Some plants must have markedly acid conditions, although lime-tolerant cultivars have been developed; tea (Camellia sinensis) can grow in soil as acid as рНц., but this is exceptional. In contrast, plants derived from those indigenous to chalk and limestone regions do best when the pH value exceeds 7. Soils under intensive cultivation can become sour, i.e. excessively acid, and then require liming to bring pH nearer to 7 to restore fertility. Acidity can be increased by the admixture of peat or acidic inorganic fertiliser such as sulphate of ammonia. Adjustment of a limy soil to accommodate calcifuge (lime-intolerant) subjects is difficult; if such plants must be grown in a limy district it is better to line a pit with polythene sheet and import naturally-acidic soil. The lime-tolerance of many plants can be improved by application of sequestered iron. Adjustment of soil pH must be regarded as temporary; the acidity of intensively-cropped soil should be kept under review, using a simple test kit available in garden shops. Soil Nutrients The major nutrients, nitrogen, phosphorus and potassium, and the dozen or more minor and trace nutrients can be taken up by roots only when in solution. Slow-acting fertilisers are those which dissolve slowly and therefore need only occasional replenishment; quick-acting, readily-soluble fertilisers need annual or more frequent supplementation. As described later, good drainage is essential for good growth; this means that during heavy rainfall water drains through the soil, leaching out nutrients. Detailed fertiliser recommendations are made in Chapter 34 (see pp.297 to 303). Soil Water When water falls on the soil surface it enters the spaces between particles. Exceptionally, certain materials, e.g. mica, absorb water inside individual particles, between the flat plates of which they are composed, so that the total stored is more than would be expected from the dimensions of the spaces between the particles. Micaceous growing media such as vermiculite are used for rooting cuttings which require large quantities of readily-accessible water. Water does not move downwards through soil until all the pores above the advancing front have been filled. Therefore, the more water which falls on the surface the deeper is the layer whose pores are filled to the exclusion of air. Reducing the volume of water applied merely reduces the depth of soil which becomes saturated; it is not possible by simple watering to raise the moisture content of soil to any level other than saturation. One cannot merely "dampen" a soil. After precipitation ceases, gravity continues to pull water downwards while air enters the partially-emptied pores. Eventually gravity is balanced by surface tension holding the layer of water round each soil particle. Drainage ceases and the soil has reached the equilibrium state of Field Capacity (FC)*, holding its maximum amount of water against free drainage.