Potassium: Are your levels slipping?

 Potash levels are falling in some Ontario fields according to soil and tissue samples.

K depletion is often a result of not fully accounting for K removal during forage years of the rotation. 3 years of alfalfa can remove more than 900 lb K2O /ac (Table 1). That’s 1500 lb/ac of muriate of potash (0-0-60). For example; in a loam soil with one application of dairy manure per year on a 3-cut alfalfa system, soil test K dropped from 132 to 73 ppm within 3 years.

Accounting for removal is an easy way to track nutrient budgets over time.  This is what the NMAN3 software (available FREE at http://apps.omafra.gov.on.ca/NMAN/  ) demonstrates, as shown in Table 1.  In the software, the user enters crop to be grown, expected yields, and nutrient additions from manure and fertilizer. The program calculates nutrients removed off the field from the harvested portion of the crop. The nutrient budget indicates whether an increase or decrease in soil test level over time can be expected. Soil sampling can confirm whether the actual changes match the predictions. To monitor soil K, collect samples at similar soil moisture content and time of year so that values will be comparable. Both soil moisture content at sampling and movement of K out of crop residues by rains after harvest can significantly affect K soil test (Mallarino et al 2011; Franzen 2011).

 Time & Place

Fall is a good time to apply potash in forage years of the rotation. For row crops, where soil test is medium to low (< 100 ppm), some K should be banded near the seed with the starter. Safe rates depend on crop, row spacing, fertilizer source, quantity of N + K, and placement distance from the seed. Consult the Guidelines for Safe Rates of Nutrients Applied at Seeding to check if the amount will injure the seedling.

 Bonnie Ball & Christine Brown

Table 1. Nutrient budget (lb/ac) from NMAN software  
  N  P2O5  K2O  
Fertilizer spring 2011 151 20 30 Addition
Corn, grain 190 bu/ac -157 -80 -55 Removal
 fall 2010-2011 -6 -60 -25 Balance
Fertilizer spring 2012 0 0 0 Addition
Soybeans @ 60 bu/ac -233 -50 -84 Removal
fall 2011-2012 -233 -50 -84 Balance
Dairy 4000 gal/ac 2013 150 60 100 Addition
Alfalfa 5.5 t/ac -346 -71 -330 Removal
fall 2012-2013 -196 -11 -230 Balance
fall 2010-2013 -435 -121 -339 Multi-Yr balance