The Sunday Mail
Agriculture
Peter Gambara
While going about my work in town last week, I came across many farmers who sought to buy foliar sprays to address nitrogen deficiencies in their crops in the face of the current incessant rains.
Most parts of the country have been receiving rains since mid-December last year, with few breaks in between.
These rains have been intensified by two cyclones, “Anna” and “Batsirai” that have or are about to hit Zimbabwe from Mozambique.
Plants require a total of 16 nutrients for their growth.
While three of them, carbon, hydrogen and oxygen are available abundantly from the atmosphere, the remaining 13, which include nitrogen, phosphorus, potassium, magnesium, calcium, sulphur, iron, zinc, boron, copper, manganese, molybdenum and chlorine can be applied to the soil or through the foliage of the plants.
This application of the nutrients to the foliage is called foliar spray.
Leaching and yellowing of leaves
Too much rain tends to leach nitrogen in most soils, but this is worse in sandy to sandy-loam soils.
Crops like soyabeans and sugar beans respond by showing yellow leaves.
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.
Out of the three macro-nutrients, nitrogen, phosphorus and potassium (commonly indicated as N, P and K on fertiliser packs), nitrogen is the most water-soluble plant nutrient and therefore leaches easily.
This article, therefore, seeks to provide answers and general information on the use of foliar sprays to address nitrogen deficiencies in major crops, like soyabeans and sugar beans.
Nitrogen fixation by leguminous crops
Both soyabeans and sugar beans are classified as legumes and can fix their own nitrogen.
This is the reason why farmers are encouraged to add some rhizobium to their seed at planting.
The rhizobium will aid the nitrogen fixation process.
However, several environmental factors can limit the symbiotic relationship between legumes and rhizobium.
These include nutrient deficiencies, insufficient or excessive moisture, unfavorable pH, among many other factors.
All these factors are known to reduce nodulation or the nitrogen fixation process.
Farmers are known to apply foliar sprays to address both micronutrients (minor elements like boron, calcium etc) and macronutrients (nitrogen, phosphorus and potassium) deficiencies in both crops and fruit trees.
Whilst micronutrients are usually applied in small amounts, they are essential to some plant processes.
Their absence might lead to deformities.
Farmers should, therefore, be careful as to which foliar spray they are applying.
Is it for macro-nutrients or macro-nutrients like nitrogen?
It is important to stress here that most macro-nutrients should ideally be applied at planting.
However, most basal fertilisers have low nitrogen levels and hence, it is desirable in most crops to come in with a top dressing application of a nitrogenous fertiliser after the crop has already emerged, usually towards flowing/tasselling or when the crop shows nitrogen-deficient characteristics, like yellowing of leaves.
Understanding the absorption process of foliar applications
Farmers should be aware that plants, in general, prefer to absorb nutrients through the root system and that leaves were never meant to be used for the uptake of nutrients.
Leaves are, therefore, designed in such a way that they minimise exchange of water with the environment.
Leaf surfaces are therefore equipped with water repellent attributes, which resist the penetration of mineral nutrients.
The pores on the leaves (called stomata) are also protected against the infiltration of liquids like water.
This means your foliar-applied nutrients have to overcome these natural barriers of the leaves.
Leaves will only absorb these nutrients if there is a concentration gradient across the leaf surface.
This means there is a limit to which leaves can absorb nutrients that are applied on the crop as a foliar spray.
However, foliar sprays offer rapid means of addressing nitrogen deficiencies in crops, hence the scramble for some by farmers recently.
If the required penetration levels are too high, the leaves might actually get scorched.
Most nutrients are applied as salts and if applied in too high concentration, they might burn the leaves.
In simple terms, you should therefore know how much to apply, so that the penetration levels are not too high for the leaves.
Farmers are therefore recommended to consult an agronomist of the foliar spray manufacturing company to be given the rate of application, as well as how frequent the foliar spray can be applied.
Frequency and rates of application
The frequency and rates of application of foliar sprays depend on the needs of the crop and this is in turn determined by the deficiency symptoms, the age of the crop, and the growth rate of the plants.
Most companies recommend that you apply 2kg of a granular foliar spray per hectare or one litre per hectare of a liquid foliar spray and this should be spaced at least two weeks apart.
Again, this is just a general recommendation, read the label and understand it or consult an agronomist before applying the foliar spray.
Effectiveness of foliar applications
Farmers should also be aware of the factors that influence the effectiveness of foliar applications. If runoff from the leaves is high, that means a lot of the foliar is going to waste.
However, it might not be a complete loss, if it is nitrogen-based because the liquid that falls to the ground might still get absorbed through the root system.
Just like in the use of herbicides, farmers should time their applications, such that there is a minimum time period before the rains come back.
Therefore, seek information on what the weather forecast says on that day before you move in to apply a foliar application.
Farmers should also avoid applying foliar sprays during periods of high temperatures.
High temperatures will cause the sprayed foliar to quickly dry off the leaves and that means the liquid had very little time to be absorbed by the leaves.
It is advisable, therefore, for farmers to apply these foliar sprays early in the morning before temperatures start going up.
Alternatives to foliar sprays
Nitrogen deficiencies in crops like soyabeans and sugar beans can also be addressed through the use of nitrogenous fertilsers like Ammonium Nitrate and Urea.
Several studies have established that mineral nitrogen (as supplied by AN for example) in the soil affects both nodulation and nitrogen fixation.
Therefore, should you wish to use fertilisers like AN to address nitrogen deficiencies in soyabeans or sugar beans, use them sparingly, so that they do not adversely affect the nodulation process. The recommended rate in soyabeans is not more than 50kg of AN per hectare.
Maize top dressing
Whilst this article was mainly on the use of foliar sprays in filed crops like soyabeans and sugar beans, let me just also take the opportunity to talk briefly about top dressing in maize.
Maize is the major crop grown in Zimbabwe and obviously the recent rains have also had an effect on the crop.
Whilst most of the early planted crop ranges from knee height to tasselling, the late December planted crop is still below knee height and most of it has been badly affected by the rains.
This year, there was a shortage of top dressing fertilizers like AN and where it was available, it was very expensive.
Fertilizer manufacturing companies claim the materials used to manufacture AN have since gone up globally and therefore the price of a 50kg bag is now around US$42, and the Zimbabwe dollar price depends on the exchange rate being used by the retailer.
Whilst many farmers have been using AN for some time now and are familiar with its placement, the same cannot be said of Urea.
Whilst AN can be placed onto the ground next to the maize plant, urea has to be used differently.
The Urea available on the market is prilled or has been made into granular form and it looks whitish like AN.
Some farmers, therefore, tend to think it can also be placed just like with AN.
Urea that is placed like that, will just volatilize 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 conditions.
Urea should therefore be covered as much as possible or placed on a wet surface.
I have heard many farmers say it is best to use urea when we have incessant rains like now.
Well, let me explain a bit.
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 urea to take effect.
When there is some moisture on the ground, the 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 the slow effect of urea after it has been applied.
Therefore, urea tends to release nitrogen more slowly than AN.
Peter Gambara is an agricultural economist and consultant based in Harare.
