INFLUENCE OF N FERTILISATION ON SOIL HEALTH AND PASTURE PRODUCTION

Discipline: pastures; Keywords: soil health, fertilisation, nitrogen mineralisation, pasture yield, area differences.

Soil health is essential for sustainable agriculture and pasture production; however, there are limited studies that have directly examined the relationship between soil health, pasture productivity as well as the influence of fertilisers concurrently. Therefore, the research by the author cited below addressed the gap in the literature by examining the said relationship.   

Data was collected on 79 fields across 8 irrigated farms in the Eastern Cape. These farms are in the Tsitsikamma (Area 1) and Cradock (Area 2) areas, respectively. The fields were measured for changes in the soil fertility and biological indicators, nitrogen fertiliser management as well as pasture growth for four years between 2016 and 2019. Phosphorus, potassium, and lime applications were also noted as explanatory factors to yield response and soil fertility changes. The fields were planted with multispecies mixtures of lucerne (Medicago sativa L.), ryegrass (Lolium perenne), grazing chicory (Cichorium intybus), red and or white clover (Trifolium pratense or T. repens). Descriptive statistics were calculated for soil, fertiliser, and pasture yield data for samples of both the Tsitsikamma and Cradock combined, as well as separately for the two areas. Analysis of variance was used to determine whether fields differed in years and between the two areas.

The two areas were found to be significantly different, and differences were observed between years as well. The observed differences in Area 1 include fertility indicators of concentration and base saturation of exchangeable cations (potassium, calcium, magnesium, sodium), phosphorus Bray 1, sulphur, as well as pH (KCl). The biological indicators that showed significant differences included total carbon %, total nitrogen %, C/N ratio, active carbon, inorganic ammonium, and nitrate as well as the percentage of nitrogen mineralised. It was interesting to note that, nitrogen mineralised decreased over the years in area one when compared to the increased total nitrogen in the soil. The negative correlation between the two indicators suggests that farmers must pay caution to nitrogen imports to the soil as their accumulation would affect the quantity of nitrogen mineralised. The observed differences in fertiliser applied included nitrogen, potassium, and phosphorus. The response variables were pasture yield and nitrogen use efficiency and also differed significantly. 

The differential responses were fewer in Area 2 compared to Area 1. The soil fertility indicators that differed significantly comprised pH (KCl), exchangeable sodium and potassium both in concentration and percentage, exchangeable calcium percentage as well as sulphur concentrations. Interestingly, phosphorus Bray 1 showed no statistical significance as opposed to Area 1. The biological indicators that showed significant differences consist of total carbon %, total nitrogen, C/N ratio, active carbon, inorganic ammonium, and nitrate as well as potentially mineralizable nitrogen. As opposed to Area 1, the percent of nitrogen mineralised were not different between years in Area 2, however, the quantity of nitrogen mineralised was different. The mineralisation rates in Area 2 increased over the years as opposed to declining rates that were observed in Area 1.

Area 2 had a lower average nitrogen application rate (152 ± 95.5 kg/ha/year) compared to Area 1 (248 ± 92.7). Although Area 2 had applied more nitrogen it produced fewer tons of pasture (17.0 ± 4.89 ton/ha) compared to (17.1 ± 5.56) produced with less nitrogen applied. As a result, the nitrogen use efficiency of Area 2 is far superior to that of Area 1, i.e. 11kg N/ton versus 17kg N/ton of pasture produced. These findings suggest that high nitrogen fertiliser application rates do not equate to increased yields. Furthermore, the findings indicate that there is a point of diminishing returns with regards to pasture yield response to nitrogen fertiliser applied.

The amount of total nitrogen in Area 2 declined over the years whereas in Area 1 it increased. Apart from the higher rates of nitrogen applied in Area 1, the other reason for the increase in total nitrogen can be attributed to the increase in the feed fed to dairy animals triggered by the decline in yield between year 3 and 4. The feed fed at the parlour was then deposited in the soil by the animals as manure while grazing. In contrast, the yield in Area 2 was increasing, and thus fewer imports of feed were made resulting in less nitrogen being brought to the farm, which would eventually end up in the soil. 

This study found that total nitrogen in the soil and nitrogen mineralisation has an inverse relationship. The soil dynamics that trigger this relationship could not be well understood. Further research needs to be done to assess the relationship between the two variables and further evaluate what other soil variables explain the relationship observed. 

Recommendations: Dairy farmers need to use nitrogen fertiliser as a management tool and avoid triggering the excessive effect of diminishing returns which may result in lower yield. Farmers need to observe the soil indicators related to nitrogen such as nitrogen mineralisation, total nitrogen, total carbon, active carbon, and C/N ratio to determine the optimal nitrogen fertiliser application rate.

Reference:

Swanepoel, P.A., 2021. Influence of nitrogen fertilisation on soil health and pasture yield.

Quarter 1 Popular Report of Project to Milk SA, Pretoria.