Mark Plunkett: How soil sampling can save you up to €390/ha in fertilizer costs

One of the key changes in the review of the Nitrates Action Plan is that all arable land sown from January 1st will require up to 5ha updated soil samples every four years or adopt a soil P index of 4.

With fertilizer prices at all-time highs, recent soil test results provide a solid basis for planning fertilizer needs and are the first step in controlling costs.

Soil analysis is associated with a low cost of €1.25/ha/year for a standard soil sample compared to a kilo of phosphorus costing up to €5 and could reduce fertilizer costs from €190/ha to €390/ha on soils with reduce high fertility.

Soil P & K Index 4

A recent review of arable soil test results shows that 19% and 38% arable soil are Index 4 for P (> 10 mg/L) and K (> 150 mg/L), respectively.

Possibility to reduce/omit K applications. Take a winter wheat crop with a yield of 10 t/ha: A maintenance fertilization with K (10 x 10 kg/t) costs about 200 €/ha (100 kg K/ha x 2 €/kg).

Now is the time to identify high fertility (Index 4) soils to adjust fertilizer applications and tap into the soil’s P&K reserves, especially for winter cereal crops.

In addition, if P and K applications were reduced or eliminated in 2022, take soil samples from those fields and examine the impact of eliminating P or K applications on the soils on your farm.

In general, depending on the soil test result and soil type, soil P values ​​decrease relatively slowly.

However, soil K levels can drop more rapidly, particularly when removing high-yielding plants on light to medium soil types, while heavier soil types have greater soil K supply.

Lime & Soil pH

The soil database shows that about 40 percent of arable soil requires lime. Optimal soil pH is key to nutrient availability and efficiency for such important nutrients as N, P, K, Mg, Ca and S.

Now is a good time to apply recommended amounts of lime based on soil analysis to correct soil pH. Ground limestone is the most cost-effective way to correct and maintain soil pH over the long term.

straw incorporation

In 2022, about 52,000 ha of straw was chopped, resulting in higher retention of P and K in cereal and OSR fields.

Grain straw retains about 10 and 50 percent of the total harvest P and K, respectively. For example, a grain harvest of 10 t/ha winter wheat gives about 4 kg P and 50 kg K/ha in straw. This reduces the crop’s P&K need for the next crop and cuts cultivation costs by around €120/ha.

Where straw has been chopped in recent years it is good practice to test the soil and assess the impact on soil fertility, particularly K.

Winter crops and P&K advice

Apply P and K to soils with low fertility (P & K Index 1 and 2). Incorporate or combine seed drill 20 kg P/ha or 10 kg P/ha. This can be delivered from 0-10-20 with 100 to 200 kg/ha (0.8 to 1.6 bags/ac) at sowing time.

Soils with a P index of 1 would benefit the most from an application of fresh P at the time of sowing, especially for sensitive crops such as winter barley. Placing P will increase the availability of P in the root zone, which is required for root and tiller development in the early stages of plant cultivation.

Construction earth P and K values ​​up to soil index 3 is currently questionable. However, a recent trial by Oak Park shows that increasing soil P from index 1 to 3 increased winter wheat grain yield by 1.5 t/ha.

When fertilizer prices are high, consider alternative P&K sources such as organic fertilizers (FYM, cattle manure, mushroom compost, or milk sludge). as an inexpensive source to build up soil fertility.

Mark Plunkett is a Teagasc soil and plant nutrition specialist in Johnstown Castle Mark Plunkett: How soil sampling can save you up to €390/ha in fertilizer costs

Fry Electronics Team

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