Fertilizer placement and uptake of nutrients
By Kobus van Zyl (Senior Agronomist: North West Business Unit)
Plants react not only to the amount of fertilizer applied, but also to the type of fertilizer, time of application as well as how the fertilizer is applied. In this article we will look at how plants take up nutrients and also the effect of this uptake on the eventual crop yield. There is specific reference to pre-plant application of fertilizer, application during planting and top dressing.
The pre-plant application of fertilizer, especially on a controlled traffic system, is already very popular and mostly delivers good results, if the following very important rules are observed:
- The fertilizer must be placed within reach of the plant roots.
- The fertilizer should not be placed too close to the seed. This can cause burn damage or retard growth due to the salinisation effect of fertilizer.
- If urea is applied pre-plant, it should be band placed at least 20-30 cm deep in wet soil. The conversion reaction to ammonium and nitrate is delayed in dry, cold or too wet soil. Therefore you should make sure at planting that this conversion has already taken place and that placement is not a problem. Great damage can occur if placement is not accurately away from the seed (20 cm) and also if the conversion takes place during planting.
- If you are fertilizing to achieve a very high yield potential, it will be better to apply the nutrients (NPK and others) before planting, at planting and by top dressing, as this practice increases the nutrient use efficiency substantially and lowers the risk considerably.
To ensure that plants utilise the fertilizer and efficiently convert it to yield, it is essential to know how plants move nutrients to the roots. Nutrients move to the plant roots in three main ways: by mass flow, diffusion and root interception. Mass flow is the uptake of dissolved plant nutrients, driven by the transpiration process. In Table 1, an indication is given of the mechanism involved with the movement of plant nutrients to the roots. Diffusion is the uptake mechanism driven by a concentration gradient. This means that plant nutrients move in the soil from high concentrations, fertilizer band,, to low concentrations in the soil around the root.
Root interception is the mechanism during which the roots reach the plant nutrients through normal growth. In other words the roots grow into the soil zone enriched with nutrients. Healthy soil biology plays a very important role in root interception. Mycorrhiza in particular perform an important function in this matter to assist the plant with nutrient uptake.
|Nutient||Mass Flow||Diffusion||Root Interception|
Source: Barber, Stanley A. Soil bionutrient availability. John Wiley & Sons, New York, NY (1984).
Table above shows clearly that phosphate needs to be placed as close as possible to the plant roots to encourage good root development. A general practice is to place nitrogen and phosphates together 20 - 30 cm deep and 20 cm away from the seed. Nitrogen placed 20 cm away from the roots poses no problem, but this is too far from the roots for phosphates. The yield on soils with a high phosphate status [P (mg/kg) Bray I > 30] and optimal pH should not be affected by phosphates placed 20 cm away from the seed. However, this is on condition that the planter mix contains enough phosphates to establish a strong, vigorous seedling. Yield differences of up to 1 tonne/ha have been observed between phosphates in the row and 20 cm away from the row. In both instances the depth of application was ± 25 cm.
Planter mixes have to ensure that plants have a good start without any deficiencies. The ideal placement of chemically granulated, ammonium nitrate based planter mix is 5 cm under the seed and 5 cm away from the seed. If in doubt, contact your Omnia agronomist for expert and valuable advice.
Soil moisture, soil type and clay content can vary from field to field. It often happens that the planter mix was placed 5 cm away, but as deep as up to 15 cm under the seed. In cold, wet conditions during planting, the young plants will yellow because the fertilizer is placed too deep. If the fertilizer is not placed deep enough, but rather above the seed, yellowing can also occur, especially when planted in dry soil using a seed drill. The correct placement of planter mixes cannot be emphasized enough, especially on wet, sandy soil and also on more clayey soils planted early in October. Liquid fertilizer has an obvious advantage with regards to placement above granular mixtures. A special coulter attached to the planter unit follows the ground contour and ensures a much more uniform fertilizer depth. Inspect the fertilizer placement carefully twice or three times per day on liquid as well as granular planters.
There are no substitutes for a good planter mix, as the example in Photo 2 illustrates. In this case the maize on the left of the photo was deliberately planted without a planter mix. The plants received a pre-plant fertilizer, but no top dressing. The plants on the right in the photo received pre-plant fertilizer, planter mix and also top dressing. If you look at the total plant nutrients, a comparison cannot be made. The purpose however, was to see the impact of a good planter mix. Before the top dressing was applied, there was already a very large difference in plant growth. The additional top dressing made this difference even greater and at harvest there was a difference of 3 tonnes/ha.
Top dressing should be used as a management tool to ensure the maximum nutrient use efficiency. Over the long term, three applications of nitrogen on sandy soils (pre-plant, planting and top dressing) have always given the best performance. This effect is pronounced in wet years, when top dressing can be applied, ammonium nitrate fertilizer performs extremely well as top dressing. The effect of potassium, sulphur and even some phosphates combined with nitrogen, should not be under-estimated as a top dressing.
If you think that your soil is not producing according to its potential, speak to your Omnia agronomist to help you to unlock the true potential of your soil.
This article was published in the Nutriology® Newsletter, Summer 2013