Forage yields are disappointing this year in many parts of the province. Demand for hay is very strong and hay prices are at record highs. Good fertility is essential to both yields and persistence of forage crops.  Phosphorus (P) and potassium (K) fertility management is often overlooked, but is fundamental to producing enough forage to feed livestock and being competitive in today’s forage market.

Crop Removal Of P and K

Forage crops remove a lot of nutrients and therefore have high nutrient requirements. With an alfalfa-grass mixture, a typical amount of P and K removed per tonne of hay harvested is equivalent to 13.5 lbs (6.1 kg) of P₂0₅  and 54 lbs (24.6 kg) of K₂O. As an example, assuming a mixed stand with a modest yield of 3.2 tonnes per acre per year, hay will remove about 43 lbs (20 kg) of P₂0₅ and 173 lbs (78 kg) of K₂O every year.

Unlike nitrogen, forage crops cannot generate P or K out of thin air. Without replacing P and K with manure or commercial fertilizer, the soil tests will drop quickly. Assuming that it takes about 35 lbs/ac of P₂0₅ and 20 lbs/ac of K₂0 to move the soil tests by 1 ppm on some soils (these amounts will vary according to soil type), after only 4 years the P soil test could drop by 5 ppm and the K by 35 ppm. This is easily enough to significantly reduce forage yields if soil test levels drop below optimum levels. We also need to maintain soil nutrient levels for the next crops in the rotation. At lower soil test levels, this “soil mining” is not acceptable. Yet it goes on in many hay fields every year.

There is a wide range of soil fertility levels found in hay fields across the province. Dairy farms that apply a lot of manure typically have high P and K levels. However, K deficiency has become more common. Hay fields that are infrequently (or never) rotated that seldom (or never) receive manure or commercial fertilizer are typically very low in soil fertility and yield.

Soil Testing

Soil testing is essential. Knowing how much P and K is available in the soil to start with is critical. Take a representative soil sample, send it to an accredited lab and use the results to determine optimum fertilizer rates. Keep records. Monitor whether fertility is increasing, decreasing or staying in an optimum range over time.  Soil samples should be taken at least every 3 years. The time and effort it takes to do the soil sampling seems to be a obstacle, but with the cost of fertilizer there is likely no greater potential return on the cost and extra effort. Compare your fertilizer bill with lab and mailing costs plus an incentive for the kids to do the sampling for you!  Refer to OMAFRA Factsheet 06-031 “Soil Sampling & Analysis” www.omafra.gov.on.ca/english/engineer/facts/06-031.htm.

Soil Analysis Report Interpretation

When you get your report, check the sodium bicarbonate phosphorus (P) and ammonium acetate potassium (K) soil test levels (ppm). (Use only these tests, as other tests (Bray or Mehlich) cannot be interpreted using our calibration data.) How do the P and K soil test levels look?

Research shows that the yield loss of alfalfa is significant when P soil test levels are much below 12 ppm and K soil tests are below 120 ppm. The yield curves at low soil test levels are steep. A positive yield response from applying fertilizer will be seen when soil tests are below these optimum levels. On the flip side, the yield curve at high soil fertility levels is flat. Don’t expect any extra yield from applying fertilizer once the soil test have been built up to higher levels. In these cases, you can choose to apply fertilizer to replace the nutrients removed by the crop to prevent future nutrient deficiencies, but don’t expect extra yield from that maintenance application.

P & K Recommendations For Established Stands

Tables 1 and 2 provide the OMAFRA P and K recommendations on established forage stands. These recommendations use the “sufficiency approach” to supply the optimum needs of the current crop by giving the greatest short term (1 year) net returns to fertilizer.

A longer term method also used to determine fertilizer applications is the “buildup and maintenance approach”. This includes the amount of nutrient removal of the crop. It adjusts this by an amount required to increase (or decrease) the soil test levels  to targeted optimum levels, amortized over several years. Applying nutrients to soils that test much above these critical levels (12 ppm P and 120 ppm K) rarely results in a profitable response.  Refer to OMAFRA Publication 611, Soil Fertility Handbook.

If manure is applied, reduce the fertilizer application according to the amount of P and K in the manure. For P and K recommendations at seeding (banded or not, with or without a nurse crop), or information on nitrogen rates, pH, manure application and micronutrients (boron, sulfur), refer to the Forage Fertility section of OMAFRA Publication 811, Agronomy Guide.  www.omafra.gov.on.ca/english/crops/pub811/3fertility.htm

Table 1 – Phosphate Recommendations For Established Forage Stand (Based on OMAFRA-Accredited Soil Tests)

 Sodium Bicarbonate Phosporus Soil Test (ppm)

¹ HR, MR, LR, RR & NR denote respectively – high, medium, low, rare and no probabilities of profitable crop response to applied nutrient. Profitable response to applied nutrients occurs when the increase in crop value from increased yield is greater than the cost of the applied nutrient.

Table 2 – Potash Recommendations For Established Forage Stand (Based on OMAFRA-Accredited Soil Tests)

 Ammonium Acetate Potassium Soil Test (ppm)

¹ HR, MR, LR, RR & NR denote respectively – high, medium, low, rare and no probabilities of profitable crop response to applied nutrient. Profitable response to applied nutrients occurs when the increase in crop value from increased yield is greater than the cost of the applied nutrient.

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