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Soil Test Results

As I mentioned earlier, I got a proper soil test kit recently. Well the results are in and they’re pretty dismal. I took three samples all from under the turf, to allow for exceptional results. In fact the results are all very similar, which is also reassuring as it rules out testing errors.

The test kit enables me to measure the pH (acidity) and the NPK levels in the soil. There’s a series of articles on the site about fertilisers here: Fertiliser Use for Best Vegetable Results

pH Level (acidity)

This was the only test to show any variation between the three samples analysed. It’s actually a little better than I feared. Two samples showed a pH level of 5 and one showed slightly higher at 5.5. So it’s reasonable to assume a level of 5.17 (15.5 divided by 3)

Since vegetables generally grow best at around pH 6.5 and it’s a sandy soil, I’m going to need around 750 grams to a kilogram per square metre of ground limestone. That’s an awful lot, especially in one go, so it make take a while to get things up where they should be.

Nitrates

The next test was for nitrogen. No wonder the grass is slow growing, it was so low as to barely register! To take it up to something reasonable, I’d need to add 10.5 grams per square metre. With say, sulphate of ammonia which is 20% nitrogen, that’s 52.5 grams per square metre. Nitrogen is vital for leaf growth.

Phosphorus

Once again the reading was as low as the kit could show. 5mg/litre. To get this up to a reasonable level would take around 17 grams per square metre. Since superphosphate is 17% phosphorus that would be an application rate of 100 grams per square metre. Phosphorus is needed for good root growth, healthy buds and stems.

Potassium (Potash)

You guessed it, next to nowt. Lower than 50mg / litre. To take this up will need something like 16 grams per square metre. 100 grams of sulphate of potash which is 50% K. Potash is good for flowers and also helps plants resist infection.

Fertiliser Plan

Now I know that increasing the pH will make nutrients available that are locked up by the soil acidity, however there appears to be precious little to unlock. So the plan is lime heavily where I’ll be growing this autumn. Then fertilise in the spring.

I could make up a fertiliser by combining the elements above to the exact specification required. The problem here is nitrogen tends to wash out of the soil and so is best applied just before growing and intermittently through the season.

I like fish, blood and bone fertiliser which is NPK 5:5:5 so top dressing with that at a rate of 200 grams per square metre will give me 100% of required nitrogen, 60% of the phosphorus and about the same of potassium.

To cover the P (phosphorus) & K (potassium) will actually overdose the nitrogen. This is wasteful and not environmentally sound practice. So I’ll add some artificials to the organic FB&B to balance it and apply that.

That’s just for the land where I’ve no compost or manure to apply yet. Compost and manures don’t just supply NPK but also micro-nutrients and humus. It’s going to be a few years before I’ve enough and everything is in balance again.

Oh well, Rome wasn’t built in a day!

Posted in Allotment Garden Diary
4 comments on “Soil Test Results
  1. michael rolle says:

    Hi John I am quite new to growing veg, I have 5 large raised beds. I have tried to grow veg for 2 years now, some good results were achived like carrots but some did not grow very big like parsnips and onions. I compost kitchen waste which goes on and the other beds i used manure. Reading your article I have realised that I sould be adding other stuff as well, could this be the problem. The soil I got from a top soil suppler which I fear was lacking most things. I want to keep things organic so would adding those things you mention be non organic? I am now adding the straw and sawdust etc from my chicken coop to the compost will that help. Sorry for the long question.

    michael

  2. john foster says:

    Any wood or paper will reduce the free nitrate and ammonia in a compost for at least a year whilst it is rotting down.

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