The Sunday Mail
Peter Gambara
It is that time of the year again when maize in most parts of Zimbabwe is due for top-dressing fertiliser.
This year has, however, witnessed a shortage of top-dressing fertilisers as the area planted to maize increased mainly due to the Command Agriculture programme.
Demand for such fertilisers has also increased on the back of heavy rains as some crops are leached.
One of the top-dressing fertilisers available on the market is urea. It seems, though, that many farmers are not quite aware of how it should be used.
I have met a lot of farmers who have either used or intend to use urea as top-dressing fertiliser, but are not conversant with its application.
Here, I give the low-down in terms of use and risks involved.
Urea, also known as carbamine, is an organic compound widely used as a nitrogen release fertiliser in the agriculture industry.
At 46 percent (compared to 34,5 percent in ammonium nitrate), it has the highest nitrogen content among all materials used as nitrogenous fertilisers.
It is, therefore, more economical than AN if correctly used.
While a lot of farmers are familiar with the use of AN as a top-dressing fertiliser, which can easily be placed on the ground next to the maize crop, urea has to be used differently.
Some of the urea available on the market is prilled, or has been made into granular form and looks as white as the usual AN.
A lot of farmers are tempted to just place it on top of the soil next to plants as they do with AN. If used in that manner, urea risks volatilising into the air as ammonia gas.
This is especially true if it remains on the soil surface for extended periods during warm and dry weather.
Urea will start to breakdown as soon as it is applied to the soil. However, if the soil is dry, no reaction occurs.
The ammonia in the urea will simply escape into the air.
If one comes back onto that land a few hours after application, one will smell the ammonia gas as it vaporises. It smells like some rotten substance.
It is thus important for farmers to note that urea should only be applied under moist conditions.
With a bit of moisture, urea will be converted to ammonia (NH3) or ammonium ions (NH4+). While plants readily absorb the latter, the former can easily escape into the air as ammonium gas.
This conversion will take two to four days to happen.
It is, therefore, true that under extremely moist conditions, urea will slowly release nitrogen, unlike AN, which is easily soluble and will have quick effects.
Whereas in most instances the effects of AN are visible within a week from application of the fertiliser, as the crop turns green, some farmers have wondered why it seems to take ages for the applied urea to take effect.
Where the urea has been applied correctly, the above explanation details that slow release.
However, it could also be because the urea was not incorporated and will have vaporised and very little or no nitrogen found its way into the soil.
The key to efficient use of urea is to incorporate it into the soil and to apply it under moist conditions.
One option is for a farmer to dig a small hole next to the plant or, alternatively, use a sharpened long stick to make a hole next to the plant and place the urea into that hole.
Some small-scale farmers use an ox-drawn cultivator or plough to cover if after placing it on the ground.
That way it is unlikely to escape into the air and will react with the moisture in the soil, producing the nitrogen required by the plants.
Holes should be covered with soil after placing the urea.
The stick method becomes more economical as the person placing the urea into the hole can simply step on the hole to close it.
That minimises the labour required to apply the fertiliser.
However, it remains that the use of urea instead of AN is indeed more labour-intensive and not ideal for farmers with large areas.
A lot of farmers have struggled to secure AN this season and have to apply it when the crop is almost tasselling.
It, therefore, becomes impossible to get in with a tractor-drawn vicon fertiliser spreader.
In such situations, farmers have to resort to the “Chola” method, where workers fit a plastic tube on the corner of a fertiliser bag and simply drill the AN as they walk through the field.
However, it also becomes risky, as workers who are engaged to apply the fertiliser are not visible in the field.
If these workers are not closely monitored, one risks having fertilisers hidden inside the field and then later taken for other purposes.
Farmers should estimate the amount of fertilisers that a worker needs to go through to the end of the field and come back to the re-filling point.
Farmers should also be aware of the number of bags they intend to apply per hectare and continuously assess the situation.
A worker working for eight hours a day can apply a minimum of four bags per day or 10 workers can apply 200kg per hectare of AN to 10 hectares in a day using the “Chola” system.
Ideally, supervisors should also follow up to ensure fertilisers are being applied correctly and that no thieving is occurring.
Heavy rains call for split application of top-dressing fertilisers. If applied once, crops will definitely change to green.
However, with incessant rains, the crop might still turn yellowish later as nitrogen is leached downwards.
I know some farmers have faced challenges in securing top-dressing fertilisers and will therefore argue that they cannot split application as their crop is almost tasselling.
With the current moisture, splitting applications even one to two weeks apart will make a big difference.
The crop will definitely be able to absorb more nitrogen if it is split, rather than applied all at once and it is raining heavily.
It will be easily leached.
It is essential that a maize crop is greenish at cob formation and filling. The green leaves will be able to absorb more light and manufacture more food for the developing grain and hence higher yield will be achieved.
A crop that fails to get any top-dressing fertiliser before tasselling risks having cobs with very little grain or small cobs altogether and hence low yield.
Mr Peter Gambara is an agricultural economist and consultant based in Harare
