Potash is a source of potassium: the element of fruit quality, water regulation and heat tolerance. In Iran, potassium sulphate is usually safer than the chloride, and it should be split from fruit set until just before ripening.
Potash fertiliser means a fertiliser that delivers available potassium to the soil or to a nutrient solution. On the label it is usually written as K2O. Unlike nitrogen, potassium is not part of the chlorophyll molecule, but without it the stomata, the movement of sugars, the filling of grain and tolerance of drought and cold do not work properly. Iranian growers often know potash as the fertiliser of colour and sweetness; that reputation is not baseless, but it is incomplete. Potassium makes both growth and cropping durable, provided it is not lost in competition with the sodium of saline water and provided it arrives during fruit filling rather than only in March. Farkesht does not write the same potash programme for grapes in Qazvin, greenhouse tomatoes, dates in the south and pistachio in Kerman; the source and the splitting change with the salinity of the water.
What potash fertiliser is and what the plant does with potassium
Potassium is a free cation inside the cell. It maintains osmotic pressure, opens and closes the stomata, and activates the enzymes that build starch and sugar. When it is sufficient, the plant manages water better, the phloem carries sugar to the fruit, the fruit skin is firmer and it rots later in store. When it is short, leaf margins burn, fruit stays soft or tasteless, and in the heat of Isfahan and Khuzestan the plant shuts down earlier at midday. Potash fertiliser brings this element as chloride (muriate of potash), as sulphate, as nitrate, or from organic sources. The figure on the sack is the percentage of K2O, not the percentage of potassium metal; do not confuse this with the nitrogen and phosphorus of a complete fertiliser.
The effect of potassium on growth, resilience and cropping
In vegetative growth, potassium does not play nitrogen's role; it does not build leaves, but it makes the growth already built efficient. A plant high in nitrogen and low in potash has large leaves and soft tissue and is prone to pests and lodging. On cropping the effect is more direct: fruit set, size, colour, sugar and evenness of ripening. In tomato, potassium deficiency causes yellow shoulder and hollow fruit. In grapes, the berries stay small and the colour weak. In dates, filling is incomplete, and in citrus the skin becomes coarse and juice quality falls. Potassium also raises tolerance of drought and salinity because it competes with sodium; in orchards with high-EC water, potassium deficiency appears even when fertiliser has been applied, because sodium has taken its place on the colloid.
Do not mistake potassium deficiency for salinity
Potassium is mobile too; the deficiency shows first on old leaves. The familiar pattern is scorch on the margin and tip of an older leaf while the middle of the blade may stay green. The leaf looks like salt scorch, and telling them apart without a test is difficult. If the scorch is exactly along the drip row and the edge of the wetting bulb, look at local salinity first. If it is scattered through the crown and on old leaves, potassium is more likely. Soft fruit, cracking, failure to colour and fruit drop may come with it. In maize the leaf margin turns brown, and in potato the tuber loses cooking quality. A leaf test during fruit growth turns a guess about a scorched margin into a number.
- Scorched margins on older leaves; the middle of the blade is often still green.
- Weakness in heat and earlier wilting than a neighbouring tree on the same water.
- Small fruit, poor colour, low sugar, or spotting and yellow shoulder in tomato.
- Greater sensitivity to drought and, in some orchards, to late frost.
- On saline water: deficiency symptoms even after a chloride fertiliser, because sodium competes.
Potassium sulphate or potassium chloride in Iran
Potassium chloride is cheaper and has a higher K2O percentage, but it adds chloride to orchards whose water or soil is already saline or high in chloride. In the pistachio of Rafsanjan, the dates of Bushehr, in olive and in many orchards of the plateau, potassium sulphate is the safer source. Potassium nitrate is useful for greenhouse fertigation and the fruiting stage because it supplies both nitrogen and potassium; at the end of the season on a deciduous tree, the nitrogen that comes with it has to be counted. Give potash with the irrigation water in several splits so it is not leached at once in light soil. Spreading a sack over a crust in summer wastes much of the effect.
Do not buy potash for the sweetness of the fruit; buy it to move sugar and water into the fruit. If the sodium in your water is high, a chloride sack only makes the competition worse.
When to apply potash in orchards, greenhouses and field crops
For fruit trees, part of the potassium can be placed in the soil in autumn or winter to build a spring reserve; the more important part is split with irrigation from fruit set until just before ripening. Heavy potash immediately before harvest is late and sometimes disturbs ripening. In greenhouse tomato and pepper, raise the potassium ratio from the start of flowering and keep the nitrogen in check. In potato, onion and sugar beet, potassium is a matter of tuber quality and storage life and must be in the programme before the rows close. Foliar potassium is a supplement for the critical days of heat, not a replacement for soil fertiliser. Farkesht's Chalkood Cake Macro places part of the season's base potassium at root depth; the peak cropping demand usually has to be completed with a soluble top dressing.
- Read the EC and chloride of the irrigation water; then choose between sulphate and chloride.
- Take a soil test for exchangeable potassium and a leaf test during the fruiting period.
- Give the source in several applications with irrigation, not one sack in the midday heat.
- Keep potassium in the programme from fruit set until just before full ripening.
- Check the scorched margin against the dripper map so you do not fertilise what is actually salinity.
Grape, tomato, date, citrus and pistachio depend on potassium during fruiting more than leafy vegetables do. In the vineyards of Qazvin and Fars, tie potash to the bunch rather than only to shoot growth. In the citrus of the south, potassium is juice quality and heat tolerance. For the date palm, the article on palm nutrition on this site has its own calendar; here it is enough to know that you do not give a palm potash the way you give it to an apple tree in March. An excess of potassium is not harmless either: it disturbs the uptake of magnesium and calcium, and in tomato it makes blossom end rot worse. Farkesht writes potash against the ratio of K to Mg and Ca, not on the principle that more means sweeter. For a prescription for your own orchard, bring the test so the sulphate source and the fertiliser-pit dose line up with the fruiting top dressing.
