Page 62 - ALG Issue 1 2017

Nitrogen – how we manage it on our site
Selwyn Richardson is a retired Soil Scientist and Chairman of Hale Road A&GA at Swavesey in Cambridgeshire. He spent over 40 years with ADAS, the Government’s former advisory service for farmers and growers. His philosophy on crop nutrition is “keep it simple”.
Keep it simple
Nitrogen is an essential constituent of protein on which all life depends, whether human, animal or plant and the two inputs that have the greatest effect on crop yield and quality are i) water and ii) nitrogen.
The easiest and most straightforward way to supply all the nutrients required by crops on an established site is to use a dressing of 5 to 10 kg/sq. metre (9 to 18 lbs/sq. yard) of horse manure, cattle manure, pig manure or compost (soil improver) applied every four or ve years in the rotation, preferably in front of potatoes or brassicas. Then boost the nitrogen supply by using nitrogen fertiliser each year. It really is that simple, as we know from four years of experience on our site between Cambridge and Huntingdon.
These bulky organic manures will
supply all of the phosphorus, potassium, magnesium, sulphur and trace elements needed by all the crops grown over the rotation, but not enough nitrogen for most crops. At the same time they will maintain a suitably high level of organic matter in your soil which will improve workability and drainage on heavy soils, improve drought resistance on light soils and support biological activity on all soils.
Be aware that although it is commonly assumed that legumes do not require added nitrogen, this is certainly correct for peas and broad beans as they don’t need any but dwarf beans and runner beans actually require as much nitrogen fertiliser as any other vegetable crop.
Nitrogen availability to crops
Crops take up their nitrogen from the soil and they absorb most of it in the form of nitrate (NO3), although they can absorb some as ammonium (NH4). Farmers in the UK mainly use ammonium nitrate fertiliser because of its high content of active ingredient (34.5% N) but this is not easily obtained by gardeners. It contains equal amounts of nitrate-N and ammonium-N
and the ammonium fraction is converted
to nitrate by soil bacteria within days of spreading. However, nitro-chalk (26% N) and ammonium sulphate (21% N) can easily be obtained and nitro-chalk is just a physical mixture of ammonium nitrate with chalk. Ammonium sulphate (sulphate of ammonia) contains all its nitrogen in ammonium form but again, this is quickly converted to nitrate in the soil.
Bulky organic manures contain a large amount of nitrogen but it is slowly released to crops because most of it is bound up with the organic fraction such as lignin and cellulose. Certain types of soil bacteria have to work hard on the organically bound N which is slowly released, mainly in ammonium form then their colleagues quickly convert it to nitrate. Actually, poultry manure starts with a much higher
ammonium-N content than other manures so it releases its N relatively quickly while
at the bottom end of the scale, compost releases N very slowly. Thus when manures, other than poultry manure, are used it is necessary to also add nitrogen fertiliser because the release of nitrogen is too slow to keep pace with crop requirement. When poultry manure is dried and pelleted most of the ammonium-N is driven off and the residual nitrogen content is only slowly available to crops.
Organic fertilisers
Organic fertilisers such as dried blood,
hoof and horn, and blood, sh and bone also, contain their nitrogen mainly bound in organic form so it is released slowly, except that some batches of dried blood can release nitrogen relatively quickly. Some of the organic binding is so complex that the rate of release of nitrogen is unpredictable. Added to this is the disadvantage that their nitrogen content is signi cantly lower than that of manufactured nitrogen fertilisers (sometimes called arti cial fertilisers). Thus, dried blood and hoof and horn both contain around 13% N plus some phosphorus, whereas blood, sh and bone contains
only around 5% N plus phosphorus and potassium. They are also commonly more expensive per kg to purchase and hence more expensive per unit of active ingredient. Although they add some organic matter, the amount added is very small when spread
at recommended rates of around 140 gms/ sq. metre (4 oz./sq. yard). Since it makes good sense to use a source of nutrients which is both effective and cheap, we prefer manufactured nitrogen fertiliser.
Editor’s note
De ciency symptoms in crops
On most crops the symptoms of nitrogen de ciency are not obvious because the crop simply lacks vigour and has smaller, pale green foliage. However, on brassicas such as cabbage, Brussels sprouts, cauli ower, swede and turnip it is more distinct with the older, lower foliage turning yellow then purple or pink as in the image of spring cabbage.
Which nitrogen fertiliser to use?
The two obvious, low cost nitrogen fertilisers which rapidly and ef ciently supply nitrogen to crops are nitro-chalk and sulphate of ammonia. Ammonium sulphate has a mildly acidifying effect on soil so if your soil pH
is above 7.0 it is marginally better to use ammonium sulphate.
How we apply nitrogen at
Hale Road allotments
Although gardening books usually recommend thoroughly mixing either sulphate of ammonia or nitro-chalk into the soil before planting, typically at about 1 to 2 oz./sq. yard (35 to 70 gms/sq. metre), we nd that potatoes are the only crop where it is important to mix in N fertiliser before planting. For all other crops we nd it is better targeted and more economical to dissolve the N fertiliser in water and apply it as a liquid feed during the growing season.
Make a concentrated stock solution
by putting about ve or six handfuls of N fertiliser into a 1.5 litre bottle, ll with water and shake to dissolve. Don’t worry about any precipitate that forms - nitro-chalk
will produce a sludgy chalk deposit in the bottom of the container but the nitrogen itself will fully dissolve. Drill your parsnips, beetroot, lettuces, leeks, carrots, sweetcorn etc. into a seedbed without added N. When they are half an inch to an inch tall, put two or three tablespoons of the stock solution into a watering can and ll with water to make the diluted liquid feed. Water fairly heavily over the top of your plants along the row with this liquid feed. When the plants are young it is essential to quickly wash
off the feed using plain water otherwise it can scorch them, especially on a hot day. Most semi-mature plants will tolerate this feed without washing off except on very hot, sunny days. Potato and lettuce foliage is sensitive so always wash it off these two crops.
For transplanted crops, water fairly heavily around the stem base with this liquid feed immediately after planting. Most vegetables will bene t from at least one more N feed during the season but all brassicas, peppers, courgettes, sweetcorn, tomatoes and runner beans will need two or three more additional feeds.
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Great care should be exercised when applying nitrogenous fertilisers. Nitrogen is very mobile and can leach out of the soil, contaminating ditches, watercourses and rivers. If it is applied at too great a level, it can cause accelerated growth in plants, but at the same time can make some micro nutrients de cient. Always carry out a simple soil test before applying any nitrogenous fertiliser to determine the quantity required to support the type of crop being grown.