P and K Fertility in Forage-Dominated Rotations: Are You Keeping Up? By Adam Parker, Maizex Territory Manager

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P and K Fertility in Forage-Dominated Rotations: Are You Keeping Up?

Corn silage is an essential part of many livestock operations as a primary feed ingredient. Maximizing silage crop performance is key to profitability as a result. As we approach corn planting time, it is a good idea to refresh and review your corn fertility program. Many of us may be rusty on soil fertility or assume that our manure applications are sufficient in feeding the crop. The best way to determine if there is additional need for nutrients is to soil test on a regular basis, as this will provide an indication if your base fertility is low, medium, or high. If you continue to test on a regular basis, it will also give you a fertility trendline. That is a reliable way to evaluate if your fields are increasing or decreasing in fertility and if you should adjust your fertility plan going forward.

Many livestock producers underestimate the total nutrient removal when they harvest forage crops of alfalfa or corn silage. When compared to a standard grain crop rotation, in which most of the stover is not removed, the removal rates are significantly higher when whole-plant forage crops are harvested. Below are three examples of crop rotations and the total Phosphorus (P) and Potassium (K) removal over three years for each.

Corn-Soy-Wheat

Crop Yield

(bu/ac)

Phosphorus Removal

(P2O5) lbs/ac

Potassium Removal

(K2O) lbs/ac

Corn (grain only) 200 80 52
Soybean (grain only) 50 43 70
Wheat (grain)

Wheat straw (removal)

100

3,300 lbs of straw

60

5

36

30

Total   188 188

 

Alfalfa-Alfalfa-Corn Silage

Crop Yield

(MT/ac @65%)

Phosphorus Removal

(P2O5) lbs/ac

Potassium Removal

(K2O) lbs/ac

Alfalfa 10 52 224
Alfalfa 10 52 224
Corn Silage 25 91 207
Total   195 655

 

Corn Silage-Corn Silage-Corn Silage

Crop Yield

(MT/ac @65%)

Phosphorus Removal

(P2O5) lbs/ac

Potassium Removal

(K2O) lbs/ac

Corn Silage 25 91 207
Corn Silage 25 91 207
Corn Silage 25 91 207
Total   273 621

 

There are three main approaches to consider for P and K management. The OMAFRA recommendation is based on the “Sufficiency Approach” or what level of nutrients need to be supplied to maintain yield potential and provide an economic return for that year. The crop removal rate calculates what is removed off the field with harvest, and those nutrients are accounted for and re-applied for that season. The goal with crop removal is to maintain the soil fertility at the current level and not let it decline.

The “Build and Maintain” approach is generally used on fields below optimum levels. The goal over time is to build the base fertility levels to some pre-determined level where economics, crop yield, and environmental considerations are all factored in. In recent Ontario research, reasonable optimum levels of 20 PPM for P and 120 PPM for K have been suggested. The grower calculates how much fertilizer is needed to bring the soil fertility levels up to the optimum level and includes the annual crop removal as well. In many cases, the build component may be more than practical for a single application. In this case, a timeframe of several years may be needed to apply the extra fertilizer. It should be noted that in some cases the correct approach may be to “draw down” and maintain if soil test levels are very high, particularly in the case of elevated soil test P values.

Case Study

In this case study, a grower’s field is set for corn silage in the coming year. It gets soil tested, and the results come back at 15 PPM for Phosphorus and 100 PPM for Potassium. These levels are considered moderate and are slightly below the optimum soil test levels selected of 20 PPM P and 120 PPM K. Below are three fertility recommendations for this grower’s field using 1) OMAFRA’s sufficiency approach, 2) replacing crop removal, and 3) a build and maintain approach.

Table 1. Various fertilizer recommendation strategies for a 25 MT per acre corn silage crop.

Recommendation OMAFRA Sufficiency Crop Removal Rate Build & Maintain1
Nutrient P2O5 K2O P2O5 K2O P2O5 K2O
Application rate (lbs/ac) 18 44 91 207 (B)175+(M)91

=266*

(B)400+(M)207

=607*

 

1Assumptions: building soil test P by 1 PPM requires 35 lbs/ac P205, while building soil test K by 1 PPM requires 20 lbs/ac K2O.

*These rates are too high to be practical for a single-year application. The grower will need to select a longer timeframe to build soil test levels to the desired level.

Some Key Reminders

  • Soil testing is the key to allow for a sound soil fertility management strategy. Paying attention to your soil fertility levels on a frequent basis will pay dividends through yield performance and yield consistency over time.
  • Forages can remove large amounts of P and K. These removal rates need to be addressed by way of manure, fertilizer, or other amendments.
  • Short-term response to soil test levels and maximizing economic returns for that year are accomplished by following a sufficiency approach (i.e., OMAFRA recommendations).
  • Longer-term strategies for P and K management and increasing crop productivity should consider crop removal rates and optimum soil test levels (i.e., Build and Maintain approaches).

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