What’s in Your Soil: Making Sense out of Macronutrients

Sally Krueger

Nov 15, 2018

After sampling your soil and sending it off to the lab for analysis, you’ll receive a report that can help you to guide one of your most important agronomic decisions—fertility. And your soil analysis will report on the amount of macronutrients and micronutrients that are present in the soil to help you improve your fertility plan. 

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All plants rely on 16 essential elements to grow properly.

These elements can then be divided in mineral elements and non-mineral elements: hydrogen, oxygen, and carbon.

These three elements are found in the earth’s atmosphere and from rainfall to help plants grow. During photosynthesis, plants convert carbon dioxide (CO2) and water (H2O) using energy from the sun to produce starches and sugars, which provides them with food.

The remaining 13 elements are mineral elements.

They are divided into macronutrients and micronutrients. Macronutrients by definition are the mineral elements that are used in large quantities for healthy plant growth and seed production.

Macronutrients include nitrogen, phosphorus, potassium, calcium, magnesium and sulfur.

Here’s how each mineral functions to support plant growth:

1. Nitrogen (N)
  • Nitrogen plays a key role in photosynthesis and protein production

  • Too little nitrogen can limit plant productivity

  • Soils are generally deficient in the total nitrogen needs for most crops

  • Symptoms of nitrogen deficiency include stunted plants, yellow and aborted leaves, poor grain quality and lower yields

  • Nitrogen is mobile in the soil and within plants, which can lead to leaching of nitrogen beyond the root zone, making it unusable to the plant

  • Nitrogen demands a sound fertility management plan to prevent nitrogen loss and water contamination

  • N management includes using nitrogen stabilizers and split N applications

2. Phosphorus (P)
  • Phosphorus is a primary component of cell membranes and enzymes of plants; it also plays a crucial role in photosynthesis

  • Phosphorus is used in large quantities and requires annual monitoring

  • Phosphorus deficiency appears as purple to red leaves accompanied with stunted plant and root growth

  • Lower yields and late plant maturation can be found where phosphorus was in low supply

  • Phosphorus is considered immobile in the soil; it is added annually to the soil to retain soil fertility levels

3. Potassium (K)
  • Potassium is the third most common mineral element to be deficient in soils (after nitrogen and phosphorus)

  • Potassium plays an vital role in stomata formation which regulates plant moisture loss and stress from heat and cold temperatures

  • Plants rely on potassium for translocation of sugars

  • Symptoms of P deficiency are difficult to see until after yield loss has occurred

  • Potassium moves easily within the plant and is slightly mobile in the soil

4. Calcium (Ca)
  • Most soils typically have adequate calcium to maintain plant growth

  • Calcium is immobile within plants and shows low mobility in the soil

  • Calcium plays an important role in nutrient transport and plant membrane strength

  • Deficiency symptoms include stunted roots and poor plant standability

  • Soil pH can be adjusted upward with applications of calcium (in any of its forms)

5. Magnesium (Mg)
  • Magnesium is sufficient in most soils; however there are many soils that may be a bit low and they could see yield bumps a magnesium application

  • Magnesium is significant in enzyme production, chlorophyll structure and photosynthesis

  • When magnesium is short in availability, you’ll see evidence of chlorosis in the oldest leaves

  • Magnesium is considered a immobile in the soil

6. Sulfur (S)
  • Sulfur is a mobile nutrient in the plant and the soil

  • Sulfur helps plants to take up other nutrients

  • Sulur is essential to amino acid and protein synthesis

  • Symptoms of sulfur deficiency include young leaves demonstrating severe necrosis and premature leaf loss

Soil pH is a critical part of your soil test.

Soil pH is a measure of how acidic or alkaline the soil is—it is the most important soil characteristic that controls nutrient availability, plant growth and microbial health. A pH of 7.0 is neutral.

Most row crops prefer the pH between 6.3 and 7.2 for maximum yield potential. These pH levels are where the mineral elements are most available to the plants.

Interpreting a soil analysis report is critical to making an efficient nutrient plan, so it is important to understand how the nutrients that show up on the report can impact crop growth.

Sally Krueger

Nov 15, 2018