Lime, Phosphorus, and a Trout Stream: Allocating a Tight Fertility Budget

Priority order: lime, then banded starter phosphorus, then potassium, and application method woven through all three.

Lime is first because pH 5.7 with a buffer pH of 6.5 means exchangeable aluminum is limiting root growth and phosphorus availability, and corn is more sensitive than soybean. Lime is also cheap per acre relative to the yield it protects.

Full lime at 2.5 tons x $32 = $80 per acre, which leaves $15 and buys no phosphorus. A defensible split:

• Lime, 1.5 tons/acre = $48. Splitting lime on a low-CEC soil (11.5 meq/100 g) is agronomically acceptable; the rest goes on after next harvest.
• MAP, 100 lb/acre = $32, supplying 11 lb N and 52 lb P2O5. Band this as a 2x2 starter at planting, which is about half the lab's build-plus-replacement P2O5 rate but places it where the seedling roots reach it.
• Potash, 68 lb/acre = $15, supplying 41 lb K2O. Total: $95.

The logic is that lime delivers the largest yield response per dollar at pH 5.7, banded P protects early vigor and limits runoff, and potassium at 118 ppm (medium) can be finished with a maintenance application after harvest. Nitrogen should be sidedressed, not applied preplant.

Late March on this field means the highest-risk combination: no-till, so no incorporation; a 4% slope; ground that may be frozen or saturated, so infiltration is near zero; and dissolved phosphorus sitting on the surface until the next rain. Phosphorus leaves fields mainly attached to eroded sediment and as dissolved P in runoff, and dissolved P is the form most available to algae in the stream. A high phosphorus index rating is very likely, and the 35-foot buffer will intercept some sediment but not dissolved P moving in shallow flow.

Changes:

• Drop the broadcast. Put the MAP in the starter band at planting, where it is incorporated in the seed trench and unavailable to runoff.
• Time lime for ground that is firm and not frozen, and avoid spreading immediately before a forecast rain.
• Apply phosphorus at replacement rate rather than build-up rate on the acres nearest the drainage, since 14 ppm is only slightly below optimum.
• Keep the 35-foot buffer intact and add a grass waterway or filter strip at the low edge if concentrated flow develops.
• Plant a cereal rye cover crop ahead of corn where the calendar allows, to hold soil over winter.
• Document the P index rating in the nutrient management plan, since the field is a regulated condition in many watersheds.

It saves one trip but shifts the loss risk in the wrong direction. October application means five to six months of exposure before the corn crop needs the nutrients, including freeze-thaw cycles, snowmelt, and spring storms, all on a no-till surface with a 4% slope. Fall-applied phosphorus broadcast on the surface has more opportunities to move off-site than a spring band placed at planting, and the phosphorus index will rate it accordingly.

Potassium adds a second problem. On a soil with a CEC of 11.5 meq/100 g, potassium is not strongly held, and fall-applied K can leach in coarse or wet conditions before the crop roots reach it.

A better cost saving is to keep the potassium as a single post-harvest application after the corn is off, spread on ground that is dry and firm and immediately followed by a cover crop, while keeping phosphorus in the planter band. If the operator truly wants one pass, the defensible version is a spring preplant band of MAP plus potash applied in a single operation ahead of the planter, not an October broadcast.