[] [] [index]

                                                                      How Green is your Grass?

Popular general purpose grass conditioners are sold as Spring and Autumn types.  They usually contain NPK in various proportions, but the Spring dressing contains more nitrogen than the Autumn type to provide fast, fresh green leaf growth. The Autumn dressing contains more phosphate to encourage root growth.  The proprietary compounds also usually contain ferrous sulphate (sulphate of iron) as a moss killer. The nitrogen part is often ammonium sulphate.

Now ammonium sulphate (sulphate of ammonia), and another high nitrogen fertiliser sodium nitrate (nitrate of soda) is also available. The ammonium sulphate is used to promote top growth in leafy vegetables, exactly as you would expect from a high nitrogen source.

 The interesting thing is that sodium nitrate  is promoted as good for beetroot.  I didn't understand this, but a little research quickly established that beetroot is a cultivated variety of the wild plant Beta vulgaris. Now, Beta vulgaris thrives on the sea shore, and is therefore very happy in saline environments.  Salt is a sodium compound, hence the connection with beetroot. (Beetroot is indeed a very interesting plant. It is really a biennial, and the 'seed' is actually a little ball of several seeds).

Sodium nitrate is commonly known as saltpetre, originally obtained from mines in Chile.

I wondered how effective each compound would be for promoting lush green grass, so I set up a little test on my lawn. The doses were not well controlled, but treating two small patches one with ammonium sulphate (21% nitrogen) and the other with sodium nitrate (16% nitrogen) soon produced nice green growth, especially after all the rain we have been having. Initially the  sodium nitrate seemed to produce a lighter green colour, but soon the difference between the two patches was negligible.

Ammonium sulphate is a brown hygroscopic powder, whereas the sodium nitrate  is supplied as small white granules, and is much easier to apply dry. Both dissolve readily in water. The ammonium is not as user friendly, and you have to be careful to use plastic containers to mix with water or it will leave a nasty stain. Price is comparable.

When the ammonium sulphate dissolves in water, it produces a very dilute sulphuric acid, so over a long time will also tend to acidify the soil and reduce the pH. This is fine for some plants, but you need to be careful if lime loving plants are grown. The result of using sodium nitrate tends be neutral, as dissolving it in water produces sodium hydroxide(an alkali) and a dilute acid.

I did a little experiment to assess the effects of the two chemicals on acidity. I dissolved a heaped dessert spoonful of each in about 240mL of distilled water.  The measured pH for the ammonium sulphate produced a pH of about 6.1, clearly acidifying the water.  The sodium nitrate solution had a pH of about 7.1, indicating a slight alkilinity.   So if you want a minimal effect on the soil it would be best to use sodium nitrate, but the advantage of using ammonium  sulphate on 'lime hating' plants is clear.

I should add here of course that the Soil Association is against the use of 'artificial fertilisers' on organic produce. They do permit the use of trace elements in some circumstances, and sodium in the form of ground salt is also allowed. They point out that a large proportion of the nitrogen in artificial fertilisers gets washed away before the plant can make use of it.  So always use anything sparingly!

Whilst doing a bit of research for this note, I found out that the 'Park Grass Experiment' was started at Rothamsted in 1856. This experiment divided an area of grass into many small plots and subjected them to different treatments. As far as I know, this is still operating, and the results are data for many research reports. There isn't much that's new!