The Sunday Mail
Farming Peter Gambara
WEATHER forecasts say most areas are likely to receive rains almost daily from now until Christmas.
That might mean some areas might receive too much rain and crops might end up being leached or become yellowish. This leaching is worse where a farmer did not apply enough basal fertiliser to a crop.
Leaching is the loss of water-soluble plant nutrients from the soil, due to excessive rain or irrigation and the heavier the rain or irrigation, the worse the leaching.
The most water-soluble plant nutrient that is easily leached is nitrogen. Whilst most basal fertilisers contain nitrogen, the nitrogen level in most basal maize fertilisers is usually low.
It is therefore necessary for farmers to add nitrogen to a crop as it grows so that it remains green.
Even if a farmer used adequate basal fertilisers at planting, heavy rains are still likely to leach the fertiliser and it becomes necessary for farmers to come in with an additional top-dressing fertiliser.
Where a maize crop shows signs of leaching, a farmer can apply some top dressing at two weeks after emergence.
Besides addressing the yellowing, this should also boost the growth on the crop. A farmer can then apply additional top dressing at four, six or eight weeks’ stage as required.
The crop will definitely be able to absorb more of the nitrogen if it is split, rather than applied all at once as the expected heavy rains this year are likely to easily wash or leach it down.
Nitrogen requirements in maize
Most farmers associate top dressing a maize crop with tasselling and cob formation. It is true that maize requires only a fraction of its nitrogen requirements during its early growth stages, but its needs escalate rapidly once the crop reaches 8-leaf growth stage (knee height).
Maize can grow from knee height stage to shoulder level (approximately 12 to 14-leaf stage) in about two weeks and reach tasselling/silking stage in another two weeks if conditions are favourable.
Such rapid growth requires a large supply of nitrogen; hence farmers should ensure that their maize gets enough top dressing by the time it gets to knee height.
Maize generally requires over half its nitrogen supply between knee height stage and tasselling stage, a period of approximately 30 days. This means if you fail to top dress the crop between knee height and tasselling stage, you will have deprived the crop of over half its nitrogen requirements and that has serious consequences on the yield level.
The maize stalk becomes thin from knee height upwards and a thin stalk will not be able to carry a heavy load of a well-developed cob let alone two cobs per plant.
A maize plant’s need for nitrogen does not end at tasselling. In fact one third of the plant requirements must still be met by uptake during the kernel/grain development stage.
Nitrogen is actually crucial during ear and kernel development. If the plant fails to get this nitrogen from the roots, it cannibalises the lower leaves to provide nitrogen for the kernel development.
This explains the yellowing of leaves that is sometimes seen during that stage.
Choice of top-dressing fertiliser
There are mainly two fertilisers normally used as top dressing by farmers in Zimbabwe, Ammonium Nitrate (AN) and Urea. Urea has a very high nitrogen content of 46% (compared to 34.5% for AN).
Whilst a lot of farmers are familiar with using AN as a top-dressing fertiliser, not many are knowledgeable about the proper use of Urea as a top-dressing fertiliser.
Whilst AN can be easily placed onto the ground next to the maize plant, Urea has to be used differently.
Some of the Urea that is available on the market is prilled, or has been made into granular form and it looks whitish like AN.
A lot of farmers have therefore been tempted to just place on top of the soil next to the plants as they do with AN. Urea that is placed like that, risks just volatising 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 applying the urea, one will be able to smell the ammonia gas as it vaporises.
It smells like some rotten substances. It is therefore important for farmers to note that Urea should only be applied under moist conditions.
Whereas in most instances the effects of AN are visible within a week from application of the fertiliser, as the crop turns green, it takes time for the applied Urea to take effect.
With some moisture, Urea will be converted to ammonia (NH3) or ammonium ions NH4+).
Whilst 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 and explains that slow effect. It is therefore true that under extremely moist conditions, Urea will slowly release nitrogen, unlike AN, which is easily soluble and can be easily leached by heavy rains.
The key to the efficient use of area is therefore to incorporate it into the soil and to apply it under moist conditions.
Top dressing application rates
The amount of top-dressing fertiliser required in a sandy or sandy loamy soil is generally higher than what would be required in a red or clay soil and the best guided by soil analysis.
Most farmers in the high rainfall areas are applying 300 to 450kg of AN per hectare. Higher rates should be accompanied by either irrigation or higher rainfall received.
Higher rates will definitely give better yield responses.
Application methods
Under wet conditions, a farmer can use a tractor drawn Vicon fertiliser spreader to apply AN to a maize crop. However, it is important to stress that it is best practiced where some rains are expected, or a farmer can apply water through irrigation, immediately after application, as AN can burn the maize leaves if it stays on the leaves for long without being washed away. Farmers can also use the “Chola” method, where one fits a plastic tube at the corner of a fertiliser bag and simply drills the AN as one walks through the field.
However, it is risky, if workers who are engaged to apply the fertiliser are not visible whilst inside the field, especially if the crop has passed knee height stage.
If workers are not closely monitored, there is a risk of them hiding fertilisers inside or at the end of the field, which they will later come and collect and use on their small pieces of land.
With the “Chola” method, farmers should estimate the amount of fertilisers that a worker needs to go to the end of the field and come back to the re-filling point.
Workers who continuously fail to return to the refilling point are either applying too much fertiliser or are hiding some away. Ideally, some supervisors should follow these workers to ensure that the fertilisers are being applied correctly and that no thieving is occurring.
A worker working for eight hours a day can apply a minimum of four bags per day or 10 workers can apply 200kg AN per hectare to 10 hectares in a day, using the “Chola” system.
Farmers applying fertiliser to small plots are safer using fertiliser cups. Firstly, farmers should determine the number of bags they intend to apply per hectare.
Where a farmer is splitting 400kg/ha into two applications, it means the farmer is only applying 4 bags per hectare (200kg/ha) at a time.
To arrive at the size of the fertiliser cup to be used, one should first convert the fertiliser rate into grammes e.g. 200kg/ha X 1000grammes =200,000grammes/ha, then divide this by the plant population i.e. 200, 000 grammes/ha divided by 50,000 plants/ha =4 grammes per plant or Cup no. 8 applied per two stations or Cup no. 5 per plant to the nearest.
Since this is being done on a small plot, this is not likely to result in a major difference in the total fertiliser applied. However, at a large scale, this would result in a big fertiliser rate change from 400kg/ha to 500kg/ha.
Fertiliser cups are available for sale at most outlets that sell fertilisers.
They normally cost US$1 per any five cups chosen and are normally available in cup numbers five, eight, 16 and 32.
The proper timing, placement, choice and rate of application of top-dressing fertilisers to a maize crop can make a big difference in your yield at the end of the season. I hope this enables you to make the right decisions on top dressing your maize crop this season.
Peter Gambara is an Agricultural Economist and Consultant based in Harare.
