The 12 days of Crop Nutrients

Is anyone else missing the Christmas carols? Because we’re in the midst of the 12 Days of Christmas, and because the lack of seasonal music has me feeling a little deflated, I’m going to borrow the theme of the beloved Christmas carol “The 12 Days of Christmas,” and turn it into the 12 days of crop nutrients.

Partridge in a Pear Tree – or Phosphorus (P)

The alliteration here demands that we feature phosphorus first.

Phosphorus is an essential plant nutrient and very important for numerous plant processes and crop production. It is a vital component of DNA and RNA, the building blocks of proteins and protein synthesis. The adenosine triphosphate molecule (ATP) molecule is responsible for storing and transferring all of the energy produced and needed by the plant. At the core of this ATP molecule are phosphates, responsible for all of the activity of ATP. Phosphorus also plays a major role in the stimulation of new root growth.

So, P is Important

Our crops clearly need phosphorus to thrive. So, what do we need to worry about when supplying P? “Tie up” within the soil is the primary concern with phosphorus fertilizers. In acidic soil conditions, P will tend to get tied up by iron, aluminum, and manganese. In basic soil conditions, calcium will be the major component of phosphorus tie up.

Phosphorus deficiency symptoms in corn
Phosphorus deficiency in corn

Phosphorus is most available to the plant in a soil pH range of 6.3-6.8. Common liquid fertilizers, such as ammonium polyphosphate (10-34-0) and orthophosphate (9-18-9), applied in the early spring will also have a likely chance of being tied up if a gypsum application was made in the fall.

Choosing a phosphorus fertilizer that is protected from tie up will ensure that you get the most out of your fertilizer investment and that your crop will receive the required amount of phosphorus needed.

Available P versus Usable P

Not to mention, applying phosphorus as a crop nutrient can be tricky. Just because phosphorus was applied to the soil does not mean that it is doing what you want it to do: feed the plant!  AgroLiquid founder, Douglas Cook, was known to say that all applied fertilizer is available, but not all applied fertilizer is usable. Sounds funny, but it’s true. What’s the difference? All fertilizer is available to plants — it’s right there for the taking. But it may not be usable. In order for a nutrient to be usable, it must be close to the roots and it must be in a form that the plant can absorb.

Nutrients like nitrogen can be lost to leaching or volatility before absorption. Potassium can be strongly held by clay in the soil and not able to be taken up by roots since it is not in the soil solution. Phosphorus can also become unusable. Phosphate is negatively charged and can react with, or be fixed, by positively charged elements in the soil (cations). Plants cannot take up these compounds of calcium phosphate, aluminum phosphate or iron phosphate. Estimates are that the crop will utilize only around 20% of applied phosphate fertilizer during the season after application, and in following years, the amount becomes progressively less as it reverts to mineral forms. Again, the nutrients are there and available, but they are not always usable.

Out of Sight, Out of Mind

You cannot actually see the fate of phosphate molecules in the soil, so it’s not necessarily something growers are thinking about. If only a small percentage of your planted seed came up, you would probably be mad because you can see that loss. Similarly, only a small percentage of the applied phosphate is usable. However, you cannot see this, so it is not a concern.  But it should be.

Placement is Key

Phosphate fertilizer works best if it is placed close to the seed at planting. In the picture at the left, it is apparent that phosphate fertilizer placement is affecting growth. Five rows of the plot had 5 gal/A of Pro-Germinator applied through the planter, and the sixth row had no planter fertilizer.

Phosphorus source comparison in field corn
Fertilizer placement comparison using Pro-Germinator in corn

The rows with the In-furrow placement are tasseling, whereas the 2×2 placement has yet to tassel. Close inspection shows that the corn with the 2×2 placement is taller than the row with no fertilizer, but it is behind the rows with the in-furrow placement. This shows that phosphorus placement for earliest root access affects plant growth and yield. Additional testing at the North Central Research Station has shown that in furrow placement can out-yield 2×2 placement by almost 5 bu/A.

In order for phosphorus fertilizer to be most effective, it needs to be usable. Usability is increased by placement close to the seed row and protection from fixation losses. Pro-Germinator is the only fertilizer that does both.




Common phosphorus deficiency symptoms:

  • Stunted plants
  • Leaves may be darker green or begin purpling
  • Leaves may curl upward
  • Maturity can be delayed
  • Poor seed set
  • Poor fruit quality
Phosphorus deficiency in citrus fruit
Phosphorus deficiency in citrus fruit can result in poor fruit quality.
Purpling leaves, like those in this canola plant, can be a symptom of phosphorus deficiency
Purpling leaves, like those in this canola plant, can be a symptom of phosphorus deficiency



The Nutrients That Matter To Citrus Production

What are the nutrients that are most critical to citrus fruit color, weight and size; juice content and color; acid and soluble solids content; and peel thickness — the most important characteristics of a good citrus crop?

It’s about applying the right nutrients at the right time, says Bob Rouse, associate professor and citrus horticulturist at University of Florida IFAS. Citrus trees are most active and develop new growth the first six months of the year, so that would be the time to apply about 2/3 of nutrients applied to a crop.

“Rains always work against you,” Rouse says. “That’s another reason to get 2/3 of fertilizer out in the first half of the year. Florida’s rainy season doesn’t start until June or early July. The tree stores fertilizer nutrients really well before then.”
According to Rouse, there are four nutrients that most affect these key characteristics of citrus: nitrogen, phosphorus, potassium and magnesium.

#1: Nitrogen
Of course, the number one element is nitrogen and the amount taken up by trees can positively or negatively affect citrus fruits. There is a balance between under and over application that can make the difference between a successful citrus harvest and a disappointing one.

• Adding nitrogen increases fruit juice content.
• Increases soluble solids (sugars) and increases acid slightly. Growers need to balance the increases from the sugars and the slight increase in the acid, which affects the flavor of the fruit and can lead to an acidic-tasting juice.
• Increases the juice color. Increasing solids or sugars on per box basis is what growers are paid for. “[Increasing nitrogen] will increase the sugar, so that means growers get more money,” Rouse says. “There’s a relationship there between profit and use of nitrogen.”
• Too much nitrogen can make the fruit green, preventing it from coloring up, and also makes an undesirable thicker peel.
• Too much nitrogen will make fruit puffy inside with less juice, as well.

#2: Phosphorus
Phosphorus applied during citrus production can work to counteract too much nitrogen, and a healthy balance between them is important.

• If you exceed the recommended ranges of phosphorus, it can negatively affect the acid content, pulling it down.
• Excess phosphorus can also increase the ratio of sugars to acid, since acid is decreasing during fruit maturity.
• Phosphorus has no effect on fruit size or weight, but will make fruit stay green if over applied, as nitrogen also does.
• Phosphorus can also help to keep peel thickness to a minimum.

#3: Potassium
Potassium does have quite an effect on the external part of the fruit.

• Potassium can increase your fruit size and weight.
• Over application of potassium can cause the fruit to stay green and not color up at the end of the season.
• Potassium can keep the peel thickness from being too thick.
• Potassium may increase acids, but it can also increase sugars.

4: Magnesium
Magnesium is the last nutrient that has much effect on the fruit directly. Chlorophyll is the driving force for making sugars in fruit. The magnesium is the center of the chlorophyll molecule, so plenty of magnesium means plenty of chlorophyll.

• In addition to increasing sugars, magnesium will also increase the ratio of the amount of sugars to the amount of acid and will result in a better tasting fruit or juice. That’s the main function of magnesium.
• Externally, magnesium can increase size and weight of fruit, because you’re increasing sugar content, which results in a heavier fruit.
• Magnesium can negatively affect the peel thickness, however.

To apply the optimum amount of nitrogen to citrus, refer to the ranges set forth by University of Florida Extension for leaf content for nitrogen, potassium on page 27 of Nutrition Of Florida Citrus Trees.