Emerging trends show soil fertility levels are declining with the reduction in fertilizer usage in recent years. This is occurring across all farm types, according to Teagasc.
Stocking rates have been increasing on dairy farms with the impending end of milk quotas. As soil fertility at farm level is poor, not enough grass is being grown to sustain the increased stock so extra feed is being bought. Soil fertility should be rectified before the extra stock arrive on the farm.
A soil test will indicate the soil fertility levels of pH, phosphorus, P and potash, K. Soil pH is the first thing to get right. Due to the high rainfall in Ireland, it is a natural process for the soil surface to become acidic and the soil pH to drop. Optimum soil pH for grassland is at or above 6.3 except where molybdenum is an issue. The release of nutrients from the soil and the response to applied fertilisers will be reduced if the soil pH is low (or high) leading to a waste of costly inputs. Soils with low pH will tend to lock up P making it unavailable for plant uptake. There is little benefit in applying additional fertilizer to soils where the underlying problem is soil pH.
Soil test results for each nutrient are divided into one of four categories called an Index, index 1 being the lowest and 4 the highest. The long-term objective should be to have soils in Index 3 for both P and K, for optimum grass production. The fertilization rate should replace the nutrients removed in product, be that milk or meat, or in losses such as leaching in the case of K. Only approximately 30% of soils are in Index 3 for P and K.
At low soil P levels (Index 1 and Index 2), additional nutrients are required to build up reserves to index 3. This normally takes a number of years .
When soil P levels are high (Index 4) plant responses to fertiliser applications are rare. The speed with which soils will return from Index 4 to Index 3 depends on land use and soil type. Regular soil testing is essential for monitoring the soil nutrient status. At current fertiliser prices investment in P&K fertiliser is giving 150% annual return making it the single most important item to rectify.