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The rate of photosynthesis is primarily limited by light level and carbon uptake. However, resource availability and competition can also limit tree growth. Trees need water and nutrients in order to grow. Specifically, trees need nitrogen and phosphorus in large amounts. These nutrients are often needed in greater quantities than available in soil. Nitrogen, which is needed for growth, reproduction, and photosynthesis, exists in organic and inorganic forms. Trees can only uptake two major inorganic forms, nitrate and ammonium. Therefore, trees have to rely on nitrogen-fixing organisms in order to obtain the large quanity of nitrogen necessary for survival. Likewise, phosphorus can exist in organic and inorganic forms and is not always available for uptake by the tree. Phosphorus binds tightlyto particles in the soil that make it hard for the trees to access, so trees also have to rely on water flow in order to receive most of their phosphorus. They can also get it from microbial activity or decomposition of microbes or other organisms. In addition to nutrients, soil composition effects the amount of water a tree can access.  Large soil particles hold a greater amount of water than coarse soils. Since nitrogen is taken up by roots mainly through bulk transport, particle size can limit the amount of water and minerals taken up by trees. Soil also effects tree growth through cation exchange capacity. A soil that has a large cation exchange capacity is able to attract and hold ammonium, reducing the amount of nitrogen leached from the soil.