Many corn fields suffer from poor early plant health, particularly during cool springs like we have experienced this year. These growth problems can be intensified by a multitude of factors, but can normally be attributed to nutritional limitations and/or poor root development.
Many initially believe poor growth results from inadequate or poor nitrogen availability, but this is rarely the case. Nitrogen is very mobile in the soil and corn requires relatively little nitrogen until rapid growth begins, so nitrogen fertilizer placement and amount rarely limit early-season corn growth.
However, other nutrients, including phosphorus, potassium, magnesium, and zinc and soil pH commonly limit early-season corn growth in Mississippi.
Poor root development will also severely limit nutrient uptake and stunt development, but it is often overlooked in terms of crop diagnostics and management.
Field scouting will frequently reveal symptoms indicating specific problems. The best method to diagnose fertility limitations is to collect soil and plant tissue samples from stunted and adjacent healthy field areas and submit the samples to a soil-testing laboratory for analysis and recommendations. This method is particularly useful for identifying marginal nutrient levels, which may not show up with a composite soil-sampling technique.
Young corn plants appearing stunted with purple lower leaves are likely suffering from phosphorus deficiency. This deficiency is especially prevalent when corn is grown following a rice crop, because the absence of a flood reverts soluble ferrous phosphates to unavailable ferric phosphates.
It also often occurs when corn is grown in rotation with cotton or soybeans, because corn uses nearly double the amount of phosphorus required for cotton and soybeans.
Phosphorus deficiency symptoms appear on lower leaf tips and progress along leaf margins until the entire leaf shows purpling. New leaves emerging from the whorl are usually green, but may turn purple shortly thereafter.
Corn hybrids often differ considerably in the degree of purple symptomology they express — so don't be alarmed if one hybrid exhibits more vivid symptomology than others. Hybrids differ in the presence of genes that trigger production of anthocyanins, which are the pigments responsible for the purple coloration.
Phosphorus deficiency symptoms often occur as young plants are exposed to warm, sunny growing conditions following cool and wet conditions. This results in a lag phase where rapid vegetative growth exceeds the roots' ability to supply phosphorus. Young plants are especially vulnerable because their root systems are small and phosphorus is immobile in the soil solution.
Low soil pH will also severely limit phosphorus availability to plants, which may cause deficiency even where high soil test phosphorus levels exist.
Any cultural or environmental factors capable of limiting root growth will magnify nutritional limitations and stunt growth. One common condition, which is particularly prevalent when rainfall limits corn planting progress, is known as “rootless corn syndrome.” This problem occurs when the nodal root system, which develops from the lower nodes in the crown of the stem, fails to develop.
There are several reasons why nodal roots may not develop, including shallow planting, wet planting and soil erosion. Corn planted less than 1 inch deep will move the nodal root origination point up to or above the soil surface, subjecting these roots to many factors that can retard growth (hot, dry and/or compacted soil, herbicide injury or carryover, and surface-feeding insects).
Corn planted at the recommended seeding depth (1.5 to 2 inches deep) will develop the nodal roots below the soil surface, generally avoiding these potential problems.
Corn planted in marginally wet soil will also compact the soil around the seed-furrow, especially on heavy clay soils, substantially impeding root penetration and often cause the furrow to open and present problems to shallow planting. This scenario is quite common when rainfall hampers planting progress during the spring, pushing producers to plant into soils that are wetter than optimum.
Heavy rainfall or wind erosion may also move soil away from the crown of emerged corn plants, especially on light-textured soils with raised beds, exposing the nodal root system as well.
Phosphorus deficiency symptoms normally slowly disappear when favorable growing conditions promote more root growth.
Treatment options to remedy phosphorus deficiency produce gradual results, particularly compared to nitrogen application, because phosphorus is immobile in the soil solution. Thus, plant roots must grow into the zone where fertilizer was applied before phosphorus uptake and plant response will occur.
Surface application of phosphorus fertilizer will limit availability to the top couple inches of soil. Thus, broadcast phosphorus application would be best-suited to irrigated and/or minimum tillage fields with substantial crop residue on the soil surface, where these factors would promote soil moisture, root activity and nutrient uptake in the upper few inches of soil.
Phosphorus injected as a side-dress treatment would generally increase availability to roots in most situations, particularly in dryland fields. However, be extremely careful not to prune roots when sidedressing. Corn root diameter is generally similar to plant height, so don't sidedress much closer to the row than the plant height.
Erick Larson is a grain crops specialist with Mississippi State University. e-mail: firstname.lastname@example.org