Soil fertility

Correct soil fertility is critical for optimal plant growth.

There are 16 nutrients that are essential for pasture plants to grow to their potential, 13 of these are obtained from the soil. There are five nutrients N, P, K, S and Mg (pumice soils) that are added to soils in the form of fertilizer and the remaining nutrients are abundant in most New Zealand soils. Adding elements other than the 16 listed below will not enhance plant growth and some may be toxic to pasture plants and animals.

Assessing nutrient status

Capital fertiliser inputs can be much greater than the maintenance input, especially if a rapid increase in the soil nutrient status is required.

It is important to measure the current soil nutrient status to assess whether a farm is in the maintenance or development stage. Soil testing and taking into account fertiliser history is the only way to assess nutrient status.

Trial showing half a paddock with nutrient limiting.

Soil testing

A robust soil fertility monitoring program is the best way to monitor nutrients in the topsoil of any farm. Fertiliser costs are a major investment and need to be monitored. Even in the maintenance stage, more precise rates can be calculated by establishing trends in soil test levels over time through the use of annual soil testing.

Pasture testing

Pasture testing is a back up to soil testing. While soil testing determines available soil nutrient status, pasture analysis should be used to assess how much nutrient has been taken up from a fertiliser application and check trace element status.

There are two key reasons to test and analyse pasture samples

1. Concerns about animal health

• Insufficient concentrations of: Mg, Na, Cu, Co and Se,
• Excessively high concentrations of K, Mo and Mn.

2. Clover Nutrition

• Clovers have a higher requirement of all nutrients compared to grasses; they are a sensitive indicator of underlying soil fertility.

Nitrogen

• Nitrogen (N) fertiliser can be used to overcome seasonal pasture deficits by increasing pasture supply.
• Similar to any purchased input, N requires careful use to optimise economic return.
• Use appropriate applications rates and apply to pasture with some regrowth e.g. 1600-1800kg DM/ha or better.

For more information please see managing nitrogen fertiliser.

Seasonal nitrogen use

• The amount of additional pasture grown in response to the application of N is influenced by climate, with a low response from cold temperatures, waterlogged soils, and dry conditions.
• The largest and most reliable response to N is when the growth rate of pasture is greatest i.e., in mid-late spring in most regions.
• Typical pasture growth response rates during early spring range from 10 to 15 kg DM/kg N applied.
• Autumn responses are generally smaller and less reliable than those in spring, while winter responses are lowest and the risk of direct loss of fertiliser N by leaching is greatest.
• Mid-to-late summer applications of N fertiliser are not recommended where low soil moisture limits growth. However good responses can occur on irrigated farms or regions which have reliable summer rainfall.

N deficient pasture

Lime Soil Acidity

Liming is carried out to mainly overcome aluminium toxicity and increase molybdenum and phosphorus availability. On pasture, liming generally encourages more productive pasture species (legumes and ryegrasses) at the expense of low fertility species.

Phosphorus(P)

Phorphorus is a key nutrient for the successful growth of pasture and crops. Soil phosphate (P) moves slowly through soils, reducing opportunity for leaching losses.

• In a maintenance situation P can be applied at any time of the year but applying soluble P fertilisers outside the high risk months of April to October will reduce the risk of P runoff.
• If the soil test levels are low and an immediate increase in production is required, the sooner P is applied the sooner there will be benefits.
• Application rates of greater than 100 kg P/ha (1100 kg superphosphate/ha) in a single application are not recommended. If capital inputs higher than this rate are required, then the dressing should be split.
• Animals should not graze pastures where phosphate fertiliser has been applied until at least 25mm of rain has fallen. This is especially true where capital rates of P have been applied.

Sulphur(S)

Clover has a higher requirement for all nutrients including sulphur, relative to the grasses. Thus, if a soil is S deficient the clover vigor and abundance will be poor.

For pastures the normal distinction between capital S and maintenance S inputs does not apply. This is because the amounts of S to eliminate the most severe S deficiency are similar to the amounts required to maintain soil S levels.

There are two forms of S fertiliser

• Sulphate S (the form present in superphosphate) is readily available to plants and moves rapidly through the soil
• Elemental S is not immediately available to the plant (nor prone to leaching) as must first be oxidised by soil microorganisms to sulphate-S before it is plant available.

Potassium(K)

Potassium fertilisers are often referred to as ‘potash fertilisers’, a name originally given to an early form of potassium fertiliser produced by extracting a pot of wood ashes (pot ash). However, potassium is now mined from abundant mineral deposits.

Potassium is removed and lost through animal excretion in dairy sheds and farm tracks. Potassium can be easily leached especially under heavy rainfall. Potassium fertilisers are generally applied in low rates to pasture and the percentage lost by leaching is small. Potassium fertilisers applied to crops usually remains in the root system during the growing season.

• Before and during calving, avoid applying K as it can worsen cow metabolic problems.
• After calving, apply K when clover growth is increasing.
• Ryegrasses are very efficient at extracting K from the soil and can usually grow to potential without K fertiliser over the early spring period.