Drip irrigation with hard water for cotton: what scale does

Drip irrigation with hard water for cotton has a particular failure, and it arrives at the worst moment: the far-end drippers slow through July and August as chalk forms inside them, and the rows that flower last are the rows that go dry at flowering. No filter stops it, because the calcium that becomes the chalk is dissolved in the water and passes any screen.

Drip irrigation with hard water for cotton is still the right system for the crop; it simply needs a flushing regime, a lateral that survives the regime, and an honest plan for the acid treatment that we do not sell.

This guide covers what hard water does across a cotton season, what the filter answers and what it cannot, the flushing that keeps the far end alive, the wall thickness that tolerates it, and the early-season job that rain pipe does better on this water.

What hard water does across a cotton season

Hard borewell water carries calcium and magnesium bicarbonate in solution. It stays dissolved while the water moves and stays cool; it comes out as a chalky crust wherever the water warms, slows and dries. In a drip line those three happen together at the emitter outlet, where a drop sits in the sun between runs, and at the far end of each lateral, where the flow is slowest and the last of each run evaporates in the pipe.

Cotton makes this worse than any other crop we size. The system is laid dry before the monsoon and commissioned in heat; the laterals lie in full sun until the canopy closes in August; and the season is long enough, June to November, for a crust that is harmless in week four to close an outlet by week sixteen. The timing is the cruelty: the far end fails in the flowering and boll-filling weeks that decide the bales.

• Weeks one to four: nothing visible; the water runs clear and the flow is even.
• Weeks five to ten: a pale ring at the outlets of the last few drippers; the far-end cup reading drops a little.
• Weeks eleven onward: far-end drippers at half flow or closed, the head pressure a little higher than week one, and a dry stripe along the tail of each row.

What the filter does, and what it cannot

A screen or disc filter at the head, which we do not stock, removes sand, silt and algae, and every drip system on borewell water needs one. It does nothing to hardness: dissolved calcium passes a screen as easily as the water does. A grower who fits a good filter and still loses the far end by August has not been sold a bad filter; the fault is dissolved, and the answer is chemical and procedural.

Have the water tested before the season, at the horticulture department's laboratory or a private one: hardness, bicarbonate and pH are the three numbers. The laboratory reads them against the published irrigation water classes, which is a better guide than any rule of thumb from a supplier, including us. A bore that tests moderately hard can be managed with flushing alone; a bore that tests very hard needs the acid treatment described below as a planned cost, not a rescue.

The flushing regime for hard water

Flushing moves water through the far end fast enough to carry loose crust out before it bonds, and on hard water it is weekly through the hot months, not the every-few-weeks that clean water allows. Take the end caps off a few laterals at a time, run the pump until each tail runs clear, and cap them again. A 16 mm end cap is ₹6 and a box of 100 is ₹600, so the caps are not the cost; the hour is.

The acid treatment is the real cure, and it is not ours to sell. A diluted acid run through the lines takes chalk back into solution and out through the open tails; the chemical, its strength and the safety rules come from the treatment's supplier and the label, and the injector it needs is bought elsewhere too.

Two habits reduce how much there is to flush. Water on a steady daily schedule rather than long gaps, because every gap is a drying cycle at every outlet; the black-soil scheduling guide sets out the daily minutes for cotton. And flush after every fertigation, because fertiliser salts in hard water precipitate faster than the water alone.

Plan it twice on very hard water, once before the season and once when the far-end cup reading drops, rather than once in despair in September. The blocked-line guide covers how to tell chalk from silt before treating anything.

The wall that survives the regime

Every drip line we list carries the same 4 litre per hour dripper every 40 cm, so there is no larger-outlet emitter on our shelf to choose; what differs is the wall. The round inline Aquaflow in 500 metre coils at ₹4,482 and the 0.4 mm flat tape at ₹3,366 both take a season of weekly flushing, repeated uncapping and an acid run without splitting.

The 0.2 mm tape at ₹3,960 per 1,000 metres does not: on hard water it is handled too often for its wall, and the tails crack at the fold by August.

So on a hard bore the economy of the thin tape is false for cotton, and the cotton drip guide reaches the same wall decision from the season's length alone. Buy from the drip range by the water you have, and lay the far ends where they can be reached for the weekly flush rather than under the headland bund.

The early-season job that rain pipe does better

Cotton's first watering job, bringing the stand up evenly before the monsoon arrives, is one that rain pipe does well, and its holes are large enough that hard water does not close them in a season.

A grower on a very hard bore can run the 40 mm rain pipe for the sowing weeks and the drip for the flowering and boll-filling weeks, which halves the time the drip laterals spend crusting in the sun. The rain pipe guide for cotton sets out where that job ends and the canopy takes the spray.

If you want the lateral and the flushing plan matched to your bore, send us the water test, the acreage and the pump and we will say which wall to buy and how often the tails must run, marking the filter, the acid and the injector as items from elsewhere.

• Flushing alone: a moderately hard bore, a daily schedule, weekly tail flushes through July and August.
• Flushing plus acid: a very hard bore, the treatment planned before the season and repeated when the far end slows.
• Rain pipe for sowing, drip for flowering: a very hard bore where the drip lines would otherwise lie crusting from June.
• Rethink: no filter, no acid plan and the thin tape, which on this water is a system bought to fail in its first August.