What do I use to fertilize my maize: Liquids or granules?
By Jan Bezuidenhout
For many years farmers have been asking this question, with justification for their preference for liquid or granular fertilizer based on very specific reasons. The type of application equipment on the farm is usually the main driving force. If the farmer only has planters equipped for liquid fertilizer, this will remain his primary choice. The same will apply for a farmer who prefers granules to liquid. To change from liquid to granules, or vice versa, will obviously involve a capital expense. A farmer should always make sure of the cost of the two systems in terms of appropriate storage, maintenance and repair costs. The advantage of liquids is mainly in the ease of handling and reduced risk for theft. Both liquid and granular systems have advantages and disadvantages. With that in mind, farmers have to distinguish between the different forms of liquid and granular fertilizers.
Liquid planter mixes are usually made available to the farmer as suspensions or clear mixtures. Liquid top dressings are usually all clear mixtures. Both the suspension and clear planter mixtures are manufactured from the same basic raw materials as the granules. As in the case of granular fertilizer, there are big differences between the composition of the nitrogen (N), phosphate (P) and potassium (K) of the mixtures. Clear mixtures with a higher concentration will usually contain fairly high levels of urea. The clear mixtures, depending on the P content, usually have a low acidity (pH), which can be corrosive.
Band placement of the clear mixtures can cause problems during the conversion process of urea in the soil, especially in cold, wet conditions. The dissolved urea can also leach during heavy rains after planting. The urea leaches before conversion to ammonium can occur. The same problem can also occur where urea is the nitrogen source in bulk blended granular fertilizer. The plants could then turn yellow as there is not enough plant available nitrogen. The problem is common when low levels of fertilizer are applied at planting or when the placement of fertilizer, whether liquids or granules, is too far from the seed.
Image below shows the difference in the 2010/2011 season between a urea-based liquid planter and an ammonium nitrate-based planter mixture.
Difference between a urea-based and ammonium nitrate-based liquid planter mixture on sandy soils in Viljoenskroon (five row 1:52 m) 2010/2011 season.
Denitrification (conversion of nitrogen to a nitrogen gas in low oxygen conditions) and physiological ammonium overload (toxicity) can also occur when wet soil conditions continue. The yellowing is usually temporary and the maize plants should recover quickly during warmer weather and enough sunshine. If the plants do not recover satisfactorily, the plant and root development should be explored. If a nitrogen deficiency becomes visible in the plant, one should start the application of an immediately available nitrogen source, such as ANO (21) (liquid) or LAN (28) (granules), as a top dressing as soon as possible.
Some liquid fertilizers do not contain urea, and will not have the same problems as urea-based liquid fertilizer under cool, wet conditions. These mixtures contain ammonium nitrate instead, which is the same nitrogen source used in chemically granulated planter mixtures. Some bulk-blended products also have an ammonium nitrate (LAN) nitrogen base, but each fertilizer granule does not contain nitrogen, phosphate, potassium and sulphur (S). The nitrate component in the ammonium nitrate can indeed leach, but it is already in a form that can be absorbed by plants. Check the source of nitrogen in both liquid and granular fertilizer products.
Does liquid fertilizer leach easier than granular fertilizer?
The question should be addressed very carefully as assumptions are often made. The chemical (eg. pH, P status, K status, Ca and Mg status) and especially the physical condition of the soil, fertilizer placement, amount and type of fertilizer, planting conditions, temporary waterlogged soil conditions, herbicide damage, nematode infestation, weak seedlings, wind damage, drastic temperature changes (cold, wet conditions) and insect damage can put the plant under stress, and often the fertilizer application practice is blamed. The primary products from which the mixtures are composed will determine the soil reaction. Liquids and granules have the same soil reaction.
The pre-plant application of fertilizer (liquids or granules) in a two-track system is becoming increasingly popular and the use of satellite navigation (GPS) makes the practice easier. The placement of the pre-applied fertilizer is very important to achieve the desired effect on plant growth. Often the pre-plant fertilizer is applied directly after or during the primary tillage. Weed pressure then necessitates chemical or mechanical control. During mechanical weed control the rows often move as far as 70 cm from the centre of the primary tillage (if tilled on the row), or too close to the planter row which could cause fertilizer burn damage. Cold, wet soil conditions may delay the root development, and if the roots are very far from the fertilizer, or if densification occurs, the nutrients cannot meet the plant's demand. Often there are plants that look better; usually these plants have a better root system and have already reached the pre-plant fertilizer.
Liquid fertilizer definitely has an advantage compared to bulk blended granules, simply because of the fact that no segregation occurs in the liquid. It can be safely assumed that each plant root will come into contact with nitrogen, phosphate, potassium and sulphur. The same degree of certainty is not present in the case of a bulk blended granule product. Salt shock with bulk blended products happens often in this case, especially because of poor distribution of salts such as potassium chloride.
A lot of yellowing of maize plants occurred during the 2010/2011 season. An example where the leaching of nitrogen has been investigated is discussed in graphs 1 to 3 below. The soil samples were drawn at intervals of 10 cm (0-50 cm) on the planter row and 0-20 cm in the middle of the row. Graph 1 gives a clear indication that most of the nitrogen, regardless of the colour of the plant, was already available in the nitrate form for uptake by plants. It is clear that the green plants at Locality 2 had more nitrogen available than the yellow plants. However, the total available nitrogen from Locality 2 did not differ much from the green and yellow plant nitrogen levels at Locality 1.
There was much less movement of nitrogen from the root zone at both the localities than expected. The total plant available nitrogen is shown in intervals of 10 cm in Graphs 2 and 3. There were differences between the nitrogen levels of the yellow and green plants. At Locality 2, there was a very clear difference between the nitrogen levels of the plants displaying a green and yellow colour. The question is whether the differences were large enough to cause the yellowing of the maize. Leaf analyses of the green and yellow plants confirmed that no nitrogen deficiency occurred in the leaves. No other prominent nutritional shortages occurred. On investigation, it came to light that there was a big difference in the nematode count on the roots of the green and yellow plants. The yellow plants had a much higher count than the green plants.
Nitrogen soil analysis at two localities (1 - stronger soil, and 2 - sandy soil) to determine the composition of plant available N in the soil (in the case of an ammonium nitrate-based liquid).
Nitrogen soil analysis that was done (Locality - 1, stronger soil) to determine to what depth the planter mix (in the case of ammonium nitrate-based liquid) has leached.
Nitrogen soil analysis that was done (Locality 2 - sandy soil) to determine to what depth the planter mix (in the case of ammonium nitrate-based liquid) has leached.
Any constraint that limits the plant's normal growth and development could eventually have a determining impact on the yield. The question is, do we always know what caused the constraints, or does drought, water logging, etc. get blamed for poorer yields?