Pasture nutrient applications

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 Application descriptions

  • Capital applications of fertiliser are aimed at lifting soil nutrient levels to target levels determined by the manager. They involve making one or more fertiliser applications over and above the level required for replenishing nutrients lost through export off the farm or paddock in agricultural products. Capital applications are designed to increase yearly pasture growth and quality. Typical fertilisers involved include phosphorus (P), potassium (K) and lime. Sulfur (S) is usually supplied to varying levels with P.
  • Maintenance applications of fertiliser are aimed at replacing the amounts of key nutrients either tied up in the soil or exported off the farm or paddock through agricultural production practices and products (eg meat, wool, grain, hay and silage where the nutrients are not replaced). Maintenance applications are designed to maintain existing patterns of pasture growth and quality. Maintenance applications usually involve P, K and S.

Guidelines for making capital applications

  1. Expenditure on capital applications must be regarded as investments of financial capital.

a) There is a negative return on investment unless increased pasture growth is converted to increased profit.

b) Conversion to profit can only come from increased margins ($/kg sold) or increased volume of turn-off (kg sold /animal, kg sold/ha or kg run/ha if agisting).

c) The higher the marginal return per unit increase in turn-off or stocking rate the higher the return from capital applications.

d) Co-investments such as purchase of stock, improved pasture species, water and fencing to improve utilisation may need to be made to maximise returns.

  1. Ensure that targets set for minimum nutrient levels are appropriate for each pasture zone, as well as your financial base.

a) Environmentally fragile zones, for example zones with steep slopes or zones adjacent to riparian areas with high risk of sideways movement of P and N, may not be suitable for capital applications of fertiliser.

b) Similarly, raising P levels in stable native pastures may disturb pasture stability while failing to give optimum return on investment through the absence of highly responsive species such as clover.

  1. Timing is vital.

a) There is evidence that achieving targets over a number of years is just as effective in lifting soil nutrient levels as a one off application.

b) All P applications should be made in periods when soil is unlikely to be waterlogged (eg mid-spring to mid-autumn in winter rainfall regions). P is rapidly fixed and becomes unavailable to the plant when soil moisture reaches saturation levels.

c) Sulfur-based fertilisers can contain both elemental S and sulfate S. Elemental sulfur is not plant available and is only slowly converted by microbial action to water soluble and readily plant available sulfate S. Apply before the end of summer to gain benefit in autumn if S content is largely in the elemental form.

  1. All limiting factors should be corrected where economic responses are indicated.

a) P should come before lime except where indicated below.

i)  P is a major driver of pasture quantity and is also associated with quality.

ii)  The amount needed to increase P by one unit varies with soil type. Heavier textured soils usually require more P than lighter, sandier soils.

iii)  As a rule of thumb, 6–13kg P/ha will increase Olsen P by 1 unit or Colwell P by 3 units depending on the soil’s phosphate buffering index.

b) Sulfur is normally applied in fertilisers with a mix of P and S in varying proportions.

i)  When P and S are both low, use blends with 1:1 ratios (P/S).

ii)  When P is low and S is within limits, use 2:1 blends.

iii)  When P is within limits (or above target) and S is low use sulfur fortified products for capital S while supplying maintenance levels of P.

iv)  Base final choice of product within the above guidelines on cost of P or S applied. High analysis fertilisers can be cost competitive after allowing for transport, handling and spreading costs.

c) Liming is indicated when the percentage of aluminium is above maximum or when pH is below minimum for the existing and/or desired species.

i)  Note that phalaris is sensitive in its establishment phase. Most cultivars cope with slightly raised aluminium or low pH when established.

ii)  Speed of response to liming is normally inversely proportional to soil cation exchange capacity (CEC), ie quickest responses occur in soils with low CEC.

iii)  It should also be noted that liming to raise pH has a beneficial effect on soil health through increased biological activity of soil organisms.

Phoshporus (P) and potassium (K) soil factors (kg/ha) for capital applications based on Phosphorus Buffer index (PBI)
PBI value P required to raise Olsen P by one unit P required to raise Colwell P by one unit K required to raise Colwell K by one unit
0–50 6 2.2 1
51–100 8 2.3 1
101–300 9 2.5 1
301–400 10 2.8 2
401–500 11 3.0 2
501–600 13 3.2 2
>601 15 3.2 2
Source: Accounting for Nutrients www.accounting4nutrients.com.au

 

Sulfur soil factors (kg/ha) for capital applications
  Sulfur required to raise soil test (KCI 40 test) by one unit
Soil test level 0–4 4–9 9–13 Above 13
Sulfur (kg/ha/year) 30 15 7 1
Source: Accounting for Nutrients www.accounting4nutrients.com.au

 

Guidelines for maintenance applications

  1. To determine the amount of phosphorus (P), potassium (K) and sulfur (S) to be applied annually as a maintenance dressing.

a) A rough rule of thumb is to base the amount of P required on the phosphorus buffering index (PBI) of the soil. PBI is related normally to soil texture (clay content).

i)  When PBI is low (0–100), apply 0.8kg P/DSE.

ii)  When PBI is moderate (100–300), apply 1.0kg P/DSE.

iii)  When PBI is high (> 300), apply 1.2kg P/DSE.

b) Alternatively, base application rate on rainfall, applying 1kg P/ha for every 25mm annual rainfall (eg if annual rainfall is 700mm, apply 28kg P/ha annually).

c) Alternatively use the following tables which take into account current fertility levels and your soil type (based on PBI)

Olsen phosphorus (P) soil factors (kg/ha) for maintenance nutrient applications based on Phosphorus Buffer index (PBI)
PBI value Current Olsen P level (mg/kg)
  2–4 5–7 8–10 11–13 14–17 18–25 26–35+
0–50 0 3 6 8 9 10 10
50–100 0 5 10 15 18 20 20
100–200 0 6 13 20 23 25 25
200–400 0 7 15 22 26 28 28
400–600 0 8 16 24 28 30 30
> 600 0 10 18 26 31 35 35
Source: Healthy Grazing Pastures Manual, Victorian Department of Primary Industries

 

Colwell phosphorus (P) soil factors (kg/ha) for maintenance nutrient applications based on Phosphorus Buffer index (PBI)
PBI value Current Colwell P level (mg/kg)
  ≤ 20 20–34 35–59 60–79 80–94 95–114 ≥ 115
0–50 1.2 4.0 7.0 9.0 10.0 10.0 10.0
51–100 2.3 7.0 12.0 17.5 20.0 20.0 20.0
101–300 3.0 8.0 14.5 19.5 22.0 22.5 22.5
301–400 3.0 8.0 14.5 19.5 22.0 22.5 22.5
401–500 3.5 10.0 19.5 27.0 29.0 30.0 30.0
501–600 3.5 11.0 20.0 28.5 31.0 32.0 33.0
> 600 3.5 22.0 29.5 32.0 31.0 34.0 35.0
Source: Accounting for Nutrients www.accounting4nutrients.com.au

 

Potassium (K) and sulfur (S) soil factors (kg/ha) for maintenance nutrient applications based on Phosphorus Buffer Index (PBI)
PBI value Soil factor K Soil factor S
0–50 25 12
51–100 20 12
101–300 15 12
301–400 15 12
401–500 10 12
501–600 10 12
> 600 10 12
Source: Accounting for Nutrients www.accounting4nutrients.com.au

 

d)  Choose a P–S fertiliser blend consistent with the guidelines established for capital applications and unit cost of P.

2. Molybdenum should be applied as an additive to P-based fertilisers:

a) When soil pH is less than 5.5CaCl2 or tissue testing indicates levels are lower than critical limits.

b) Whenever legumes are reseeded and/or when poor nodulation and rhizobia activity is suspected.

c) At least once every 4–5 years when applying maintenance P on low pH soils.

Seek advice on suitable rates for your region, but rates used are generally in the range of 50–100 g/ha.

3.  Consider maintenance applications of phosphorus, potassium and sulfur in paddocks regularly cut for forage conservation. The table below indicates typical extraction rates for P, K and S in kg/t fresh weight (FW) of hay or silage. For example, if you cut and remove 10t of lucerne hay per hectare from a paddock, you will need to apply 20kg of P, 240kg of K and 26kg of S to maintain existing nutrient levels on each hectare cut. Note the large amount of K removed. Producers who frequently cut forage from the same paddock should consult an agronomist or other adviser to determine the best method of maintaining K levels. Also be aware that nutrient composition, especially minerals, can vary considerably (in some instances more than two-fold).

Typical extraction rates for phosphorus (P), potassium (K) and sulfur (S) in kilograms per tonne of fresh weight (kg/t FW) hay or silage
Type of fodder Moisture (%) Mean P conc. (kg/t FW) Mean K conc. (kg/t FW) Mean S conc. (kg/t FW)
Legume hay (clover, medic) 89 1.7 18 1.6
Lucerne hay 87 2.0 24 2.6
Legume–grass hay 88 2.0 18 1.7
Oaten hay 90 1.6 17 1.1
Pasture hay 88 1.8 15 1.6
Grass silage 44 2.8 24 2.2
Maize silage 62 1.9 15 1.0
Pasture silage 48 2.8 26 2.3
Oaten silage 45 2.5 23 1.8

 

An alternative approach is to have hay or silage tested for P, K and S as well as N and MJ ME/kg DM (M/D).

4. Nitrogen (N) is found mainly in the soil organic matter fraction and as protein in plants and soil organisms. The N present in the soil occurs in two major forms:

  • organic N found in the soil organic matter and soil biota (organic N is not available to plants); and
  • mineral N found in soil as either ammonium (NH4+) or nitrate (NO3-).

Only 2–3% of soil N is in the mineral form and hence available to plants. Organic N is converted to mineral N (mineralisation) during the breakdown of soil organic matter by soil microbes. Nitrogen is reconverted from mineral to organic forms (immobilisation) when it is taken up in plant growth.

The amounts of N removed by sale of livestock products are modest but plants require large amounts of N for growth. Nitrogen application may be considered to boost pasture growth in some grazing situations but can be lost through rapid evaporation if application is not immediately followed by precipitation sufficient to dissolve the N into the plant root zone. A partial budget analysis is recommended when considering an application of nitrogen to assess the benefits against the costs.

Guidelines for lime applications

Lime can be applied as a top dressing if a paddock is to remain in the pasture phase for several years. 

Most pastures on very acidic soils (less than pH 4.2(CaCl2)) will respond to surface applied lime over a period of time. Pastures on soils with very high levels of aluminium or manganese will also respond.

When surface-applying lime; a maximum rate of 5t/ha is recommended to avoid smothering the plants and to avoid possible animal health problems.

Calculating your lime requirement:

Target pH(CaCl2) – soil test pH(CaCl2) )/conversion factor = t/ha lime required

Conversion factors

  • 0.26 for clay
  • 0.37 for clay loam
  • 0.47 for sandy clay loam
  • 0.57 for sandy loam.

If you soil test shows organic matter level is above 2%, add an extra 0.4t/ha.

For example, your soil is a clay loam with a soil test pH(CaCl2)  result was 4.5 and your target level is pH(CaCl2) 5.0.  Your organic matter level in your soil is 3%. 

How much lime should you apply?

(5.0 – 4.5)/0.37 = 1.35 t/ha lime, but with a higher organic matter level.

1.35 t/ha lime + 0.4 t/ha lime = 1.75 t/ha lime

Note:  the lime rates calculated assume a pure limestone product.

If the neutralising value of the lime is not 100%, then:

Lime to spread (t/ha) = (lime required (t/ha) x 100)/ neutralising value.

Source: Healthy Grazing Pastures manual, Victorian Department of Primary Industries

Guidelines for taking a soil test

How to take soil tests properly

It is important to collect soil samples correctly to ensure a meaningful test result:

  1. Representative samples – Establish monitor areas or transects that represent each of the major classes of land (land management units) across the farm. The objective is to adequately represent the differing areas of the farm that are to be fertilised whilst ensuring a reasonable soil testing load and expense. Using a soil corer, sample in the monitor area or along the transect in a systematic way and record the sampling interval and pattern used so that the sampling pattern can be replicated at later times. To ensure samples reflect the paddock as a whole, avoid stock camps, fence lines, water troughs, fertiliser dumps, burnt timber rows, wet gullies, gateways, tracks or dung patches and sample from different soil types separately.
  2. Mark the site – Keep a record of the monitor area or transect for future testing. You may do this by noting where you started and finished and the route taken, by taking a series of GPS readings, etc.
  3. Depth – Extractable P is measured in topsoil samples using a soil sample depth of 10cm. P is typically more concentrated in the top few centimetres of soil so it is very important to obtain the full volume of soil to 10cm depth to avoid biasing the concentration of P in the soil sample.
  4. Sample number and handling – Take a minimum of 30 soil cores along the transect or monitor area and combine to give a sample that is representative of the paddock. Send the sample to the testing laboratory promptly. Use a NATA or ASPAC-accredited laboratory to take advantage of the quality control that this accreditation represents.
  5. Timing – Always sample at the same time every year. It is potentially feasible to take annual samples at any time of the year, but soil samples are most commonly taken in late spring. At this time soil is usually moist, but not wet, allowing soil cores to be taken quickly and easily. Moist soil holds together in the corer and this helps to ensure the sample is the full 10cm depth. Never sample within the first few months after fertiliser application.
Source: 5 easy steps to ensure you are making money from superphosphate booklet.