[adri123]

How long should a 25kg bag of Growmore last?  I've got 8 x 8ft by 4ft beds and wouldn't mind applying some fert through the season but not if it's going to cost an arm and a leg.

TIA

Adri

[Salmo]

OK - I wasn't great at maths, even when I needed to be (which was a long time ago), but I think this is correct:

Formula (per 100g of made-up fertiliser): grams of each ingredient =  required %/actual % X 100

For the 6:10:10 potato fertiliser this equates to 29/56/20 grams of the NPK ingredients respectively (total 105 grams)

For the 6:8:15 onion fertiliser it equates to 29/44/30 (total 103 grams)

NPK analysis is based on the proportions by weight, so given that the ingredients are likely to be of different densities they would need to be weighed rather than measured by volume, although it's not an exact science and would probably make no difference in practice - how much you apply, your soil, and the weather will all have a bearing on how it's actually used by your plants.

Thanks, great stuff.

The sums are correct.
Fertilizer is not an exact science and the NPK ratios are only a guide.
In practice the 6-10-10 could be achieved with 3kg of amminium nitrate / 6kg of superphosphate / 2kg of sulphate of potash. Similarly 6-8-15 with 2kg/3kg/2kg. It is the ratios that are important.

[JimB]

Another reply

A few years ago I got from a farmers supply company (sadly shut down now) 25kgs of Special potato fertilizer   ( N:P:K 15:21:24.5 ) it appeared to work O'K , though I have never been able to get any more!

One final thought, it would appear that when putting on ferts etc, we and that includes myself, are heavy handed and apply far to much, we do not read the labels and follow instructions about how much to use!

[Nobbie]

How long should a 25kg bag of Growmore last?  I've got 8 x 8ft by 4ft beds and wouldn't mind applying some fert through the season but not if it's going to cost an arm and a leg.

TIA

Adri

At 4oz per sq yard you would get about 8 applications from a 25Kg bag if my maths is correct

[Steve.B]

Hi All, thanks for the past info from above.....

Next question, am I right in saying......

Sulphate of Ammonia adds Nitrogen and lowers Ph
Nitrate of Soda adds Nitrogen and raises Ph
Lime raises Ph

So instead of Liming for Brassicas and then adding Nitrogen (Sulphate of Ammonia)

Just add Nitrate of Soda? or is that too simple.

[JayG]

Understanding soil chemistry requires a good knowledge of organic and inorganic chemistry, microbiology, geology, climatology and probably several other "ologies" I can't think of at the moment!

I only know a little about one or two of those, but my guess would be that using sodium nitrate to raise the pH for brassicas by the amount usually recommended to deter clubroot would seriously overdose them with nitrogen and might well upset the balance of the soil in other ways too.

[Yorkie]

JayG has a good point - overdosing with one element can have a detrimental effect on the plant's ability to unlock / take-up other elements from the soil.  e.g. if you apply too much potash, the plant cannot access nitrogen.

[Steve.B]

Thanks for that.

[Goosegirl]

Hi All, thanks for the past info from above.....
Next question, am I right in saying......
Sulphate of Ammonia adds Nitrogen and lowers Ph
Nitrate of Soda adds Nitrogen and raises Ph
Lime raises Ph
So instead of Liming for Brassicas and then adding Nitrogen (Sulphate of Ammonia)
Just add Nitrate of Soda? or is that too simple.

Yes - lime increases soil pH - good for brassica family but only if the soil needs it. Giggling this for you, I have found that "the primary use of ammonium sulfate is as a fertilizer for alkaline soils. In the soil the ammonium ion is released and forms a small amount of acid, lowering the pH balance of the soil, while contributing essential nitrogen for plant growth. The main disadvantage to the use of ammonium sulfate is its low nitrogen content relative to ammonium nitrate." Sodium nitrate seems a bit more complicated, but see what you can find on it. Peronally, what I would do to make life more simple for you is to get some Growmore, then put a measured quantity into a couple of buckets. Then add more nitrogen or whatever as appropriate. You can easily see the ratios on pre-packaged specific feeds so you can use the same measure to add the correct ratio of phosphate and/or potash as needed. With the best of intentions, is there anyone near you who grows good veg and from whom you can get local advice from? It may be the easiest option for you.

[Steve.B]

Thanks Goosegirl,  the only reason was to see if you could raise the Ph without liming. My soil is around 7.5-8 Ph so good for Brassicas.
But last year was very bad for my beans and peas, very weak and yellowing leaves and not a great deal of growth. As they say these do not need Nitrogen since they fix most from the air. But I had to buy a test kit to find out what was the possible problem was/is so I can try to fix it. Put a 6" covering of compost on most beds and a 6" cover of well rotten manure on the potato bed, this I think will need some Sulphur to drop the Ph a little.
My readings are Ph 7.5/8 Nitrogen 0 Phosphorous med and Potassium med not too bad. Did notice that the beds I composted for the last two years held the water better and my pea/bean bed was a bit dryer as it was not composted the year before. Can't wait to get back on the plot, when is this rain ever going to end........
Bye the way our plot came 1st on our site of 178 plots so must be doing some thing right, always room for improvement.