Soil pH is often hypothesized to be a major factor regulating organic matter turnover and inorganic nitrogen production in agricultural soils. The aim of this study was to critically test the relationship between soil pH and rates of C and N cycling, and dissolved organic nitrogen (DON), in two long-term field experiments in which pH had been manipulated (Rothamsted silty clay loam, pH 3.5–6.8; Woburn sandy loam, pH 3.4–6.3). While alteration of pH for 37 years significantly affected crop production, it had no significant effect on total soil C and N or indigenous mineral N levels. This implies that at steady state, increased organic matter inputs to the soil are balanced by increased outputs of CO2. This is supported by the positive correlation between both plant productivity and intrinsic microbial respiration with soil pH. In addition, soil microbial biomass C and N, and nitrification were also significantly positively correlated with soil pH. Measurements of respiration following addition of urea and amino acids showed a significant decline in CO2 evolution with increasing soil acidity, whilst glucose mineralization showed no response to pH. In conclusion, it appears that changes in soil pH significantly affect soil microbial activity and the rate of soil C and N cycling. The evidence suggests that this response is partially indirect, being primarily linked to pH induced changes in net primary production and the availability of substrates. In addition, enhanced soil acidity may also act directly on the functioning of the microbial community itself.

PH regulation of carbon and nitrogen dynamics in two agricultural soils

Soil acidification is caused by a number of factors including acidic precipitation and the deposition from the atmosphere of acidifying gases or particles, such as sulphur dioxide, ammonia and nitric acid. The most important causes of soil acidification on agricultural land, however, are the application of ammonium-based fertilizers and urea, elemental S fertilizer and the growth of legumes. Acidification causes the loss of base cations, an increase in aluminium saturation and a decline in crop yields; severe acidification can cause nonreversible clay mineral dissolution and a reduction in cation exchange capacity, accompanied by structural deterioration. Soil acidity is ameliorated by applying lime or other acid-neutralizing materials. 'Liming' also reduces N2O emissions, but this is more than offset by CO 2 emissions from the lime as it neutralizes acidity. Because crop plants vary in their tolerance to acidity and plant nutrients have different optimal pH ranges, target soil pH values in the UK are set at 6.5 (5.8 in peaty soils) for cropped land and 6.0 (5.3 in peaty soils) for grassland. Agricultural lime products can be sold as 'EC Fertiliser Liming Materials' but, although vital for soil quality and agricultural production, liming tends to be strongly influenced by the economics of farming. Consequently, much less lime is being applied in the UK than required, and many arable and grassland soils are below optimum pH.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/sum.12270

Soil acidification and the importance of liming agricultural soils with particular reference to the United Kingdom.

Interaction among plant nutrients can yield antagonistic or synergistic outcomes that influence nutrient use efficiency. To provide insight on this phenomenon, peer-reviewed articles were selected ...

https://www.tandfonline.com/doi/pdf/10.1080/00103624.2017.1407429

Effects of Nutrient Antagonism and Synergism on Yield and Fertilizer Use Efficiency

Biology of Apples and Pears is a comprehensive reference book on all aspects of pomology at the organ, tree and orchard level. It provides detailed information on propagation, root and shoot growth, root stock effects, canopy development in relation to orchard design, flowering, pollination, fruit set, fruit growth, fruit quality factors and quality retention in store. It also deals with mineral nutrition, water-relations and irrigation, diseases and pests and biotechnology. The book emphasises the scientific basis of modern tree and orchard management and fruit storage. It describes key cultivar differences and their physiology and genetics and environmental effects and cultivar x environment interactions in tropical and sub-tropical as well as temperate zone conditions. It is written for fruit growers, extension workers, plant breeders, biotechnologists and storage and crop protection specialists as well as for researchers and students of pomology and horticulture.

The Biology of Apples and Pears

Samples of a sandy loam soil taken from a long-term liming experiment in southeast England were amended with solutions of metal sulfate salts. Soils with a range of pHs were amended to contain Cu, Cd, or Zn at concentrations around the maximum permissible values for these metals in agricultural land receiving sewage sludge. After a 3-year equilibration period, the microbial biomass was determined by the fumigation-extraction technique. These results were compared with data from substrate utilization patterns of microbial populations extracted by using a weak salt solution. There was no reduction in microbial biomass due to pH or metal treatment in any of the soils except the Cu treatment. Principal-component analysis of the respiration patterns in Biolog plates demonstrated effects of both pH and metal treatment on the extracted microbial population which were independent of gross biomass size. pH and soil amendments with Cu and Zn were found to reduce the metabolic potential of the extracted soil microbial population.

Biomass Carbon Measurements and Substrate Utilization Patterns of Microbial Populations from Soils Amended with Cadmium, Copper, or Zinc

Soils were analysed from two long-term liming experiments on a sandy-clay loam at Rothamsted and a loamy sand at Woburn. Plots given four levels of limestone factorially combined with phosphate and potassium fertilizers (with magnesium subplots in 1974) were cropped with beans, barley, potatoes and oats from 1963 to 1974.The smallest limestone applications (5 t CaCO3/ha) increased soil pH the following year to values predicted by lime-requirement determinations using a standard advisory method. The larger limestone applications (10 and 20 t/ha) increased pH proportionally less. Soil pH decreased after the first year with 5 t/ha in both experiments but increased at the 20 t/ha rate for 6 years in the sandy-clay loam and for 3 years in the loamy sand before starting to decline.Exchangeable calcium (soluble in N ammonium acetate) decreased at approximately linear rates in all plots of both experiments from the first year. Slopes of the regressions were smaller at low than at higher rates of liming, depending primarily on the average pH. Rates of CaCO3 losses from the surface 23 cm of soil ranged from 225 to 823 kg/ha per year at Rothamsted and from 307 to 852 kg/ha per year at Woburn.Observed rates of Ca loss were compared with an empirical relationship suggested by Gasser (1973) between annual Ca losses and soil pH under average rainfall conditions and estimates based on a model system.

Changes in soil pH and exchangeable calcium in two liming experiments on contrasting soils over 12 years

In an 8-year field experiment, potassium sulphate and to a lesser extent magnesium sulphate increased yields of all crops both when applied alone and together. Although K/Mg interactions did not affect yields they considerably affected the ratio of concentrations of these elements in the dry matter of the crops. Sodium chloride increased yields of kale but not of barley harvested at ear-emergence.Percentage yield response to potasium followed the orderPotatoes (218%) < clover = barley < sugar beet < kato; < ryegress (17%).Magnesium increased yields from 3 to 10%, most with potatoes.Changes in exchangeable magnesium in the soil reflected differences between applied magnesium and crop uptakes. Changes in exchangeable potassium were less than expected, probably because non-exchangeable potassium was released on plots without added potassium and ‘fixed’ in non-exchangeable forms on plots where much fertilizer potassium had been given.Increase in the incidence of magnesium deficiency symptoms reported recently in South. East England are attributed to the local liming materials containing only small amounts of magnesium and to less F.V.M being applied to crops than previously.

The effects of potassium and magnesium fertilizers on yield and composition of successive crops of ryegrass, clover, sugar beet, potatoes, kale and barley on sandy soil at Woburn

Chalk in the 0–9 in topsoil of Broadbalk has declined from nearly 5% in 1865 to less than 1 % in some plots in 1966, and fastest in plots given ammonium sulphate. Losses were equivalent to 5–8 cwt limestone/acre annually. The paper relates these rates to estimated anion losses, especially bicarbonate, from the plots. Af-ammonium acetate dissolved Ca and Mg from chalk particles in the soils. After correcting for this, exchangeable magnesium had increased during the first 50 years but decreased since 1914 in the F.Y.M. plot. In plots given none or 10 lb/acre of magnesium in fertilizer annually, exchangeable magnesium had changed little since 1865. Thissuggests that equilibrium was soon established between additions and losses of Mg in all except the F.Y.M. plots. Estimates of annual additions and losses of Mg from each plot show that an equilibrium is feasible without large gains from non-exchangeable soil resources. A method of calculating losses of Mg in the drainage using activity ratios and annual calcium losses was developed.

Changes in magnesium and calcium in soils of the Broadbalk wheat experiment at Rothamsted from 1865 to 1966

In two long-term liming experiments on a sandy-clay loam at Rothamsted and a loamy sand at Woburn the responses of potatoes in 1974 and oats in 1975 to cumulative dressings of superphosphate and potassium chloride applied since 1962 and magnesium sulphate applied in 1974 were measured at four pH levels within the range pH 4–pH 7.Yields of both crops were similar over the whole pH range when standard rates of NPKMg fertilizers were given. Without added P and Mg, yields of potatoes were much smaller on the more acid soils, but without K, yields were larger. A similar pattern of yield responses to P and Mg fertilizers occurred with oats but there were no differences between the small responses to K additions over the whole pH range.Analyses of potato leaves sampled at the 10% flowering stage showed differences related to the yield effects for K and Mg but not for P. Yield responses were associated with concentrations less than 4% K and 0·18% Mg in the dry matter of topmost fully mature leaves.The differences in yield response caused by liming were not associated with differences in ‘available’ nutrients in the soils (determined using standard methods).

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0021859600027222

Liming effects on the response of potatoes and oats to phosphorus, potassium and magnesium fertilizers

Ryegrass was used to remove potassium from two acid soils limed to different pH values. Most non-exchangeable potassium was removed from the unlimed soils (pH 4·5) but differences in removal between pH 5·5 and 7·0 were small. Air-drying the soils after cropping released further potassium into the exchangeable form in amounts independent of soil pH.Equilibrium potassium activity ratios (ARK) after each out declined to small constant values characteristic of the soils. A sandy soil (Woburn) initially contained less exchangeable potassium than a soil with more clay (Sawyers), but after a few crops, ARK, % K in the grass and K uptakes per cut were larger from Woburn soil, showing that non-exchangeable potassium was being released faster than in the other soil.

J. Bolton

Papers

Changes in soil pH and exchangeable calcium in two liming experiments on contrasting soils over 12 years

The effects of potassium and magnesium fertilizers on yield and composition of successive crops of ryegrass, clover, sugar beet, potatoes, kale and barley on sandy soil at Woburn

Changes in magnesium and calcium in soils of the Broadbalk wheat experiment at Rothamsted from 1865 to 1966

Liming effects on the response of potatoes and oats to phosphorus, potassium and magnesium fertilizers

The effect of soil pH on potassium intensity and release of non-exchangeable potassium to ryegrass