- Author: Michelle Leinfelder-Miles
It is the time of year when the harvest of our summer crops has concluded. Some growers may be planting small grains, and let us hope that winter rains nourish these crops. I received an inquiry from a grower at the end of harvest regarding nutrient removal with the harvest. This grower, in particular, had questions about how much potassium (K) is removed with the grain and straw/stover of a crop. Alternatively, we can think about how much K is added to the soil when crop residues are incorporated. This grower farms on a low K soil in the Delta and wants to know how much K could be available for his tomatoes next season, a crop that has high K demand.
The California Department of Food and Agriculture Fertilizer Research and Education Program (CDFA FREP) provides crop fertilizer guidelines for nitrogen (N), phosphorus (P), and K. These guidelines were developed by Daniel Geisseler, Nutrient Management Specialist at UC Davis, using research results from California and elsewhere when California information was not available. The guidelines for wheat state that the concentration of K in the grain is 0.4-0.5%, and the concentration of K in the straw is approximately 1.5%. What this means is that for a wheat field that yields 3 tons/acre, 24-30 lbs K/acre would be removed with the grain (0.004 or 0.005 * 6000 lbs grain/acre = 24-30 lbs K/acre). Likewise, 90 lbs K/acre (0.015 * 6000 lbs straw/acre = 90 lbs K/acre) would be removed with the straw if the straw is not incorporated, assuming that approximately one ton of straw is produced per ton of grain. For corn, the grain is approximately 0.4% K, and the stover is about 1.5-2.5% K. Whether the corn is harvested for grain or silage, approximately 40 lbs K/acre will be removed with the grain at harvest (assuming a 5-ton crop). If the corn is harvested for silage, or if the stover is removed, then approximately 150-250 lbs K/acre will also be removed with the stover (again, assuming a 5-ton crop). If the stover is incorporated, then the grower is returning this amount back to the system.
The grower also asked how soon the K would be available for a subsequent crop (like his tomatoes) if he incorporates the crop residues. To get an answer for this question, I consulted with UC Davis graduate student, Jordan Wade, and UC Davis professor of biogeochemistry, Will Horwath. Both of them surmised that the K should become available fairly quickly because K is not a structural component of the plant. Contrasting K and N, for example, K is floating around in the cells in a soluble form; whereas, N is bound with carbon, forming structural parts of the plant. K is in a form that plants can use (it does not need to be mineralized into a plant-available form, as with N), so when the crop residue is incorporated and breaks down, the K should be readily available for plants, unless it gets adsorbed to soil particles.
In agricultural areas where little-to-no “maintenance” K has been applied over the years, it is possible that crops have depleted soil K. The FREP guidelines recommend applying K to wheat fields if the pre-plant soil test is less than 40 ppm. In corn, it is recommended to apply K if the pre-plant soil test is less than 50 ppm. It is recommended to take pre-plant soil samples for nutrient testing before each new crop.
- Author: Michelle Leinfelder-Miles
In late August, I was requested by a crop consultant to visit an alfalfa field in the Delta that was exhibiting symptoms like those in the picture. The leaves had little white spots near the margins, and generally, the plants had not grown well after the previous cutting. These symptoms are characteristic of potassium (K) deficiency. Delta soils are often deficient in K, so the consultant was going to follow-up with a soil nutrient test.
Fall is the season in which to address alfalfa fertilizer needs. Two nutrients that alfalfa frequently needs are phosphorus (P) and K. [Nitrogen fertilizer (N) is seldom required or profitable in alfalfa because root nodules contain N-fixing Rhizobium bacteria.] P and K should be applied between October and February because it could take 60-90 days for the crop to fully respond to fertilizer application.
A deep-rooted crop, alfalfa can scavenge nutrients from where other crops cannot, but because it has a long growing season, alfalfa has a long season of nutrient demand. Additionally, each cutting removes large amounts of nutrients with the plant tissue. Therefore, proper nutrition is important in maintaining alfalfa yield and quality year after year.
P – important for seedling vigor, root development, and early season growth – is the most commonly deficient nutrient because it can get tied up by the soil. P deficiency may be difficult to diagnose because it can be mistaken for moisture stress, but in general, plants are stunted and have smaller leaves. K is often required because large amounts of it are removed with each cutting. Additionally, sandy soils and/or soils with a long history of alfalfa production can be low in K.
Soil sampling is a reliable way to assess P and K deficiency, but tissue sampling is another way to test for deficiency. P is deficient when soil levels are 4-P (when plant samples are taken at 1/10 bloom). K is deficient when soil levels are 1/10 bloom).
Use a granule (0-45-0, 11-52-0) or liquid (10-34-0) fertilizer to correct P deficiency. These sources are the most economical. If soil or plant tissue tests show deficient levels, then apply P at a rate of 120-180 lbs P2O5/acre (if yield was around 8 tons/acre) and at a rate of 180-270 lbs P2O5/acre (if yield was around 12 tons/acre). Use muriate of potash (0-0-52) to correct K deficiency, or use potassium sulfate (0-0-52, 18% sulfur) if sulfur was also deficient. Correct K deficiency by applying it at a rate of 300-400 lbs K2O/acre (if yield was around 8 tons/acre) and at a rate of 400-600 lbs K2O/acre (if yield was around 12 tons/acre). Single applications of P should not exceed 100-150 lbs P2O5, and single applications of K should not exceed 200-300 lbs K2O. If soil or tissue tests indicate that higher rates are needed, then apply half of what is needed in late fall/early winter and the other half after the second or third cutting. Both P and K are effectively taken up by plants whether pre-plant incorporated or surface applied in established stands. Use these rates to guide your fertilizer applications – remembering that soil type, climate, and yield will influence fertilizer needs – and keep good records of all laboratory results, fertilizer applications, and crop observations. These records will be helpful in developing a long-term, economical fertilization program.
Additional information on how to soil and tissue sample, as well as tables to guide adequate nutrition, are available from this handout.