Sulphur
Superphosphate contains 11% sulphur, which is mainly as immediately available sulphate. Rates of superphosphate sufficient to meet the P requirements of pastures in most districts will also meet the S requirements on moderately S deficient soils. However, concentrated forms of P fertiliser, such as triple super and DAP, often cannot meet the S requirements in some soils. These fertilisers can be fortified with S by mixing in elemental (yellow) sulphur. This form of S has to be converted to sulphate by organisms in the soil before it can be used by pasture plants; this can delay the availability of sulphur to the plants.
Timing the application of fertiliser containing elemental S can be important. Because the S has to be converted by microorganisms, it should be applied well before periods of rapid growth when S deficiency in plants is likely to occur. When the S is required quickly after application, particularly after cold periods, which slow the rate of conversion, use fertilisers with sulphate S.
Which source of P should I apply?
This will be influenced by various factors:
The relative price of the N, P and K components from the various fertiliser sources.
- The number of applications required to deliver the total fertiliser.
- The cost (bag fertiliser is dearer than bulk fertiliser).
To decide which source to use, you need to determine the amount of nutrients in kg/ha that you plan to use over a year:
- Estimate the kg/ha of N, P and K required for each paddock for the whole year.
- Adjust for other nutrients such as sulphur, molybdenum and trace elements.
- Decide the timing of application for each nutrient.
A combination of the price of the fertiliser types that can supply the required nutrient amount and the cost of applying it will decide the most efficient combination of fertiliser types to use.
Potassium
Potassium (K) is usually deficient in dairy pastures. This is because cows transfer K in their urine and manure to unproductive parts of the farm, dairy farmers remove it in hay and silage, and the milk leaving the farm takes the K with it.
Keep the following points in mind when applying K:
- Potassium will burn seedlings if it is applied at more than 62kgK/ha. Broadcast the K before sowing and work it in, or sow a maximum of 30kgK/ha with the seed.
- Potassium has a big effect on the ultimate population of clover and lucerne stands. Application of K before sowing will produce a good plant population. Application of K when clover and lucerne populations decline gives a poor response because the reduced plant population cannot compensate.
- Lighter or sandy soils are more prone to K deficiency.
- Dairy waste water is high in K.
- Pasture can take up excess applied K; this can cause milk fever in freshly calved cows.
- Excess applied K can be leached out of the soil and wasted on sandy soils.
- Calcium and magnesium compete with K for entry into plants. A soil high in one or both of these will require extra K for satisfactory crop nutrition.
- Conversely, high amounts of K can reduce the uptake of magnesium and thus cause grass tetany, particularly when K fertiliser is applied with N fertiliser in early spring.
- Responses to K can be disappointing if N and P are deficient.
Using Fertiliser Blends
Fertiliser blends can be useful for applying small amounts of K regularly:
N+P+K: Used mainly for summer crops. Can be used for pastures where K is deficient and N is required.
P+K: Very good for pastures that have a high clover component.
N+K: Ideal for topdressing a pasture. Maintains a balance of N and K and ensures that the grasses do not dominate the clovers.
The balance of K and P is critical if a blended fertiliser is to be sown with the seed. The balance will depend on soil concentrations and paddock history. The most important consideration is that excess K will burn seedlings.
Nutrient Deficiencies
In the early stages nutrient deficiencies might be evident.
Nitrogen Deficiency
If clovers and lucerne are growing actively but grasses are light green to yellow then nitrogen is deficient. Heavy rain at sowing can leach nitrogen below the root zone. Topdress with urea at 125kg/ha or nitram at 187kg/ha.
Phosphorus Deficiency
Clovers are slow to establish. In extreme phosphorus deficiency grass leaves become purple. Apply any P fertiliser at 30kgP/ha.
Potassium Deficiency
Hard to detect in the early stages. Clover and lucerne plants die out in the first 12 months. Apply potassium chloride before sowing or at 125kg/ha when symptoms first appear.
Molybdenum Deficiency
Often seen on lighter soils. The young clover and lucerne seedlings have red stems. The plants remain stunted, the reddening affects the leaves and the young plants die.
Molybdenum deficiency causes nitrogen deficiency. Because molybdenum is necessary for nodulation of the roots, the plant is unable to produce nitrogen. Spray with sodium molybdate at 1 00g/ 100L applied at 70L/ha, or apply either urea at 1 25kg/ha or nitram at 187kg/ha with either Mo single superphosphate at 250kg/ha or Mo Goldphos at 125 kg/ha.
Fertilisers and the Environment
There is concern that fertiliser use in intensive farming systems could pollute surface water and groundwater. The Agfact Fertilisers and the Environment (AC.21) discusses this problem.
