Nitrogen: the key to high cabbage yield

A shortage of nitrogen is the single most common reason for a cabbage crop not reaching its full yield potential. This applies equally to the quantity applied as to how to conserve it and maintain its correct level.

Applying sufficient nitrogen will help the lower leaves grow. In turn, this will ensure a good-sized head.
Photo: Bill Kerr

Nitrogen (N) comes in different forms and can easily be leached from the soil. We should anticipate leaching and take appropriate measures when it happens. A wait- and-see approach will cause a loss in yield. All the required nitrogen can be applied at planting and some farmers do so successfully. But they may lose out badly if heavy rain leaches the nitrogen from the soil.

How much nitrogen is required? Usually around 200kg N/ha. It depends on how much is already present in the soil before fertilising as well as how much nitrogen is tied up in organic matter and becomes available when the organic matter mineralises, which in turn depends on weather conditions. Many variables are involved. But with a significant difference in profitability between an average crop and a good crop, rather disregard these variables and stick to the recommended dosage. In other words, don’t stint on nitrogen!

A major mistake
Some farmers believe that half the nitrogen should be applied before transplanting, and that all of it must be applied before 28 days after transplanting. They also think that any nitrogen applied subsequently will have no effect. All of this is nonsense emanating from flawed trial work. It has led to great production losses over a number of years. In the trial where the ‘advice’ originated, applying a large amount of nitrogen at the start in reality gave the same effect as applying a small amount near the plant at a young age.

You can apply 100kg N over the entire area, mixed in to a depth of 20 cm. But it will take many weeks for the roots to reach all of this. Once converted from the ammonia form to the nitrate form, which may take place in a matter of days, it is vulnerable to leaching with heavy rain, even in a day or two. If it does not rain heavily during the trial, you can expect good results. But next year, under the same management, the yield might be disastrous. It all depends on the rain and on irrigation. It is possible to lose all the nitrogen before the roots can get near it.

The amount of nitrogen that can be taken up by a plant’s roots depends not on the amount of N in the soil but on its concentration in the soil solution. Band-placing 30kg N in a strip 10cm wide near the plant five days from transplanting is equivalent to 180kg broadcast in its effect on the concentration in the soil solution where the plant roots reach. Applied as LAN, half will be in the ammonia form, which is immobile and not accessible to the plant until converted, while the nitrate fraction will be immediately available to the plant. The two forms provide an element of safety in that half the nitrogen cannot be leached immediately.

A simple method
When a root system has spread widely, the individual roots can reach more nitrogen. The newly transplanted cabbage requires very little nitrogen as its biomass is so low, but it still needs the higher concentration in its root zone to stimulate it to form a frame that can develop to a maximum size head. The most economical way to work with nitrogen is to apply 2g to 3g of LAN near to, but not touching, the stem of the young plant. The saving seldom justifies the cost of labour of doing so, however.

Rather cut a hole in the corner of an empty fertiliser bag, tie a length of pipe into the hole, and roll the bag down halfway. Then walk down the row and direct the trickle of fertiliser to a band near the plants. One worker can do one hectare in a day this way.Alternatively, devise an implement for a tractor to band-place the fertiliser alongside the row. You can even apply it over the plants if the leaves are dry and irrigation follows shortly afterwards. Some farmers simply broadcast a large amount of fertiliser over the entire area, using enough to get the concentration right and taking a chance on not losing fertiliser between the rows to leaching.

How to apply LAN for the best results
Three applications of LAN are required. The first must be within the first week of transplanting. Apply 100kg/ha LAN close to the plants.The second application should come two weeks later. It does not have to be close to the plants; broadcasting will be effective at this stage. Apply about 200kg/ha. The third application should follow two to three weeks later. Apply 200kg/ha to 250kg/ha.

A fourth may be necessary depending on rain and conditions. More applications provide a greater level of protection against leaching. Keep an eye on the colour of the crop at all stages. This will show if you are not providing enough nitrogen. In summer, if you apply LAN 10 days before heavy rain, you should assume that the ammonia fraction has already converted
to nitrate and there is a good possibility that it has all leached away. Rather than wait and see if this is so, re-apply nitrogen. This is better than losing yield.

Manure and nitrogen

If you are using cattle manure, even if you have calculated how much nitrogen you have applied, take into account that this nitrogen will become available over a long period. The length of this will depend on soil and climatic conditions. On the plus side, the nitrogen leaching is slowed down. Most farmers prefer to work with recipes and formulas to make management and planning easy. With nitrogen, however, we have to work with experience and observation to ensure successful cabbage farming.