Most of the dry weight of the plant consists of organic carbonaceous materials resulting from photosynthesis and subsequent processes. At least 12 nutrient elements must be taken up for the corn plant to grow and develop normally. An adequate supply of each nutrient at each stage is essential for optimum growth at all stages.

The seasonal pattern of nutrient accumulation in the plant is similar to that of dry matter accumulation (see fig. 54). However, nutrient uptake begins even before the plant emerges from the soil. The amounts of nutrients taken up early in the growing season are small, but the nutrient concentrations in the soil surrounding the roots of the small plant at that stage often must be high.

Uptake of potassium is completed soon after silking (see fig. 55), but uptake of the other essential nutrients such as nitrogen and phosphorus continues until near maturity. Much nitrogen and phosphorus and some other nutrients are translocated from vegetative plant parts to the developing grain later in the season. This translocation can result in nutrient deficiencies in the leaves unless adequate nutrients are available to the plant during that period.

A large portion of the nitrogen and phosphorus taken up by the plant is removed in the grain that is harvested But most of the potassium taken up is returned to the soil in the leaves, stalks, and other plant residues. unless these plant parts are removed for silage or other forms of feed.

Fertilizer Applications

Although only relatively small amounts of fertilizer nutrients are required in the very early stages of plant growth, high concentrations of nutrients in the root zone at that time are beneficial in promoting early plant growth. This is the period when all the different plant parts are being initiated and begin to grow. Even though the amount of nutrients taken up is relatively small, final size of the leaves, ear and other plant parts depends to a large degree upon having an adequate supply of nutrients available to the plant during this early part of the growing season.

During early growth, the root system is limited and the soil is often cold. The seminal (seed) root system which is elongating when the plant emerges from the soil serves as the main root system during the first weeks after plant emergence. Fertilizer placement in a band about 5.1 cm (2 inches) to the side and slightly below the seed is important so the seminal roots may intercept the band of fertilizer. Roots will branch and proliferate in and near the band of fertilizer after they contact it; but they are not attracted toward the fertilizer band, so fertilizer must be placed where the roots are going to be. Placing the fertilizer too near the seed can result in salt injury to the young plant.

At later stages of growth, the plants require much larger amounts of nutrients. These nutrients must be in moist soil for effective root uptake. For all tillage systems, a soil testing high in P and K should provide adequate nutrients and placement is not a major concern. For conservation tillage systems on soils low in P and K, a starter band or injection 6 to 8 inches deep may be advantageous over broadcast applications. Time and placement of nitrogen fertilizers should be based on greatest efficiency, minimizing potential loss, and fitting into the production system.

The nodal roots begin to develop above the coleoptile node at about VE. Within 2 or 3 weeks after plant emergence. the nodal roots become the main root system of the plant. This nodal root system becomes well distributed in the soil, so precise fertilizer placement is less critical after these roots have developed.

Care should be taken that roots are not destroyed by cultivation or by sidedressing of fertilizer after the root systems of the plants become established.

figure 54

figure 55

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