Vegetable News April 2005

Nitrogen fertilisers

Nitrogen is an essential plant nutrient. Different nitrogen sources have different effects on crops and on soil.

Before applying nitrogen, growers should know crop needs, response time, effect on soil pH and how much nitrogen is likely to be lost by leaching.

Crops take up nitrogen from the soil either as ammonium (NH4+) or nitrate (NO3-). Both forms are present in the soil profile either from mineralisation (release) of organic matter eg crop residues, or as a result of soil transformations (nitrification). In the soil, microbial activity breaks down organic nitrogen or urea into ammonium-N, which is then converted to nitrite and finally to nitrate. Provided there is adequate soil moisture, breakdown will be quicker at higher temperatures.

Evidence suggests that NO3- N (nitrate nitrogen) is taken up less readily than NH4+N (ammonium nitrogen) under cold conditions. Nitrate fertilisers, eg calcium nitrate, are also more prone to loss by leaching or denitrification (microbial transformation to gaseous N). Non-nitrate fertilisers (eg those containing ammonia or urea) are therefore more suitable in base dressings, or when soils are cold, than nitrate forms.

However nitrate only fertilisers do not need nitrification so are available immediately to the crop. These fertilisers are best suited to top dressings in warm conditions when the crop is growing rapidly.

Effect of different forms of nitrogen on uptake of other nutrients

Nutrients dissolved in soil solution carry an electrical charge.

Those with a positive charge - NH4+ (ammonium), K+ (potassium), Ca++ (calcium), Mg+ (magnesium) and are called cations. Those with a negative charge, NO3 -, _ H2PO4_-, are called anions. When an anion is absorbed it carries with it a cation attracted by the electrical charge. Uptake of nitrate will therefore favour uptake of nutrients such as potassium, calcium or magnesium. Uptake of ammonium often competes with and suppresses uptake of these nutrients.

Effect on pH

Application of fertiliser containing ammonium-N, or urea, tends to acidify the soil due to both microbial nitrification and root uptake of ammonium-N. Over time use of such fertilisers will cause a lowering of soil pH, whereas calcium nitrate will, over time and repeated use, increase soil pH.

Leaching

Nitrate leaching occurs due to: -

mineralisation of organic matter in the autumn. With soil still warm and, especially if cultivation, which stimulates microbial activity, is undertaken, nitrate is released. This is leached out by rain washing through bare soils.
excessive application of nitrogen fertilisers, particularly at times when root activity is low and rainfall high.

Top dressing on an actively growing crop minimises nitrate loss as it is efficiently taken up by the crop.

Spray application on carrots and sprouts.

Syngenta’s spray technologist Tom Robinson has researched spray penetration and retention in both carrot and sprout crops.

From this work he has concluded the following: -

boom height needs to be 40 cm above target - at 70cm drift is double that at 40cm.
·wider angles/smaller nozzle /higher pressure all produce small droplets.
pressure has no effect on drop speed – so high pressure does not improve spray penetration in the crop
narrow drop speed is an important factor in spray retention
narrow fan angle reduces droplet speed
the lower the amount of water the better the retention
variable pressure nozzles work well at all pressures to maintain spray pattern (unlike conventional nozzles which don’t hold pattern at low pressures)

In carrots - there was no significant difference in spray retention at different water volumes but the best volume for foliage is probably 100l/ha. To get down to the crown of the carrot 300l/ha gave better results.

Narrow angle fan jet nozzles gave better retention of spray than hollow cone nozzles

In sprouts - 65º twin cap nozzles are recommended rather than conventional flat fan nozzles. Drop legs were found to give good spray coverage.

Lower water volumes (100l/ha) are advised during the early growth stages of sprouts.

Cabbage root fly and carrot fly forecast online

Forecast for egg laying of cabbage root and carrot flies will be available this season on the Internet through http://www2.warwick.ac.uk/fac/sci/hri2/hdcpestbulletin

DARD MTR workshops on Single Farm Payments

DARD is holding workshops for arable, potato and vegetable growers to deal with issues arising from the Single Farm Payments and IACS forms. Those most convenient for vegetable growers will take place on: -

Tuesday 26 April in the Strangford Arms Hotel

Monday 25 April in The White Horse Hotel, Limavady

Tuesday 3 May at Greenmount Campus.

All the meetings will begin at 8p.m.

This will be an opportunity for growers to clarify areas of confusion and resolve specific queries.

Pesticide Update

The following off label approvals have been issued since the last Vegetable News

Product	Active ingredient	Use	Crop	Off label Number
Favour 600SC	metalaxyl + thiram	downy mildew control	herbs including parsely	0312/05
Butisan S	metazachlor	kale, collard, chinese cabbage	weed control	0344/05
Plenum WG	pymetrozine	control peach-potatoe aphid	collards and kale	0385/05
Dow Shield	clopyralid	weed control	lettuce and leafy herbs including parsely	0473/05
Stomp 400SC	pendimethalin	weed control	peas	0520.05
Stomp 400SC	pendimethalin	weed control	lettuce and leafy herbs including parsely	0549.05
Aliette	fosetyl-aluminium	downy mildew	scallions	0548.05

Joan Hamilton

Edible Crops Development Adviser

Greenmount Campus, College of Agriculture, Food and Rural Enterprise

April 2005

Telephone (office) 028 9442 6683

Mobile number 077 7575 7251