Potash on pasture: when is it worth spreading?

Is potash good for grass?

Only if the soil is short of potassium. The Corangamite Brown Book (Victoria) notes most soils hold a large store of K, and deficiency usually shows up where plants remove a lot of it.

• Grasses are deeper rooted than clovers and compete harder for K. A grass-dominant pasture with plenty of weeds can itself be a sign of K deficiency.
• In WA dairy studies, ryegrass responses to fertiliser K were rare, and routinely applying K after each grazing was unlikely to pay (DPIRD).
• Legumes respond to K more than grasses (Incitec Pivot), and lucerne has responded in spring even where the topsoil test looked adequate.
• In clover, look for whitish spots along the leaf edges that later die off. In grass, K deficiency looks like N deficiency or frost (pale leaves, browning tips), so you cannot diagnose it by eye.

Where potash is most likely to pay

• Sandy soils, especially in higher rainfall areas where K leaches (Soil Quality, WA).
• Paddocks cut for hay or silage, where K leaves the farm in every bale. See fertiliser for hay for how much each cut removes.
• 'Day' paddocks and runs where stock graze but camp elsewhere. About 80–90% of the K stock eat comes back in urine, so it piles up in camps, laneways, feed-out areas and dairy night paddocks instead.
• Clover-based pastures. Soil Quality (WA) says pasture legumes can go short of K even when cereal yields are unaffected, and DPIRD says deficiency is most likely for legumes on sandy soils in wet years.
• In NSW, Soil Quality notes K deficiency is not widespread but occurs on some volcanic and heavier soils, especially in the southern highlands, and in paddocks cut for hay or silage.

If paddocks near the dairy or feed-out areas already test high, the Brown Book says K can be cut back or left out until levels fall.

Check the soil test first

Use Colwell K or exchangeable K. The right target depends on soil texture, because clays hold and supply more K than sands.

• For clover in WA dairy pastures, DPIRD found clover always responded below soil test K 30 mg/kg and never above 100 mg/kg; in between, the test was not reliable. Its crude rule is to apply K below 100 mg/kg.
• Soil Quality's WA factsheet gives pasture legumes as deficient below 80 and sufficient above 100 (Colwell K).
• Tissue tests help, especially in intensively grazed ryegrass, where DPIRD found soil K tests unreliable.

More on the numbers: reading a soil test.

How much, which product and when

• Rates (Brown Book): 50–100 kg K/ha every 5–10 years on clay soils, where one dressing lasts 6–8 years; about 20 kg K/ha a year on lighter soils, where it lasts 1–2 years.
• Cap each dressing at 60 kg K/ha (Agriculture Victoria). Our arithmetic: that is 120 kg/ha of muriate of potash at 50% K.
• After silage or hay on WA dairy farms, DPIRD recommends 50–100 kg/ha K where deficiency is likely.
• Timing: 1972 WA trials found autumn topdressing beat spring on most South-West soils; spring was better only on deep sands with very little clay or organic matter.

The Brown Book calls muriate of potash (MOP) the most cost-effective K source and says its chloride should not be a concern on pasture. The full comparison is in sulphate of potash vs muriate of potash. Straight MOP examples: Incitec Pivot, Summit and Impact. If you need P and S as well, super and potash blends such as Summit Super Potash 2:1 do it in one pass: compare it with straight MOP, or browse the potassium list.

Do not overdo it: stock health

• High soil K is linked with grass tetany in cattle and grass staggers in sheep, with the most risk for cattle in late pregnancy and early lactation (Brown Book).
• Agriculture Victoria warns that if exchangeable K is more than 10% of total cations it may cause magnesium deficiency in plants and animals, and a magnesium to potassium ratio below 1.5:1 raises the grass tetany risk.
• Pasture takes up more K than it needs (luxury uptake) when it is freely available, so big dressings waste money as well (Incitec Pivot).
• A GippsDairy article warns that a high-K diet for cows near calving can upset calcium and cause metabolic problems at calving.

NSW Local Land Services also lists grass tetany among the livestock risks fertiliser can raise. Always check with your agronomist before acting.