There are sixteen essential elements that are required by every plant for growth and development. Three of these nutrients are supplied by air and water.

They are:

  1. carbon
  2. hydrogen
  3. and oxygen

Thirteen of these nutrients are supplied by the soil.

The three primary macronutrients are:

  1. nitrogen
  2. phosphorus
  3. and potassium

We call these nutrients the big three because they are required by the plant in largest amounts.

The three secondary macronutrients are:

  1. sulfur
  2. calcium
  3. and magnesium

These secondary macronutrients are required by the plant in large amounts but not is the quantity as the primary macro nutrients.

There are 7 micronutrients which are required by the plant in small amounts:

  1. iron
  2. zinc
  3. manganese
  4. copper
  5. molybdenum
  6. chlorine
  7. and boron

If any of these plant nutrients are lacking, the plant will not grow to its full potential. This is called Liebig’s Law of the Minimum.

Plant nutrition usually is not a problem in forested areas and pastures with native grasses because of nutrient recycling and lower nutrient needs of the plants. In garden areas and improved pastures and hay meadows where we remove the growth for production by harvesting. This removes the nutrients and takes them away from the natural recycling of nutrients. With that being said, we need to add additional fertilizers to supplement the crop that is being grown.

The 3 important Macronutrients in Fertilizer

What is in a fertilizer that makes it so important, and what do the three numbers mean? The three numbers are the three primary macronutrients which are nitrogen, phosphorus, and potassium, and always in that order. The number indicates how much of each nutrient is present as a percentage of the total weight of the fertilizer. Thus, a 50 pound bag of 15-5-10 fertilizer contains 15% nitrogen (7.5 lbs.), 5% phosphorus (2.5 lbs.), 10% potassium (5 lbs.), or their chemical equivalents. That is only 15 pounds total. The rest of the fertilizer bag weight is simply an inert carrier or filler, such as sand, clay or other materials.

1. Nitrogen

Nitrogen is necessary for vegetative plant growth of the roots, leaves, stems, flowers and fruits. It is the main component in chlorophyll production which gives the plant it’s green color, and amino acid production which increase protein content up to a point. Nitrogen is highly mobile in the plant which means that the plant will remove nitrogen from the older leaves and translocate it to the new growing tissue. The deficiency symptoms show up on the older leaves first and looks light green to yellow.

2. Phosphorus

Phosphorus is essential to cell division, root formation, flowering and fruiting. It is also involved in the storage and transfer of energy vital to all plant growth processes. Consequently, a deficiency causes stunted growth and poor flowering and fruiting. Also, the leaves will have a purplish color to them. Phosphorus uptake is by root interception. That means that it needs to be applied very close to the roots of the plant. If you are growing a garden, apply fertilizer in the furrow or very close to the roots. In pastures, the phosphorus is broadcasted over the entire soil surface and the grass feeder roots will take the phosphorus up. Phosphorus is very pH dependent and can become tied up with other elements (made plant unavailable) if the pH falls below pH 6 or above pH 7. So, try to keep your soil pH between 6-7.

3. Potassium

Potassium is another one of the big three plant nutrients. It is often called potash. It has many roles in the plant with being an osmotic regulator being the main part. It regulates what goes in and out the plant cell. It makes the stem very hardy and adequate amounts prevent lodging (falling over) of the plant. It regulates the opening and closing of the stomatal openings in the leaves. A plant that is deficient in potassium will not be able to open the stomatal opening all the way and this will not allow the plant to transpire (cool itself) properly. A plant will take up more potassium than it needs. This is called luxury consumption. A potassium deficiency will start at the outer edges of the leave and move inward toward the center of the leaf. It starts off as a discoloration and turns brown to black. We call this marginal leaf scorching, because it looks like a torch was applied to the outside of the leaf.

It is very difficult to determine how much fertilizer to apply to your soil without first conducting a soil test. I usually recommend the routine analysis. I would send it to one of the State Soil Testing Laboratories in Texas to have your soil tested. The two state labs are at Texas A&M University and Stephen F. Austin State University. We have the soil bags and forms in our office. A soil test needs to be conducted every 3-4 years. You will need to get a spade or a soil probe and take a soil sample from 0-6 inches in depth, put each core in a bucket. Mix the soil in the bucket well and fill the soil bag half full and mail it to the soil testing lab. The more samples you take of your property, the more accurate the soil test will be. It is not an exact science, but it gets you very close to what is and is not needed in your soil. I always use this analogy, you do not put oil in your vehicle without checking the dipstick first”. A soil test can save you money by not over or under applying a nutrient that is or is not needed.

If you have any questions about conducting a soil test, please contact Clint Perkins with the Texas A&M AgriLife Extension office in Smith County located at 1517 West Front Street, suite 116 Tyler, TX 75702 or call 903-590-2980