Why is my maize yellow, purple or uneven?

Earlier in the season, it is not uncommon for maize plants that previously looked green and uniform to suddenly become yellow, purple or uneven. While insects, pests, root diseases, nutrient or chemicals are usually blamed as the main culprit, more often the problem tends to be due to other factors and is normally short term with very little or no impact on yields.

Even though maize plant yellowing and purpling early in the growing season is mostly attributed to nitrogen (N) and phosphorus (P) deficiency respectively, the primary cause for the problem could be due to soil saturation which affects root activity, inhibiting nutrient uptake. Unless the soil is highly depleted of nutrients, N and P deficiency at this stage is very unlikely because prior to the V8 stage (about knee high) N and P requirements are very low. Cool temperatures combined with wet conditions are usually the main reason for the phenomenon. The healthy green colour should be regained once weather conditions improve and yield should usually not be affected.

Yellowing and purpling are generally more pronounced on lighter or more exposed soils, which can be attributed to cooler conditions. If plants on the south facing slopes, wetter ground or exposed areas are more yellow or purple than the rest of the paddock, chances are the problem is not nutrient related. Other stress factors which inhibit normal plant or root development such as soil compaction or herbicide injury can also make the effect more pronounced. In short, factors that inhibit root growth can cause plant purpling and some hybrids are more prone to the syndrome than others.

The V3 stage (approximately 12 - 15 cm plant height) marks the end of the maize plant’s reliance on the seed as the main nutrient source, transitioning to a process where roots supply all nutrient requirements from the soil. Unevenness at this stage is common, particularly in paddocks with different soil characteristics which may cause temperature variability particularly in cooler and/or wetter springs. Under these conditions, small temperature differences in the root environment can significantly influence plant growth either through restricted root development or a slower conversion of nutrients into a form that is available for plant use (mineralisation).

Not only does surface trash lower soil temperature but it can also lock up soil N such that it becomes unavailable to the plant. The combination of surface trash and variable soil types can cause soil temperature differences resulting in differential growth rates. Plants tend to be taller and greener in places with less or no trash, or where soil temperatures are higher. Provided the soil nutrient status is optimum, this phenomenon is usually temporary, with evenness eventuating as temperatures improve.

Maize seedlings weakened by cold weather generally have greater susceptibility to herbicide injury which may require a delay in postemergence herbicide application until plants begin to exhibit normal growth and development signs. Where surfactants or oils are used it may be best to consult a chemical representative to make the right decision where plant growth doesn’t appear normal.

Until the V5 development stage (almost red gumboot high), growers should be patient and not rush into taking remedial actions, particularly nutrient application. If plants remain purple or yellow closer to or past the V5 stage, assessment (soil and herbage tests) is needed to determine if it is due to nutrient deficiency or other factors. Even though nutrient deficiency symptoms may seem obvious, it is good practice to conduct herbage and/or soil tests to ascertain the real issue before taking remedial action. 

A reflection on the season to date: the number of growers applying boron (B) as part of their fertiliser program seems to be increasing every year. Whereas growers can get away with applying excess amounts of other nutrients, the range between correct and toxic B rates is very small, making it easy to apply excess B. Similarly, B requirement is negligible and should only be applied if the soil test result proves it is deficient, in which case uniform mixing and application are critical to reduce chances of over-application. Broadcasting would be preferable to row or foliar application.