It is important to ensure that sufficient nutrients are available to the young pasture in the early stages of growth. Seedlings need nitrogen, phosphorus, sulphur and molybdenum to be available nearby.
When sowing into a prepared seedbed, the fertiliser is often sown with the seed. When heavier rates of fertiliser are required, they can be prespread or applied soon after the pasture has effectively established.
High rates of nitrogen or potassium fertiliser placed in contact with or very close to the seed can restrict germination and reduce establishment.
Nitrogen
Small amounts of nitrogen at sowing (10– 20 kgN/ha) can be beneficial for establishing pasture seedlings, especially on lighter-textured soils. The nitrogen stimulates early growth of pasture seed¬lings and helps their rapid establishment. Additional nitrogen can be topdressed soon after establishment to continue the rapid growth of grass-based pastures.
Previous nitrogen fixation by legume- based pastures will add to the nitrogen supply available in the soil for pasture establishment and growth. As shown in Nitrogen fixed by various pasture legumes different legumes vary in their capacity to fix nitrogen. A high proportion of this fixed nitrogen is avail-able for cycling and and can stimulate the establishment and growth of the pasture.
Phosphorus
Phosphorus is taken up rapidly in the early stages of seedling growth. It should therefore be applied near the establishing seedling. Phosphorus fertilisers can be drilled in contact with seed. The rhizobia in legume inoculum can be killed when mixed with acidic phosphorus fertilisers, but lime-pelletting inoclated seed protects them from damage.
Phosphorus is more readily available to establishing pastures when placed in a drill row than when broadcast. Follow-up topdressings are effective when applied in subsequent autumns and springs.
Soil tests help in deciding the best rate of phosphorus to apply. Take samples as close as possible to the planned sowing date.
Potassium
Potassium is not usually applied in the drill row at sowing as it can reduce germination if placed in close contact with the seed. Potassium-deficient soils are best treated with a separate application before or after sowing with muriate of potash (potassium chloride). The rate will vary from 62 to 125 kg/ha, depending on the deficiency and the soil type.
Molybdenum
Molybdenum (Mo) deficiency in young legume seedlings inhibits rhizobia and reduces nitrogen fixation, leading to nitrogen deficiency.
Molybdenum can be applied in prepared mixes with superphosphate and other fertilisers. Super Mo 0.02 at 250kg/ ha and Super Mo 0.04 at 125 kg/ha both supply 50g/ha of Mo (the required rate).
Treating legume seed to apply 100g of molybdenum trioxide per hectare is an effective method of applying Mo that guarantees sowing the seed in contact with the Mo. A foliar spray of sodium molybdate at 420g/ha will overcome molybdenum deficiency. Do not use sodium molybdate to coat seeds as it kills rhizobia.
Sulphur
Certain soils, for example those derived from basalt, are low in sulphur (S). Apply sulphur fertiliser at sowing to meet deficiencies. Sulphur is more likely to be needed for direct-drilling than when conventional cultivation has been used to prepare a seedbed. Apply at least 10kg/ha of sulphur at sowing.
Sulphate S, such as in gypsum, is more quickly available than elemental yellow sulphur, which has to be changed to sulphate by microorganisms before it becomes available. This change takes longer.
Fertiliser Placement
Place the fertiliser under the seed, where the roots of the seedlings can use it. Fertiliser burn of seedlings can be caused by direct osmotic effects (because of high concentrations of dissolved salts in the soil solution) and specific ion effects (for example, ammonium from urea or DAP). Different fertilisers have different effects on soil pH in the immediate vicinity of the granule when they dissolve. Some products form an acidic solution, and might induce aluminium or manganese toxicity in seedlings on soils that are already strongly acidic.
Fertiliser burn is most likely to occur with nitrogen, then potassium, then phosphorus fertilisers. To avoid fertiliser burn, don’t exceed 20kg/ha of N or 10kg/ ha of K when seed is in direct contact with the fertiliser. Potassium is usually not used near the seed when establishing pastures. The salt effects of nitrogen and potassium fertilisers are additive.
Rhizobia and Fertiliser
Rhizobia bacteria can be killed when the seed is mixed directly with fertiliser. They are suceptible to phosphate fertiliser, nitrogen, sulphur as sulphate of ammonia, and potassium.
Lime-pelleting legume seed will improve rhizobium survival, the nodulation of the seed and the growth of young seedlings. Lime-pellet and inoculate legume seed not more than 24 hours before sowing.
Commercial Fertilisers
A wide range of fertilisers is available from the various fertiliser companies for establishing pastures. Fertilisers and rates are chosen on the basis of the nutrients required to successfully establish the pasture, and on the cost to buy the fertiliser, deliver it to the farm and apply it.
The table below shows several fertilisers available from the major fertiliser suppliers Incitec, Pivot, Hi-Fert and W. Paton.
Which Fertilisers are the Best Buys?
A fertiliser can be valued by estimating the quantities of N, P, K and S and then valuing the components at current market prices. For example, calculate the value of each in terms of the nitrogen it contains. Another way is to value all of the components and then estimate the value of the most important one.
The following table shows typical examples. The two most popular NP fertilisers are DAP and Starter 15.
