Take care to apply the right amount of fertiliser when placing with seed

There are a number of factors to consider when placing fertiliser with seed according to Dr. Mike Stewart from the International Plant Nutrition Institute in Norcross, Georgia, USA.

Placing fertiliser in-furrow with the seed during planting is a common practice in small grain production and to some extent in row-crop production. Placing fertiliser with the seed can be an effective and beneficial management practice, but over- application and mismanagement can result in seedling damage, and ultimate stand and yield loss. The type of crop, fertiliser source, row spacing, and soil environment all affect how much fertiliser can be safely applied with seed.

Type of crop: Some crops are more susceptible to injury from in-furrow fertilisation than others. Oil seed crops are particularly sensitive; therefore most guidelines allow no fertiliser placed with the seed of these crops. The general order of sensitivity (most to least) among major crops grown on the Great Plains in the United States is soybeans > sorghum > corn > small grains.

Type of fertiliser: Fertilisers are salts, and these salts can affect the ability of the seedling to absorb water… too much fertiliser (salt) and seedling desiccation or “burn” can occur. Some fertiliser materials have a higher salt index or burn potential than others. Salt index values are usually included in basic agronomic texts, or are available from fertiliser dealers or extension resources such as government bodies or universities. As a general rule, most common nitrogen (N) and potassium (K) fertilisers have higher salt indexes than phosphorus (P) fertilisers; therefore, a common predictor for the potential for salt damage is the sum of N+K2O per acre (0.4 ha) applied with the seed. For example, most guidelines for corn (maize) in 30 inch (76.2 cm) rows will allow for no more than 10 lb (4.5kg)/A of N+K2O in medium to fine textured soils — assuming no urea-containing products are used.

Ammonia formation potential of fertiliser: Fertilisers that have the potential to release free ammonia can cause ammonia toxicity to germinating seeds or young emerging seedlings. Thus, extra caution must be used with in-furrow placement of urea-containing fertilisers. In some cases urea-ammonium nitrate (UAN) or urea can be applied successfully in-furrow in small grain production, but this requires careful consideration of several factors including those discussed below.

Row spacing: For a specific set of circumstances (i.e. crop, soil conditions, etc.) the safe rate of in-furrow fertiliser increases as row spacing narrows or decreases. A narrowing row space has the effect of diluting fertiliser over more linear feet (metres) of row.

Soil type and environment: Soil conditions that tend to concentrate salts, or stress the germinating seed, increase the potential for damage. So, the safe limit for in-furrow fertilisation is reduced with sandier soil texture and in drier soil conditions. Also, environmental conditions that induce stress and/or slow germination (e.g. cold temperature) can prolong fertiliser-seed contact and thus increase the likelihood of damage.

Seed bed utilisation: The more scatter there is between seed and fertiliser in the seed band or row, the more fertiliser can be safely applied. The type of planting equipment and seed opener influences the intimacy of seed-fertiliser contact. The concept of “seed bed utilisation” (SBU) has been used to address this factor. SBU is simply the seed row width divided by the row width (i.e., proportion of row width occupied by seed row). The wider the seed row for a specific row width the greater the SBU. As SBU increases so does the safe rate of in-furrow fertilisation.

* Reprinted from the International Plant Nutrition Institute, Plant Nutrition Today Series by Dr Mike Stewart. http://www.ipni.net/pnt