Agronomy Ag Fundamentals

3 Components to Nitrogen Uptake

Root Growth, Nitrogen Band Movement, Growth Stage and Nitrogen Requirements

1. root growth;
2. nitrogen band movement and;
3. growth stage and nitrogen requirement of the plant.

1. Root growth

Cereal Roots

Figure 2

Canola Roots

Figure 1

Canola plants employ a tap root that grows downward with smaller roots radiating outward. These roots grow at a rate of .75”/day in adequate growing conditions after emergence. At full maturity, canola roots can reach depths of 3-6’ feet. ⁽¹⁾

The Bourgault Agronomy team did a root dig and washed off roots of a 3 leaf canola plant 28 days after seeding, and 14 days after emergence. At this stage, the roots are at least 4” to each side and 5” down. (Figure 1)

Cereal roots do not have a defined tap root, but more a fibrous rooting system. They also grow rapidly in adequate soil conditions. It has been documented that at the 2 leaf stage, wheat roots can grow laterally up to 5” and as deep as 6.5”. At full maturity, cereal roots depths are typically between 3 to 5’ deep. ⁽²⁾

The Bourgault agronomy team dug and washed off the roots of a 2 leaf wheat plant. The wheat plant was found to have a 3” spread and to be 4” deep in our conditions at the 2 leaf stage. (See Figure 2)

2. Nitrogen Band Movement

PRS™ Probes

Figure 3

Nitrogen must be in the form of nitrate (NO^3-) to be taken up by the plant via mass flow through water flow in the roots. Most synthetic fertilizers are not applied in this form and must go through one or two conversions after application to be plant available. It is in the nitrate form that the nutrient is most mobile, radiating from regions of higher concentrations to lower concentrations.

The Bourgault Agronomy Team conducted a three year study using PRS ^TM probes to determine the time it takes urea to convert to nitrate and move in the soil.

The urea in the MRB began to convert to ammonia (NH₃) within the first 5 days (refer to Figure 4). After 10 days, the amounts of nitrate in the band began to rise exponentially. At 15 days, nitrate was detected between 1” and 2.5” from the initial band. After 31 days, nitrate levels began to be detected 5” away from the band. (refer to Figure 5)

This shows how a nitrogen band spreads in the soil. Within 31 days of application, the band had spread out almost 5” in each direction. It is crucial that the fertilizer be placed into soil moisture for this to occur.

PRS - NH3 & NO3

Figure 4

NO3 - Distance from Band

Figure 5

3. Growth Stage and Nitrogen Requirements

Plants take up nutrients as  required. Typically, lower quantities of nutrients are needed earlier in plant development, but accelerates as the plant matures, peaking at the height of the flowering or elongation stage⁽³⁾. (Refer to figure 6)

Cereal Plant Growth

Figure 7⁽³⁾

Canola Plant Growth

Figure 6⁽³⁾

Timely Access to N

Based on our research, and cited information from reliable sources:

Timely Access to N

Figure 8

1. Root Growth – By the second leaf, spring crops will develop roots systems as deep as 5” and 4” to each side;
2. Nitrogen band movement – Nitrogen fertilizer will convert to a plant available form within days of placement, and radiate out as far as 5” from the band within 30 days of placement;
3. Nitrogen uptake – Most of the nitrogen uptake occurs at the bolting stage or elongation stage, which is typically 40-50 days after seeding.

Positioning the bulk of the nitrogen mid row, or 5-6” away from the seed allows for optimal safety and uptake timing for the plant. Nutrient interception between the growing root and expanding nitrogen band occur in the soil between where the seed was placed and nitrogen was banded. When the root intercepts the nitrogen source, that particular root grows in size to accommodate higher nutrient uptake while all other roots continue to grow in every direction. (refer to Figure 8).  By the time the plant has reached the state of peak nutrient demand, the root system and radiating band of nitrate will have completely overlapped.

An additional and major benefit to this seeding practice is seed safety.  Nitrogen fertilizer begins its process in the form of ammonia (NH₄), which is highly toxic to most living things.  It also has a strong affinity for water, and will outcompete any nearby seeds for moisture.  Positioning the bulk of the nitrogen mid row, or 5-6” away from the seed allows for complete safety during plant germination and emergence.  The plant will draw nutrients from its own supply, the surrounding soil, and placed starter fertilizer to carry it through the initial growth stage.