The following 15 pointers will help you get the most from your phosphorus (P) and potassium (K) fertilizer investment. They come from Dan Kaiser, University of Minnesota Extension nutrient management specialist.
1. Soil-test regularly and follow those recommendations. Although soil tests aren’t perfect, they’re your best indicator of nutrient sufficiency.
2. Monitor soil pH, the master variable that affects nutrient availability. A 6.0 pH is sufficient for most nutrients to be available. However, it doesn’t really pay to lime unless soil pH nears 5.0 when growing corn or soybeans.
3. Know that, at best, a corn crop takes up 30% or less of applied P. The remainder joins a soil storage pool that’s released over time, or it binds to soil ions in plant-unavailable form. Most P taken up comes from the soil.
4. Remember that P binds to calcium, aluminum, and iron soil ions, becoming less available to plants. The decision to apply liquid in-furrow starter fertilizer (which typically costs more per unit of nutrient than dry fertilizer)depends where you are on your soil test requirements. If you apply starter, be sure to subtract what’s applied from your broadcast or full-season application. A rate of 5 gallons per acre of 10-34-0 represents 20 pounds per acre phosphate (the rate you want for an early growth response from corn), Kaiser says.
5. Don’t exceed 7.5 to 8 gallons per acre of a starter fertilizer rate of 10-34-0. For clays and loams, 5 gallons per acre is at the top of the economic payback range, he says. “Cut back by at least half that rate on sandy soils. Placing fertilizer on the corn seed always presents some risks, so keep rates low.”
6. Broadcast P if soils are testing low in P. “Low-input, in-furrow starter strategies may work on medium- to high P-testing soils and may return a higher return per nutrient unit applied,” he says. “It’s important to monitor soil tests for P when using low-input strategies, to detect low levels that would benefit from a broadcast or band fertilizer application away from the seed row.”
7. Be aware that starter fertilizer doesn’t pay off in all circumstances. In-furrow starter can produce the same corn yield as a higher broadcast P application.
8. Avoid starter fertilizer if the broadcast application supplies all of corn’s required P. “Starter’s P needs to be accounted for to avoid wasting a lot of money,” he says.
9. Don’t use in-furrow starter for soybeans. They’re less tolerant of seed-placed fertilizer.
10. Cover starter’s cost with decreased grain moisture at harvest. Where grain moisture is 20% to 25% and above, it’s possible to cover the starter’s cost with decreased grain moisture at harvest, Kaiser says. (Remember, you still need to take the in-furrow starter’s nutrients into account when setting your overall P rates.)
11. Apply phosphorus on early-season corn plants. Choose a starter formulation that can supply at least 10 pounds per acre P2O5 economically. There’s no difference in fertilizer phosphorus sources relative to their ortho- or poly-phosphate content.
12. Use lower-cost nutrient formulations. “A straight starter N and P mix like 10-34-0 is more cost-efficient than a low-salt nutrient formulation,” Kaiser says.
13. Keep starter N, K, and S rates low to avoid salt burns. Don’t exceed 10 pounds per acre N plus K20, and you’ll avoid too high a salt load.
14. Scrutinize chelated micronutrients. “Chelated micronutrients tend to increase costs and will likely not result in a positive return on investment if your soil test shows sufficient nutrient levels,” Kaiser says.
15. Don’t count on in-furrow inputs to last all season. “In-furrow inputs should be limited to an early-season kick,” he says. “You can’t apply a full season’s worth of inputs safely in furrow in low-testing soils.”