Soil fertility is crucial when reseeding

With 50% of soil sample results coming back severely deficient, have Irish farmers stolen from their soil bank once too often and could soil fertility be the new constraint to increasing farm output and profit? I visited a Tipperary discussion group to discuss soil fertility results.

The amount of chemical phosphorus and potassium being applied on Irish farms dropped by about 40% over the last 10 years. Results from soils sampled in 2013 show that nine out of 10 samples are now deficient for lime and/or phosphorus and/or potassium.

While the Nitrates Directive has legislated for the use of nitrogen and phosphorus (P), no such limitations exist for lime or potash (K).

This indicates that for a lot of farmers the focus has shifted away from managing soil fertility.

The Tipperary group started a project early last year looking at the soil fertility status of members’ farms.

The project had two objectives: firstly, to get a baseline of where each member was in terms of soil fertility and secondly, to design and implement a plan to improve soil fertility where required. The overriding objective was to grow and utilise more grass over the coming years.

Soil samples

Table 1 shows the percentage of each member’s farm at the various P and K indices. These soil samples were taken in early 2014. Roughly one-third of farms in the group are at Index 1 and 2 for P and K, with a further third at Index 3 and the remainder at Index 4, although surprisingly, more farms are at index 4 for P than K.

As you would expect, soil type and previous farming history played a big role in the breakdown of the results, with farmers on heavier type soils having a higher proportion of their farm at indices 1 and 2.

Farms that previously had access to pig slurry tended to have far higher results for P and K. For example, farmer 3 used to have a piggery on the farm where most of the pig slurry was applied on the dairy farm land, so all his land is at index 4, despite exiting pig production a number of years ago.

On the other hand, Farmer 10 had almost half of his farm between index 1 and 2. A high proportion of this farmer’s land was previously in long-term tillage and as a result the organic matter and soil P and K levels are low.

The next step for the group was to sit down with their adviser to discuss and interpret the results and to devise a plan to improve the fields that were below target.

Soil pH is critical

The first piece of advice the group received was that soil pH is critical and should be the first to be corrected as pH regulates the availability of other nutrients. So if the soil pH is low, no amount of any other nutrient will improve growth as it won’t be made available to the plant.

The optimum pH for most soils is between 6.2 and 6.3. As can be seen in Table 2, only 34% of the total land area farmed by the group was between pH of 6 to 6.5, with a large proportion (46%) less than pH of 6.

The group was told soil pH decreases over time due to the use of chemical nitrogen and by leaching through rainfall. For every 100kg of nitrogen applied per acre, 200kg of lime is required to balance the acidity in the nitrogen. For every tonne of lime spread per acre, pH should rise by 0.2 or 0.3, but the group was advised not to spread any more than three tonnes of lime per acre in any one application.

Moving on to potassium, the adviser said that it’s a big issue on silage fields with silage taking up about 20 units of K per tonne of silage dry matter. He suggested that since the Nitrates Directive was introduced, a lot of farmers simply ignored potash and as a result many fields that are regularly cut for silage are now very low for K, especially on fields that are not getting slurry.

He recommended 100 units of K per acre for first-cut silage and 80 units of K for second-cut silage, but said that slurry is high in K and should be targeted at silage fields to reduce the amount of chemical K required.

Typically, every 1,000 gallons of cattle slurry contains seven units of nitrogen, five units of P and 28 units of K, which is worth about €25 per 1,000 gallons when compared with chemical fertilizer prices.

On phosphorus, the Nitrates Directive is clearly limiting what can be spread. However, the adviser outlined that the changes made to the directive in January 2014 meant that the allowance for chemical P has increased for nearly all farms.

The two main changes for grassland farmers were that the first 300kg of concentrates fed to dairy cows is now discounted as a source of P and the P availability in slurry is now reduced by 50% where it is spread on soils with P index of 1 and 2.

Obviously, how much P each member can spread depends on their own fertilizer plan and whether or not they are in a derogation, but the adviser suggested that where P can be spread, 50% should be spread in springtime with the remainder spread out over the summer to maintain the phosphorus levels in the grazing animals.

Case study

Part of the project is to set aside time to discuss soil fertility at every farm visit. A meeting was held recently on farmer 10’s farm. During 2014, he spread 3t of lime per acre on the 25% of his farm that was lowest in pH and 18 units of P and 98 units of K was applied on average across the farm.

This farmer took soil samples again this year but was very disappointed with the results achieved. Despite spreading more chemical P and K fertilizers, no improvement in his soil indices was observed. In fact, when looking at the mg/l figure for P, most fields actually dropped slightly.

The group made a number of suggestions. Firstly, they all acknowledged that building up soil reserves is a long-term project, especially when starting from a low level. However, they did suggest that the level of P being spread was not sufficient to cover the maintenance and the build-up requirements of the soil, particularly at high stocking rates because the nutrient offtake is greater and that if possible, more chemical P needs to be spread to balance for the P leaving the farm in milk solids and stock sales.

The group suggested that slurry should be targeted at all Index 1 and 2 fields, where the P availability in the slurry would be reduced by 50%, thereby allowing for more chemical P to be applied and remain in compliance with the Nitrates rules. It was suggested that at least 10 units of P and 30 units of K was required per acre to build up soil reserves enough to move from one index to the next.

The farmer was also encouraged to be more aggressive with his lime spreading by covering more area of the farm this year.

This group is being proactive by identifying an area of weakness on their farms and taking measures to overcome it. While the Nitrates Directive has limited the quantities of nitrogen and phosphorus that can be spread, it is not an excuse to stop managing soil fertility.

Increasing grass growth on farms involves having the right grass species with the correct soil fertility and good drainage. Reseeding older pastures, without correcting underlying soil fertility problems, will not work in the long term. As is evident on many farms throughout the country, the percentage of perennial ryegrass in the reseed falls over time and the sward is quickly dominated by poorer grass species.

As the case study shows, improving soil fertility is slow and costly but the research shows that it is paid back in extra production within the year.