to answer u fancylad, that would be a deficiency.
Here is some more detailed information about some of these deficiencies which comes directly from the book... Marijuana Horticulture The indoor/outdoor medical grower's bible:
S-Sulfur Deficiency: Young leaves turn lime green to yellowish. As shortage progresses, leaves yellow interveinally and lack succulence. Veins remain green and leaf stems and petioles turn purple. Leaf tips can burn, darken, and hook downward. S deficiency resembles a N deficiency, acute S deficiency causes elongated stems that become woody at the base. S deficiency occurs indoors when the pH is too high or when there is excessive calcium present & available.
Progression is as follows=older leaves turn a pale green, leaf stems turn purple and more leaves turn pale green, entire leaves turn yellow, intervienal yellowing, acute deficiency causes more and more leaves to develop purple leaf stems and yellow leaves.
Treat: fertilize with hydroponic fertz that contain S. Lower pH to 5.5 to 6. Add inorganic S to a fertilizer that contains magnesium sulfate (Epsom salts) Organic sources of sulfur include mushroom composts and most animal manures. Make sure to apply only well-rotted manures to avoid burning roots.
Fe-Iron: I deficiencies are common with pH is above 6.5 & uncommon when the pH is below 6.5. Symptoms may appear duing rapid growth or stressful times and disappear by themselves. Young leaves are unable to draw immobile iron from older leaves, even though it is present in the soil. The first symptoms appear on the smaller leaves as veins remain green and areas between the veins turn yellow. Interveinal chlorosis starts at the opposite end of the leaf tip, the apex of the leaves attached by the petiole. Leaf edges can turn upward as the deficiency progresses. Leaves fall off in severe cases. I deficiency is sometimes traced to an excess of copper.
Progression is as follows: younger leaves and growing shoots turn pale green and progress to yellow between the veins starting at the petiole but the veins remain green. More and more leaves turn yellow and develop interveinal chlorosis. In acute cases, leaves develop necrosis and drop.
Treat: lower the soil pH to 6.5 or less. Avoid fertz that contain excessive amounts of manganese, zinc, copper which inhibit iron uptake. High levels of phosphorus complete with uptake of iron. Improve drainage, excessively wet soil holds little oxygen to spur iron intake. Damaged or rotten rooots also lower iron uptake. Increase root-zone temperature. Apply chelated iron in liquid form to root zone. Chelates are decomposed by light and must be thoroughly mixed with the growing medium to be effective. Exposing the nutrient solution to light causes depleted iron. Sterlizing the nutrient solution with UV light causes iron to precipitate. Leaves should green up in four or five days. Complete, balanced, hydroponic nutrients contain iron, and deficiencies are seldom a problem. Organic sources of iron, as well as chelates, include cow, horse, and chicken manure. Use only well-rotted manures to avoid burning plants.
P-Phosphorus: A lack of P causes stunted growth and smaller leaves; leaves turn bluish-green and blotches often appear. Stems, leaf steams (petioles), and main veins turn reddish-purple starting on the leaf's underside. Note: the reddening of the stems and the veins is not always well pronounced. The leaf tips of older leaves turn dark and curl downward. Severely affected leaves develop large purplish-black necrotic (dead) blotches. These leaves later become bronzish-purple, dry, shrivel up, contort, and drop off. Flowering is often delayed, buds are uniformally smaller, seed yeild is poor, and plants become very vulnerable to fungal and insect attack. P deficiencies are aggravated by clay, acidic, and soggy soils. Zinc is also necessary for proper utilization of P. Deficiencies are somewhat common and are often misdiagnosed, and are most common when growing medium pH is above 7 and P is unable to be absorbed properly; the soil is acidic (below 5.8 and/or there is an excess of iron and zinc, the soil has become fixated (chemically bound) with phosphates.
Treat: by lowering pH to 5.5-6.2 in the hydroponic units; 6 to 7 for clay soils; and 5.5-6.5 for potting soils so P will become available. If the soil is too acidic, and a excess of iron and zinc exists, P becomes unavailable. If you are growing in soil, mix a complete fertilizer that contains P into the growing medium before planting. Fertigate with an inorganic, complete hydroponic fertilizer that contains P. Mix in the organic nutrients-bat guano, steamed bone meal, natural phosphates, or barnyard manure, to add P to soil. Always use finely ground organic components that are readily available to the plants.
