Foliar fertilizing - to spray, or not to spray?

Many of us at Seven Springs subscribe to a number of excellent farming newsletters from various universities, research institutes and other organizations around the country. A recent MOFGA mailing contained an article from Crop Specialist, Dr. Caleb Goossen on foliar fertilization, which reminded me of a conversation we have frequently. 

Foliar fertilization is commonly practiced, and for certain situations (see below) it is an excellent option. But should it be the default option that it often seems to be? Dr. Goossen raises this questions and draws on his experience, technical understanding, and some central principles of organic farming to support the case for a soil-first mentality when it comes to crop nutrition.

We encourage you to read Dr. Goossen's article, and we'll share some additional thoughts below.

A basic soil amending approach that we recommend for most typical vegetable cropping situations is to

• Begin with dry fertilizer amendments based on soil report recommendations, applied at or just prior to planting and again as necessary or prescribed
• If crops need additional nutrients, side or top dress with more dry fertilizer as advised in the soil report recommendations or as your experience dictates
• If side or top dressing is impractical because of access issues like a closed canopy or plastic mulch, then use liquid or soluble fertilizers (preferably through drip irrigation, with foliar sprays as a second option) to make corrections based on known demands, tissue tests or deficiency symptoms

The two primary reasons for this approach are that

• Plants typically take up nutrients through their roots much more efficiently than in any other way
• Liquid and soluble fertilizers are invariably more expensive than dry fertilizers

A couple of years ago we were at a university research farm, and the researcher we were visiting described a trial they were conducting on foliar feeding. The researchers were aiming to understand, on a practical level, how well foliar macronutrient sprays work in an organic setting.  To help determine whether foliar-applied macronutrients were being absorbed through leaves or through runoff, the plants in the trial were either left with bare soil underneath, or "skirted" to cover the soil and prevent any nutrients from making their way down to the roots. The plants with the skirts invariably showed lower nutrient levels in tissue tests, and had less growth and lower yields. This led to the pretty clear conclusion that the nutrients making their way into the plants were primarily doing so through the soil and roots. 

So does this mean that there is no place for foliar nutrient applications? Not at all! Sometimes a foliar or liquid application can be the most efficient or appropriate means of getting a nutrient where it needs to be, when it needs to be there.  

Foliar applications are often the best option when aiming to apply small amounts of a specific nutrient evenly over a large area, as is commonly necessary with micronutrients like boron, zinc, copper, manganese, etc.

Likewise, when soil pH is much above 7, the availability of micronutrients may be limited, and in these situations we often recommend foliar application of chelated micros like the Biomin or MilAgro lines.

It may also make sense for to include soluble nutrients when making other types of foliar applications, such as a pest or disease controls, although it's important to review product labels to ensure compatibility.

Grain farmers often like to use soluble sprays for targeted nutrient applications at strategic points in the development of their crops, while many vegetable growers find that the simplest way to keep plant starts in trays well fed is to spray them with liquid fish or some other soluble nutrients every week or so. 

However you like to fertilize, we always recommend getting regular soil and/or tissue samples from a reputable lab, and working with experienced folks to help guide your decisions.