Interpreting Grassland P & K Soil Analysis

25 June 2024

This article is produced as a part of the FAS Crops & Soils Bulletin. Subscribe now to receive the full report in your inbox monthly.

With many farmers having utilised the funding for soil analysis through Preparing for Sustainable Farming (PSF), now is the time to act on the recommendations and begin to rectify any low values. Here we explore what to consider when choosing phosphorus (P) and potassium (K) products, calculating an application rate, and working out the optimum timing of applications for optimum soil and crop response.

A Note on pH

Maintaining the optimum pH in the topsoil is important to achieve optimum nutrient availability, yields and consistent forage quality. The target for an optimum soil pH is 6.0 to 6.2 on mineral soils, or higher at 6.5 if growing a sward with a high legume content. If soil pH is allowed to drop lower than 5.5 then grass yield potential would be expected to be limited by up to 20%.

Phosphate

Phosphate applications to established grass are primarily aimed at maintaining target P Status in the soil rather than increasing grass growth. Having said this, phosphate applications aid root development and early growth when the plants roots are not developed enough to access soil phosphate reserves, so phosphate application can be important for growth when establishing reseeds. The target P status for clover is higher than that of grass at swards due to the differing root structures between the species that allow grass to compete more effectively. The target P status is therefore 9 mg/l for clover (the middle of Moderate Status) compared to 6 mg/l for grass only swards (the lower end of Moderate Status).

Phosphate deficiency is more common in wetter and upland soils because acidic conditions limit the availability of phosphate for plant uptake. The optimum pH for phosphate availability is between 6.5 and 7.5. If conditions are too alkaline phosphate availability will also be decreased as it will react with calcium and get ‘locked up’. For this reason, it’s best not to apply phosphate within a month of a lime application to prevent the phosphate from reacting with the calcium in the lime. Soil P availability is reduced at low temperatures such as in early spring. A fresh boost of soluble P at this time can have a greater impact on growth until soil temperatures rise sufficiently to increase P-release from the soil reserve. Trials have shown that an enhanced response to phosphate applied in early spring is achieved on soils with low and very low soil status if combined with nitrogen.

Potash

The aim for potash status on grassland is within the lower half of moderate (M-). On grazed grass it is assumed that about 80% P2O5 and 95% of the K2O is recycled back to the grass. However, when the crop is removed for conservation, the offtake is far greater. If the soil is of moderate status, then maintenance applications (enough to compensate for the offtake) are required. Offtakes of potash in conserved grass can be considerable (equivalent to 180 – 330kg K2O/ha from 2 or 3 cuts totalling 10t DM/ha) and soil potash reserves can quickly become depleted, especially on light sandy soils.

For maximum efficacy it is important to balance the use of nitrogen fertilisers with potash. Typical application rates for 1st cut silage should around 60 – 90 kg K2O/ha for first cut and 50 – 60 kg for 2nd cut. Exact rates will vary with soil status, nitrogen application rates and anticipated grass yields but a useful rule of thumb is to apply two thirds of a kilogram K2O/ha for every kilogram of N applied. Where grass offtakes and soil status indicate that application rates should exceed 90 kg K2O/ha for 1st cut, any surplus above 90kg K2O/ha should be applied later in the season to prevent luxury uptake. This is where an excess of potash is taken up by the plant which prevents the plant from taking up sufficient magnesium. This can lead to hypomagnesemia/grass staggers in livestock. The best time to correct soil potash status is after silage is cut. Heavy applications of potash should also be avoided on grazed grass in the early spring for the same reason.

Organic Manures

Organic manures can be a good source of P and K and can help to meet a good proportion of crop requirement. Some typical values of the nutrient content of manures are listed below. For a more accurate representation of the nutritive value of your organic manures it can be useful to have the manures analysed. This will allow you to accurately adjust your fertiliser application rates accordingly.

Table 1: Typical dry matter (DM%) and nutrient contents of livestock manures as kg/t (solids) or kg/m²(slurries)