From soil test to fertiliser rate: working out what to apply

Fertiliser is often the biggest single input cost on a crop farm, and on many smallholder farms it is the input most likely to be under-applied or applied to the wrong field. Getting the rate right means knowing three things: how much of each nutrient the crop needs for a realistic yield, how much the soil can already supply, and how much of the nutrient is in the product you are buying.

Step 1: Set a realistic yield target

The crop's nutrient needs rise with yield, so start with a yield you can realistically expect. Base it on your own records from that field over several seasons, the rainfall and soil depth, and the hybrid. Setting the target too high wastes money on fertiliser the crop cannot use when water runs short; setting it too low leaves yield on the table in a good year.

Step 2: Work out the nutrients needed

Nitrogen (N)

Soil tests are not a reliable guide to nitrogen, because it moves and changes quickly in the soil. Nitrogen rates are therefore usually based on the yield target, adjusted for the previous crop. For dryland maize, advisers in Southern Africa often work from a rule of thumb in the order of 15 to 20 kg of N per ton of expected grain, but the right figure varies with soil, rainfall and region. After a good legume crop such as soybeans or groundnuts, less nitrogen may be needed.

Phosphorus (P) and potassium (K)

These are where the soil test earns its keep. The laboratory measures how much P and K the soil can supply, and your adviser compares that with thresholds for the crop. Where the soil level is low, you apply more than the crop removes to build it up; where it is high, you may apply little or none. The thresholds depend on the extraction method the lab used (Bray 1, Olsen, Mehlich 3 and others give different numbers for the same soil), so always interpret results with guidelines made for that method.

Lime comes first

If your soil is acid, fix that before worrying about the last few kilograms of fertiliser. On acid soils, phosphorus is locked up and roots are damaged, so fertiliser gives a poor return. The soil test will show pH and acidity; your adviser will recommend a lime type and rate, and the lime needs time and moisture to react before planting.

Step 3: Understand the fertiliser label

Fertiliser grades are shown as percentages of nutrients by mass. There is an important difference across Africa in how they are written:

• In South Africa, labels normally show elemental N, P and K. A mix labelled 2:3:4 (30) contains 30 percent total nutrients in a 2:3:4 ratio.
• In much of East and West Africa and internationally, labels show N, P₂O₅ and K₂O, the oxide forms. DAP is sold as 18-46-0, meaning 18 percent N and 46 percent P₂O₅.
• To convert: P = P₂O₅ × 0.44, and K = K₂O × 0.83. So DAP's 46 percent P₂O₅ is about 20 percent elemental P.

Step 4: Convert nutrient to product

Product needed in kg per hectare = nutrient needed in kg per hectare ÷ (nutrient percentage ÷ 100).

When one product supplies two nutrients, like DAP or a compound blend, count the nitrogen it supplies and reduce the top-dressing nitrogen accordingly.

Step 5: Compare prices per kilogram of nutrient

The cheapest bag is not always the cheapest nutrient. Work out the cost per kilogram of the nutrient you are buying. A 50 kg bag of LAN at 28 percent N holds 14 kg of N; a 50 kg bag of urea at 46 percent holds 23 kg. Divide each bag price by its kilograms of N to compare. Then consider other factors: urea can lose nitrogen to the air if it is left on the surface of warm, moist soil and not worked in or rained in, and some soils or crops suit one product better than another.

Step 6: Split and place it well

• Apply phosphorus and potassium at or before planting, placed near the seed row but not in contact with the seed, as fertiliser salts can damage germinating seed.
• Split nitrogen: a smaller amount at planting and the rest as a top-dressing, commonly before the crop's fast growth stage (for maize, around knee height). This reduces losses if heavy rain leaches nitrogen early in the season.
• In a season that turns dry, you can reduce or skip the top-dressing and save the money.
• Micro-dosing, a small measured amount placed in each planting hole, can give smallholders a good return where cash for fertiliser is limited.

Key points

• Base nitrogen on a realistic yield target; base phosphorus and potassium on the soil test.
• Correct acidity with lime before fine-tuning fertiliser.
• Know whether figures are elemental or oxide, and convert if needed.
• Product rate = nutrient needed ÷ nutrient fraction; compare prices per kg of nutrient.
• Have your plan checked by an agronomist or extension officer who knows your soils.