Many early-season vegetable transplants are put in with starter fertilizer in the transplant water. This is primarily for the response to phosphorus often seen when transplanting in cool soils. Small amounts of nitrogen are often included to improve early vigour. Ammonium-nitrogen actually enhances the absorption of phosphorus by the plant.

In using starter fertilizer at transplanting, we want to gain the benefits to early growth, but avoid injuring the plant. Fortunately, the primary nutrient of interest in a starter - phosphorus - is fairly safe. Nitrogen and potassium salts can pose more of a risk. Including micronutrients in a starter solution increases the potential toxicity to the transplants. Different brands or batches of starter with the same fertilizer analysis could vary in salt levels, depending on the ingredients used to formulate them.

Many factors, including soil texture, soil moisture, organic matter, plant condition, temperature, form of nutrient applied, and even crop variety will determine what rates are safe under field conditions. Be aware that the salt level of the transplant solution can vary through the day, as the tanks are refilled. Fertilizer concentration can also pulse up and down when using a metering system.

We often think about salt levels or cost of product based on a certain fertilizer concentration, such as 1.5 L in 100 L of water. This table shows a comparison of some starter fertilizers I tested a few years ago.

Table 1: Comparison of the EC of 1.5% and 2.5% starter solutions and nutrient content of the fertilizer concentrate.
Fertilizer1

EC (mS/cm)

1.5%

EC (mS/cm)

2.5%

g/L or g/kg of fertilizer concentrate

N

g/L or g/kg of fertilizer concentrate

P2O5

g/L or g/kg of fertilizer concentrate

K2O

g/L or g/kg of fertilizer concentrate

N + K

3-18-18
7.31
12.01
41
244
244
285
9-18-9
7.42
11.86
117
234
117
234
6-24-6
7.84
13.32
82
328
82
164
6-24-6
8.34
13.12
82
328
82
164
10-31-4 IBA & PHCA
10.12
16.16
131
407
52
184
10-34-0
10.22
15.16
131
446
0
131
10-50-10 (dry soluble)
10.80
16.81
100
500
100
200
Water
0.00
0.00
 
 
 
 

1EC for a sample of a particular batch of each formulation. Fertilizer products of the same analysis can vary greatly in salt levels. Always test your own solution with an EC meter.

However, the key nutrient in a starter fertilizer is phosphorus. In the table above, P2O5 ranges from 244 to 446 g in 1 L of liquid fertilizer concentrate. Some have a low EC, but they also deliver low levels of phosphorus. When we compare products based on delivering equivalent phosphorus, the picture changes substantially.

Table 2: Comparison of the EC of starter solutions delivering equivalent phosphorus to a 1.5% solution of 10-34-0
Fertilizer

EC (mS/cm)

% solution1

3-18-18
13.5
2.86%
9-18-9
11.2
2.21
6-24-6
11.8
2.21
6-24-6
13.9
2.98
10-31-4 IBA & PHCA
10.8
1.65
10-34-0
10.2
1.65
10-50-10 (dry soluble)
10.0
---

1Calculated based on measured density of a particular batch of each formulation. Density of fertilizer products can vary.

Look for a starter that will give you an adequate rate of phosphorus at a low EC and an acceptable price (notice how you need twice as much of low analysis products to deliver equivalent phosphorus to a 1.5% solution of 10-34-0). Remember, the presence of some ammonium nitrogen provides a benefit, but potassium and micronutrients are not particularly beneficial in a starter and can increase the risk of damage.

It's also important to monitor the EC of your starter solution. There are handheld EC meters that are reliable and relatively inexpensive. In-line meters that can measure continuously are also available. Having said that, it's hard to find a definitive answer on EC level is safe. There are so many factors that influence the potential for salt injury. It has been suggested to me that 4 to 5 mS/cm is a good target for field tomato transplant solutions. Grower experience is very valuable in making a decision on starter rate, so keep good records so that if you see injury, you can look back at the rates, planting conditions, product used, EC level, etc.