	0.7-2.0	>2
	Ca	0-2	2-5	5-10	10-20	>20
	Mg	0-0.3	0.3-1.0	1-3	3-8	>8

The cations manganese, iron, copper, and zinc are usually present in amounts that do not contribute significantly to the cation complement

Table 13 - Desirable percentage range of exchangeable cations for soils

	Cation	Range
	Calcium	65-80%
	Magnesium	10-20%
	Potassium	3-8%
	Sodium	Less than 6%
	Aluminium	Less than 1%

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Calcium to Magnesium Ratio (Ca:Mg) - What is it meant to be?

Early research suggested that high soil exchangeable Ca:Mg ratios may induce Mg deficiency, leading to the view that the Ca:Mg ratio should be between 2 and 7
Later work from overseas and Australia indicates that yield will be unaffected over a very wide range of soil exchangeable Ca:Mg ratios
It has been suggested that provided the exchangeable Mg is high enough, the ratio can vary over a wide range and will be of little consequence when there is not a livestock nutritional problem
The following table shows possible desirable levels for the Ca:Mg ratio

Table 14 - Ca:Mg Ratio.

	Ca:Mg ratio	Description
	<1	Ca deficient
	1-4	Ca (low)
	4-6	Balanced
	6-10	Mg (low)
	>10	Mg deficient

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Magnesium to Potassium Ratio (Mg:K) - What is it meant to be?

The Mg:K ratio can be an important factor under some conditions, e.g. fertilising with potassium can reduce the uptake of magnesium by grasses being grazed by livestock, resulting in grass tetany
Low soil temperature and adequate soil moisture in the presence of only moderate amounts of potassium result in higher potassium uptake, compared to magnesium and the development of tetany-prone grass pastures
Mg:K ratio � less than 1.5 indicates possible grass tetany problems (Target 10)

Trace Elements - What are they meant to be?

Soil tests for trace elements are not recommended in Australia because they cannot reliably predict pasture or crop responses. They are a tool to assist in assessing whether further investigation is required
Tissue testing is a far more accurate test of trace element levels and it usually takes a combination of local knowledge, tissue testing and strip tests to resolve exactly which elements are required. Fertiliser test strips are good for determining which fertiliser to use. Be aware that growth responses to molybdenum may not be apparent until the year after application. Test strips can be put down any time between May and late August
For most soils in South West Victoria there is no clear data regarding responses by pastures to the application of the trace elements zinc, copper, cobalt, boron or manganese. There are, however, some special cases where experience has shown that some trace elements are necessary. Molybdenum is the main trace element of interest for pasture growth in South West Victoria but the level of other elements can be of value if investigating poor pasture performance and trace element problems with stock, particularly copper
Total soil content of trace elements does not indicate the amounts available for plant growth. For example in spite of high amounts of iron being present in soils, iron deficiency is very common on calcareous and alkaline soils

Table 15 - Desirable levels of trace elements.

	Trace element	Preferred level in soil (mg/kg)
	Boron	0.5-4
	Copper	2-50
	Molybdenum	2
	Sulphur	10-20
	Zinc	1-200
	Manganese	2-25
	Chloride	<120 Low, >1200 High
	Iron	Locks up P applied to pastures and crops. Lower the better

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Other related questions in the Brown Book

Brown Book content has been based on published information listed in the Resources and References sections below

Resources

What Nutrients do Plants Require? - Department of Environment and Primary Industries, Victoria

Essential Nutrients - Victorian Resources Online, Department Environment and of Primary Industries, Victoria

Dairy - Nutrient Planning - Department of Environment and Primary Industries, Victoria

Pasture Maintenance Factsheet (2008) - Heytesbury District Landcare Network

Soil Assessment Component - HDLN Soil & Water Dairy Action Program - June 2008 � Victorian Resources Online, Department of Primary Industries, Victoria

References

Baxter, N., and Williamson, J., (2001) Know Your Soils: Part 1 � Introduction to Soils. Department of Natural Resources and Environment, Vic.
Bluett, C., and Wightman,B. (1996) Cropping in South-West Victoria. Depatment of Primary Industries, Victoria.
Field, D.J., McKenzie, D.C., and Koppi,, A.J., Development of an Improved Vertisol Stability Test for SOILpak,.Australian Journal of Soil Research, 35:842-52, CSIRO Publishing.
Gourley, C.J.P., Melland, A.R., Waller, R.A., Awty, I.M., Smith, A.P., Peverill, K.I., Hannah, M.C., (2007) Making Better Fertiliser Decisions for Grazed Pastures in Australia. Department of Primary Industries, Victoria.
Hazelton, P., and Murphy, B., (2007) Interpreting Soil Test Results � What do all the numbers mean. 2nd edition, CSIRO Publishing, Melbourne.

Nie, Z., and Saul, G., (2006) Greener Pastures for South West Victoria,. 2nd Edition, Department of Primary Industries, Hamilton.
Price, G., (2006)Australian Soil Fertility Manual. 3rd edition, CSIRO Publishing, Melbourne.
Peverill, K. I., Sparrow, L.A., and Reuter, D.J. (1999) Soil Analysis an Interpretation Manual. CSIRO Publishing, Melbourne.
Ried, G., and Dirou, J., (2004) How to Interpret your Soil Test. Department of Primary Industries, NSW.
Target 10 (2005) Fertilising Dairy Pastures, 2nd edition. Department of Primary Industries, , Melbourne.

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