FAQ: Fertilisers

Should I test for soil Aluminium levels and what depth should I take the soil sample?

Yes – in areas where aluminium is known to be a problem. Although aluminium testing is expensive in more marginal climates (like the high country), the upfront costs of rigorous soil testing are an investment in decision making. I would be looking to sample the soil down to at least 1.0 m because if the lucerne roots are not going down at least to that depth the purpose of lucerne is lost. We recommend taking a 0-15 cm test for your normal full soil test plus aluminium (Basic+sulphur+Al tests for topsoil layer). Then every 15-20 cm for just pH and Aluminium to 1.0 m – deeper is better.

Some farmers may have a pit or cutting on a road that could be sampled (provided it was cleaned back appropriately) but for others we have advised they get a digger out and go down up to 2.0 m if they can –then testing the lot but bulking samples from 1.0-1.5 m and 1.5 to 2.0 m (this way it adds only 2 extra samples for pH and Al).

Should I check magnesium and boron levels when soil testing?

These nutrients are important to check with soil tests if you think there may be a deficiency. Boron is usually readily available if your soil pH is between 5.7 and 6.5. Likewise magnesium is less plant available until the soil pH is above 6.0 – unless you know you are on a soil that is historically low in these nutrients then they should be available.

Boron is deficient on soils of high pH (7.0+) or those with low soil organic matter (e.g. sandy and pumice soils) so these will need checking with a soil test. Deficiency symptoms show up as a reddish tinge around the edge of leaves at the top of the plants.

Magnesium is deficient on soils of high potassium, or also sandy soils with low organic matter. Deficiency symptoms will show up as yellowing in older leaves between the veins mid canopy rather than at the top of the canopy.

So, if you are doing a regular herbage test because that is an efficient way of working out fertilizer requirements then Bo and Mg are probably not needed.

But…if you are doing a test to solve a problem on sandy soils or those with low organic matter they may be the issue. Molybdenum (Mo) can also be deficient especially when the pH is below 6.0 –deficiency symptoms will appear as yellowing herbage that looks nitrogen deficient (as does sulphur deficiency –which is most apparent after heavy winter rain).

In all cases, remember the lucerne is probably growing more herbage than previously so will place more demand on the soil, especially in a cut and carry regime – which is why grazing is always best.

When is the best time to take a herbage nutrient test in lucerne?

A herbage test in November (before water stress restricts growth), can aid in decision making if you think you have nutrient issues. At the time when lucerne growth is fastest, it is putting the most pressure on the soil to supply nutrients for growth. Therefore, any nutrient deficiencies will show up at this time.

For fertiliser, apart from soil testing, is there a basic requirement?

No just do your testing. Annual soil testing should have been done in May – see txt alerts from Beef + Lamb New Zealand. For grazed lucerne apply just basic requirements to maintain the stand with an Olsen P in high teens. For cut and carry paddocks keep an eye on potassium (K) and for insurance do a herbage test if you want – take a sample of the top 15 cm of leaf and stem together for this. Do the herbage test when the plant is growing at its fastest to tell you if you have any deficiency.

What fertiliser should I use when sowing?

Depends on where you are. In Marlborough Doug applies 20 kg of nitrogen as Cropmaster 20. At Lincoln we don’t use any nitrogen but we always have the pH and P levels adequate at establishment for conventional cultivation. If direct drilling superphosphate down the spout is key (see Kearny et al. 2010 for details). Don’t direct drill lucerne without phosphate.

Does lucerne require fertiliser nitrogen during its first season/ establishment to assist it with early development until it can fix its own?

Note: this question related to establishment of lucerne into land previously dominated by low fertility browntop, sweet vernal and hieracium. Two years of ryecorn crops were planned prior to direct drilling the lucerne with DAP at 100 kg/ha.

The planned use of the ryecorn crops and then direct drilling lucerne with DAP is adequate for the situation described here.

The high carbon from the browntop stubble actually needs to be broken down and to do that you need nitrogen – so if anything I would ensure the first ryecorn crop receives more N (urea) than usual – you may find that first crop has a lower than expected yield. This is because the N you put on for ryecorn crop growth has actually been used by the soil microbes to break down the roots and dead material from the high carbon browntop thatch. The “hoof and tooth” winter feeding then mechanically breaks that thatch as well and the concentrated number of animals adds N to the system from their urine which also helps it break down the remaining browntop. The second year ryecorn crop is usually higher yielding because of the first year N. Then in the third year of the pasture renewal phase the lucerne should be sown with only about 20 kg/ha of N and only because you are coming out of that N deficient period and are in a cold area (inland Canterbury). The DAP at sowing is ideal for that.

Let the lucerne grow in the first year until it is flowering – it needs to grow 5 tonne per hectare of root material underground and does that as a seedling. Make sure you freshly inoculate – especially on the virgin ground where there is no history of lucerne (in the previous 5 years). Seed coated within the 1-2 prior to sowing should be fine – after that ask for it to be freshly inoculated. The lucerne will use up any available soil N before it starts fixing N so you may not see nodules immediately.

Why is my young lucerne turning yellow in spring?

Note: this question relates to an inoculated ‘Kaituna’ lucerne stand at sown into a paddock with pH = 6.0• in autumn but symptoms can also be seen in established stands.

Pale yellow leaves can indicate sulphur deficiency. It is usually first seen as a yellowing of the young newly emerged leaves at the top of the growing stem and can often be confused with nitrogen deficiency.

If you’ve experienced a wet winter it is possible that sulphur may have leached from the soil. This can occur because sulphate (a plant available soluble form of sulphur in the soil) is a negatively charged nutrient and is repelled by the negatively charged soil. In a wet winter, soil water drains through the soil and the soluble Sulphate moves with it (however, it must maintain a neutral charge so it binds with positively charged ions such as hydrogen, potassium or sodium etc. in the process).

Do a herbage test now while the lucerne is actively growing – because deficiency symptoms are most easily identified when the plant is putting the most demand for resources on the soil system.

for overseas readers: In New Zealand pH is usually determined in water not calcium chloride.

How much potassium do I need to replace if I cut and carry lucerne off a paddock?

In a cut and carry system you are removing a lot of potassium – about 20 kg/tonne of dry matter removed. If you take 3 cuts of 4.5 t/ha which would total 13 t/ha removed, you would need to replace about 260 kg/ha of potassium. On greywacke soils some potassium will come from the parent material but because there is limited grazing, you will need to keep an eye on the potassium levels from soil and herbage tests.