pH and soil nutrients - The Accidental Smallholder

What is pH?

pH is a measure of acidity and alkalinity in the soil and ranges from pH1 (pure acid) to 14 (pure alkali), with neutral being pH7.

Artificial fertilizer has an acidifying effect on soil; urine is also acid, so heavily grazed pasture will decrease in pH over time. Grass grows best at a pH of about 6 – 6.5. If your soil pH is lower than this, spreading limestone, ground chalk, calcified seaweed or basic slag (mainly to provide phosphate but its liming action is additional) will raise it.

A tonne of lime on one acre will raise the pH by about 0.3 but you shouldn’t spread more than two tonne to the acre in any year. Using calcified lime will also increase the amount of calcium in the soil and calcified seaweed will add other micronutrients to the soils as well. The pasture can continue to be grazed after the application of calcified seaweed; other liming agents have to be washed in before stock can be allowed to graze.

Soil nutrients

Grass also needs a range of nutrients to grow well. The three main ones are:

• Nitrogen (N)
• Phosphorus (P)
• Potassium (K)

All are available in bags and in various proportions – you’ll see fertiliser advertised as 10:20:20 for example, meaning that there is twice the amount of P and K than N. However, bagged fertilizer is expensive both in terms of pounds and in its impact on the environment. More farmers are now looking at other sources of nutrients to reduce their carbon footprint.

Nitrogen

Nitrogen (N) promotes green, leafy growth. Grass species like the ryegrasses have been developed to respond to high nitrogen inputs and in these circumstances, will outcompete other grasses and plants (including clover) in the sward. Of course, clover is a natural producer of soil nitrogen, which can replace artificial nitrogenous fertiliser. The use of artificial nitrogenous fertilizer is not recommended on grazing for horses and it should never be applied to mixed species grassland.

Unlike nitrogen, the action of phosphorus and potassium is slower and longer acting. Applications should only be required every five years or so. Hay meadows are most likely to need P and K, especially if there is no aftermath grazing so no manure deposition.

Phosphorus

Phosphorus (P) is required for plant tissue growth, to help the plant take up nutrients and for root development. It is usually applied from a bag as phosphate or rock phosphate but grazing animals will return phosphorus to the soil through their dung.

Potassium

Potassium (K) is required for good flower and seed development. It is usually applied from a bag as potash, rock potash or basic slag. Like phosphorus, it is found in the dung of grazing animals. Potash is most likely to diminish on light soils, especially those with a high pH.

Micro-nutrients

Plants also need other micronutrients to either grow well or to provide these micronutrients to the stock grazing the plants. These include:

• Calcium
• Magnesium
• Selenium
• Copper
• Cobalt
• Manganese
• Molybdenum

If you are removing grass as hay or silage, you will have to add fertility either as bagged fertiliser or as farmyard manure (FYM).

FYM is the traditional way to return fertility to the soil. As well as providing slow release nutrients, it also adds organic matter. Given its bulk, it is difficult to over-enrich the soil.

On semi-natural grasslands, it is recommended that no more than 20 tonnes of FYM be applied per hectare no more than one in every four years. It is best applied in spring or late summer (before the autumn flush of grass) so that grass can immediately take up the nutrients. Applied in autumn, nutrients may be leached out and contaminate watercourses.