What fertilizers are used on Inga agroforestry plots?

Fertilizer, mulch, and mineral supplements in the Inga system

Note: This is a bricolage protocol; which means we are building it in public. This article is still under construction, and information is being corroborated. Especially the mineral supplements.

Mulch and Inga alley cropping

Great news. Inga is a system where stripped and growing foliage gives mulch and weed abatement. Which is why we love it!

Inga works for both because its leaves are recalcitrant (slow to rot). So they stick around and suppress weeds, while giving nutrients.

To quote the Inga Foundation, "the foliage is stripped from the branches and mulched onto the soil surface...The mulch will settle at about 100–150 mm in depth within a few days." (Hands 2021)

Fertilizer and Inga alley cropping

Savimbo does not use any synthetic fertilizers, although we do use bokachi and biochar. Hands uses rock phosphate rather than soluble synthetic forms (ammonium nitrate, urea, triple superphosphate), citing the leaching risk in acid ultisols.

Inga has two important mechanisms. Nitrogen-fixation matters; Phosphorus-retention is the breakthrough.

• Nitrogen. Inga is a nitrogen fixing tree, and will function as a natural fertilizer.
• Phosphorus. Hands' core finding across 37 years is that 1995, 1998, and 2021 is that Inga's recalcitrant mulch retains and recycles phosphorus in the surface soil layers — which is the actual limiting nutrient in tropical ultisols.

Pretty much any legume fixes N. Only Inga maintains a permanent mulch cover on acid soils for P-cycling.

Mineral supplements and Inga alley cropping

There are competing reports from Amazonian and Andean ecosystems about mineral supplementation. Both Ecuador and Colombia report no use of mineral supplements. It's possible that Inga Foundation's work in depleted soils at higher altitudes in Honduras required more mineral amendments than these plots, or that the information was simply not shared.

We're A/B testing mineral amendments on our plots and will share field observations here. Here is a summary of what we got from the Honduras literature.

Mineral supplements from Hands literature

The Inga Foundation working in Costa Rica and Honduras has been consistent that mineral supplements are essential, not optional, in degraded tropical soils:

• Rock phosphate at planting. This has a slow release onto the mulch layer. Hands 1998 and 2021 both document that Inga alleys without rock-P fail. The San Juan responses to rock-P were "immediate, significant, and persistent in every ecosystem component" across the entire 7-year trial.
• Dolomitic lime + K-Mag (MgSO₄ + K₂SO₄) for very degraded soils. This is a mineral amendment. Hands 2021 describes chlorotic I. edulis saplings stuck at 1–1.5 m height that grew to 3–4 m in three months after dolomite/K-Mag application. He calls this "one late lesson from the Cam Projects."
• Ongoing top-up: Hands 2021, closing line: "The process can be repeated annually, provided that minerals extracted in the grain crops are topped up from time-to-time and that gaps from occasional tree mortality are refilled."

Hands' position, condensed: Inga + rock-P = sustainability; Inga alone = eventual failure on degraded latosols.