Fertiliser for lucerne: phosphorus, sulfur, potash, moly and lime

What lucerne needs, and what it usually does not

NSW DPI's lucerne Agfact says lucerne in NSW can run short of phosphorus (P), sulfur (S), molybdenum (Mo), potassium (K), calcium (lime), boron, zinc and nitrogen (N). Fertile soil gives faster regrowth after cutting or grazing and helps stands persist.

• Nitrogen: if lucerne is properly inoculated (or the soil has the right rhizobia) and P and Mo are adequate, NSW DPI says nitrogen fertiliser is not necessary.
• A small dose of 5–10 kg N/ha can help seedlings where cold or wet weather is likely to slow nitrogen fixation, or the soil is low in N.
• More than that can encourage weeds and too much N can upset nodulation, as plants use fertiliser N before fixing their own.
• Hay paddocks lose more nutrients than grazed pasture or cereal crops, especially P, K and calcium.

Phosphorus and sulfur

Most soils are low in P, so phosphorus fertiliser is usually needed. Rates depend on soil type, fertiliser history and rainfall.

• At sowing: up to 30 kg P/ha (about 375 kg/ha of single superphosphate) on low-P soils in higher rainfall areas, falling to 10 kg P/ha in drier districts. Under irrigation, up to 40 kg P/ha for hay. Use less if the soil test shows medium to high P.
• Maintenance (irrigated, or above 600 mm rain in southern NSW or 700 mm in the north, on low to medium P soils): 10–20 kg P/ha a year, plus up to 5 kg P/ha for each hay cut.
• Placement: band fertiliser 2–3 cm below the seed. In drier areas, the top 5 cm often dries out, so place it 7–15 cm deep.
• Sulfur: low-S soils on the slopes and tablelands may need up to 25 kg S/ha a year; low rainfall areas with a good fertiliser history may need only 3–10 kg S/ha.

Where both P and S are short, NSW DPI says single super has the right balance; double and triple super do not carry enough sulfur. Examples: SuPerfect, Impact SSP and Summit SuperPhosphate. See the superphosphate rate guide for pastures and the sulfur fertiliser list.

Potash for hay stands

Lucerne holds more potassium than any other soil-supplied nutrient except nitrogen. NSW DPI says K deficiency is not widespread but is increasing in paddocks cut for hay for many years. Look for white or yellow spots around the edges of older leaves.

• One tonne of lucerne hay removes about 21 kg K (NSW DPI). Incitec Pivot estimates a 3 t DM/ha lucerne hay cut can remove 70–80 kg K/ha.
• To check, NSW DPI suggests strip-testing growing lucerne with muriate of potash (50% K) at 250–800 kg/ha; 250–375 kg/ha usually fixes a deficiency.
• Where hay is made regularly, 125–375 kg/ha of muriate of potash a year may become necessary.
• The NSW Soil Quality factsheet names irrigated lucerne paddocks as a place where K can run low.

Straight potash options include Incitec Pivot MOP and Summit MOP. P and K blends such as Summit Super Potash 2:1 do both jobs in one pass; compare it with single super. For MOP vs SOP, read sulphate of potash vs muriate of potash. For working out what each cut removes, see fertiliser for hay.

Soil pH, lime and molybdenum

Lucerne is one of the most acid-sensitive common legumes. Sour soils (acidic) are often short of calcium and molybdenum, which holds back nodules and roots, and very acidic soils can have toxic aluminium.

• Best growth: topsoil pH (Ca) of 6.0–7.0. For good yields NSW DPI gives pH (Ca) 5.2–7.5.
• Lucerne is hard to establish if the top 10 cm is below pH (Ca) 5.2 with more than 5% exchangeable aluminium.
• Mildly acid (pH (Ca) 5.1–5.4): inoculate and lime-pellet seed, or band 100–250 kg/ha of lime mixed with super.
• Very acid (around pH (Ca) 4.5): more than 2.5 t/ha of lime may be needed, worked in at least 6–8 weeks before sowing.
• Each tonne of hay removed acidifies the soil by about the same as 70 kg/ha of lime, so keep testing hay paddocks.

Molybdenum is needed in tiny amounts but is essential for nodules. NSW DPI recommends molybdenised fertiliser on all soils below pH (Ca) 5.0, at about 50 g Mo/ha every four or five years; 250 kg/ha of super with 0.02% Mo supplies that. Liming can also make more Mo (and P) available, but too much lime can lock up P, K and boron. Do not overdo it: too much Mo can cause copper deficiency in grazing stock. Summit lists 0.06% Mo in Super Copper Zinc Moly. For lime rates see how much lime per hectare; one granulated lime is Calciprill.

Before you order

• Soil test and use tissue tests to confirm K, S, Mo, boron or zinc problems. See reading a soil test.
• Do not wait for severe symptoms: NSW DPI warns production may already be checked by then.
• Boron and zinc: NSW DPI suggests 2–3 kg/ha boron only if symptoms persist (the margin to toxicity is small), and zinc sulfate strip tests at 20–25 kg/ha; zinc deficiency is more common on high-pH soils.
• Too much fertiliser can favour weeds such as barley grass where lucerne competition is low (NSW DPI).

Always check with your agronomist before acting.