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

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.