Fertilizer placement and optimal time of application
By Derick Wessels and Kobus Coetzee (Agronomists: Western Free State)
The recent droughts and increasing input costs have forced the modern grain farmer to work more efficiently. Fertilizer is one of the greatest expenses and its application in terms of placement should therefore be done as effectively as possible.
Because they want to plant as quickly as possible during the optimal planting period, farmers are motivated to apply more fertilizer before planting and less during planting. This practice has many disadvantages such as, for example, sub-soil acidification, the risk of not planting near enough to the fertilizer band and the danger of nutrient losses. The pre-application of fertilizer also has great benefits. For example, nutrients placed in wet soil will have enough time to be converted into a plant available form.
It has been established that the first yield potential of maize is determined at an early stage (V5) of its development (Labuschagne and Van Zyl, 2015). It is therefore critical to use a well-balanced planter mix during planting. Sufficient phosphates concentrated near the seed (5 cm away, 5 cm deep) has also been proven to lead to stronger seedlings. The maize plant develops slowly up to V5, but increases drastically thereafter (Pannar, 2009).
Due to the increasing growth rate, the maize plant’s nutrient requirements also multiplies (Figure 1). The final yield potential of a maize plant is determined during V12. Nutrient deficiencies during this growth stage can lead to serious consequences in terms of yield loss. The practical application of top dressing is sometimes problematic and time is also limited. Nevertheless, the splitting of nitrogen fertilizer (a third before planting, a third during planting and a third after planting) has been proven by many scientists to give the best yields (Nemati and Sharifi, 2012). Factors such as the correct nitrogen source that is immediately available, the application method (band placement versus broadcasting) and soil moisture during top dressing is important to keep in mind. Top dressing of potassium will also be beneficial on soils with low potassium levels.
One of the stumbling blocks for today’s grain farmer is to limit his risks as much as possible and to do everything in his power to fertilize the maize plant for optimal yields during the right growth stage. Omnia offers various tools and advice to limit the farmer’s risk. Speak to an Omnia agronomist about soil analyses for nutrient status, soil potential, soil health (OmniBio™), production functions for each area, leaf analyses, OmniSap® analyses and the processing of yield data to ensure that every farm reaches its full potential.
Figure 1: Cumulative uptake of N, P and K as a percentage of the total uptake by maize (FSSA, 2007; Pannar, 2009)
Broadcast versus band placement
When the time of application is considered, the placement of fertilizer should also receive attention with the focus on band placement versus broadcast application. Again, there are two main factors that play a role. The first is to make nutrients optimally available from seedling to harvest. Secondly, the farmer should be able to do this as practically and efficiently as possible without causing yield losses.
Optimising of nutrients for plant uptake
With regards to nutrient availability, various core influences have to be considered. These include nutrient characteristics and uptake, fixing risk and soil fertility.
Each of the macro- and micronutrients are taken up in a specific manner (Barber, 1984). Table 1 shows the percentage of the specific uptake methods of each element.
Table 1: Indication of how each element is taken up by plants (Barber, 1984).
Nitrogen (N) is mostly taken up by mass flow, which means that the plant takes it up together with water (FSSA 2007). Therefore, if the root environment is favourable for the uptake of water, the plant should also be able to easily absorb nitrogen. However, cultivation, nitrogen source and the environment has to be taken into consideration.
Urea nitrogen volatilises when it is broadcast on the soil and it should therefore be worked into the soil. Even then, there is still a high risk of volatilisation (Botha, 1986). If urea is band placed into damp soil and the soil is then sealed, the risk of volatilisation is reduced. But high concentrations of urea can cause ammonium toxicity. Bornman (2013) stated that in cool, acidic, wet soils with low organic matter contents, the nitrification process is inhibited. High concentrations of ammonium will then build up to toxic levels in the soil, to be taken up by plants. Concentrations of as low as 150 mg.kg-1 can become toxic if ammonium is the only source of nitrogen (80 kg.ha-1 on 1 meter row widths). To prevent toxicity, the soil pH and basic cation levels (especially calcium) should be maintained and sources containing nitrate nitrogen should be used.
There is no risk of volatilisation with ammonium nitrate based nitrogen sources (Botha, 1986). However, the root environment where the fertilizer ends up has to be favourable. Therefore, the soil moisture, temperature and oxygen should be sufficient for nutrient uptake.
Phosphorus (P) is taken up by diffusion. This is the process where the nutrient moves from an environment with a high concentration of the element to an environment with a low concentration. Phosphorus is also one of the elements most easily fixed and the least movable in the soil (Havlin et al., 2004). Research shows that the fixing of phosphate takes place at either very low soil pH (through iron and aluminium) or at high pH (through calcium). The optimal pH (KCl) for phosphorus availability is between 5 and 6 (Figure 2). Low soil temperatures inhibits phosphate uptake and therefore band placing in cool conditions will be beneficial. Even though greater reactions are achieved when band placing on soils low in phosphate, good reactions are also seen when sufficient phosphate is band placed, even when the soil has a good phosphate status. By considering the soil analysis and environmental conditions, a decision can be made regarding the portion of phosphate that should be band placed and the portion that should be broadcast. Considering all of the variables, the band placement of phosphates remains the safest option.
Figure 2: Graphical representation of phosphate fixation at different pH levels (Havlin et al., 2004)
Potassium (K) is also mostly taken up by diffusion. The element is usually present in the soil in large quantities. Therefore, a high concentration gradient between the root and the soil particle should already exist. Moisture and temperature are factors that will play a role. With diminishing moisture, the distance that the potassium has to diffuse becomes increasingly greater (Havlin et al., 2004), the drying of the topsoil (0 to 20 cm) under dryland conditions is a reality and the uptake of potassium is impeded. On soils with potassium levels of less than 80 mg.kg-1, about double the amount of potassium should be broadcast than what would have been applied with band placement. The difference becomes smaller as the potassium status increases: at 120 mg.kg-1 about a third more potassium is broadcast to get the same reaction. However, care should be taken as certain clay minerals can fix potassium, in which case band placement will be the safer option (Rehm, 2002).
Other elements such as sulphur, calcium magnesium and micronutrients, which are essential plant nutrients, are each also taken up in unique ways by the plant.
Sulphur is mostly taken up by mass flow, therefore the same principles as for nitrogen apply.
Calcium and magnesium are elements mostly corrected by broadcast liming. However, there are benefits when these elements are applied in concentration with the primary macro nutrients.
The plant needs micronutrients in small quantities. Micronutrients can easily be fixed in the soil, in which case band placement of the chelated form is the recommended practice. Zinc uptake can be negatively affected by excessive phosphates (Havlin et al., 2004) and therefore a zinc containing fertilizer should be used.
The practical and time efficient approach – the risks
The rates of fertilizer used during planting should be kept in mind. To speed up the planting process, producers tend to use less and less fertilizer during planting (band placed near the seed). This is especially problematic when insufficient amounts of phosphates are applied during planting, as the seedling needs energy to establish a strong root system that can optimally utilise soil moisture and withstand pathogens which attack roots. Row widths can play a role in that narrower row widths causes a lower concentration of nutrients.
A seedling transpires very little in the first 20 days after planting. The same applies to nitrogen; therefore nitrogen in the planter mix is crucial during this time. The nitrate form of nitrogen is critical for the young seedling (Errebhi and Wilcox 1990). Sufficient amounts of an ammonium nitrate based product should therefore be used during planting.
Figure 3: An indication of plant biomass in gram 12 days after planting with different ammonium to nitrate ratios (Errebhi and Wilcox 1990).
Band placing the product will ensure that the nutrients are better protected when conditions become unfavourable and nutrients become more readily available. The depths at which the fertilizer is band placed is critical. Especially where reduced forms of nitrogen such as urea or ammonium is band placed, acidification takes place. Products should therefore be placed in such a way that acidification can be rectified by liming.
Take soil analyses, the environment, product, row widths, plant population, crop and growth stage into consideration when a decision between broadcast application and band placement has to be made.
- Barber, Stanley A. 1984 Soil bionutrient availability. John Wiley & Sons, New York, NY.
- Fertasa Fertilizer Handbook. 2007 Seventh revision.
- Botha, A.D.P. 1986. Nitrogen losses from Urea, UAN and LAN when applied to different soils. Soil and Irrigation Institute.
- Havlin, J.L., Tisdale, S.L., Beaton, J.D., Nelson, W.L. 2004 Soil Fertility and Fertilizers. Seventh edition. Pearson education
- Errebhi, M. & Wilcox, G.E. 1990 Plant species response to ammonium-nitrate concentration ratios. Journal of plant nutrition. Volume 13, 1990 – Issue 8.
- Labuschagne A. & Van Zyl K. 2015 Bemesting van mielies – waar, wanneer en hoekom? SA Graan Julie 2015.
- Pannar. Ken die mielieplant. 2009.
- Nemati, A.R. & Sharifi, R.S. 2012 Effects of rates and nitrogen application timing on yield, agronomic characteristics and nitrogen use efficiency in corn. Intl J Agri Crop Sci. Vol., 4 (9), 534-539, 2012.