Declining Potato & Root Crop Yields – Why?

A healthy, balanced soil will grow healthy balanced crops. When yields start to fall, identifying the cause of the problem is the the first step to resolving it. Here I look at the steps to work out what is deficient in this reader’s soil.

I had an interesting puzzle arrive in my inbox from Trevor

… I wonder if you can help me. I have been growing vegetables, with suitable rotation, for over 20 years on my patch. Over recent years whilst leaf vegetables, i.e. beans, peas, brassicas etc. are fine, root crop yields including potatoes are diminishing.
Any ideas what could be the cause?

Possible causes for the decline in crop yields

Well first of all, it seems the most likely reason will be a developing deficiency. Lack of water will cause a decline in potato yield but this year has been pretty wet so we can rule that out.

The leaf vegetables continuing to do well tells us that there isn’t a shortage of nitrogen which is necessary for leaf growth. It’s possible that too much nitrogen has been applied either as fertiliser or in manures. That would cause excessive leaf growth at the expense of tubers. It seems you can have too much of a good thing.

That leave phosphorus and potassium of the macro-nutrients. A shortage of potassium (potash) would be my feeling although phosphorus shortage would reduce yields on root crops.

There are other possible causes, a shortage of a micro-nutrient will have an effect even if there are plenty of macro-nutrients available.

The last suspect is the pH. If the soil is too acid or alkaline this will prevent nutrient uptake so mimicking a shortage. The ideal for potatoes is a pH between 5.5 and 6.5. Brassicas much prefer a high pH so possibly the soil is too alkaline.

Identifying The Soil Deficiency.

We need to identify the cause of the problem to rectify it so my advice is:

Check the pH of the soil. Testing kits are readily available, very cheap and easy to use. If necessary, rectify the pH.

Assuming the pH is within bounds then the next thing to look at is the macro-nutrients, Full test kits are a little more expensive than pH test kits but should show if there is a shortage to rectify.

Finally we have the micro-nutrients to consider. Testing for these is a job for a laboratory. The cost of professional testing is going to be higher than applying a trace element fertiliser like S-Chelate Cultiv-8. This will resolve any trace shortages. It’s organically approved and inexpensive.

See the following for more information:

NPK – Basic Components of Fertiliser Explained

Lime – the Vital Fertiliser!

Trace Elements in Plant Nutrition

Elements of Plant Nutrition: Calcium, Sulphur, Magnesium

Also of interest:

Gypsum, A Valuable Input for Agriculture & Gardeners

Posted in Allotment Garden Diary
12 comments on “Declining Potato & Root Crop Yields – Why?
  1. Philip Foxe says:

    ‘ This year has been pretty wet’ no it most certainly hasn’t been! although we had significant rainfall there were weeks without rain during hot spells. I didn’t have time to water much as I was busy so my potatoes are pretty small and onions too.

    • John Harrison says:

      Depends where you are, of course. I tend to forget things are different elsewhere. However, an experienced gardener will know to water dry soil and lack of water would have hit his beans hard. He mentions all root crops being in decline which goes to a common requirement for all root crops.

    • Julie Huckell says:

      Our first and second earliest were fine. But the main crop were dismal. Same as last year. The plants died off early. I think it was the extremely hot weather we had in June. Then the complete change to cool and wet conditions.
      Next year we are going to plant the main crop later and in beds that have produced good earlies in the past. We are going to dig in a soil enhancer.

      • Sandy Mathers says:

        When I plant my tatties I always put in some farm yard manure take out a trench (not to deep) put in some potato manure then put in some farm yard manure then plant the potatoes In that and cover them up in a drill.

  2. Ian Trueman says:

    I have a 30 year old 5-course rotation on lightish soil. This year my second early potatoes (mostly Charlotte) are small and the yield quite low (if delicious). However the maincrop (mostly Sarpo Mira) were large and the yield high. The Charlotte were flowering in June, when it was hot and dry with us. I should have watered them but that part of the plot was beyond my hoses. I think that this was when they were also filling their tubers. The maincrop flowered – and probably filled their tubers – from July, when there was plenty of water here in the midlands.

  3. Margaret E Johnson says:

    Without going to the expence of soil testing on a large area of land such as an allotment or field I would say add lime at the recommended rate and also Bonemeal . Lime will help with an acidity problem and Bonemeal promotes root growth.
    Living in Sheffield Yorkshire We have experiences both soggy and draught conditions this year with a damp and misty patch in August causing blight on Potatoes and outdoor tomatoes. I pulled up the tomatoes and compost binned them covering immediately with other waste and potatoes got dug early and carefully sorted. I found the Maris piper were affected more by the blight but Charlotte has a lower yield, We had consistent drought last year and the dams in the nearby valleys ran dry, as did my rain barrels, though I only watered in the green house.

    • John Harrison says:

      PH tests are available from £2.50 for 2 tests, hardly expensive. Soils vary and Trevor may well be on a calciferous soil so liming would not be helpful.

      • bob HASTINGS says:

        Hi John

        Ive been looking at soil testing probes on Amazon , there is one

        Are these any good ?

        • John Harrison says:

          Hi Bob – they must work as they’re readily available in different makes etc. but I’ve never got on with the meters. I think the chemical kits are more accurate.

          • Derek Bull says:

            I’ve tried various PH meters and they usually work, but they are not accurate as far as I’m concerned because you can go from acid to alkaline within inches apart with them.
            The only way is to bite the bullet and do a proper NPK test, which will take the weekend by time you take samples, dry them, grind them and then test each one! but by God it makes a difference!
            I found my whole plot was depleted of everything which was the reason that my crops were scrawny at best and a waste of time overall.
            Last winter I set about remedying it with garden compost 3 inches deep, dried manure 2 inches deep, the correct amount of separate minerals per square meter all rotovated in to the top 6 inches and then in the spring a week before planting a dusting with lime. This resulted in, not exhibition quality crops but sooo much better than the last two years I’ll be doing that again this winter as it’s written that it’ll take 3 years to get your ground back after depletion.
            As far as rain in Wiltshire goes, I used 3 thousand litres from my tanks in a fortnight! just as the plants were getting going which saw them through to full size producing a shed full of squashes and onions for this winter.

  4. David Shaw says:

    A common cause of poor potato crops is Potato Cyst Nematode. This sucks the food out of the roots of the potato and multiplies. Roots become infested with brownish cysts and the potato plants yellow and die. Each year potatoes are grown further increases the population of the nematode and symptoms get worse. No cure but best not to grow on that land again as it survives for more than 10 years and often a lot more. Another cause can be Early Die Back caused by a soil fungus. This causes the land to become potato sick as the fungus survives to infect the following year so a long rotation is called for. And do not save your own seed.

    • John Harrison says:

      I hadn’t thought of PCN which could hit other root crops but I think early die back is pretty specific to potatoes.
      Nice to have another potential suspect! Thanks

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