nitrogen – Texas A&M AgriLife Organic

This past fall, we let our warm-season legumes—Sunn Hemp, Tepary Bean, and Cowpea—grow as long as the season allowed. Since we planted late, we hoped for a long fall, and thankfully we got one. Each crop reached full maturity, including seed set, about two weeks before a freeze finally stopped all aboveground growth. That’s when we collected the forage samples—at peak biomass and nutrient accumulation, just before decomposition began.

These samples, along with earlier soil tests from the same plots, gave us a snapshot of how each forage performed in terms of nutrient cycling and potential soil improvement.

Crude Protein: A Quick Look at Forage Quality

One of the easiest ways to relate cover crop nitrogen content to livestock feed value is by converting it to Crude Protein (CP) using the standard multiplier:

Crude Protein (%) = Nitrogen (%) × 6.25

Crop	Nitrogen (%)	Crude Protein (%)
Sunn Hemp	3.04	19.0
Tepary Bean	3.02	18.9
Cowpea	2.62	16.4

All three crops delivered respectable CP levels. Sunn Hemp and Tepary Bean were just under 19%, making them excellent options for grazing, hay, or green manure. Cowpea was slightly lower but still solid at 16.4%.

Nutrient Accumulation: What the Plants Took Up

These legumes not only fixed nitrogen—they also scavenged and stored other critical nutrients. Here’s a quick summary of key nutrients in the biomass at the time of harvest:

Nutrient	Sunn Hemp	Tepary Bean	Cowpea
Phosphorus %	0.21	0.17	0.15
Potassium %	1.62	1.37	1.16
Calcium %	1.25	1.41	0.94
Magnesium %	0.47	0.35	0.27
Sulfur (ppm)	2,483	1,766	1,603
Zinc (ppm)	34	21	16
Iron (ppm)	59	243	68
Manganese	30	22	27
Copper	10	9	8
Boron	24	16	19

What Stood Out

Sunn Hemp was the most balanced nutrient accumulator, with strong numbers in phosphorus, potassium, magnesium, and trace minerals like zinc and boron. It had the highest sulfur and copper as well.
Tepary Bean stood out for iron, with 243 ppm in the tissue—much higher than the others.
Cowpea showed lower uptake in most nutrients but still delivered usable protein and respectable mineral content.

These Cover Crops Don’t Just Grow—They Mine the Subsoil and Feed the Topsoil

Our recent soil tests confirm this. For example:

Magnesium and calcium levels rose significantly under Cowpea, even though the forage tissue levels were lower than in Tepary Bean or Sunn Hemp. This suggests slow, steady mineralization of residues.
Sunn Hemp left higher sulfur, zinc, and boron levels in the plant tissue, and five months later, those nutrients are becoming more available in the soil, contributing to fertility for the next season.
Phosphorus availability dropped slightly, especially in Cowpea plots, indicating that some P may still be tied up in decomposing roots or surface residues—but will likely continue releasing over time.

This delayed but strategic nutrient release is one of the reasons we emphasize cover crops not only as temporary fixes, but as seasonal tools for long-term soil fertility.

Takeaways for Organic Producers

These results highlight how cover crops can build both soil and have forage value. Even after seed production, these legumes held onto good nutrient density. If you’re grazing, cutting for hay, or planning a green manure termination, these crops offer real value beyond just nitrogen fixation.

Sunn Hemp may be the best all-around soil builder.
Tepary Bean could be ideal where iron uptake is a priority.
Cowpea, while slightly behind in nutrient concentration, is still a useful, fast-growing legume that fits well in diverse rotations.

One of the lesser-appreciated benefits of warm-season legumes like Sunn Hemp, Tepary Bean, and Cowpea is their ability to act as biological nutrient pumps. With their deep and aggressive root systems, they draw up nutrients from deeper layers of the soil profile—nutrients that would otherwise remain unavailable to shallow-rooted crops.

What makes this process even more valuable is the timing of nutrient return. These plants don’t immediately release what they’ve gathered. Instead, after several months, the decaying plant material released those deep-stored nutrients onto the soil surface, right where the next crop can use them.

There are all kinds of ways and amounts to fertilize melons and each of you has their own “special mix” that works just right for you. Even though each producer does it differently there is some interesting information about amounts based on growth that might be useful. Dr. Don Maynard, University of Florida edited the Watermelon Production Guide for Florida for years and in it he listed a fertigation schedule for a seeded and plastic mulched watermelon crop. I am asked occasionally how much nitrogen I should be applying, and the answer is “it depends.” I use the number 120#’s as the total N that you will probably apply in the season minus what you’ve already put out, say 40#’s which leaves 80#’s to apply in 8 weeks or 10#’s per week. This is simple, but it doesn’t consider that sometimes the plants need more nitrogen than at other times. Dr. Maynard includes this schedule which considers the plant’s needs.

This chart assumes that you will apply 120#’s of total N and that 20% or 24 lbs. of N was already put down as a starter N before planting. Assuming it takes 15 weeks from planting a seed to final harvest then you just follow along with each week applying the amount of N recommended per day for seven days then go to the next week. They recommend applying N through the drip each day but not many producers do that so just use the chart to calculate how much N you need and when you need it. He also adds that if you are using transplants then start on week 3 just shortly after you set out the plants.

So, what this means is that by week 8 you should have applied 28% of the fertilizer plus the 20% you put down as starter or 48% of the 120#’s. In week 8 you would apply 1.4 lbs. of N per acre per day or 14#’s of liquid 10% (1 gallon is 10.5 lbs.) per acre per day or 98#’s per week which is about 9.33 gallons per week per acre of liquid 10%. Just remember that we are talking pounds of actual nitrogen, but no nitrogen source is pure but is a percentage. Divide the pounds of N needed by the percentage to get amount of fertilizer to apply. 1.4/0.10 = 14#’s. Now an acre is a physical planted acre with plants having about 24 square feet per plant. Reminder: in organic you may want to adjust the schedule earlier, but the principal is that melons need lots of N right before and just after they start sizing melons.

You can see that in weeks 9 and 10 the plants are really using nitrogen and after that the plants begin to concentrate on making fruit instead of plant growth, so nitrogen is used less. Those big plants also have stored nitrogen in the leaves which they can use for maintenance and fruit giving you a safety net. We see the same kind of response in most field crops like corn where we would put most of the nitrogen out before tasseling to insure it is available when the ears are made.

How do you do a fertilizing schedule like this in Organic Production? There are several companies that make fertilizers approved for organic production that can supply lots of somewhat readily available nitrogen. As an example, Nature Safe has a pelleted 11-1-0 for organic production and Ferticell has Active 13-2-2, Ferticell has an Explorer Liquid 10-0-0 and an Explorer 16-0-0. I am sure there are many other companies that sell products like these that will add to your fertilizer needs beyond what you get from using high quality compost.

Always check your soil and your compost for nutrient content before you buy and apply these high nitrogen organic fertilizers. As always, I do recommend you check with your certifier before adding any amendments or soil fertilizers.

Tag: nitrogen

What Do Cover Crops Leave Behind? Comparing Sunn Hemp, Tepary Bean, and Cowpea