Using fertilizers to satisfy crop nutrient needs

Fertilizers play an important role in satisfying crop nutrient needs, even when manure is applied to fields. Take a look at a general approach to using fertilizers as a supplement to manure applications.

1. Determine crop nutrient guidelines, based on the crop, soil test, soil type, manure and/or sod histories, etc.
2. Find out what fertilizer materials are available to you.
3. Strategic use of fertilizer #1: Plan banded starter fertilizer applications on plantings and new seedings.
4. Allocate manure based on nutrient risk indices, manure inventories, and remaining nutrient guidelines (i.e. after the starter fertilizer plan).
5. Satisfy remaining fertility needs with sidedress N applications on corn (strategic use #2) or topdressed fertilizer on perennial forages and small grains (strategic use #3).

Let's look into the 3 major strategic uses of fertilizers to support manure nutrients on dairy farms.Consider the following two fields for the example.

Field 1: a 4^th year corn field that has received some manure in the past will also receive manure before corn planting this year. It has medium soil tests for both P and K. The nutrient management planner determined that based on sod and manure histories, the crop type, and soil test levels, it requires an additional 110 lbs N/acre, 40 lbs of P₂O₅/acre, and 50 lbs of K₂O/acre. Based on the Phosphorus Runoff Index, we'll apply manure to meet the N requirement.

Field 2: a 4^th year field of reed canarygrass (a perennial cool season grass). It's intensively managed, with 3 cuts per year.

Strategic Use #1: Starter Fertilizer

Let's focus on Field 1 for now. How should we use fertilizer and manure to satisfy the nutrient requirements? Start by thinking back to the basic fertilizer guidelines for corn, below.

Fertilizer Approaches with Corn
	N	P2O5	K2O
Manure applied this year.	20-30 lbs of N in the band. PSNT and, if deemed necessary, side-dress any additional N.	Very Low to Medium Soil Test, then 25 lbs P2O5/acre in the band with the rest through manure. High or Very High Soil Test, then 0 lbs P2O5/acre, period.	Usually can catch it all with manure, but not a problem in the starter band as long as <80-100 lbs N+K2O are applied to avoid salt injury.
No manure to be applied this year.	20-30 lbs of N in the band. PSNT and, if deemed necessary, side-dress any additional N.	Very Low to Medium Soil Test, then 25 lbs P2O5/acre in the band with the rest through manure. High Soil Test, then 0-25 lbs P2O5/acre. High or Very High Soil Test, then 0 lbs P2O5/acre, period.	Banded (<80-100 lbs N+K2O) or broadcast applied.

Given your choice, the fertilizer supplier offers 6-24-24, 15-15-15, 46-0-0, 18-46-0, and 0-0-60 as dry fertilizers and cannot make blends on site.

We want 25 lbs each of N, P₂O₅, and K₂O to be applied per acre in the starter band. The best option in this case would be a fertilizer with a 1-1-1 N, P₂O₅, and K₂O ratio.

We also need to ensure that the starter fertilizer application won't hurt the emerging corn plants. If we assume that the 15-15-15 blend that you used was made from Ammonium Sulfate, MAP, and Muriate of Potash, does our rate and blend pass the following test?

Guidelines for safe and effective use of starter fertilizer:

• For row crops, band 2 inches below and 2 inches to the side of the seed furrow.

• Ideally, minimal Urea and DAP in band:
• High pH may cause low grade NO₂ toxicity.
• No more than:
  • 30 lbs/acre of actual N from urea,
  • 30 lbs/acre of actual P₂O₅ from DAP, or
  • a 30 lb/acre combination of these from fertilizers containing urea and DAP.
• No more than 80-100 lbs/acre any combination of N + K₂O in band to avoid salt injury to the emerging plant.

A plan has been made for a starter fertilizer application: 167 lbs of 15-15-15, supplying 25 lbs N/acre, 25 lbs P₂O₅/acre, and 25 lbs K₂O/acre. Let's say that 15 tons of manure will also be applied in the spring, giving 30 lbs N/acre, 60 P₂O₅/acre, and 25 lbs K₂O/acre. The balance of nutrients remaining is:

	N (lbs/acre)	P2O5 (lbs/acre)	K2O (lbs/acre
Nutrient Requirement	110	40	50
-less nutrients from this year’s manure	30	60	25
-less starter nutrients	25	25	25
= Balance	55	-45	0

The K₂O requirement is satisfied, the P₂O₅ requirement is surpassed because of the imbalance of N and P₂O₅ in the manure, and we still need 55 lbs of N/acre, perhaps through a sidedress application. We're doing OK, as the starter fertilizer N and, in this case, P will provide a yield response relative to the no starter option. The use of the starter fertilizer will be very efficient, because we banded the fertilizer near the seed at planting.

Strategic Use #2: Sidedress N Applications

We might need to supply 55 lbs N/acre to reach the N requirement. "Might?" you ask. Now that the corn is planted and starting to grow, we can wait and see how much N will become available during the spring and early summer. Reach back to the N Cycle and remember what processes convert N from organic forms to mineral, plant available forms, like nitrate (NO₃^-).

Since the degree of N mineralization and nitrification depends not only on springtime conditions, but also the types of organic matter in the soil, soil compaction, soil type, soil biota, etc., the Pre-Sidedress Nitrate Test (PSNT) has been developed for corn to help us gauge the degree of N mineralization and nitrification from organic sources. The PSNT is a soil test taken when the corn is 6-12 inches tall and can indicate whether a corn crop will respond to additional sidedress N.

read more: Nitrogen Guidelines for Field Crops in New York (Section 6 for PSNT)

Let's say that the PSNT indicated that the corn crop would not respond to additional N. In that case, we could abandon the original plan to apply the 55 lbs N/acre when the corn is ~12-24 inches tall. We'll save money, keep excess nutrients out of the field, and not lose any yield.

If the PSNT indicated that the corn crop does need the sidedress application of N, then we would set the 55 lbs N/acre recommendation in motion.

Strategic Use #3: Topdress application.

Now consider Field #2, a 4^th year field of reed canarygrass (a perennial, cool season grass). It's intensively managed, with 3 cuts per year. The field is properly limed and has medium/high soil test P and K. Given the intensive harvest schedule, 225 lbs N/acre is recommended in order to achieve optimum yields and quality. Manure applications on the grass at green-up in the spring leave ruts in the field, so N fertilizer applied with a high-flotation spreader is the best way to go. The 225 lbs N/acre recommendation should be split into three applications 100 lbs N/acre at green-up in the spring, 75 lbs N/acre after 1^st cutting (discounted if any manure is applied), and 50 lbs N/acre after 2^nd cutting (discounted if any manure is applied).

Urea is a fairly concentrated (46-0-0), relatively inexpensive form of nitrogen. Although it can cause seeding injury if not managed very carefully in the starter fertilizer band, for a topdress application on an existing grass stand, it poses little risk of injury.

Subsequent topdress applications of urea can be adjusted if manure is applied after the cuttings.

When used strategically, fertilizers on dairy farms can lead to higher yields, better quality crops, and greater nutrient use efficiency. Three common ways to use moderate amounts of fertilizer to complement manure are:

• Banded, starter applications for corn or new seedings,
• Sidedress N applications in corn, and
• Topdress applications on perennial forages and small grains.

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