11 Aug 2022

Get your soil tested

A VERY informative event on soil fertility was held by Teagasc in Kildalton College last week. Four important steps in achieving and maintaining good soil fertility were explained by Teagasc advisers and researchers.

A VERY informative event on soil fertility was held by Teagasc in Kildalton College last week. Four important steps in achieving and maintaining good soil fertility were explained by Teagasc advisers and researchers.

Soil Testing - Have soil samples taken for the whole farm. Unless you know what is in the soil, it is impossible to know how much extra it needs. Therefore, by taking soil analysis and putting the results into practice, the fertilizer programme can be tailored to the needs of the soil and the crop. Repeating soil analysis over time is also critical to monitor the effectiveness of the fertilizer strategy.

Taking a sample for standard pH, lime, P and K analysis for every 4 ha of the farm every 5 years will cost approximately €1.25 per hectare per year. This annual cost is equivalent to less than the cost of 1 kg of P in fertilizer.

Having very low soil fertility and not identifying it with soil tests will restrict yields on the farm. Similarly, not identifying soils high in nutrients can result in excess applications that are not required by the crop and an unnecessary expense.

Soil pH and Liming - Apply lime as required to increase soil pH up to target pH for the crop. Maintaining the soil pH at the optimum level will increase the microbiological activity of the soil, and result in better soil nutrient recycling and release. Soil pH is also critical for maximising the availability of nutrients applied in organic and chemical fertilizers.

Soil pH should be the first thing to get right if soil test results show a lime requirement. Lime should be applied to neutralise acidity and raise the pH. For mineral soils, a pH of 6.3 is recommended for grassland, and 6.5 for cereals. Acid soils will result in reduced nutrient release from soil, and poorer response to fertilizers. Apply lime as a priority as per the lime advice.

Getting best value from slurry - Organic fertilizers are a valuable source of N, P and K. Cattle slurry is by far the most common form of organic fertilizer applied in Ireland. Over the last number of years there has been a major drive on Irish farms to improve the utilization of slurry and reduce fertilizer costs. This has been achieved on farm by better facilities for the management of slurry during the winter and more slurry being applied on grassland in early spring when N utilisation is highest.

One of the biggest problems with using slurry within a nutrient management plan is the variability in the dry matter (DM) and nutrient contents. Up to tenfold variation has been found in the DM, and total N P and K contents of different cattle slurry samples. Although factors such as animal type, production system and diet will affect the slurry, the variability is mainly attributable to varying DM content caused by the dilution of slurry with water from dairy parlour washings or rainfall collected on open yards. Where slurry is diluted with water, the nutrient content will be lower. The degree of slurry dilution with water sources can be used as a guide to estimate the nutrient content of slurry. On average, slurry in Ireland is approximately 7% DM. However, the variation in slurries, even between two tanks in the same shed or yard, can be quite considerable.

Estimating nutrient content

To make best use of any organic manure it is important to know the actual nutrient content (N, P and K). Laboratory analysis is the most accurate way to determine the dry matter and nutrient content of the slurry. However, the farmer needs to know the nutrient content of the slurry on the day of application. This can be difficult as a representative sample of slurry, usually only obtained after agitation, is needed well in advance of the day of spreading in order to have results back from the laboratory. Since cattle slurry is generally agitated and spread on the same day, the usefulness of laboratory analysis when making decisions about applications is limited. However, occasional analysis of slurry, even if not every year, can still be a good guide to the nutrient content that one might expect from similar animals on similar diets in the same sheds.

In order to get more rapid information on the day of application, there are a number of on-farm measurement tools to estimate the nutrient content of slurry. The slurry hydrometer is the simplest and most effective on-farm tool which estimates the slurry DM content. Since, the DM content of the slurry is closely related to the N, P and K content of the slurry, this can be a very useful tool to estimate nutrient contents in slurry easily and cheaply on the day of slurry application. This will allow adjustment of slurry application rates based on the slurry nutrient content.

The extent to which slurry can vary in its fertilizer value based on dilution can have a big impact on the nutrient application rate. For example silage ground receiving 33m3/ha (3000 gals/acre) of slurry at 7% DM will supply 20 kg (16 units) of P and 142 kg (114 units) of K by comparison, 4% DM slurry would supply 13 kg (10 units) of P and 83 kg (66 units) of K. Incorrect estimation of nutrient value based on average values can lead to significant under- or over-supply of nutrients. Therefore a tool like the hydrometer that can help estimate nutrient content quickly and cheaply on farm would be beneficial.

Organic fertilizers on tillage crops

It is even more important where organic fertilizers are applied to tillage crops that the first load is of similar nutrient content to the last load. Organic fertilizers need to be applied evenly and at a consistent rate to ensure consistent nutrient supply across the field. Immediate incorporation of slurry will significantly increase N recovery. On tillage farms organic fertilizers should also be targeted to fields that are the longest in tillage, particularly those with low organic matter contents. As well as supplying N, P and K, organic fertilizers also supply organic matter along with a range of secondary and micro nutrients such as magnesium, sulphur, manganese, copper and zinc.

Providing a balanced nutrient supply - The final step in the soil fertility management programme is to select a fertilizer that will deliver sufficient N, P and K in a cost effective way. Nutrients need to be applied in the correct balance. Over-supplying one nutrient will be money wasted if the output is being limited by another nutrient that is in short supply. The fertilizer products selected should complement the remaining N, P and K required after soil test results, production potential and earlier organic and chemical fertilizer applications have been accounted for. On grassland farms, nutrient advice has to be adjusted for the nutrients supplied in slurry and the P coming onto the farm in concentrate feeds. On many intensively stocked livestock farms, chemical P rates allowed are either very low or are not permitted at all. In this case, the main source of P on the farm is cattle slurry. It is therefore critical to target slurry applications to fields with a P requirement. The reduction in P fertilizer application rates has resulted in a reduction in K fertilizer, as P and K are usually applied together in NPK or PK compounds. This has resulted in an increase in the application of straight N products as the main fertilizer, potentially neglecting the K requirements on the farm. Potassium has a key role in the efficient use of N. Where K is limiting, it will reduce grass yield potential and reduce N efficiency.

When choosing a fertilizer product for grassland, consider the ratio of P and K required based on the stocking rate and system. Grazing systems typically require P and K in the ratio of between 1:2 and 1:4. By contrast, silage has a P and K requirement closer to 1:6. Therefore, the P:K ratio in the fertilizer should reflect the requirements of the field. This ratio will change depending on the soil index, concentrate feed usage, or where slurry is applied. It may also be appropriate to use a straight K or NK product where P is not required. Similarly, a straight P or NP product should be used where no K is required.

Important Events

One 2 One Forestry Clinics

Are you seeking independent advice and up to date information on planting and managing forestry? Then attend this free clinic in your local Teagasc office and avail of a one to one consultation with an experienced Teagasc forestry adviser. Benefits of Planting Trees on the Farm. Forestry Figures: financial assessment of forest establishment / management. Preparation for Thinning. A clinic will be held in the Teagasc Office, Kells Road, Kilkenny on Thursday February 23. Telephone 056 7721153 for an appointment.

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