Fertility and Nutrients
Determining safe rates of seed-placed fertilizer
By Ross H. McKenzie PhD P. Ag.
Fertilizer research has shown clear benefits when modest amounts of some fertilizers are seed-placed. However, when commercial fertilizers are placed with seed, injury can result to germinating seeds. The amount of fertilizer that can be safely seed-placed depends on the types of fertilizers used, crop type, seedbed utilization (SBU), soil texture and soil environmental conditions, with soil moisture being very important. To understand seed-placed fertilizer safety, we first need to understand how fertilizer injury occurs.
Understanding fertilizer injury
Fertilizer injury can occur by two main mechanisms: ammonia toxicity and salt injury. Ammonia (NH3) toxicity occurs when urea [(NH2)2CO] is placed with or very near seed. During the urea breakdown process, ammonia is released near germinating seedlings, and high concentrations of NH3 near germinating seed can be very toxic. When soil moisture conditions are good, hydrogen (H+) ions from water (H20) will rapidly attach to ammonia to convert to ammonium (NH4-) which minimizes potential ammonia injury. The ammonium is then gradually converted to nitrate-N (NO3-) by soil bacteria for plant uptake. Ammonia toxicity is greater when: higher rates of urea are seed-placed, causing higher NH3 concentration levels; NH3 persists longer in soil which usually occurs when soil moisture is poor; NH3 persists longer in soil when soil pH conditions are higher (>7.5).
Salt injury occurs when higher rates of fertilizers are placed near germinating seeds. The salts in the fertilizer cause injury or death of the seedling. Injury occurs when the concentration of fertilizer salt in the seedrow is greater than the concentration of natural salts within the cells of the germinating seed, resulting in higher osmotic pressure in the soil versus the seedling. This causes water to move out of the seedling cells. When water moves out of plant cells, the tissue desiccates, causing injury or death of the seedling. The term “fertilizer burn” comes from the visual appearance of blackened seed and roots.
Soil environmental conditions, particularly seedbed soil moisture, will vary each spring. As a result, fertilizer injury may occur in one year but not another year, in the same field with the same crop and seed-placed fertilizer, due to soil moisture and temperature differences during seed germination and emergence. Under conditions with good soil moisture, dissolved fertilizer salts are less concentrated in the soil and will diffuse away from the fertilizer band and become diluted in the soil solution. When soil moisture conditions are very good to excellent, this will greatly reduce the osmotic pressure and little or no fertilizer salt injury will occur to germinating seedlings.
When seedbed soil moisture conditions are marginal or very poor, fertilizer salts become more concentrated in the soil solution. This results in a higher osmotic pressure, causing a greater potential for injury to germinating seedlings. Fertilizer injury is an increasing concern in drier spring soil moisture conditions immediately after seeding. Also, when soil temperature is cooler, roots grow more slowly, causing roots be exposed to the higher concentration of fertilizer for a longer period of time.
Salt index (SI) of a fertilizer is a measure of the salt concentration that fertilizer induces in the soil solution. Salt index is the ratio of the increase in osmotic pressure produced by a fertilizer material. Nitrogen (N), potassium (K) and sulphate-sulphur (SO4-S) fertilizers generally have higher SI values than phosphorus (P) fertilizer (see examples in Table 1). Salt index can be used to compare fertilizer materials but cannot be used to determine the amount of fertilizer that will cause injury.
Further, seedbed soil moisture conditions and soil characteristics are often variable in fields due to variable topography and variable soil texture conditions. When soil environmental conditions are less favourable, the negative effects of seed-placed fertilizer are greater in drier, upper-slope positioned soils and knolls versus lower-slope areas, which often have better soil moisture conditions.
Safe seed-placed N rates
Urea N is the most commonly seed-placed N fertilizer. Tables 2 and 3 summarize general seed-placed N recommendations from Alberta, Saskatchewan and Manitoba agriculture ministries for wheat, barley and canola. Recommendations in Tables 2 and 3 assume an adequate amount of phosphate (P2O5) is also placed with the seed. From Table 2, for cereal crops grown on a medium textured (loam) soil using a 11 per cent SBU, about 25 lb/ac of N in urea form can be safely placed with cereal crops based on Alberta and Saskatchewan recommendations, but Manitoba recommends only 15 lb N/ac. Generally, Manitoba recommendations for urea N placement are more cautious than Alberta and Saskatchewan recommendations. Differences in recommendations are the result of slightly different research results and interpretations. It is important to note that recommendations are based on good environmental conditions during seed germination and emergence. If soil moisture conditions are very good to excellent during germination and emergence, higher seed-placed urea N rates can be tolerated. However, if soil moisture is poor or very poor with just enough moisture to initiate germination, the safe N rates in Tables 2 and 3 should be reduced by about 50 per cent to avoid significant seedling injury.
ESN (Environmentally Smart Nitrogen, 45-0-0), is a polymer-coated, slow release urea-N fertilizer that can be seed-placed at about three times the safe rate of conventional urea-N. When seed-placed, ESN is released slowly over about 60 days under normal soil moisture and temperature conditions. It is important to note that if the protective polymer coating is fractured, or granules are cracked or broken, the polymer-coated urea will convert to ammonia relatively quickly which would affect seed safety.
Safe seed-placed P, K and S rates
Phosphate is often limiting in most Prairie soils and often placed with or near the seed for best crop response. Phosphate fertilizer has a relatively low salt index (Table 1). Table 4 provides the safe seed-placed rates of P2O5 at a 10 per cent SBU for various crops as recommended by Alberta, Saskatchewan and Manitoba agriculture ministries. The recommended rates are somewhat different due to slightly different research results and interpretations. The safe rate for cereal crops is about 50 lb/ac of P2O5 for wheat and barley in all Prairie provinces. For canola, the safe seed-placed rates of phosphate at a 10 per cent SBU for Alberta, Saskatchewan and Manitoba are 15, 25 and 20 lb/ac of P2O5, respectively. In Alberta, injury has been observed at rates as low as 15 lb/ac of P2O5, therefore the recommended rate is more cautious compared to those for Saskatchewan and Manitoba. Conversely, recommendations for safe rates of P2O5 with peas are higher in Alberta versus Saskatchewan due to differences in observed field research trials.
When considering seed-placing other fertilizers, such as potassium or sulphate sulphur, very careful consideration is needed. Many farmers place phosphate with the seed and even some N fertilizer; therefore, great care is needed when considering the combined effect of seed-placed P coupled with K and/or S fertilizer. Remember, both 0-0-60 (potassium chloride) and 21-0-0-24 (ammonium sulphate) have a relatively high salt index per unit of nutrient (Table 1).
As a general rule, total pounds of (P2O5) plus total pounds of K2O should not exceed the maximum safe rate of seed-placed phosphate. For example, for cereal crops, the maximum safe rate is 50 lb/ac of P2O5. If 30 lb/ac of P2O5 is seed-placed with wheat, then an additional 20 lb/ac of K2O could be seed-placed safely. This rule applies only under good soil moisture conditions for cereal crops. The injury risk for canola and other small seeded crops is much higher. Often, the safest plan for small seeded crops is to seed-place the safe rate of P2O5 and side or mid-row band any K fertilizer away from the seed to minimize potential salt injury.
Sulphate-sulphur is the form of S taken up by crops, and it is the fertilizer form needed to correct S deficiency. Seed-placed SO4-S can be very beneficial for early crop growth and development when surface soil (0 to 6 inches) is low in plant-available sulphate. The maximum safe rate of SO4-S that can be applied with the seed for cereal crops with a 10 per cent SBU is 20 lb/ac of SO4-S, when phosphate fertilizer is seed-placed at a rate of 30 lb/ac of P2O5. For canola, the safe seed-placed SO4-S rate at a 10 per cent SBU should not exceed 10 lb/ac, if the seed-placed phosphate fertilizer rate is not more than 10 lb/ac of P2O5. Rates of seed-placed ammonium sulphate near or above 20 lb/ac of S have been shown to reduce the emergence of canola under some conditions.
For more detailed information, it is important to refer to your provincial agriculture ministry website, as well as to seek advice from knowledgeable, experienced soil and crop specialists.