Farm Ireland

Tuesday 26 September 2017

Don't underestimate the value of soil tests

Growing as much grass as possible is the best opportunity to reduce on-farm costs this year
Growing as much grass as possible is the best opportunity to reduce on-farm costs this year

Pat Minnock pminnock@

Talking with dairy farmers over the last month they agree that growing as much grass as possible is the best opportunity to reduce on-farm costs this year. Fertile soils are critical to optimise grass growth on your farm. Dr Stan Lawlor presented an excellent paper on soil fertility at the recent Positive Farmers Conference.

Fertiliser costs account for 20pc of variable costs on dairy farms. This is good value for money, but only if used correctly. It is vital that fertiliser is managed efficiently, allowing maximum return in grass growth and milk production.

Teagasc recently published a report showing a national and regional breakdown of soil fertility trends. While some stabilisation of soil fertility levels has occurred since 2012, only 11pc of soils tested achieved good overall fertility in 2014.

This means that almost 90pc of soils are deficient in at least one of the three critical criteria that contribute to soil fertility and optimal grass production. These are soil pH, phosphorus (P) and potassium (K). Where any of these are deficient you are not getting the full benefit from applying the other nutrients.

Therefore, applying additional nitrogen where soil pH, P or K are low will give a lower than expected return on your investment.

Soil sampling

The levels of pH, P and K in your soil are relatively easy to test for at a cost of about €25 per sample. Soil samples should be taken over five to 10ac every three to five years. These are essential when planning the fertiliser and lime applications on your farm for the year. This is particularly true in the case of lime and K, which are not restricted by the Nitrates regulations. Use your fertiliser plan to decide on your fertiliser purchases for the coming year.


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Soil pH is the first thing to correct. Only 35pc of dairy samples nationally are at the optimum pH of 6.3 for grassland. Due to the relatively high rainfall in Ireland it is a natural process for the soil surface to become acidic and the pH to drop. Regular application of lime is required to counteract this.

Both the chemical availability and the natural recycling in soil organic matter of many nutrients are reduced in soils with low pH. For example in the case of P, soils with a low pH tend to lock up P making it unavailable for plant uptake. Applying additional P in this case is poor value for money. The low pH means that the potential of the soil to release P is not fully realised and some of the P in the fertiliser applied is locked up.

A recent experiment showed that soil test P levels after 12 months were over twice as high where fertiliser P and lime were applied together compared to when P was applied on its own. The same experiment also showed that lime application had a positive effect on soil test P even where no P was applied.

Optimising pH can be worth as much as 60 units/ac of N fertiliser. Lime is a cheap input relative to the cost of fertilisers. Spread the cost by liming a portion of the farm annually over four to five years to maintain the optimum soil pH.

P and K

Soil test results for P and K are classified in an index system, with Index 1 being very low and index 4 being high. Index 3 is the target level required for optimum grass production. On soils of index 3 the fertiliser applied should replace the nutrients removed in meat and milk in the year. Where soils are low in P or K additional fertiliser will be required to build the soil reserves to Index 3.

Long-term studies have shown that there are gains of as much as 1.5t/ha per year of grass dry matter production to be gained by improving soil P from index 1 to index 3.

The fact that most of this increase in grass yield will come in the spring when grass dry matter is most valuable makes it even more attractive. Similar responses have been shown in the case of K.

Slurry is a valuable source of nutrients so target those fields which have the highest requirement for P and K. Every 1,000 gallons of thick slurry spread per acre will give roughly 6 units of N, 5 units of P and 30 units of K.

Nora O'Donovan is a Teagasc business and technology advisor based in Tralee, Co Kerry.

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