Carrot Country

Optimizing Nitrogen Fertilizer Application

Fall 2006
Carrot Country

By Sean Westerveld, Alan McKeown and Mary Ruth McDonald

Strange as it may seem, carrots can often be grown without nitrogen fertilizer and still produce high yields. This happens most often in temperate climates such as the northeastern United States, Canada and
northern Europe. However, nitrogen fertilizer may still be important. Applying nitrogen fertilizer usually reduces carrot leaf blight. Furthermore, applying nitrogen fertilizer can increase yields if carrots are grown on sandy soils and if there is heavy rainfall. Consequently, to get the most out of nitrogen fertilizer, we need to understand when the carrot plant takes up nitrogen and when is the best time to fertilize.

Five Years of Research
We have conducted experiments over a five-year period on the effects of applied nitrogen on carrot yield, quality, rooting depth, nitrogen uptake and growth. In addition, we
have done tissue and soil nitrogen analyses to track nitrogen availability in the soil and uptake over the various growing seasons. The timing of nitrogen application has also been examined. This research has been used to propose a timeline of nitrogen uptake and carrot development over the growing season. Carrot growth is slow and nitrogen uptake minimal during the fi rst month of growth. However, this is the time that the eventual length of the harvested root is determined. The taproot can reach 40 cm depth by one month after seeding and it is essential to avoid root damage during this period. While nitrogen is required by the plant at this early stage, there is usually suffi cient residual nitrogen in the soil during this period without the need for nitrogen fertilizer.

During the second month, leaf growth takes priority. Nitrogen uptake begins to increase and much of the nitrogen is used for leaf growth. During this period, the leaves that form the canopy later in the season get their start. The research suggests that if the plant does not have suffi cient nitrogen at this stage, leaf blight damage can be more severe later on. The plant obtains this nitrogen from
the top foot (30 cm) of the soil, where most of the roots are located at this time. Our research suggests that more nitrogen is required for the leaves to defend against leaf blight than for optimal yield. It is the fertilizer nitrogen that is applied before seeding that seems important for optimal leaf blight defence. Even though most of the roots are in the top 30 cm of the soil at this time of the season, the taproot can
extend to 60 cm depth by 60 days after seeding.

Beginning at 60 days after seeding the carrot root begins to grow at a rapid rate, and this growth continues steadily until harvest (Figure 1). It is at this time that most of the nitrogen is required for maximal growth of the carrot, since nitrogen uptake also increases rapidly at 60 days after seeding (Figure 2). Since the root system extends well below 1 ft (30 cm) depth at this time, and preplant-applied nitrogen is
mainly located in top foot (30 cm) of the soil, nitrogen deep in the soil that is left over from previous seasons is the most important source of nitrogen for this stage of growth. It is only if there isn’t enough of this deep nitrogen that preplant or sidedress nitrogen has an effect on carrot yield.

Advantages of Preplant N
Preplant-applied nitrogen always has more effect on carrot yield than sidedress-applied nitrogen. This is because there is more time for preplant nitrogen to leach down to the depth of the active root system by 60 days after seeding, when the carrot requires the nitrogen. However, sidedress nitrogen application can increase yield if an early to mid-season deficiency in nitrogen occurs. To be effective there must be sufficient rainfall or irrigation to leach the nitrogen into the active root zone.

By 100 days after seeding, foliage production decreases and the plant is entirely focussed on the harvested portion of the root (Figure 1). The taproot can reach 5 ft (150 cm) depth during this period, and more than half of the nitrogen uptake occurs below 1 ft (30 cm) depth if suffi cient nitrogen is available from this depth. Leaf blight can become severe during this period, but is reduced in plants supplied with optimal preplant nitrogen earlier in the season. There is nothing that can be done at this stage to correct nitrogen defi ciencies in time to have an effect on yield.

This research suggests that it is important to know how much nitrogen is available from the soil before seeding, in order to adjust nitrogen application rates. However, a traditional soil nitrogen test from
the top foot (30 cm) of the soil will not tell us how much nitrogen is available to the crop. A soil test to 2 ft (60 cm) depth is required to assess the available nitrogen. Regardless of soil test N levels, preplant-applied nitrogen is necessary to reduce leaf blight damage. Alternatively, leaf blight can be controlled with fungicides, and nitrogen can be applied only if necessary to produce optimal yield. Further research is required to develop a preplant soil nitrogen test to 2 ft (60 cm) depth. Sidedress nitrogen should be applied as a means of correcting nitrogen defi ciencies that are evident later in the season, but rainfall or irrigation is required for these to be effective. However, growers should be cautious when applying sidedress N or mid-season irrigation, because research has shown that the roots can split with a sudden increase in growth.

Benefits of Improved Management
Through more intensive nitrogen management of the carrot crop, carrot growers can often get high yields, and keep leaf blight at low levels, with less nitrogen or fewer fungicide
sprays. This can increase profi t, and the environment will benefi t as well. However, more research is required to identify the best way to incorporate deep soil sampling into a
carrot production system.

This research was funded by the Agricultural Adaptation Council through the support of the Ontario Fruit and Vegetable Grower’s Association, the Ontario Ministry of Agriculture, Food and Rural Affairs/University of Guelph Plant Program, and the Natural Sciences and Engineering Research Council of Canada. For more details about this study please contact Dr. Sean Westerveld or Dr. Mary Ruth McDonald at the University of Guelph, Dept. of Plant Agriculture, Muck Crops Research Station, 1125 Woodchoppers Lane, RR#1 Kettleby, Ontario, Canada L0G 1J0 or by email at swesterv@uoguelph.ca or mrmcdona@uoguelph.ca.